| 1. |
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| 2. |
- Ahlman, Daniel, 1978-
(author)
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A study of turbulence and scalar mixing in a wall-jet using direct numerical simulation
- 2006
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Licentiate thesis (other academic/artistic)abstract
- Direct numerical simulation is used to study the dynamics and mixing in a turbulent plane wall-jet. The investigation is undertaken in order to extend the knowledge base of the influence of the wall on turbulent dynamics and mixing. The mixing statistics produced can also be used to evaluate and develop models for mixing and combustion. In order to perform the simulations, a numerical code was developed. The code employs compact finite difference schemes, of high order, for spatial integration, and a low-storage Runge-Kutta method for the temporal integration. In the simulations performed the inlet based Reynolds and Mach numbers of the wall jet were Re = 2000 and M=0.5, respectively. Above the jet a constant coflow of 10% of the inlet jet velocity was applied. A passive scalar was added at the inlet of the jet, in a non-premixed manner, enabling an investigation of the wall-jet mixing as well as the dynamics. The mean development and the respective self-similarity of the inner and outer shear layers were studied. Comparisons of properties in the shear layers of different character were performed by applying inner and outer scaling. The characteristics of the wall-jet was compared to what has been observed in other canonical shear flows. In the inner part of the jet, 0 ≤ y+ ≤ 13, the wall-jet was found to closely resemble a zero pressure gradient boundary layer. The outer layer was found to resemble a free plane jet. The downstream growth rate of the scalar was approximately equal to that of the streamwise velocity, in terms of the growth rate of the half-width. The scalar fluxes in the streamwise and wall-normal direction were found to be of comparable magnitude.
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| 3. |
- Ahlman, Daniel, 1978-
(author)
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Numerical studies of turbulent wall-jets for mixing and combustion applications
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Direct numerical simulation is used to study turbulent plane wall-jets. The investigation is aimed at studying dynamics, mixing and reactions in wall bounded flows. The produced mixing statistics can be used to evaluate and develop models for mixing and combustion. An aim has also been to develop a simulation method that can be extended to simulate realistic combustion including significant heat release. The numerical code used in the simulations employs a high order compact finite difference scheme for spatial integration, and a low-storage Runge-Kutta method for the temporal integration. In the simulations the inlet based Reynolds and Mach numbers of the wall-jet are Re = 2000 and M=0.5 respectively, and above the jet a constant coflow of 10% of the inlet jet velocity is applied. The development of an isothermal wall-jet including passive scalar mixing is studied and the characteristics of the wall-jet are compared to observations of other canonical shear flows. In the near-wall region the jet resembles a zero pressure gradient boundary layer, while in the outer layer it resembles a plane jet. The scalar fluxes in the streamwise and wall-normal direction are of comparable magnitude. In order to study effects of density differences, two non-isothermal wall-jets are simulated and compared to the isothermal jet results. In the non-isothermal cases the jet is either warm and propagating in a cold surrounding or vice versa. The turbulence structures and the range of scales are affected by the density variation. The warm jet contains the largest range of scales and the cold the smallest. The differences can be explained by the varying friction Reynolds number. Conventional wall scaling fails due to the varying density. An improved collapse in the inner layer can be achieved by applying a semi-local scaling. The turbulent Schmidt and Prandtl number vary significantly only in the near-wall layer and in a small region below the jet center. A wall-jet including a single reaction between a fuel and an oxidizer is also simulated. The reactants are injected separately at the inlet and the reaction time scale is of the same order as the convection time scale and independent of the temperature. The reaction occurs in thin reaction zones convoluted by high intensity velocity fluctuations.
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| 4. |
- Ahlström, Anders, 1973-
(author)
-
Aerolastic simulation of wind turbine dynamics
- 2005
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Doctoral thesis (other academic/artistic)abstract
- The work in this thesis deals with the development of an aeroelastic simulation tool for horizontal axis wind turbine applications. Horizontal axis wind turbines can experience significant time varying aerodynamic loads, potentially causing adverse effects on structures, mechanical components, and power production. The needs for computational and experimental procedures for investigating aeroelastic stability and dynamic response have increased as wind turbines become lighter and more flexible. A finite element model for simulation of the dynamic response of horizontal axis wind turbines has been developed. The developed model uses the commercial finite element system MSC.Marc, focused on nonlinear design and analysis, to predict the structural response. The aerodynamic model, used to transform the wind flow field to loads on the blades, is a Blade-Element/Momentum model. The aerodynamic code is developed by The Swedish Defence Research Agency (FOI, previously named FFA) and is a state-of-the-art code incorporating a number of extensions to the Blade-Element/Momentum formulation. The software SOSIS-W, developed by Teknikgruppen AB was used to generate wind time series for modelling different wind conditions. The method is general, and different configurations of the structural model and various type of wind conditions can be simulated. The model is primarily intended for use as a research tool when influences of specific dynamic effects are investigated. Verification results are presented and discussed for an extensively tested Danwin 180 kW stall-controlled wind turbine. Code predictions of mechanical loads, fatigue and spectral properties, obtained at different conditions, have been compared with measurements. A comparison is also made between measured and calculated loads for the Tjæreborg 2 MW wind turbine during emergency braking of the rotor. The simulated results correspond well to measured data.
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| 5. |
- Ahlström, Anders
(author)
-
Simulating Dynamical Behaviour of Wind Power Structures
- 2002
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Licentiate thesis (other academic/artistic)abstract
- The workin this thesis deals with the development of anaeroelastic simulation tool for horizontal axis wind turbineapplications.Horizontal axiswind turbines can experience significanttime varying aerodynamic loads, potentially causing adverseeffects on structures, mechanical components, and powerproduction. The need of computational and experimentalprocedures for investigating aeroelastic stability and dynamicresponse have increased as wind turbines become lighter andmore flexible.A finite element model for simulation of the dynamicresponse of horizontal axis wind turbines has been developed.The simulations are performed using the commercial finiteelement software SOLVIA, which is a program developed forgeneral analyses, linear as well as non-linear, static as wellas dynamic. The aerodynamic model, used to transform the windflow field to loads on the blades, is a Blade- Element/Momentummodel. The aerodynamic code is developed by FFA (TheAeronautical Research Institute of Sweden) and is astate-of-the-art code incorporating a number of extensions tothe Blade-Element/Momentum formulation. SOSIS-W, developed byTeknikgruppen AB was used to develop wind time series formodelling different wind conditions.The model is rather general, and different configurations ofthe structural model and various type of wind conditions couldeasily be simulated. The model is primarily intended for use asa research tool when influences of specific dynamic effects areinvestigated.Simulation results for the three-bladed wind turbine Danwin180 kW are presented as a verification example.Keywords:aeroelastic modelling, rotor aerodynamics,structural dynamics, wind turbine, AERFORCE, SOSIS-W,SOLVIA
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| 6. |
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| 7. |
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| 8. |
- Andrén, Peter
(author)
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Development and results of the Swedish road deflection tester
- 2006
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Licentiate thesis (other academic/artistic)abstract
- A project to construct a high-speed road deflection tester was initiated in the 1991. A mid-sized truck was used as a carrier for the first prototype. The results were promising and it was decided to build a full-size truck system. The new vehicle, based on a Scania R143 ML, was completed in 1997. The Road Deflection Tester (RDT) is equipped with two arrays of twenty noncontact laser sensors that collects transversal surface profiles at normal traffic speeds. One profile, placed between the wheel axles, constitutes an unloaded case. The other profile, just behind the rear axle of the vehicle, constitutes the loaded case. By subtracting the front cross profile from the corresponding rear one, the "deflection profile" is assessed. The deflection is assumed to vary with the stiffness of the road. In order to produce a large load on the rear wheels the engine was mounted in the back of the vehicle, slightly behind the rear axle. In testing mode the rear axle force is approximately 112 kN, and the front axle force is about 30 kN. An incremental wheel pulse transducer, two force transducers and two accelerometers, an optical speedometer and a gyroscope are also mounted on the RDT. The first test programme was carried out in 1998. Due to the careful choice of test sections, data from these sections still produce the best results. A smaller test programme was carried out in 2001, and a larger one in 2002 when the RDT was taken to England and France for demonstration. Promising results, both on an aggregated scale and for individual test sections, have been obtained. The RDT compares favourably with the Falling Weight Deflectometer. Short histories of road construction and road research give some historical and cultural background to the more recent developments. A more comprehensive history of rolling deflectographs presents all devices found in the literature from the start in the mid-fifties when the California Traveling Deflectograph and Lacroix Deflectograph were constructed, to the latest laser based High-Speed Deflectograph. Many references are given for further reading. The data acquisition hardware on the RDT system consist of sensors, signal converters, signal processing cards, an industrial computer for data communication, and an ordinary PC for operating the equipment and data storage. The software used to evaluate the data is written entirely in Matlab. Many levels of pre-processing make evaluation relatively fast, and the modularised design makes it easy to implement new evaluation algorithms in a clean and efficient way. A literature survey on the deformations of solids under static and moving load is presented in Appendix A. The static case started with Boussinesq in 1885, was much developed in the sixties, but since the eighties only a very limited amount of new results have been published. The moving load case, on the other hand, is still an field of active research and development.
