| 1. |
- Dadfar, Reza, 1978-
(författare)
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Active Control and Reduced-Order Modeling of Transition in Shear Flows
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis direct numerical simulation is used to investigate the possibilityto delay the transition from laminar to turbulent in boundary layer flows.Furthermore, modal analysis is used to reveal the coherent structures in highdimensional dynamical systems arising in the flow problems.Among different transition scenarios, the classical transition scenario isanalysed. In this scenario, the laminar-turbulent transition occurs when Tollmien-Schlichting waves are triggered inside the boundary layer and grow exponentiallyas they move downstream in the domain. The aim is to attenuate the amplitudeof these waves using active control strategy based on a row of spatiallylocalised sensors and actuators distributed near the wall inside the boundarylayer. To avoid the high dimensional system arises from discretisation of theNavier Stokes equation, a reduced order model (ROM) based on EigensystemRealisation Algorithm (ERA) is obtained and a linear controller is designed.A plasma actuator is modelled and implemented as an external forcing on theflow. To account for the limitation of the plasma actuators and to further reducethe complexity of the controller several control strategies are examinedand compared. The outcomes reveal successful performance in mitigating theenergy of the disturbances inside the boundary layer.To extract coherent features of the wind turbine wakes, modal decompositiontechnique is employed where a large scale dynamical system is reduced toa fewer number of degrees of freedom. Two decomposition techniques are employed:proper orthogonal decomposition and dynamic mode decomposition.In the former procedure, the flow is decomposed into a set of uncorrelated structureswhich are rank according to their energy. In the latter, the eigenvaluesand eigenvectors of the underlying approximate linear operator is computedwhere each mode is associated with a specific frequency and growth rate. Theresults revealed the structures which are dynamically significant to the onsetof instability in the wind turbine wakes.
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| 2. |
- Jesudasan, Rejish, et al.
(författare)
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PRELIMINARY AERODYNAMIC WING DESIGN OPTIMISATION FOR WING-IN-GROUND EFFECT AIRCRAFT
- 2022
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Ingår i: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022. - : International Council of the Aeronautical Sciences. ; , s. 3106-3118
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Konferensbidrag (refereegranskat)abstract
- Wing-in-Ground (WIG) effect aircraft have become an interesting concept in the context of reducing the environmental footprint and increasing the speed of coastal transport. However, obtaining an efficient wing shape in a cost-effective manner is an elusive goal in the early stages of any aircraft design. In this work, a multi-objective wing planform optimisation methodology is proposed by combining a parametric shape modeller OpenVSP, a low fidelity solver VSPAERO and Non-dominated Sorting Genetic Algorithm (NSGA-II) to support the preliminary design of wing-in-ground effect aircraft. Methodology is demonstrated by performing three different wing planform optimisations ranging from planar wing optimisation to nonplanar wingtip optimisation by improving both lift to drag ratio and static height stability characteristics of a wing planform in ground effect. The analysis of the Pareto optimal solutions suggests that when employing the Vortex Lattice Method (VLM) for aerodynamics and stability derivatives computation, the optimiser converged to drooped wing type configuration which enhances both aerodynamic efficiency and static height stability characteristics of wing-alone configuration.
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| 3. |
- Amoignon, Olivier, et al.
(författare)
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Shape optimization for delay of laminar-turbulent transition
- 2006
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Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics (AIAA). - 0001-1452 .- 1533-385X. ; 44:5, s. 1009-1024
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Tidskriftsartikel (refereegranskat)abstract
- A method using gradient-based optimization is introduced for the design of wing profiles with the aim of natural laminar How, as well as minimum wave drag. The Euler equations of gasdynamics, the laminar boundary-layer equations for compressible flows on infinite swept wings, and the linear parabolized stability equations (PSE) are solved to analyze the evolution of convectively unstable disturbances. Laminar-turbulent transition is assumed to be delayed by minimizing a measure of the disturbance kinetic energy of a chosen disturbance, which is computed using the PSE. The shape gradients of the disturbance kinetic energy are computed based on the solutions of the adjoints of the state equations just named. Numerical tests are carried out to optimize the RAE 2822 airfoil with the aim to delay simultaneously the transition, reduce the pressure drag coefficient, and maintain the coefficients of lift and pitch moments. Constraints are also applied on the geometry. Results show a reduction of the total amplification of a large number of disturbances, which is assumed to represent a delay of the transition in the boundary layer. Because delay of the transition implies reduction of the viscous drag, the present method enables shape optimization to perform viscous drag reduction.
