<正> Unsteady supersonic base flows around threeafterbodies,cylindrical(Cy),boattailed(BT)and three-step(MS),are investigated in this paper.Reynolds-averagedNavier-Stokes(RANS)and two RANS/LES(large-eddysimulation)hybrid methods,detached eddy simulation(DES)and delayed-DES(DDES),are used to predict the base flowcharacteristics around the baseline Cy afterbody.All theRANS and hybrid methods are based on the two-equationSST(shear-stress transport)model with compressible correc-tions(CC).According to the comparison of measurements,both DES and DDES can produce more satisfactory resultsthan RANS.RANS can only present the 'stable' flow pat-terns,while the hybrid methods can demonstrate unsteadyflow structures.DDES and DES results are little differentfrom one another although the latter exhibits better agree-ment with the experiment.DES is taken to investigate the5~ BT and three-step afterbodies.The mean flow data andthe instantaneous turbulent coherent structures are comparedagainst available measurements.
Zhixiang Xiao Song Fu School of Aerospace Engineering,Tsinghua University, 100084 Beijing, China
Based on Reynolds-averaged Navier-Stokes approach,a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows.The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy,ωthe specific dissipation rate and the intermittency factorγ.The particular features of the model are that:1)k includes the non-turbulent,as well as turbulent fluctuations;2)a transport equation for the intermittency factorγis proposed here with a source term set to trigger the transition onset;3)through the introduction of a new length scale normal to wall,the present model employs the local variables only avoiding the use of the integral parameters,like the boundary layer thicknessδ,which are often cost-ineffective with the modern CFD(Computational Fluid Dynamics)methods;4)in the fully turbulent region,the model retreats to the well-known k-ωSST(Shear Stress Transport)model.This model is validated with a number of available experiments on boundary layer transitions including the incompressible,supersonic and hypersonic flows past flat plates,straight/flared cones at zero incidences,etc.It is demonstrated that the present model can be successfully applied to the engineering calculations of a variety of aerodynamic flow transition.
在 1990 年代初执行的以前的研究推测了主要可压缩的效果能与速度变化的 dilatational 效果被联系。后来，主要压缩的可能性效果由于减少的压力变化来自减少的压力紧张术语，这被显示出。尽管可压缩的骚乱的更好的理解通常与增加的 DNS 和试验性的研究努力被完成，仍然在这些最近的调查结果之中有一些差异。如果可压缩的效果与狂暴的马赫数字有关， DNS 和试验性的数据的分析建议一些差异是明显的， M t 。从可压缩的流动的二个班的比较，同质砍流动并且不同类砍流动(混合的层) ，我们发现两个班上的压缩的可能性的效果砍流动能在相应于狂暴的马赫数字的三个区域的三个范畴被描绘：low-M t , moderate-M t 并且 high-M t 区域。在这三个区域，狂暴的混合层厚度的生长率上的压缩的可能性的效果是相当不同的。减少的压力紧张效果的一条简单途径不能必然减少混合层生长率，并且可以甚至处于生长率引起增加。现在的工作通过 M t 在流动上说明压缩的可能性效果。模型成功地被用于可压缩的混合层。
Even though a number of rapid pressure-strain models have been suggested and successfully tested for different flow situations by various authors,the model proposals still exhibit some apparent deficiencies when subjected to the flows with rapid distortion. From Mansour's relatively straightforward rapid distortion analysis,if an initially anisotropic flow undergoes a purely rapid rotation,the anisotropy measures will exhibit the behavior of the damped oscillations. Within the current framework of modeling the rapid pressure-strain correlation,i.e.,the models based on the assumption that the M-tensor for the rapid pressure-strain term is expand-able in the Reynolds-stress anisotropy tensor alone,all the model predictions fail to give the damped oscillations in the turbulence anisotropy. In the case of initially isotropic turbulence subjected to rapid distortion,Sj?gren and Johansson showed that all the existing rapid pressure-strain models would deliver the identical path in the anisotropy-invariant map for both homogeneous plane strain and shear flows. The rapid distortion analysis shows two distinct curves reflecting different flow physics. In this work,we try to present a possible way to create a system that can overcome these deficiencies with the aid of the rapid distortion theory (RDT).
The proper orthogonal decomposition(POD) method was applied to analyzing the database obtained from the direct numerical simulation(DNS) of supersonic plane mixing layers.The effect of different forms of the inner products in the POD method was investigated.It was observed that the mean flow contributes to a predominant part of the total flow energy,and the energy spectrum of the turbulence fluctuations covers a wide range of POD modes.The patterns of leading(high energy) POD modes reveal that the flow structures exhibit spanwise counter rotating rolls,as well as oblique vortices.These flow patterns are insensitive to the velocity of the observer.As the convective Mach number increases,the energy spectrum be-comes wider,the leading POD modes contain more complicated structures,and the flow becomes more chaotic.
