| 1. |
- Agarwal, A., et al.
(författare)
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Transition to Turbulence in Viscoelastic Channel Flow
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 519-526
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Konferensbidrag (refereegranskat)abstract
- The influence of viscoelasticity on bypass transition to turbulence in channel flow is studied using data from direct numerical simulations by Agarwal et al. (2014) 1. The initial field is a superposition of a laminar base state and a localized disturbance. Relative to the Newtonian conditions, the polymeric FENE-P flow delays the onset of transition and extends its duration. The former effect is due to a weakening of the pre-transitional disturbance field, while the prolonged transition region is due to a slower spreading rate of the turbulent spots. Once turbulence occupies the full channel, a comparison of the turbulence fields shows that energetic flow structures are longer and wider in the polymeric flow. The final turbulent state is compared to elasto-inertial turbulence (EIT), where the polymer conformation field takes the form of elongated sheets with wide spanwise extent.
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| 2. |
- Appelquist, Elinor, et al.
(författare)
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Investigation of the Global Instability of the Rotating-disk Boundary Layer
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 321-328
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Konferensbidrag (refereegranskat)abstract
- The development of the flow over a rotating disk is investigated by direct numerical simulations using both the linearized and fully nonlinear incompressible Navier-Stokes equations. These simulations allow investigation of the transition to turbulence of the realistic spatially-developing boundary layer. The current research aims to elucidate further the global linear stability properties of the flow, and relate these to local analysis and discussions in literature. An investigation of the nonlinear upstream (inward) influence is conducted by simulating a small azimuthal section of the disk (1/68). The simulations are initially perturbed by an impulse disturbance where, after the initial transient behaviour, both the linear and nonlinear simulations show a temporally growing upstream mode. This upstream global mode originates in the linear case close to the end of the domain, excited by an absolute instability at this downstream position. In the nonlinear case, it instead originates where the linear region ends and nonlinear harmonics enter the flow field, also where an absolute instability can be found. This upstream global mode can be shown to match a theoretical mode from local linear theory involved in the absolute instability at either the end of the domain (linear case) or where nonlinear harmonics enter the field (nonlinear case). The linear simulation grows continuously in time whereas the nonlinear simulation saturates and the transition to turbulence moves slowly upstream towards smaller radial positions asymptotically approaching a global upstream mode with zero temporal growth rate, which is estimated at a nondimensional radius of 582.
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| 3. |
- Ask, Anna, et al.
(författare)
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Modelling of viscoelastic dielectric elastomers with deformation dependent electric properties
- 2015
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Ingår i: Procedia Iutam. - : Elsevier BV. - 2210-9838. ; 12, s. 134-144
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Tidskriftsartikel (refereegranskat)abstract
- This work deals with electro-viscoelastic modelling and simulation of dielectric elastomer actuators (DEA), including the case of deformation dependent electromechanical coupling. A large deformation modelling framework is adopted, and specific thermo- dynamically consistent material models are established. The general framework is applied to VHB49 polyacrylic polymers which are commonly used in DEA applications. The effects of viscosity and deformation dependent electric permittivity are studied with regards to the stability behaviour and also in view of predicting experimentally observed electromechanical behaviour using numerical simulations.
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| 4. |
- Brynjell-Rahkola, Mattias, et al.
(författare)
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Global Stability Analysis of a Roughness Wake in a Falkner-Skan-Cooke Boundary Layer
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 192-200
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Konferensbidrag (refereegranskat)abstract
- A global stability analysis of a Falkner-Skan-Cooke boundary layer with distributed three-dimensional surface roughness is per- formed using high-order direct numerical simulations. Computations have been performed for different sizes of the roughness elements, and a time-stepping method has been used to find the instability modes. The study shows that a critical roughness height beyond which a global instability is excited does exist. Furthermore, the origins of this instability is examined by means of an energy analysis, which reveals the production and dissipation terms responsible for the instability, as well as the region in space where the instability originates.
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| 5. |
- Fransson, Jens H. M.
(författare)
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Transition to Turbulence Delay Using a Passive Flow Control Strategy
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 385-393
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Konferensbidrag (refereegranskat)abstract
- Since the mid-50s, until the first proof-of-concept experiment of transition delay using circular roughness elements in 2006, there was a strong consensus within the research community that roughness elements in general promoted the transition to turbulence process. From a series of wind tunnel experiments miniature vortex generators have emerged as being a coveted passive device for transition delay and hence skin-friction drag reduction. These devices are miniature with respect to classical vortex generators typically used for separation control and fall under the appealing category passive flow control device, since it uses the existing energy in the flow and hence no energy has to be added to the control system in order to accomplish the control. The underlying physical mechanism is attributed to an additional term in the perturbation energy equation, when the boundary layer is modulated in the direction orthogonal to the base flow by the control devices, which counteracts the wall-normal production term and, hence, stabilizes the flow. The present paper briefly reviews the current state-of-art of passive flow control using physical devices for transition delay with the implication of reducing skin-friction drag. This passive control strategy has potential to lead to an unforeseen positive impact on the broad spectrum of industrial applications where reducing drag is a daily challenge.
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| 6. |
- Hall, Stephen, et al.
