Numerical investigation on the structure of a confined supersonic mixing layer

Abstract

Direct numerical simulations using the high-resolution TVD scheme are performed for the study of two-dimensional confined spatially growing supersonic mixing layers. Numerical simulations, being forced by the linear instability wall modes, are justified by comparing the calculated results with both theoretical and experimental data. The numerically visualized downstream compression/expansion wave system explains the so-called ‘‘strange waves’’ observed in experiments and are found to be responsible for the rapid growth of the mixing layer in that region. The existence of shock waves would lead to mixing enhancement by accelerating the process of extracting turbulent kinetic energy from the mean stream. However, the phenomenon of shock-induced mixing improvement is found quite local around the shock impingement area, and often is followed by a saturation in turbulent energy absorption which impedes the further growth of the shear layer in the immediate downstream region. For the present cases considered, the subharmonic instability modes are not dominant, and the baroclinic-torque effect is comparatively less important for the vorticity development. These observations suggest that compressibility effects may behave in a considerably different manner as the underlying instability mechanism changes from the subsonic Kelvin–Helmholtz type to the present supersonic wall instability type.