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| 9. |
- Angele, Kristian, 1971-
(author)
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Experimental studies of turbulent boundary layer separation and control
- 2003
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Doctoral thesis (other academic/artistic)abstract
- The object ofthe present work is to experimentally study thecase ofa turbulent boundary layer subjected to an AdversePressure Gradient (APG) with separation and reattachment. Thisconstitutes a good test case for advanced turbulence modeling.The work consists ofde sign of a wind-tunnel setup, developmentofP article Image Velocimetry (PIV) measurements and evaluationtechniques for boundary layer flows, investigations ofs calingofb oundary layers with APG and separation and studies oftheturbulence structure ofthe separating boundary layer withcontrol by means ofs treamwise vortices. The accuracy ofP IV isinvestigated in the near-wall region ofa zero pressure-gradientturbulent boundary layer at high Reynolds number. It is shownthat, by careful design oft he experiment and correctly appliedvalidation criteria, PIV is a serious alternative toconventional techniques for well-resolved accurate turbulencemeasurements. The results from peak-locking simulationsconstitute useful guide-lines for the effect on the turbulencestatistics. Its symptoms are identified and criteria for whenthis needs to be considered are presented. Different velocityscalings are tested against the new data base on a separatingAPG boundary layer. It is shown that a velocity scale relatedto the local pressure gradient gives similarity not only forthe mean velocity but also to some extent for the Reynoldsshear-stress. Another velocity scale, which is claimed to berelated to the maximum Reynolds shear-stress, gives the samedegree of similarity which connects the two scalings. However,profile similarity achieved within an experiment is notuniversal and this flow is obviously governed by parameterswhich are still not accounted for. Turbulent boundary layerseparation control by means ofs treamwise vortices isinvestigated. The instantaneous interaction between thevortices and the boundary layer and the change in the boundarylayer and turbulence structure is presented. The vortices aregrowing with the boundary layer and the maximum vorticity isdecreased as the circulation is conserved. The vortices arenon-stationary and subjected to vortex stretching. Themovements contribute to large levels ofthe Reynolds stresses.Initially non-equidistant vortices become and remainequidistant and are con- fined to the boundary layer. Theamount ofi nitial streamwise circulationwas found to be acrucial parameter for successful separation control whereas thevortex generator position and size is ofseco ndary importance.At symmetry planes the turbulence is relaxed to a nearisotropic state and the turbulence kinetic energy is decreasedcompared to the case without vortices.Keywords:Turbulence, Boundary layer, Separation,Adverse Pressure Gradient (APG), PIV, control, streamwisevortices, velocity scaling.
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| 10. |
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| 11. |
- Appelquist, Ellinor, 1985-
(author)
-
Direct numerical simulations of the rotating-disk boundary-layer flow
- 2014
-
Licentiate thesis (other academic/artistic)abstract
- This thesis deals with the instabilities of the incompressible boundary-layer flow that is induced by a disk rotating in otherwise still fluid. The results presented are mostly limited to linear instabilities derived from direct numerical simulations (DNS) but with the objective that further work will focus on the nonlinear regime, providing greater insights into the transition route to turbulence.The numerical code Nek5000 has been chosen for the DNS using a spectral-element method in an effort to reduce spurious effects from low-order discretizations. Large-scale parallel simulations have been used to obtain the present results.The known similarity solution of the Navier–Stokes equation for the rotating-disk flow, also called the von Karman flow, is investigated and can be reproduced with good accuracy by the DNS. With the addition of small roughnesses on the disk surface, convective instabilities appear and data from the DNS are analysed and compared with experimental and theoretical data. A theoretical analysis is also presented using a local linear-stability approach, where two stability solvers have been developedbased on earlier work. A good correspondence between DNS and theory is found and the DNS results are found to explain well the behaviour of the experimental boundary layer within the range of Reynolds numbers for small amplitude (linear) disturbances. The comparison between the DNS and experimental results, presented for the first time here, shows that the DNS allows (for large azimuthal domains) a range of unstable azimuthal wavenumbers β to exist simultaneously with the dominantβ varying, which is not accounted for in local theory, where β is usually fixed for each Reynolds number at which the stability analysis is applied.Furthermore, the linear impulse response of the rotating-disk boundary layer is investigated using DNS. The local response is known to be absolutely unstable. The global response is found to be stable if the edge of the disk is assumed to be at infinity, and unstable if the domain is finite and the edge of the domain is placed such that there is a large enough pocket region for the absolute instability to develop. The global frequency of the flow is found to be determined by the edge Reynolds number.
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| 12. |
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| 13. |
- Appelquist, Ellinor, 1985-
(author)
-
The rotating-disk boundary-layer flow studied through numerical simulations
- 2017
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Doctoral thesis (other academic/artistic)abstract
- This thesis deals with the instabilities of the incompressible boundary-layer flow thatis induced by a disk rotating in otherwise still fluid. The results presented include bothwork in the linear and nonlinear regime and are derived from direct numerical sim-ulations (DNS). Comparisons are made both to theoretical and experimental resultsproviding new insights into the transition route to turbulence. The simulation codeNek5000 has been chosen for the DNS using a spectral-element method (SEM) witha high-order discretization, and the results were obtained through large-scale paral-lel simulations. The known similarity solution of the Navier–Stokes equations for therotating-disk flow, also called the von K ́arm ́an rotating-disk flow, is reproduced by theDNS. With the addition of modelled small simulated roughnesses on the disk surface,convective instabilities appear and data from the linear region in the DNS are anal-ysed and compared with experimental and theoretical data, all corresponding verywell. A theoretical analysis is also presented using a local linear-stability approach,where two stability solvers have been developed based on earlier work. Furthermore,the impulse response of the rotating-disk boundary layer is investigated using DNS.The local response is known to be absolutely unstable and the global response, onthe contrary, is stable if the edge of the disk is assumed to be at radius infinity. Herecomparisons with a finite domain using various boundary conditions give a globalbehaviour that can be both linearly stable and unstable, however always nonlinearlyunstable. The global frequency of the flow is found to be determined by the Rey-nolds number at the confinement of the domain, either by the edge (linear case) or bythe turbulence appearance (nonlinear case). Moreover, secondary instabilities on topof the convective instabilities induced by roughness elements were investigated andfound to be globally unstable. This behaviour agrees well with the experimental flowand acts at a smaller radial distance than the primary global instability. The sharpline corresponding to transition to turbulence seen in experiments of the rotating diskcan thus be explained by the secondary global instability. Finally, turbulence datawere compared with experiments and investigated thoroughly.
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| 14. |
- Ashwear, Nasseradeen, 1968-
(author)
-
Vibration-based Assessment of Tensegrity Structures
- 2016
-
Doctoral thesis (other academic/artistic)abstract
- Vibration structural health monitoring (VHM) uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The whole function, expressed by stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.One of the aims of the thesis was to improve the current models for resonance frequency simulation of tensegrities. This has been achieved by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness.The environmental temperature effects on vibration properties of tensegrity structures have been also investigated. Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies coming from temperature changes could of several magnitude as those from damage.Coinciding natural frequencies and low stiffness are known issues of tensegrity structures. The former can be an obstacle in VHM, while the later normally limits their uses in real engineering applications. It has been shown that the optimum self-stress vector of tensegrity structures can be chosen such that their lowest natural frequency is high, and separated from others.The environmental temperature effects on vibration properties of tensegrity structures were revisited to find a solution such that the natural frequencies of the tensegrity structures are not strongly affected by the changes in the environmental temperature. An asymmetric self-stress vector can be chosen so that the criterion is fulfilled as well as possible. The level of pre-stress can also be regulated to achieve the solution. The last part of this thesis, services as a summary of the work.