(författare)
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Experimental characterisation of (localised) deformation phenomena in granular geomaterials from sample down to inter- and intra-grain scales
- 2012
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Ingår i: Procedia Iutam. - : Elsevier BV. - 2210-9838. ; 4, s. 54-65
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Tidskriftsartikel (refereegranskat)abstract
- This paper outlines some recent advances in the full-field experimental characterisation of the mechanics of granular geomaterials (in particular, sands) using a range of methods that provide characterisation at different scales, from the sample-scale down to the inter- and intra-grain scale. The techniques used are “full-field” approaches involving in-situ x-ray micro-tomography, 3D-volumetric digital image analysis/correlation and grain ID-tracking, in-situ 3D x-ray diffraction and in-situ, spatially-resolved neutron diffraction. These methods provide new data on the mechanics of sand at different scales, including continuum measures of strain, porosity, and fabric plus discrete measures of particle kinematics and force transmission. The results of such measurements might be used to advance higher-order continuum theories, and provide the necessary input parameters, or to calibrate discrete grain-scale simulations of sand behaviour to explore loading paths that are inaccessible in the laboratory.
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| 7. |
- Hosseini, Seyed Mohammad, et al.
(författare)
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Effect of Freestream Turbulence on Roughness-induced Crossflow Instability
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 303-310
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Konferensbidrag (refereegranskat)abstract
- The effect of freestream turbulence on generation of crossflow disturbances over swept wings is investigated through direct nu- merical simulations. The set up follows the experiments performed by Downs et al. (2012). In these experiments the authors use ASU(67)-0315 wing geometry which promotes growth of crossflow disturbances. Distributed roughness elements are locally placed near the leading edge with a given spanwise wavenumber to excite the corresponding stationary crossflow vortices. In present study, we partially reproduce the isotropic homogenous freestream turbulence through direct numerical simulations using freestream spectrum data from the experiments. The generated freestream fields are then applied as the inflow boundary condition for direct numerical simulation of the wing. The distributed roughness elements are modelled through wing surface deformation and placed near the leading edge to trigger the stationary crossflow disturbances. The effects of the generated freestream turbulence on the initial amplitudes and growth of the boundary layer perturbations are then studied.
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| 8. |
- Lashgari, Iman, et al.
(författare)
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Transition to Turbulence in the Presence of Finite Size Particles
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 211-217
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Konferensbidrag (refereegranskat)abstract
- We study the transition from laminar to turbulent flow in a channel seeded with finite-size neutrally buoyant particles. A fixed ratio of 10 between the channel height and the particle diameter is considered. The flow is examined in the range of Reynolds numbers 500 ≤ Re ≤; 5000 and the particle volume fractions 0.001 ≤ Φ ≤; 0.3. We report a non-monotonic behavior of the threshold value of the Reynolds number above which the flow becomes turbulent, in agreement with previous experimental studies. The mean square velocity fluctuations and Reynolds shear stress of the fluid phase are reduced by increasing the particle volume fraction at a fixed Re=1500, while the mean square velocities of the solid phase are enhanced monotonically suggesting a transition from fluid to particle dominated dynamics at high volume fraction.
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| 9. |
- Paredes, P., et al.
(författare)
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The Nonlinear PSE-3D Concept for Transition Prediction in Flows with a Single Slowly-varying Spatial Direction
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 36-44
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Konferensbidrag (refereegranskat)abstract
- A number of flow cases of practical significance exhibit a predominant spatial direction, along which the mean properties of the flow field vary slowly while having fast variations on the cross-sectional planes. This property is taken into account when the three- dimensional parabolized stability equations (PSE-3D) are derived. These equations represent the most efficient approach for the solution of the instability problem of such flows. In this work, the linear PSE-3D are extended to predict the nonlinear development of perturbations in this kind of complex three-dimensional flows. The newly developed method is formulated and verified for different flow problems of interest. Firstly, it has been verified by computing the evolution of linear and nonlinear Tollmien- Schlichting waves in Blasius boundary layer, showing excellent agreement with traditional nonlinear PSE predictions. Also, the evolution of optimal streaks is simulated and compared against direct numerical simulations. Finally, the nonlinear development of stationary crossflow instabilities in a three-dimensional boundary layer is monitored using a non-orthogonal coordinate system to follow the instability trajectory, showing again a very good agreement with PSE results.
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| 10. |
- Rorai, Cecilia, et al.
(författare)
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Hydrodynamic focusing of an elastic capsule in stokes flow : An exploratory numerical study
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 41-49
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Konferensbidrag (refereegranskat)abstract
- We study numerically the motion of an elastic capsule in a simple microfluidic device, a sheath flow focuser, designed to align (or focus) particles at the center of a microchannel. The geometry of the device is given, while the flow conditions are varied, and the outflux is constant. The efficiency is expressed in terms of the distance required to achieve focusing and the functioning is characterized by the deformations and stresses undergone by the capsule membrane. Calculations are performed for a 'stiff' and 'soft' capsule, corresponding to a capillary number equal to Ca = 0.05 and Ca = 0.3 based on the outflow rate. We report that as the sheath flow is increased the focusing efficiency grows and plateaus beyond a certain threshold, displaying a drop in the efficiency gain. Differently, the deformations and stresses undergone by the capsule membrane grow for the entire range of the flow parameters considered. This should be kept in mind if the stresses exerted on the membrane have to be minimized or need to be below certain characteristic thresholds typical of the specific application considered. Especially large initial offsets and stiff capsules benefit from focusing.
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