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  • 1.
    Le Moigne, Yann
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Adaptive Mesh Refinement and Simulations of Unsteady Delta-Wing Aerodynamics2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This thesis deals with Computational Fluid Dynamics (CFD)simulations of the flow around delta wings at high angles ofattack. These triangular wings, mainly used in militaryaircraft designs, experience the formation of two vortices ontheir lee-side at large angles of attack. The simulation ofthis vortical flow by solving the Navier-Stokes equations isthe subject of this thesis. The purpose of the work is toimprove the understanding of this flow and contribute to thedesign of such a wing by developing methods that enable moreaccurate and efficient CFD simulations.

    Simulations of the formation, burst and disappearance of thevortices while the angle of attack is changing are presented.The structured flow solver NSMB has been used to get thetime-dependent solutions of the flow. Both viscous and inviscidresults of a 70°-swept delta wing pitching in anoscillatory motion are reported. The creation of the dynamiclift and the hysteresis observed in the history of theaerodynamic forces are well reproduced.

    The second part of the thesis is focusing on automatic meshrefinement and its influence on simulations of the delta wingleading-edge vortices. All the simulations to assess the gridquality are inviscid computations performed with theunstructured flow solver EDGE. A first study reports on theeffects of refining thewake of the delta wing. A70°-swept delta wing at a Mach number of 0.2 and an angleof attack of 27° where vortex breakdown is present abovethe wing, is used as testcase. The results show a strongdependence on the refinement, particularly the vortex breakdownposition, which leads to the conclusion that the wake should berefined at least partly. Using this information, a grid for thewing in the wind tunnel is created in order to assess theinfluence of the tunnel walls. Three sensors for automatic meshrefinement of vortical flows are presented. Two are based onflow variables (production of entropy and ratio of totalpressures) while the third one requires an eigenvalue analysisof the tensor of the velocity gradients in order to capture theposition of the vortices in the flow. These three vortexsensors are successfully used for the simulation of the same70° delta wing at an angle of attack of 20°. Acomparison of the sensors reveals the more local property ofthe third one based on the eigenvalue analysis. This lattertechnique is applied to the simulation of the wake of a deltawing at an angle of attack of 20°. The simulations on ahighly refined mesh show that the vortex sheet shed from thetrailing-edge rolls up into a vortex that interacts with theleading-edge vortex. Finally the vortex-detection technique isused to refine the grid around a Saab Aerosystems UnmannedCombat Air Vehicle (UCAV) configuration and its flight dynamicscharacteristics are investigated.

    Key words:delta wing, high angle of attack, vortex,pitching, mesh refinement, UCAV, vortex sensor, tensor ofvelocity gradients.

  • 2.
    Le Moigne, Yann
    KTH, Superseded Departments, Aeronautical and Vehicle Engineering.
    Adaptive mesh refinement sensors for vortex flow simulations2004Conference paper (Refereed)
    Abstract [en]

    This paper presents results obtained with different sensors used to automatically refine a CFD mesh for vortex-dominated flow applications. The paper focuses on the sensors used to locate a vortex in a flow field in order to refine the mesh in its vicinity. Several approaches are evaluated: the first type of methods is based on flow quantities that are commonly used to visualize vortices in a flow field e.g. ratio of total pressures and creation of entropy (in this case all the cells with values above or below a certain threshold are refined); the second approach identifies the vortex core in order to refine the cells close to it. The identification in this latter case is done by analysing the eigenvalues of the tensor of the velocity gradients. The three sensors are compared through the h-refinement of a mesh around a 70° -swept delta wing. The calculations, for inviscid conditions at an angle of attack of 20° and a Mach number of 0.2, are performed with the unstructured flow solver EDGE. All sensors capture the vortex very well and lead to improved resolution in the vortex. This produces a solution with a stronger and sharper vortex as well as an improved pressure distribution on the wing. The sensor based on the eigenvalue analysis of the velocity-gradient tensor concentrates on the vortex core and the shear layer coming from the leading-edge. An analysis of the influence of the threshold chosen to start the refinement reveals that this latter sensor is more local than the two others and less sensitive to the choice of the threshold, which makes it the most promising candidate for a fully automatic refinement process without user intervention.

  • 3.
    Le Moigne, Yann
    KTH, Superseded Departments, Aeronautical Engineering.
    CFD simulations of delta wing pitching to high alpha2002Licentiate thesis, comprehensive summary (Other scientific)
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