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| 15. |
- Ashwear, Nasseradeen, 1968-
(author)
-
Vibration Frequencies as Status Indicators for Tensegrity Structures
- 2014
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Licentiate thesis (other academic/artistic)abstract
- Applications of vibration structural health monitoring (VHM) techniques are increasing rapidly. This is because of the advances in sensors and instrumentation during the last decades. VHM uses the vibration properties to evaluate many civil structures during the design steps, building steps and service life.The stiffness and frequencies of tensegrity structures are primarily related to the level of pre-stress. The present work investigates the possibilities to use this relation in designing, constructing and evaluating the tensegrity structures.The first part of the present work studies the improvement of current models for resonance frequency simulation of tensegrities by introducing the bending behaviour of all components, and by a one-way coupling between the axial force and the stiffness. From this, both local and global vibration modes are obtained. The resonance frequencies are seen as non-linearly dependent on the pre-stress level in the structure, thereby giving a basis for diagnosis of structural conditions from measured frequencies. The new aspects of tensegrity simulations are shown for simple, plane structures but the basic methods are easily used also for more complex structures.In the second part, the environmental temperature effects on vibration properties of tensegrity structures have been investigated, considering primarily seasonal temperature differences (uniform temperature differences). Changes in dynamic characteristics due to temperature variations were compared with the changes due to decreasing pre-tension in one of the cables. In general, it is shown that the change in structural frequencies made by temperature changes could be equivalent to the change made by damage (slacking). Different combinations of materials used and boundary conditions are also investigated. These are shown to have a significant impact on the pre-stress level and the natural frequencies of the tensegrity structures when the environment temperature is changed.
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| 16. |
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| 17. |
- Bagheri, Shervin, 1979-
(author)
-
Analysis and control of transitional shear flows using global modes
- 2010
-
Doctoral thesis (other academic/artistic)abstract
- In this thesis direct numerical simulations are used to investigate two phenomenain shear flows: laminar-turbulent transition over a flat plate and periodicvortex shedding induced by a jet in cross flow. The emphasis is on understanding and controlling the flow dynamics using tools from dynamical systems and control theory. In particular, the global behavior of complex flows is describedand low-dimensional models suitable for control design are developed; this isdone by decomposing the flow into global modes determined from spectral analysisof various linear operators associated with the Navier–Stokes equations.Two distinct self-sustained global oscillations, associated with the sheddingof vortices, are identified from direct numerical simulations of the jet incrossflow. The investigation is split into a linear stability analysis of the steadyflow and a nonlinear analysis of the unsteady flow. The eigenmodes of theNavier–Stokes equations, linearized about an unstable steady solution revealthe presence of elliptic, Kelvin-Helmholtz and von K´arm´an type instabilities.The unsteady nonlinear dynamics is decomposed into a sequence of Koopmanmodes, determined from the spectral analysis of the Koopman operator. Thesemodes represent spatial structures with periodic behavior in time. A shearlayermode and a wall mode are identified, corresponding to high-frequency andlow-frequency self-sustained oscillations in the jet in crossflow, respectively.The knowledge of global modes is also useful for transition control, wherethe objective is to reduce the growth of small-amplitude disturbances to delaythe transition to turbulence. Using a particular basis of global modes, knownas balanced modes, low-dimensional models that capture the behavior betweenactuator and sensor signals in a flat-plate boundary layer are constructed andused to design optimal feedback controllers. It is shown that by using controltheory in combination with sensing/actuation in small, localized, regionsnear the rigid wall, the energy of disturbances may be reduced by an order of magnitude.
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| 18. |
- Bagheri, Shervin, 1979-
(author)
-
Stability analysis and control design of spatially developing flows
- 2008
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Licentiate thesis (other academic/artistic)abstract
- Methods in hydrodynamic stability, systems and control theory are applied to spatially developing flows, where the flow is not required to vary slowly in the streamwise direction. A substantial part of the thesis presents a theoretical framework for the stability analysis, input-output behavior, model reduction and control design for fluid dynamical systems using examples on the linear complex Ginzburg-Landau equation. The framework is then applied to high dimensional systems arising from the discretized Navier–Stokes equations. In particular, global stability analysis of the three-dimensional jet in cross flow and control design of two-dimensional disturbances in the flat-plate boundary layer are performed. Finally, a parametric study of the passive control of two-dimensional disturbances in a flat-plate boundary layer using streamwise streaks is presented.
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| 19. |
- Battini, Jean-Marc, 1968-
(author)
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Co-rotational beam elements in instability problems
- 2002
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Doctoral thesis (other academic/artistic)abstract
- The purpose of the work presented in this thesis is to implement co-rotational beam elements and branch-switching procedures in order to analyse elastic and elastoplasticinstability problems. For the 2D beam elements, the co-rotational framework is taken from Crisfield [23]. The main objective is to compare three different local elasto-plastic elements. The 3D co-rotational formulation is based on the work of Pacoste and Eriksson [73],with new items concerning the parameterisation of the finite rotations, the definitionof the local frame, the inclusion of warping effects through the introduction of aseventh nodal degree of freedom and the consideration of rigid links. Differenttypes of local formulations are considered, including or not warping effects. It isshown that at least some degree of non-linearity must be introduced in the localstrain definition in order to obtain correct results for certain classes of problems. Within the present approach any cross-section can be modelled, and particularly, the centroid and shear center are not necessarily coincident.Plasticity is introduced via a von Mises material with isotropic hardening. Numericalintegration over the cross-section is performed. At each integration point, theconstitutive equations are solved by including interaction between the normal andshear stresses. Concerning instabilities, a new numerical method for the direct computation of elasticcritical points is proposed. This is based on a minimal augmentation procedure asdeveloped by Eriksson [32–34]. In elasto-plasticity, a literature survey, mainly concernedwith theoretical aspects is first presented. The objective is to get a completecomprehension of the phenomena and to give a basis for the two branch-switchingprocedures presented in this thesis.A large number of examples are used in order to assess the performances of the elements and the path-following procedures.
