Aspects of turbulent boundary-layer separation

Abstract Features of two-dimensional and three-dimensional separating turbulent boundary layer flows are discussed. For the two-dimensional cases, the behavior and structure of strong adverse-pressure-gradient separating flows over streamlined surfaces and backward-facing step separations are reviewed. There are a number of differences from attached flows: normal stress effects in the Reynolds-averaged momentum and turbulence kinetic energy equations are strong; the backflow is dominated by large-scale unsteady motions that produce the mean velocity profile, low Reynolds shearing stresses, and the turbulence energy diffusion toward the wall, which is balanced by dissipation; the non-equilibrium turbulent structure requires stress transport equation(s) to model these flows. Recent developments in laser-Doppler anemometry permit new, more detailed measurements of mean three-dimensional turbulent flow. The turbulent structure is strongly anisotropic and simple isotropic algebraic turbulence models do not describe the shearing stresses. Non-equilibrium lags of the turbulent structure relative to the mean flow structure also require stress transport equation(s) for modeling. Features of the flow structure around a 6 : 1 prolate spheroid at angle of attack and around a wing/body junction are discussed. In the first case, an open separation with no flow reversal is examined. For the wing/body junction, an unsteady horseshoe vortex structure with flow reversal is present in front of the leading edge of the wing.