Structure of a turbulent boundary layer on a concave surface

Abstract

The effects of concave curvature on turbulent boundary-layer structure are investigated, using flow visualization and two-component laser-Doppler anemometry. Destabilizing curvature amplifies large-scale motions normal to the wall. When the boundary layer entering the curve is free of spanwise non-uniformities, the resulting large-eddy structure does not consist of distinct longitudinal vortices, as suggested by some previous studies. Rather, the visualized flow is dominated by large eddies (inflows and outflows) that have a streamwise extent of only a few boundary-layer thicknesses, are quite unsteady, and do not cause significant spanwise variations in the mean properties of the boundary layer. Mixing across the boundary layer is enhanced by the new eddy structure, bringing high-momentum fluid closer to the wall than in a normal, flat boundary layer and causing a significant increase in skin friction. Spectral results show that increases in turbulence intensities and Reynolds shear stress across the outer layer are due almost entirely to increased energy in low-frequency, large-scale fluctuations.Flow visualization suggests that the large-scale inflows and outflows have strong influence on flow structure in the near-wall region. However, when the local value of the friction velocity, uτ, is used for scaling, near-wall profiles of Reynolds-averaged quantities show relatively minor differences between the flat and concave cases.The response of the boundary layer to the sudden onset of concave curvature is found to involve two overlapping stages. First, a centrifugal mechanism causes higher-velocity eddies near the start of curvature to migrate toward the wall, while lower-velocity eddies migrate away from the wall. These negatively correlated motions produce an increase in the magnitude of the correlation coefficient within a few initial boundary-layer thicknesses (δ0) from the start of curvature. The further development of the layer requires the slower growth and amplification of the large-scale inflows and outflows. This development of the new large-scale eddy structure continues for at least 20 δ0.