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| 20. |
- Bellani, Gabriele, 1981-
(author)
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Experimental Studies of Complex Flows through Image-Based Techniques
- 2011
-
Doctoral thesis (other academic/artistic)abstract
- This thesis deals with the development of experimental techniques for the study of complex flows inspired to a large extent by the papermaking process. In particular one part of this thesis is devoted to the development of laboratory experiments based on index-of-refraction matching and imaging techniques to study the behavior of dilute and concentrated suspension of elongated particles. Another part is aimed at exploring the potential of the synergy between experiments and numerical simulations to access quantities otherwise not-measurable in complex flows. Highspeedimaging experiments have been specifically designed for this purpose. The first of the Refractive IndexMatching (RIM) experiment was aimed at studying the flow generated during the filtration of a fiber suspension using Particle Image Velocimetry (PIV) and pressure drop measurements. The experiments were performed in a vertical laboratory filtration device. Index of refraction matching of fibers and fluids allowed measurements to be performed in the proximity and, to some extent, in the forming network during filtration. The area over which the forming network induces velocity gradients has been measured and have been found to be independent of the Reynolds number but dependent on the fiber length and the structure of the network. Analysis of the flow scales in the proximity of the network showed that the signature of the mesh used to filter the suspension is never completely suppressed as the network thickness increases. Also, pressure drop measurements over a static fiber network have been performed. A linear dependence of the pressure drop with the basis weight (mass of fibers in the network per unit area) and a non-dimensional filtration resistance independent of filtration velocity and network thickness (if network compressibility is accounted for) was found. These findings can help explain characteristics that are observed on paper sheets and help improvede watering efficiency. The second RIM experiment was aimed at measuring the interactions of Taylorscale elongated particles with turbulence. RIM particles with embedded tracers and Stereoscopic PIV were combined to simultaneously measure fluid phase and particle velocity. The novelty of this technique is that it allows to measure the three-dimensional angular velocity vector of arbitrarily shaped particles. This technique was applied to study the interaction of neutrally buoyant ellipsoidal particles with stationary homogeneous isotropic turbulence. The results were compared to the case of spherical particles. The main result is that both spherical and ellipsoidal particles provide enhancement of the small scales and reduction of the large scales at volume concentrations as low as 0.1%. However, the reduction of the large scales was much more evident for spherical particles. These results highlight the fact that particle elongation introduces different mechanisms of turbulent modulation as compared to the spherical particles. The first of the high-speed imaging experiments was to provide a database for test and validation of a CFD-based flow observer for complex flows. For this purpose time resolved measurements of a turbulent confined jet have been performed with high-speed PIV. The measurements have been used both as a feedback signal and as a reference for the evaluation of a CFD-based estimator for complex flows. Furthermore, based on the measurements Kalman filters have been designed and implemented in the observer. The experimental data have also been used to compare two modal decompositions, namely Proper Orthogonal Decomposition and Dynamical Modal Decomposition and evaluate their ability to describe the global behavior of complex flow. The second of the high-speed imaging experiment was applied to study spreading of a droplet on a solid surface. These experiments have been performed with extremely high time-resolution (140000 fps), over a range of parameters (in terms of droplet viscosity, equilibrium contact angle and droplet size) larger than any other experiment reported in the literature in a single work. By combining the experiments and direct numerical simulations a dissipative mechanisms arising from the contact line movement has been identified and the corresponding macroscopic coefficient has been measured.i
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| 21. |
- Bellani, Gabriele, 1981-
(author)
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Velocity measurements in a fiber suspension flow: formation of a fiber network
- 2008
-
Licentiate thesis (other academic/artistic)abstract
- The aim of the present work is to experimentally study the dynamics of the formation of a fiber network formed from the filtration of a fiber suspension. This is relevant for all industrial applications (e.g. papermaking, productions of composite material, etc.) where a suspension of fibers has to flow through narrow gaps, and the quality of the product depends on the distribution of mass and orientation of the fibers. To study the dynamics of network formation, we developed an experimental setup where the filtration of a fiber suspension through a semi-permeable screen can be studied. In the setup, both the fluid and the solid phase can be visualized. The focus of the present thesis is to study the fluid flow generated during the filtration. Index of refraction matching, image processing and particle image velocimetry have been used to measure the velocity field in the proximity of the resulting fiber network. Experiments with varying fiber length and filtration velocity have been performed. The disturbances generated by the screen and the forming network was found to be confined in a region (boundary region), whose extension varies with time: first, after the formation of the first fiber layers, the extent of the boundary region increases; at later times, the boundary region is thinner. The extent appears to be correlated to the gap size either of the screen (at very early times) or of the fiber network, but independent of the filtration velocity. Fluctuations on a scale larger than a fiber length are also observed during the filtration process. These fluctuations are found to be correlated to the nondimensional number Se that relates the sedimentation velocity of a fiber to the filtration velocity. The governing non-dimensional parameters are derived from the equations. The parameters are used to relate the experimental observations to the dewatering process in papermaking.
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| 22. |
- Berger, Marit, 1986-
(author)
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Modelling the early to mid-Holocene Arctic climate
- 2013
-
Licentiate thesis (other academic/artistic)abstract
- In the recent past it has become evident that the Earth's climate is changing, and that human activity play a significant role in these changes. One of the regions where the ongoing climate change has been most evident is in the Arctic: the surface temperature has increased twice as much in this region as compared to the global average, in addition, a significant decline in the Arctic sea-ice extent has been observed in the past decades. Climate model studies of past climates are important tools to understand the ongoing climate change and how the Earth's climate may respond to changes in the forcing.This thesis includes studies of the Arctic climate in simulations of the early and mid-Holocene, 9 000 and 6 000 years before present. Changes in the Earth's orbital parameters resulted in increased summer insolation as compared to present day, especially at high northern latitudes. Geological data imply that the surface temperatures in the early to mid Holocene were similar to those projected for the near future. In addition, the geological data implies that the Arctic sea ice cover was significantly reduced in this period. This makes the early to mid-Holocene an interesting period to study with respect to the changes observed in the region at present.Several model studies of the mid-Holocene have been performed through the Paleoclimate Modeling Intercomparison Project (PMIP1 to PMIP3). The simulations have been performed with climate models of varying complexity, from atmosphere-only models in the first phase to fully coupled models with the same resolution as used for future climate simulations in the third phase. The first part of this thesis investigates the simulated sea ice in the pre-industrial and mid-Holocene simulations included in the PMIP2 and PMIP3 ensemble. As the complexity of the models increases, the models simulate smaller extents and thinner sea ice in the Arctic; the sea-ice extent suggested by the proxy data for the mid-Holocene is however not reproduced by the majority of the models.One possible explanation for the discrepancy between the simulated and reconstructed Arctic sea ice extent is missing or inadequate representations of important processes. The representation of atmospheric aerosol direct and indirect effects in past climates is a candidate process. Previous studies of deeper time periods have concluded that the representation of the direct and indirect effects of the atmospheric aerosols can influence the simulated climates, and reduce the equator to pole temperature gradient in past warm climates, in better agreement with reconstructions. The second part of the thesis investigates the influence of aerosol on the early Holocene climate. The indirect effect of reduced aerosol concentrations as compared to the present day is found to cause an amplification of the warming, especially in the Arctic region. A better agreement with reconstructed Arctic sea ice extent is thus achieved.
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| 23. |
- Berlin, Stellan
(author)
-
Oblique waves in boundary layer transition
- 1998
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Doctoral thesis (other academic/artistic)abstract
- Traditional research on laminar-turbulent transition has focused on scenarios that are caused by the exponential growth of eigensolutions to the linearized disturbance equations, e.g. two-dimensional Tollmien-Schlichting waves. Recent research has reveled the existence of other non-modal growth mechanisms, for example associated with the transient growth of streamwise streaks. Oblique waves may trigger transition in which the new mechanisms is an important ingredient. We have investigated the role of oblique waves in boundary layer transition, using an efficient spectral code for direct numerical simulations.In the initial stage of this transition scenario oblique waves have been found to interact nonlinearly and forte streamwise vortices, which in turn forte growing stream wise streaks. If the streak amplitude reaches a threshold value, transition from laminar to turbulent flow Will take place.In the late transition stage, large velocity fluctuations are found at flow positions associated with steep spanwise gradients between the streaks. At those positions we have also found h-vortices, structures that are also characteristic for traditional sec ondary instability transition. The h-vortices are shown to be due to the interaction of oblique waves and streaks that seem to play a more important role in the late stage of transition than previously appreciated.The numerical results are compared in detail with experimental results on oblique transition and good agreement is found.A new nonlinear receptivity mechanism is found that can trigger boundary layer transition from oblique waves in the free-stream. The mechanism continuously interact with the boundary layer and the resulting transition scenario is characterized by the growth of streamwise streaks. The same structures that are observed in experiments on transition caused by free-stream turbulente. A linear receptivity mechanism that interact with the boundary layer downstream of the leading edge is also identified. It is related to linear receptivity mechanisms previously studied at the leading edge. The nonlinear and linear mechanisms are of comparable strength for moderate free-stream disturbance levels.Two strategies for control of oblique transition are investigated, both based on spanwise flow oscillations. The longest transition delay was found when the flow oscil lations were generated by a body forte. When the control was applied to a transition scenario initiated by a random disturbance it was more successful and transition was prevented.
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| 24. |
- Birgersson, Erik, 1972-
(author)
-
Mathematical Modeling of Transport Phenomena in Polymer Electrolyte and Direct Methanol Fuel Cells
- 2004
-
Doctoral thesis (other academic/artistic)abstract
- This thesis deals with modeling of two types of fuel cells:the polymer electrolyte fuel cell (PEFC) and the directmethanol fuel cell (DMFC), for which we address four majorissues: a) mass transport limitations; b) water management(PEFC); c) gas management (DMFC); d) thermal management. Four models have been derived and studied for the PEFC,focusing on the cathode. The first exploits the slenderness ofthe cathode for a two-dimensional geometry, leading to areduced model, where several nondimensional parameters capturethe behavior of the cathode. The model was extended to threedimensions, where four di.erent flow distributors were studiedfor the cathode. A quantitative comparison shows that theinterdigitated channels can sustain the highest currentdensities. These two models, comprising isothermal gasphaseflow, limit the studies to (a). Returning to a two-dimensionalgeometry of the PEFC, the liquid phase was introduced via aseparate flow model approach for the cathode. In addition toconservation of mass, momentum and species, the model wasextended to consider simultaneous charge and heat transfer forthe whole cell. Di.erent thermal, flow fields, and hydrodynamicconditions were studied, addressing (a), (b) and (d). A scaleanalysis allowed for predictions of the cell performance priorto any computations. Good agreement between experiments with asegmented cell and the model was obtained. A liquid-phase model, comprising conservation of mass,momentum and species, was derived and analyzed for the anode ofthe DMFC. The impact of hydrodynamic, electrochemical andgeometrical features on the fuel cell performance were studied,mainly focusing on (a). The slenderness of the anode allows theuse of a narrow-gap approximation, leading to a reduced model,with benefits such as reduced computational cost andunderstanding of the physical trends prior to any numericalcomputations. Adding the gas-phase via a multiphase mixtureapproach, the gas management (c) could also be studied.Experiments with a cell, equipped with a transparent end plate,allowed for visualization of the flow in the anode, as well asvalidation of the two-phase model. Good agreement betweenexperiments and the model was achieved. Keywords:Fuel cell; DMFC; PEFC; one-phase; two-phase;model; visual cell; segmented cell; scale analysis; asymptoticanalysis.
