The effects of expansion regions on the turbulence structure of compressible boundary layers

The effects of four expansions (centered and gradual expansions of 7° and 14°) on a fully-developed Mach 3 turbulent boundary layer were investigated with Laser Doppler Velocimetry (LDV). The fiat plate boundary layer was also interrogated extensively. Although essentially full recovery of the mean velocity profile is captured downstream of the expansions, the boundary layer turbulence remains far from equilibrium at the last measurement station (s/δ = 20). Computations with the rotational method of characteristics indicate the evolution of the mean velocity profile downstream of the expansions is largely inviscid for approximately 10δ. Turbulence levels decrease across the expansion with more significant reductions near the surface. As expected, reductions are more severe for the 14° expansions. In fact, downstream of the two 14° expansions, reverse transition is indicated by sharp reductions in the turbulent kinetic energy levels and a change in sign of the Reynolds shear stress. Dimensionless turbulence parameters such as the anisotropy and shear stress correlation coefficient are also altered substantially, indicating the turbulence undergoes a complex evolution downstream of the expansions.