可压缩混合层的涡结构演化与流质混合

Vortex structures and mixing of compressible mixing layer

为研究可压缩混合层的流动结构,采用七阶精度广义紧致格式离散对流项和用显式八阶精度的中心格式离散粘性项,数值求解了非定常三维可压缩Navier-Stokes方程。用约4亿规模的网格,直接数值模拟了对流马赫数为0.7的超声速可压缩混合层的空间发展流动,获得了自初始流动失稳直至充分发展湍流流动结构的精细演化历程,所得结果表明:大尺度涡结构的生成使得混合层的动量厚度快速增长,并主宰了由被动标量质量分数展示的可视混合厚度的量级,充分发展湍流的小尺度的结构主要使得该量级厚度内的流质混合趋于均匀,对可视厚度增长的贡献非常有限。

Using the seventh-order upwind compact scheme for spatial discretization of the convective flux,the explicit central eighth-order scheme for viscous flux terms,the unsteady three-dimensional com-pressible Navier-Stokes equations are solved for the compressible spatially evolving plane mixing layer.The direct numerical simulation （DNS）is performed without resorting to any sub-grid models.The convective Mach number of the mixing layer is chosen to be 0.7 for characterizing the moderate compressibility.The fi-nite difference grid size is 4160x351x256.The spatial developing structures are shown in detail from the in-itial instability flow until the full development of turbulent flow.The results show that the large-scale vortex structures,such as Λ vortices,hairpin vortices and flower structures,make the momentum thickness grow rapidly along the streamwise flow.The discussions are focused on the capacity of the moderate compressible shear layer to mix the fluids from the two different sides.It is found that the large-scale vortex structures significantly affect the thickness of the visual layer thickness shown by the mass fraction of the passive sca-lar.Particularly,strong pumping flows between the two counter-rotating vortices of Λ vortices as well as hairpin vortices injects large amounts of fluid from one side to the other side.The small vortices contained in the flower structures also help mix the fluids through their energetic twisting motion,only at a relatively small scale.Generally speaking,the dynamic evolution of these large-scale vortex structures produces very curvy interfaces between the upper side and the lower side,and the large contact area of the curvy interfaces appreciably enhances the mixing between the fluids from the two sides.The fine scale structures in the fully developed turbulence play an important role in making the mixing of the shear layer more uniform.However the contribution of the fully developed turbulence to the visual mixing layer growth is very limited.