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| 25. |
- Bobke, Alexandra
(author)
-
Simulations of turbulent boundary layers with suction and pressure gradients
- 2016
-
Licentiate thesis (other academic/artistic)abstract
- The focus of the present licentiate thesis is on the effect of suction and pressure gradients on turbulent boundary-layer flows, which are investigated separately through performing numerical simulations.The first part aims at assessing history and development effects on adverse pressure-gradient (APG) turbulent boundary layers (TBL). A suitable set-up was developed to study near-equilibrium conditions for a boundary layer developingon a flat plate by setting the free-stream velocity at the top of the domain following a power law. The computational box size and the correct definition of the top-boundary condition were systematically tested. Well-resolved large-eddy simulations were performed to keep computational costs low. By varying the free-stream velocity distribution parameters, e.g. power-law exponent and virtual origin, pressure gradients of different strength and development were obtained. The magnitude of the pressure gradient is quantified in terms of the Clauser pressure-gradient parameter β. The effect of the APG is closely related to its streamwise development, hence, TBLs with non-constant and constant β were investigated. The effect was manifested in the mean flow through a much more pronounced wake region and in the Reynolds stresses through the existence of an outer peak. The terms of the turbulent kinetic energy budgets indicate the influence of the APG on the distribution of the transfer mechanism across the boundary layer. Stronger and more energetic structures were identified in boundary layers with relatively stronger pressure gradients in their development history. Due to the difficulty of determining the boundary-layer thickness in flows with strong pressure gradients or over a curvedsurface, a new method based on the diagnostic-plot concept was introduced to obtain a robust estimation of the edge of a turbulent boundary layer.In the second part, large-eddy simulations were performed on temporally developing turbulent asymptotic suction boundary layers (TASBLs). Findings from previous studies about the effect of suction could be confirmed, e.g. the reduction of the fluctuation levels and Reynolds shear stresses. Furthermore, the importance of the size of the computational domain and the time development were investigated. Both parameters were found to have a large impact on the results even on low-order statistics. While the mean velocity profile collapses in the inner layer irrespective of box size and development time, a wake region occurs for too small box sizes or early development time and vanishes once sufficiently large domains and/or integration times are chosen. The asymptotic state is charactersized by surprisingly thick boundary layers even for moderateReynolds numbers Re (based on free-stream velocity and laminar displacement thickness); for instance, Re = 333 gives rise to a friction Reynolds number Reτ = 2000. Similarly, the flow gives rise to very large structures in the outer region. These findings have important ramifications for experiments, since very large facilities are required to reach the asymptotic state even for low Reynolds numbers.
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| 26. |
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| 27. |
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| 28. |
- Boström, Erik
(author)
-
Boundary Conditions for Spectral Simulations of Atmospheric Boundary Layers
- 2017
-
Licentiate thesis (other academic/artistic)abstract
- An atmospheric boundary layer (ABL) is generally a very high Reynolds number boundary layer over a fully rough surface that is influenced by different external forces. Numerical simulations of ABLs are typically demanding, particularly due to the high Reynolds numbers. Large eddy simulation (LES) where the grid filtered Navier--Stokes equations are solved together with a turbulence model for the subgrid-scale motions is the most accurate and widely used technique to date for ABLs. However, high Reynolds numbers, filtered equations and rough surfaces do not support the simple no-slip boundary conditions together with a feasible grid resolution. A paramount part for the performance of an ABL LES simulation therefore lies in the quality of approximate wall boundary conditions, so called wall models. The vast majority of LES codes used for ABL simulations rely on spatial discretization methods with low order finite difference approximations for the derivatives in the inhomogeneous wall normal direction. Furthermore, the wall boundary conditions are typically chosen in a mesh-dependent, non-local way, relying on the finite differences formulation. In this thesis we focus on solving the ABL LES equations with a fully (pseudo) spectral Fourier--Chebyshev code. We present how wall boundary conditions can be formulated through Robin boundary conditions and how to implement these in the normal-velocity normal-vorticity formulation that we solve. A new idea of specifying boundary conditions directly in Fourier space where also the turbulence intensity statistics can be controlled is presented and verified. The present results show that the Robin-type formulation is effective at least in near-equilibrium boundary layers. The code and boundary conditions were tested in both low and high Reynolds number (open and full) channel flows of neutral and stable stratification. Results were validated with both low to moderate Reynolds number DNS statistics as well as with the logarithmic law. Our results indicate great potential for both the the new boundary condition formulation and the specific code implementation. Further analysis of more complex flow situations will show whether the Robin-type formulation will give similarly good results.
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| 29. |
- Brandt, Luca, 1971-
(author)
-
Numerical studies of bypass transition in the Blasius boundary layer
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- Experimental findings show that transition from laminar toturbulent ow may occur also if the exponentially growingperturbations, eigensolutions to the linearised disturbanceequations, are damped. An alternative non-modal growthmechanism has been recently identi fied, also based on thelinear approximation. This consists of the transient growth ofstreamwise elongated disturbances, with regions of positive andnegative streamwise velocity alternating in the spanwisedirection, called streaks. These perturbation are seen toappear in boundary layers exposed to signi ficant levels offree-stream turbulence. The effect of the streaks on thestability and transition of the Blasius boundary layer isinvestigated in this thesis. The analysis considers the steadyspanwise-periodic streaks arising from the nonlinear evolutionof the initial disturbances leading to the maximum transientenergy growth. In the absence of streaks, the Blasius pro filesupports the viscous exponential growth of theTollmien-Schlichting waves. It is found that increasing thestreak amplitude these two-dimensional unstable waves evolveinto three-dimensional spanwiseperiodic waves which are lessunstable. The latter can be completely stabilised above athreshold amplitude. Further increasing the streak amplitude,the boundary layer is again unstable. The new instability is ofdifferent character, being driven by the inectional pro filesassociated with the spanwise modulated ow. In particular, it isshown that, for the particular class of steady streaksconsidered, the most ampli fied modes are antisymmetric andlead to spanwise oscillations of the low-speed streak (sinuousscenario). The transition of the streak is then characterisedby the appearance of quasi-streamwise vorticesfollowing themeandering of the streak.Simulations of a boundary layer subjected to high levels offree-stream turbulence have been performed. The receptivity ofthe boundary layer to the external perturbation is studied indetail. It is shown that two mechanisms are active, a linearand a nonlinear one, and their relative importance isdiscussed. The breakdown of the unsteady asymmetric streaksforming in the boundary layer under free-stream turbulence isshown to be characterised by structures similar to thoseobserved both in the sinuous breakdown of steady streaks and inthe varicose scenario, with the former being the mostfrequently observed.Keywords:Fluid mechanics, laminar-turbulent transition,boundary layer ow, transient growth, streamwise streaks,lift-up effect, receptivity, free-stream turbulence, secondaryinstability, Direct Numerical Simulation.
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| 30. |
- Brandt, Luca
(author)
-
Study of generation, growth and breakdown of streamwise streaks in a Blasius boundary layer.
