Reynolds number effects on a turbulent boundary layer with separation, reattachment, and recovery

The effects of Reynolds number on simple equilibrium turbulent boundary layers have been the subject of intense scrutiny in recent years. Much less attention has been paid to Reynolds number effects in more complex boundary layers where relatively rapid changes in the boundary conditions cause the turbulence to be out of equilibrium with the mean flow. Additional scales beyond the standard inner and outer length scales characterize these flows, and this may lead to more complex Reynolds number behavior. Empirical input is needed to sort out how best to model non-equilibrium flows where the appropriate scaling of the Reynolds-stress components cannot be discerned from simple analyses.; This thesis addresses experimental studies of Reynolds number effects on a separating, reattaching, and recovering boundary layer. One order of magnitude range in Reynolds number is achieved with a wind tunnel enclosed in a pressure vessel by varying the air density and wind tunnel speed. A custom-built, high resolution laser Doppler anemometer with a 35 μm by 60 μm measurement volume provides fully resolved turbulence measurements over the full Reynolds number range. A high accuracy particle image velocimeter is also adapted to study outer-layer flow structures.; The experiments show that the mean flow is at most a very weak function of Reynolds number while turbulence quantities strongly depend on Reynolds number. Roller vortices are generated in the separated shear layer due to the Kelvin-Helmholtz instability. Two-point correlations indicate that the shear layer vortices dominate the outer layer in the recovery region. Empirical Reynolds number scalings for the mean velocity and Reynolds stresses are proposed for the entire flow region. The Reynolds stress scalings are different in each region and complex in the outer layer of the recovering boundary layer where the different stress components relax at different rates.; The near-wall flow recovers quickly downstream of reattachment even if the outer layer is far from an equilibrium state. As a result, a stress equilibrium layer where a flat plate boundary layer scaling is valid develops in the recovery region and grows outward moving downstream.