Analysis Of DDES-GAM Method By High-Order Self-Adaptive Dissipative Scheme

Separated flow is a common complex phenomenon in aeronautical and aerospace engineering,it has a significant influence on the flight performance and load,the char-acteristics of unsteady and multi-scale are also great challenges for numerical simulation methods.Separated flow emerges from the wall with small-scale turbulent structures,which will convert to fully-developed turbulence in further downstream.During the pro-cedure,low-dissipative numerical scheme is needed to resolve these small scale structures,however,the low-dissipative scheme may lead to divergence in complex phenomenon.Meanwhile,as for the medium separated flow,a low transition may exist in the region between RANS and LES,with unprecedented eddy-viscosity,namely the grey area.It may damp the development of Kelvin-Helmholtz instability and delay the further develop-ment.Therefore,based on the high order Hybrid Dissipative Compact Scheme（HDCS）,a Adaptive Dissipative Compact Scheme（ADCS）is proposed by detecting the vortical structures,to realize the self-adjusting numerical dissipation.Besides,a Grey Area Mit-igation Delayed-Detached Eddy Simulation（DDES-GAM）model is developed based on SST-DDES model,it has the ability of mitigating grey area by decreasing eddy-viscosity appropriately.Firstly,the introductions of separated flow,theoretical foundation of hybrid RAN-S/LES method,grey area and influence of numerical dissipation are presented.Sec-ondly,recent advances of adaptive numerical dissipation adjusting method,hybrid RAN-S/LES methods and grey area mitigation methods have been reviewed,and the necessary of the research in the field of self-adaptive dissipation adjusting methods based on high order scheme and grey area mitigation methods is emphasized.The numerical methods of current thesis are also presented,the spatial discretization is based on the fifth-order cell-node/cell-edge hybrid dissipative compact scheme（HDCS）,the implicit dual-time-step method is mainly employed for time integration,the turbulence model is constructed based on the SST-DDES model,the other methods like rigid moving grid are also pre-sented.The self-adaptive numerical dissipation adjusting methods is investigated,the theo-retical foundation of self-adaptive is analyzed and the improved accuracy in typical flow is proved.The spatial discretization is based on the fifth-order HDCS scheme,the variables are interpolated with non-dissipative central interpolation and dissipative upwind-biased interpolation.The Adaptive Dissipative Compact Scheme（ADCS）is proposed by incor-porating self-adaptive dissipation adjusting parameter,which choose strain rate and vor-ticity as major criterion.The ADCS scheme perform as low-dissipative scheme in LES region to improve the resolution of small-scale turbulent structures,and normal dissipa-tive scheme in RANS region to stabilize the simulation.Thereafter,The typical cases are simulated to evaluate the performance of ADCS,the dissipation and dispersion are theoretically analyzed firstly.The two-dimensional vortex transportation is to evaluate the influence of adjusting parameter for vortex conservation.The decay homogeneous isotropic turbulence show the improved accuracy in high wavenumber range with self-adaptive dissipation.The plate channel turbulence with pressure gradient evaluate the stability of ADCS,and the NACA0021 airfoil at large attack angle and Re=3900 circular cylinder exhibit the improvement of ADCS in typical separated flow.The grey area mitigation method is analyzed,the grey area mitigation model is con-structed and the improved performance is shown in typical flows which are influenced by grey area phenomenon.Based on SST-DDES model,the current thesis incorporate shear layer adapted length scaleΔ_SLAas an alternative to conventional length scaleΔ_max,and construct DDES-GAM model with the grey area mitigation ability.TheΔ_SLAcan detect the quasi-2D region in the early shear layer,depending on the orientation of vorticity,by decreasing the eddy viscosity,the Kelvin-Helmholtz instability can be stimulated and the influence of grey area can be alleviated.Firstly,the decay isotropic turbulence shows that the DDES-GAM can exploit the current mesh scale and improve the resolution of turbu-lent structures.Secondly,the improved accuracy of DDES-GAM model in early shear layer region and further downstream separated flow are approved,by simulating mixing layer,backfacing step and wall mounted flow.Thirdly,the performance of DDES-GAM model in 3D complex phenomenon is further approved by delta wing at high attack angle,by alleviating the influence of grey area in leading edge shear layer,the accuracy of typical separated flow can be effectively improved.The grey area in dynamic situation is investigated,the influence of grey area and the improved performance of grey area mitigation in dynamic situation are analyzed.The current thesis incorporates the rigid moving grid with high order numerical scheme,and realize the high order rigid dynamic simulation.Focus on the NACA0012 airfoil rigid forced pitching movement,firstly,the inviscid flow and non-stall viscid flow are simulated to evaluate the accuracy of computational method,the results show that the non-stall flow is unaffected by grey area.Secondly,the pitching shallow and deep separated flow are investigated,the results show that the DDES-GAM can effectively improve the resolution of small-scale turbulent structures in down-stroke separated shear layer,and the accuracy of aerodynamic coefficients can be improved more evidently in deep stalled pitching.The grey area in blunt reentry capsule is investigated,and the influence of grey area in typical reentry capsule with separated flow is analyzed.The current thesis takes Stardust reentry capsule in subsonic state as subject,firstly,the static state is investigated,it shows that the DDES-GAM model can effectively improve the resolution of turbulent near the shoulder of capsule,with a more accurate aerodynamic coefficients.Secondly,the rigid pitching state is simulated,the influence of amplitude,reduced frequency and angle of attack are investigated,the results show that with increased amplitude and decreased re-duced frequency,the influence of grey area is more evident,but it does not show a linear relationship with angle of attack.The current thesis focus on the typical separated flow,investigates the influence of numerical dissipation and grey area based on the high order HDCS scheme and DDES model.The high order self-adaptive dissipative compact ADCS scheme and grey area mitigation model based on the SST-DDES model are constructed,and then the improved performance of modified scheme and model are investigated by typical cases,the current thesis may have a range of implication towards numerical simulation for separated flow.