- 2001
-
Licentiate thesis (other academic/artistic)abstract
- Transition from laminar to turbulent flow has beentraditionally studied in terms of exponentially growingeigensolutions to the linearized disturbance equations.However, experimental findings show that transition may occuralso for parameters combinations such that these eigensolutionsare damped. An alternative non-modal growth mechanism has beenrecently identified, also based on the linear approximation.This consists of the transient growth of streamwise elongateddisturbances, mainly in the streamwise velocity component,called streaks. If the streak amplitude reaches a thresholdvalue, secondary instabilities can take place and provoketransition. This scenario is most likely to occur in boundarylayer flows subject to high levels of free-stream turbulenceand is the object of this thesis. Different stages of theprocess are isolated and studied with different approaches,considering the boundary layer flow over a flat plate. Thereceptivity to free-stream disturbances has been studiedthrough a weakly non-linear model which allows to disentanglethe features involved in the generation of streaks. It is shownthat the non-linear interaction of oblique waves in thefree-stream is able to induce strong streamwise vortices insidethe boundary layer, which, in turn, generate streaks by thelift-up effect. The growth of steady streaks is followed bymeans of Direct Numerical Simulation. After the streaks havereached a finite amplitude, they saturate and a new laminarflow, characterized by a strong spanwise modulation isestablished. Using Floquet theory, the instability of thesestreaks is studied to determine the features of theirbreakdown. The streak critical amplitude, beyond which unstablewaves are excited, is 26% of the free-stream velocity. Theinstability appears as spanwise (sinuous-type) oscillations ofthe streak. The late stages of the transition, originating fromthis type of secondary instability, are also studied. We foundthat the main structures observed during the transition processconsist of elongated quasi-streamwise vortices located on theflanks of the low speed streak. Vortices of alternating signare overlapping in the streamwise direction in a staggeredpattern. Descriptors:Fluid mechanics, laminar-turbulenttransition, boundary layer flow, transient growth, streamwisestreaks, lift-up effect, receptivity, free-stream turbulence,nonlinear mechanism, streak instability, secondary instability,Direct Numerical Simulation.
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| 31. |
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| 32. |
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| 33. |
- Brynjell-Rahkola, Mattias, 1986-
(author)
-
Global stability analysis of three-dimensional boundary layer flows
- 2015
-
Licentiate thesis (other academic/artistic)abstract
- This thesis considers the stability and transition of incompressible boundary layers. In particular, the Falkner–Skan–Cooke boundary layer subject to a cylindrical surface roughness, and the Blasius boundary layer with applied localized suction are investigated. These flows are of great importance within the aviation industry, feature complex transition scenarios, and are strongly three-dimensional in nature. Consequently, no assumptions regarding homogeneity in any of the spatial directions are possible, and the stability of the flow is governed by an extensive three-dimensional eigenvalue problem.The stability of these flows is addressed by high-order direct numerical simulations using the spectral element method, in combination with a Krylov subspace projection method. Such techniques target the long-term behavior of the flow and can provide lower limits beyond which transition is unavoidable. The origin of the instabilities, as well as the mechanisms leading to transition in the aforementioned cases are studied and the findings are reported.Additionally, a novel method for computing the optimal forcing of a dynamical system is developed. This type of analysis provides valuable information about the frequencies and structures that cause the largest energy amplification in the system. The method is based on the inverse power method, and is discussed in the context of the one-dimensional Ginzburg–Landau equation and a two-dimensional flow case governed by the Navier–Stokes equations.
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| 34. |
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| 35. |
- Canton, Jacopo, 1989-
(author)
-
Numerical studies on flows with secondary motion
- 2016
-
Licentiate thesis (other academic/artistic)abstract
- This work is concerned with the study of flow stability and turbulence control - two old but still open problems of fluid mechanics. The topics are distinct and are (currently) approached from different directions and with different strategies. This thesis reflects this diversity in subject with a difference in geometry and, consequently, flow structure: the first problem is approached in the study of the flow in a toroidal pipe, the second one in an attempt to reduce the drag in a turbulent channel flow.The flow in a toroidal pipe is chosen as it represents the common asymptotic limit between spatially developing and helical pipes. Furthermore, the torus represents the smallest departure from the canonical straight pipe flow, at least for small curvatures. The interest in this geometry is twofold: it allows us to isolate the effect of the curvature on the flow and to approach straight as well as helical pipes. The analysis features a characterisation of the steady solution as a function of curvature and the Reynolds number. The problem of forcing fluid in the pipe is addressed, and the so-called Dean number is shown to be of little use, except for infinitesimally low curvatures. It is found that the flow is modally unstable and undergoes a Hopf bifurcation that leads to a limit cycle. The bifurcation and the corresponding eigenmodes are studied in detail, providing a complete picture of the instability.The second part of the thesis approaches fluid mechanics from a different perspective: the Reynolds number is too high for a deterministic description and the flow is analysed with statistical tools. The objective is to reduce the friction exerted by a turbulent flow on the walls of a channel, and the idea is to employ a control strategy independent of the small, and Reynolds number-dependent, turbulent scales. The method of choice was proposed by Schoppa & Hussain [Phys. Fluids 10:1049-1051 (1998)] and consists in the imposition of streamwise invariant, large-scale vortices. The vortices are re-implemented as a volume force, validated and analysed. Results show that the original method only gave rise to transient drag reduction while the forcing version is capable of sustained drag reduction of up to 18%. An analysis of the method, though, reveals that its effectiveness decreases rapidly as the Reynolds number is increased.
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| 36. |
- Carlsson, Allan, 1979-
(author)
-
Near wall fibre orientation in flowing suspensions
- 2009
-
Doctoral thesis (other academic/artistic)abstract
- This thesis deals with fibre orientation in wall-bounded shear flows. The primary application in mind is papermaking. The study is mainly experimental,but is complemented with theoretical considerations.The main part of the thesis concerns the orientation of slowly settlingfibres in a wall-bounded viscous shear flow. This is a flow case not dealt withpreviously even at small Reynolds numbers. Experiments were conducted usingdilute suspensions with fibres having aspect ratios of rp ≈ 7 and 30. It is foundthat the wall effect on the orientation is small for distances from the wall wherethe fibre centre is located farther than half a fibre length from the wall. Farfrom the wall most fibres were oriented close to the flow direction. Closer tothe wall than half a fibre length the orientation distribution first shifted to bemore isotropic and in the very proximity of the wall the fibres were orientedclose to perpendicular to the flow direction, nearly aligned with the vorticityaxis. This was most evident for the shorter fibres with rp ≈ 7.Due to the density difference between the fibres and the fluid there is anincreased concentration near the wall. Still, a physical mechanism is requiredin order for a fibre initially oriented close to the flow direction at about half afibre length from the wall to change its orientation to aligned with the vorticityaxis once it has settled down to the wall. A slender body approach is usedin order to estimate the effect of wall reflection and repeated wall contacts onthe fibre rotation. It is found that the both a wall reflection, due to settlingtowards the wall, and contact between the fibre end and the wall are expectedto rotate the fibre closer to the vorticity axis. A qualitative agreement withthe experimental results is found in a numerical study based on the theoreticalestimation.In addition an experimental study on fibre orientation in the boundarylayers of a headbox is reported. The orientation distribution in planes parallelto the wall is studied. The distribution is found to be more anisotropic closerto the wall, i.e. the fibres tend to be oriented closer to the flow direction nearthe wall. This trend is observed sufficiently far upstream in the headbox.Farther downstream no significant change in the orientation distribution couldbe detected for different distances from the wall.
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| 37. |
- Carlsson, Allan, 1979-
(author)
-
Orientation of fibres in suspensions flowing over a solid surface
- 2007
-
Licentiate thesis (other academic/artistic)abstract
- The orientation of fibres suspended in a viscous fluid, flowing over a solid surface, has been studied experimentally. A shear layer was generated, by letting the suspension flow down an inclined plate. Far upstream from the measuring section the suspension was accelerated to obtain an initial orientation of the fibres aligned with the flow direction. A CCD-camera was used to visualise the fibres. The velocity profile of the fibres coincided with the theoretical expression for fully developed flow of Newtonian liquid down an inclined wall. The orientation of the fibres was analysed in planes parallel to the solid surface. At distances from the wall larger than one fibre length the fibres performed a tumbling motion in the flow-gradient plane in what appeared to be Jeffery-like orbits. Closer to the wall a difference was found between fibres of aspect ratio rp = 10 and 40. The longer fibres of rp = 40 kept their orientation, aligned with the flow, also in the near wall region. For the shorter fibres the orientation shifted gradually, to orientations closer to the vorticity axis, when the distance from the wall was decreased. In the very proximity to the wall the fibres were aligned with the vorticity, perpendicular to the direction of the flow. Another distinction, most likely related to the fibre orientation, was seen in the wall normal concentration profile. Due to sedimentation effects fibres accumulated in the near wall region. For fibres of rp = 10 a peak in concentration was found at the wall, while for r=40 the maximum concentration was found approximately half a fibre length from the wall. It is previously known that a fibre can interact with the wall in what is referred to as a "pole vaulting" motion away from the wall. It is suggested, as a likely explanation to the location of the maximum concentration, that fibres of rp = 40 perform this motion, while fibres of rp=10 do not. In another experiment the surface of the wall was modified with ridges. For fibres of rp = 10 there were no longer any fibres oriented perpendicular to the flow direction in the near wall region. The main application in mind throughout this work is papermaking. The study is considered to be of fundamental character and is not applicable in a direct sense. The difference between the flow situation in the experiments and the paper machine is discussed further.