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| 4. |
- Brynjell-Rahkola, Mattias, 1986-, et al.
(författare)
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On the stability of flat plate boundary layers subject to localized suction
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The stability of the Blasius boundary layer subject to localized suction is revisited. Using tools of global stability analysis, the leading direct and adjoint eigenpairs are determined, and novel insight into the sensitivity and receptivity of the flow is obtained. The problem is addressed through high-order spectral element simulations, which enables the inclusion of a suction pipe into the domain. Due to this, a detailed investigation of the connection between the pipe flow and the boundary layer flow is possible. For all cases investigated, the former always turns out to transition for a lower Reynolds number and suction rate than the latter, and the transition scenario is found to be due to a global instability originating inside a separation bubble at the pipe inlet. Identification of such regions, provides information that is valuable in further development of algorithms for laminar flow control.
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| 5. |
- Brynjell-Rahkola, Mattias, 1986-, et al.
(författare)
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Onset of global instability behind distributed surface roughness in a Falkner–Skan–Cooke boundary layer
- 2015
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A three-dimensional linear global stability analysis of a Falkner–Skan–Cooke boundary layer with distributed three-dimensional surface roughness is performed. The Falkner–Skan–Cooke boundary layer models the flow over swept airplane wings, and investigation of the critical roughness size for which a global instability emerges is thus of great importance within aeronautical applications. The study considers high-order direct numerical simulations and shows that such a critical roughness height exists for the Falkner–Skan–Cooke boundary layer. The roughness Reynolds number and roughness element aspect ratio for which this happens is comparable to the transition data reported in the literature for two-dimensional boundary layers. This demonstrates the importance of the local flow conditions in the vicinity of the roughness for triggering a global instability, although the resulting breakdown scenario is completely different from that of two-dimensional boundary layers. This breakdown scenario is studied in detail, and a global energy analysis is used to reveal the structures and mechanisms responsible for production and dissipation of perturbation energy.
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| 6. |
- Kleine, Vitor G., 1986-, et al.
(författare)
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Simulating airplane aerodynamics with body forces: an actuator line method for non-planar wings
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Two configurations typical of fixed-wing aircraft are simulated with the actuator line method (ALM): a wing with winglets and a T-tail. The ALM is extensively used in rotor simulations, modeling the presence of blades by body forces, calculated from airfoil data and the relative flow velocity. This method has not been used to simulate airplane aerodynamics, despite its advantage of allowing courser grids. This may be credited to the failure of the uncorrected ALM to accurately predict forces near the tip of wings, even for simple configurations. The conception of the vortex-based smearing correction showed promising results, suggesting such failures are part of the past. For the non-planar configurations studied in this work, differences between the ALM with the original smearing correction and a non-linear lifting line method (LL) are observed near the intersection of surfaces, because the circulation generated in the numerical domain differs from the calculated corrected circulation. A vorticity magnitude correction is proposed, which improves the agreement between ALM and LL. This second-order correction resolves the ambiguity in the velocity used to define the lift force. The good results indicate that the improved ALM can be used for airplane aerodynamics, with an accuracy similar to the LL.
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| 7. |
- Kleine, Vitor, 1986-, et al.
