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

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 such that two distinct regions were evident: an outer region where the turbulent fluctuations are quenched by the expansion, and an inner region where turbulence levels are magnified by the expansion. For the streamwise Reynolds normal stress and primary Reynolds shear stress, the magnitude of the peak turbulence levels increased with increasing strength of the rapid expansion; the transverse normal stress field, however, was only mildly affected by the expansion. Further downstream after the shear layer mean velocity distributions become self-similar, elevated turbulence levels for the more strongly expanded case persist although the relative distribution of turbulence energy between the Reynolds stress components appears unaffected by the strength of the rapid expansion.