Detached-Eddy Simulation Of Aircrafts At High Incidence Based On Hybrid Grids

The flow field over a modern aircraft at high angle of attack is dominated by large areas of flow separation,complex vortical structures and unsteady flows,so it is important to predict accurately these phenomenons and understand the flow physics.However,so far it is very difficult to accurately predict such flows with traditional second-order RANS solver.One reason is that the aircraft configurations are too complex,so it is hard to generate high-quality meshes.Another reason is that the unsteady,nonlinear turbulence flow mechanisms pose serious challenges to the numerical method.To deal with these issues,three key factors for numerical simulations,namely turbulence model,numerical scheme and grid generation technique,are studied in this article.The purpose is to establish a more reliable numerical method for engineering applications and improve the ability to predict aerodynamic characteristics of complex aircraft configurations at high angle of attack.Because the unstructured/hybrid grid has a good property for complex configurations,it is adopted for all computational cases in this paper.In order to simulating complex(e.g.high angle of attack)turbulent separated and hence vortical flows,the currently most promising detached eddy simulation(DES)method is chosen and developed in the second-order finite volume framework.Considering that the dissipative and dispersive errors of the traditional second order upwind schemes are relatively large and will reduce the resolution of the multi-scale turbulent structures,an improved second-order self-adaptive dissipation scheme is developed based on original Roe's scheme,and the numerical dissipation issue in DES simulations can be alleviated.Meanwhile a grid self-adaptive method is established,which can effectively increase the spatial resolution of turbulence flow field.Finally,the numerical method developed in this paper is applied in the IDDES simulations of a delta wing and a modern fighter aircraft at high angle of attack and some preliminary numerical investigation is also carried out for the large amplitude pitching motion of the aircraft.Numerical results show good agreement with the experimental data,which demonstrates a good prospective of the algorithm for realistic applications.This paper is divided into seven chapters.Chapter 1 is the introduction.Firstly,we introduce the background and significance of the research on aerodynamic characteristics of aircraft at high angle of attack.Secondly,we present a brief review of the progress in this field,including physical mechanisms,experimental techniques,numerical discretization methods,turbulence simulation approaches,grid generation techniques,and so on.Finally,the main contents of this paper are outlined.The numerical method used in this paper is proposed in Chapter 2.To improve the turbulence simulation ability of the second order finite-volume algorithm,a hybrid second order scheme is developped through modifying the dissipation term of the standard Roe's flux-difference splitting scheme,and the numerical dissipation of the scheme can be self-adaptive according to the Detached-Eddy Simulation(DES)flow field information.By Fourier analysis,the dissipative and dispersive features of the new scheme are discussed,which indicates that the second order scheme can be expected to have a good performance in DES applications.The DES methods based on both the one equation Spalart-Allmaras(SA)turbulence model and the two equation k-? Shear Stress Transport model(SST)are established in this work.Chapter 3 gives some numerical example to validate the credibility of our numerical method.First,the model constants are calibrated using the canonical test case of decaying,isotropic turbulence(DIT).Then,a wide range of numerical examples are presented,including subsonic flow over a cylinder with Reynolds number(Re)3900,NACA0021 airfoil at 60° incidence,supersonic base flow and subsonic flow over a pitching NACA0012 airfoil.The computational results demonstrate that the DES method is definitely superior over conventional(U)RANS approaches,meanwhile compared with the standard second order Roe's scheme,the hybrid scheme can effectively control the numerical dissipation in DES,resulting in higher resolution of vortex structures and more reasonable agreement with the experimental data.Chapter 4 presents the mixed element grid self-adaptation technique established in this paper.It is anticipated to carry out a combined application of the DES method,the hybrid scheme and a proper mesh density,so that the numerical resolution of flow field can increase efficiently.An h-type refinement is used in this work.The hanging-nodes generated in the adaption procedure are eliminated by inserting additional point into related cells,so that a consistent refinement pattern is obtained and the flow solver may operate on the resulting mesh without any modification.And finally,some simple test cases are shown to validate the code.IDDES simulations of vortical flow around a 65° swept delta wing is carried out in Chapter 5.For the IDDES simulation using the original Roe's scheme,only a bubble type vortex breakdown can be observed,while for the IDDES simulation using hybrid scheme,not only the bubble type but also a spiral type and a double spiral type vortex breakdown can be observed.For the IDDES simulation with grid self-adaptation technique,more small scale vortices can be captured.Chapter 6 shows the numerical simulation of a complex modern aircraft configuration,a F22-like fighter.First,we give a brief introduction of the wind tunnel test about the aircraft model.Then,IDDES computations of the model are performed in static cases at high angle of attack.The comparison of simulated aerodynamic coefficients and experimental data indicates that IDDES still has obvious advantages over RANS,and meanwhile more refined three-dimensional flow structures can be observed in the IDDES solutions with hybrid scheme.Finally,we carry out a preliminary computation of pitching motion of the fighter model.The IDDES results show good agreement with experimental data also.Chapter 7 summarizes the work of this paper,points out the deficiencies,and suggests the future research directions.