(författare)
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Simulating Airplane Aerodynamics with Body Forces : Actuator Line Method for Nonplanar Wings
- 2023
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Ingår i: AIAA Journal. - : American Institute of Aeronautics and Astronautics (AIAA). - 0001-1452 .- 1533-385X. ; 61:5, s. 2048-2059
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Tidskriftsartikel (refereegranskat)abstract
- Two configurations typical of fixed-wing aircraft are simulated with the actuator line method (ALM): a wing with winglets, and a T tail. The ALM is extensively used in rotor simulations to model the blades by body forces, which are calculated from airfoil data and the relative flow velocity. This method has not been used to simulate airplane aerodynamics, despite its advantage of allowing coarser grids. This may be credited to the failure of the uncorrected ALM to accurately predict forces near the tip of the wings, even for simple configurations. The recently proposed vortex-based smearing correction shows improved results, suggesting those limitations are part of the past. For the nonplanar configurations studied in this work, differences between the ALM with the original smearing correction and a nonlinear lifting line (LL) method are observed near the intersection of surfaces because the circulation generated in the numerical simulation differs from the calculated corrected circulation. A vorticity magnitude correction is proposed, which improves the agreement between the ALM and the LL method. This second-order correction resolves the ambiguity in the velocity used to define the lift force. The good results indicate that the improved ALM can be used for airplane aerodynamics, with an accuracy similar to the LL method.
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| 8. |
- Negi, Prabal, et al.
(författare)
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Global Stability of rigid-body-motion fluid-structure-interaction problems
- 2019
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A rigorous derivation and validation for linear fluid-structure-interaction (FSI) equations for a rigid-body-motion problem is performed in an Eulerian framework. We show that the “added-stiffness” terms arising in the formulation of Fanion et al. (2000) vanish at the FSI interface in a first-order approximation. Several numerical tests with rigid-body motion are performed to show the validity of the derived formulation by comparing the time evolution between the linear and non-linear equations when the base flow is perturbed by identical small-amplitude perturbations. In all cases both the growth rate and angular frequency of the instability matches within 0.1% accuracy. The derived formulation is used to investigate the phenomenon of symmetry breaking for a rotating cylinder with an attached splitter-plate. The results show that the onset of symmetry breaking can be explained by the existence of a zero-frequency linearly unstable mode of the coupled fluid-structure-interaction system. Finally, the structural sensitivity of the least stable eigenvalue is studied for an oscillating cylinder, which is found to change significantly when the fluid and structural frequencies are close to resonance.
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| 9. |
- Negi, Prabal
(författare)
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Stability and transition in pitching wings
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aeroelastic stability of airplanes is one of the most important aspects of airplane design. Flutter or divergence instabilities arising out of the interaction of fluid forces and structural elasticity must be avoided by design or through the limitation of the flight envelope. Classical unsteady theories have been established since the 1930s however, recent investigations with laminar wings and in transitional flows have found the theories to be unreliable in these regimes. The current work investigates the flow around unsteady airfoils in these flow regimes. A linear framework for the stability analysis of fluid-structure-interaction (FSI) problems is derived and validated. The derived formulation is then used to investigate the changes in the structural sensitivity of an eigenvalue for an oscillating cylinder, which is found to change significantly when the fluid and structural systems are close to resonance. The linear stability analysis is then applied to investigate the aeroelastic stability of a NACA0012 airfoil with a free pitch-deegree-of-freedom at transitional Reynolds numbers. The stability results of the coupled FSI system are found to be in good agreement with previously performed experimental results and were able to predict the onset of aeroelastic pitch-oscillations. The boundary layer evolution for a natural laminar flow airfoil undergoing forced small-amplitude pitch-oscillations is investigated at Rec = 7.5×105. Large changes in laminar-to-turbulent transition location are found throughout the pitch cycle which cause a non-linear aerodynamic force response. The origins of the non-linear unsteady aerodynamic response is explained on the basis of the phase-lagged quasi-steady evolution of the boundary layer. A simple empirical model is developed using the phase-lag concept to model the unsteady aerodynamic forces which fits the experimental data very well. On the other hand, the forced pitching investigation at Rec = 1.0×105 for the same airfoil found abrupt changes in transition during the pitch-cycle. A local stability analysis in the reverse flow region indicates that the stability characteristics of the LSB change character from convective to absolute, and it is conjectured that this change in stability characteristics may be the cause of abrupt changes inboundary-layertransition.
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| 10. |
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