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| 38. |
- Chandrasekaran, Dilip, 1970-
(author)
-
Grain Size and Solid Solution Strengthening in Metals
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries.For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults.Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli.The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals.Keywords:strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD
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| 39. |
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| 40. |
- Chevalier, Mattias, 1973-
(author)
-
Feedback and adjoint based control of boundary layer flows
- 2004
-
Doctoral thesis (other academic/artistic)abstract
- Linear and nonlinear optimal control have been investigated in transitional channel and boundary layer .ows. The flow phenomena that we study are governed by the incompressible Navier–Stokes equations and the main aim with the control is to prevent transition from laminar to turbulent flows. A linear model-based feedback control approach, that minimizes an objective function which measures the perturbation energy, can be formulated where the Orr– Sommerfeld/Squire equations model the flow dynamics. A limitation with the formulation is that it requires complete state information. However, the control problem can be combined with a state estimator to relax this requirement. The estimator requires only wall measurements to reconstruct the flow in an optimal manner.Physically relevant stochastic models are suggested for the estimation problem which turns out to be crucial for fast convergence. Based on these models the estimator is shown to work for both in.nitesimal as well as finite amplitude perturbations in direct numerical simulations of a channel flow at Recl = 3000.A stochastic model for external disturbances is also constructed based on statistical data from a turbulent channel flow at ReT = 100. The model is successfully applied to estimate a turbulent channel flow at the same Reynolds number.The combined control and estimation problem, also known as a compensator, is applied to spatially developing boundary layers. The compensator is shown to successfully reduce the perturbation energy for Tollmien–Schlichting waves and optimal perturbations in the Blasius boundary layer. In a Falkner– Skan–Cooke boundary layer the perturbation energy of traveling and stationary cross-flow disturbances are also reduced.A nonlinear control approach using the Navier–Stokes equations and the associated adjoint equations are derived and implemented in the context of direct numerical simulations of spatially-developing three-dimensional boundary layer .ows and the gradient computation is veri.ed with .nite-di.erences. The nonlinear optimal control is shown to be more e.cient in reducing the disturbance energy than feedback control when nonlinear interactions are becoming signi.cant in the boundary layer. For weaker disturbances the two methods are almost indistinguishable.
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| 41. |
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| 42. |
- Dadfar, Reza, 1978-
(author)
-
Flow control and reduced-order modelling of transition in shear flows
- 2014
-
Doctoral thesis (other academic/artistic)abstract
- In this thesis direct numerical simulation is used to investigate the possibility to delay the transition from a laminar to a turbulent flow in boundary layer flows. Furthermore, modal analysis techniques are used to identify the coherent structures in wind turbine wake.Among different transition scenarios, the classical transition scenario is considered where the laminar-turbulent transition occursdue to Tollmien-Schlichting (TS) waves. These waves are convectively unstable and when triggered inside the boundary layer, they grow exponentially inamplitude as they are advected downstream of the domain. The aim is to suppressthese waves using flow control strategies based on a row of spatially localised sensors and actuators distributed near the wall inside the boundary layer. To avoid the high dimensionality arising from discretisation of the Navier–Stokes equations, a reduced order model (ROM) based on the Eigensystem Realisation Algorithm(ERA) is obtained and based on that a linear controller is designed. To manip-ulate the flow, a plasma actuator is modelled and implementedas an externalforcing. To account for the restrictions of the plasma actuators, several strategies are proposed and tested within the LQG framework. We studied also the design of a faster controller based on decentralised approach and compared the performance to a more expensive centralised controller. The outcomes revea lsuccessful performance in mitigating the energy of the disturbances inside the boundary layer and suppressing the TS waves.To extract coherent features of the wind turbine wakes, modal decomposition techniques are employed. In this method a large dynamical system is reduced to a fewer number of degrees of freedom. Two decomposition techniques are employed, namely proper orthogonal decomposition and dynamic mode decomposition. In the former, the flow is decomposed into a set of orthogonal structures which are ranked according to their energy contents in a hierarchical manner. In the latter, the eigenvalues and eigenvectors of the underlying approximate linear operator of the system is evaluated. In particular each mode is associated with a specific frequency and growth rate. The results revealed the coherent structures which are dynamically significant for the onset of instability in the wind turbine wake
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| 43. |
- Dalilsafaei, Seif
(author)
-
Stiffness modification of tensegrity structures
- 2011
-
Licentiate thesis (other academic/artistic)abstract
- Although the concept of tensegrity structures was invented in the beginning of the twentieth century, the applications of these structures are limited, partially due to their low stiffness. The stiffness of tensegrities comes from topology, configuration, pre-stress and initial axial element stiffnesses. The first part of the present work is concerned with finding the magnitude of pre-stress. Its role in stiffness of tensegrity structures is to postpone the slackening of cables. A high pre-stress could result in instability of the structure due to buckling and yielding of compressive and tension elements, respectively. Tensegrity structures are subjected to various external loads such as self-weight, wind or snow loads which in turn could act in different directions and be of different magnitudes. Flexibility analysis is used to find the critical load combinations. The magnitude of pre-stress, in order to sustain large external loads, is obtained through flexibility figures, and flexibility ellipsoids are employed to ensure enough stiffness of the structure when disturbances are applied to a loaded structure. It has been seen that the most flexible direction is very much sensitive to the pre-stress magnitude and neither analytical methods nor flexibility ellipsoids are able to find the most flexible directions. The flexibility figures from a non-linear analysis are here utilized to find the weak directions. In the second part of the present work, a strategy is developed to compare tensegrity booms of triangular prism and Snelson types with a truss boom. It is found that tensegrity structures are less stiff than a truss boom when a transversal load is applied. An optimization approach is employed to find the placement of the actuators and their minimum length variations. The results show that the bending stiffness can be significantly improved, but still an active tensegrity boom is less stiff than a truss boom. Genetic algorithm shows high accuracy of searching non-structural space.
