Effect of a rapid expansion on the development of compressible free shear layers

Abstract

Detailed mean velocity and turbulence data have been obtained with a laser Doppler velocimeter for two axisymmetric shear layers downstream of rapid expansions of different strengths. A comparison of the data in the near field (immediately downstream of separation) and far field (shear layer approaching self-similarity) is presented, and the fluid dynamic effects of the rapid expansion are ascertained for each regime. In general, the rapid expansion was found to distort the initial mean velocity and turbulence fields in the shear layer, in a manner similar to that in rapidly expanded, attached supersonic boundary layers; namely, two distinct regions were found in the initial shear layer: an outer region, where the turbulent fluctuations are quenched primarily due to mean compressibility effects (bulk dilatation), and an inner region, where turbulence activity is magnified due to the interaction of organized large-scale structures in the shear layer with low-speed fluid at the inner edge. With increasing strength of the rapid expansion, the effects in both regions become more pronounced, especially in the inner region, where turbulent fluctuations and mass entrainment rates are greatly magnified. Farther downstream, the turbulence activity of the large-scale eddies remains elevated, due to the rapid expansion, even though the relative distribution of the turbulence energy between the Reynolds stress components (structure of the turbulence) is independent of expansion strength.