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| 44. |
- Danielsson, Carl-Ola
(author)
-
Continuous electropermutation using ion-exchange textile
- 2004
-
Licentiate thesis (other academic/artistic)abstract
- Increased levels of nitrate in ground water has made many wells unsuitable as sources for drinking water. In this thesis an ion-exchange assisted electromembrane process, suitable for nitrate removal, is investigated both theoretically and experimentally. A new ion-exchange textile material is introduced as a conducting spacer in the feed compartment of an continuous electropermutation cell. The ion-exchange textile have a high permeability and provides faster ion-exchange kinetics compared to ion-exchange resins. The sheet shaped structure of the textile makes it easy to incorporate into the cell. A report on the development of a new electro-membrane module, capable of incorporating an ion-exchange textile spacer, is presented. A theoretical study of the flow field through the electro-membrane module was performed using two different 2-D models. The calculated flow distributions provided by different proposed module designs were compared and a prototype module was produced. The flow field obtained with the prototype cell was visualised in a experimental cell with a transparent plexiglass cover. A steady-state model based on the conservation of the ionic species is developed. The governing equations on the microscopic level are presented and volume averaged to give macro-homogeneous equations. The model equations are analysed and relevant simplifications are motivated and introduced. The dimensionless parameters governing the continuous electropermutation process are identified and their influence on the process are discussed. The mathematical model can be used as a tool when optimising the process parameters and designing equipment. An experimental study that aimed to show the positive influence of using the ion-exchange textile in the feed compartment of an continuous electropermutation process is presented. The incorporation of the ion-exchange textile significantly improves the nitrate removal rate at the same time as the power consumption is decreased. A superficial solution of sodium nitrate with a initial nitrate concentration of 105 ppm was treated. A product stream with less than 20 ppm nitrate could be obtained, in a single pass mode of operation. Its concluded from these experiments that continuous electropermutation using ion-exchange textile provides an interesting alternative for nitrate removal, in drinking water production. The predictions of the mathematical model are compared with experimental results and a good agreement is obtained
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| 45. |
- Danielsson, Carl-Ola
(author)
-
Electropermutation assisted by ion-exchange textile : removal of nitrate from drinking water
- 2006
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Doctoral thesis (other academic/artistic)abstract
- Increased levels of nitrate in ground water have made many wells unsuitable as sources for drinking water. In this thesis an ion-exchang eassisted electromembrane process, suitable for nitrate removal, is investigated both theoretically and experimentally. An ion-exchange textile material is introduced as a conducting spacer in the feed compartment of an electropermutation cell. The sheet shaped structure of the textile makes it easy to incorporate into the cell. High permeability and fast ion-exchange kinetics, compared to ion-exchange resins, are other attractive features of the ion-exchange textile. A steady-state model based on the conservation of the ionic species is developed. The governing equations on the microscopic level are volume averaged to give macro-homogeneous equations. The model equations are analyzed and relevant simplifications are motivated and introduced. Dimensionless parameters governing the continuous electropermutation process are identified and their influence on the process are discussed. The mathematical model can be used as a tool when optimising the process parameters and designing equipment. An experimental study that aimed to show the positive influence of using the ion-exchange textile in the feed compartment of an continuous electropermutation process is presented. The incorporation of the ion-exchange textile significantly improves the nitrate removal rate at the same time as the power consumption is decreased. A superficial solution of sodium nitrate with a initial nitrate concentration of 105 ppm was treated. A product stream with less than 20 ppm nitrate could be obtained, in a single pass mode of operation. Its concluded from these experiments that continuous electropermutation using ion-exchange textile provides an interesting alternative for nitrate removal, in drinking water production. The predictions of the mathematical model are compared with experimental results and a good agreement is obtained. Enhanced water dissociation is known to take place at the surface of ion-exchange membranes in electromembrane processes operated above the limiting current density. A model for this enhanced water dissociation in presented in the thesis. The model makes it possible to incorporate the effect of water dissociation as a heterogeneous surface reaction. Results from simulations of electropermutation with and without ion-exchange textile incorporated are presented. The influence of the water dissociation is investigated with the developed model.
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| 46. |
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| 47. |
- De Magistris, Federica, 1969-
(author)
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Wood fibre deformation in combined shear and compression
- 2005
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Doctoral thesis (other academic/artistic)abstract
- Mechanical pulping for producing pulps from softwood suitable for printing grade papers, like news, is a highly energy-intensive process consuming around 2000 kWh/t in electrical energy. Due to increasing energy costs and environmental issues there is a high demand for decreasing this energy consumption. The mechanical treatment of wet wood pieces in a refiner, in the mechanical pulp plant, is a complex mechanical loading. This is a process occurring between rotating discs at high speed and temperatures of 140 °C - 160 °C, where by means of shear and compression forces the fibres are separated and then made flexible, fibrillated and collapsed for good bonding ability. In this process also fines are created giving the optical properties of the paper. In mechanical pulping only a fraction of the applied energy is used for the structural changes of the wood material. Thus fundamental studies of the loading modes of wood under refining conditions and in particular under combined shear and compression loading are desired to gain more information regarding the possibility of affecting the mechanical pulping in an energy efficient way. The possibilities to study the behaviour of wood under a combined shear and compression load were in this thesis investigated using two methods: the Iosipescu shear test and the Arcan shear test. In both apparatus different combinations of shear and compression load were achieved by different rotations of the shear test device itself. Measurements with the Iosipescu device on a medium density fibreboard showed good agreement between experimental results and numerical simulations. Finite element analysis on wood showed, however, that with the use of a homogeneous material in the model the level of strain reached would be ten times smaller than experimentally measured. This fact is probably due to the honeycomb structure of the wood cells that allows for different local deformations that could not be represented by a continuous material model. Thus to study the deformations on the fibre level of wood an experimental equipment that uses smaller samples was needed. With a modified Arcan shear device such deformations under combined shear and compression load and in pure compression were possible showing different deformation patterns. During pure compression the cell walls bend in a characteristic “S” shape, independently of the shape of the fibre cells and their cell wall thickness. Under combined shear and compression, however, mainly the corners of the fibre cells deform giving a “brick” shape to the cells. In a second deformation performed in compression, the fibre cells follow the same deformation pattern as given by the first deformation type whether in compression or in combined shear and compression. The interpretation is that permanent defects in the cells themselves are introduced already in the first load cycle of the wood samples. The energy used under the different loading conditions showed that the first deformation required the largest amount of energy, for all loading conditions. The deformation in compression required larger amounts of energy than the deformation in combined loads. For subsequent deformations less energy was needed for compression if a combined load had preceded it. Due to the fact that less energy is needed to start to deform wood in combined load than under compression load, the application of a combined load as a first cycle may thus be a way to permanently deform fibres using less energy. To investigate the critical parameters determining the permanent deformation of cells, a finite element model of a network of twelve cells was developed. Special care was given to the material properties to study how the variation of the fibril angle in the different layers affects the deformation pattern of the wood fibres under the different loading conditions. The model shows that whether modelled as homogeneous linear isotropic material or as an orthotropic material defined for every layer of the cells wall, no difference in the deformation of the network of the fibres was achieved. It is probable that the deformation type is more determined by the geometry of the fibres themselves than by their material properties
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| 48. |
- Deusebio, Enrico, 1985-
(author)
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Numerical studies in rotating and stratified turbulence
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Although turbulence has been studied for more than five hundred years, a thorough understanding of turbulent flows is still missing. Nowadays computing power can offer an alternative tool, besides measurements and experiments, to give some insights into turbulent dynamics. In this thesis, numerical simulations are employed to study homogeneous and wall-bounded turbulence in rotating and stably stratified conditions, as encountered in geophysical flows where the rotation of the Earth as well as the vertical density variation influence the dynamics.In the context of homogeneous turbulence, we investigate how the transfer of energy among scales is affected by the presence of strong but finite rotation and stratification. Unlike geostrophic turbulence, we show that there is a forward energy cascade towards small scales which is initiated at the forcing scales. The contribution of this process to the general dynamic is secondary at large scales but becomes dominant at smaller scales where it leads to a shallowing of the energy spectrum, from k-3 to k-5/3. Two-point statistics show a good agreement with measurements in the atmosphere, suggesting that this process is an important mechanism for energy transfer in the atmosphere.Boundary layers subjected to system rotation around the wall-normal axis are usually referred to as Ekman layers and they can be seen as a model of the atmospheric and oceanic boundary layers developing at mid and high latitudes. We study the turbulent dynamics in Ekman layers by means of numerical simulations, focusing on the turbulent structures developing at moderately high Reynolds numbers. For neutrally stratified conditions, we show that there exists a turbulent helicity cascade in the logarithmic region. We focus on the effect of a stable stratification produced by a vertical positive temperature gradient. For moderate stratification, continuously turbulent regimes are produced which are in fair agreement with existing theories and models used in the context of atmospheric boundary layer dynamics. For larger degree of stratification, we show that laminar and turbulent motions coexist and displace along inclined patterns similar to what has been recently observed in other transitional flows.
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| 49. |
- Do-Quang, Minh, 1971-
(author)
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Melt convection in welding and crystal growth
- 2004
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Doctoral thesis (other academic/artistic)abstract
- A parallel finite element code with adaptive meshing was developed and used to study three dimensional, time-dependent fluid flows caused by thermocapillary convection as well as temperature and dopant distribution in fusion welding and floating zone crystal growth. A comprehensive numerical model of the three dimensional time-dependent fluid flows in a weld pool had been developed. This model considered most of the physical mechanisms involved in gas tungsten arc welding. The model helped obtaining the actual chaotic time-dependent melt flow. It was found that the fluid flow in the weld pool was highly complex and influenced the weld pool’s depth and width. The physicochemical model had also been studied and applied numerically in order to simulate the surfactant adsorption onto the surface effect to the surface tension of the metal liquid in a weld pool. Another model, a three dimensional time-dependent, with adaptive mesh refinement and coarsening was applied for simulating the effect of weak flow on the radial segregation in floating zone crystal growth. The phase change equation was also included in this model in order to simulate the real interface shape of floating zone. In the new parallel code, a scheme that keeps the level of node and face instead of the complete history of refinements was utilized to facilitate derefinement. The information was now local and the exchange of information between each and every processor during the derefinement process was minimized. This scheme helped to improve the efficiency of the parallel adaptive solver.
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| 50. |
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