Local effects of large-scale eddies on bursting in a concave boundary layer

Abstract

Concave curvature has a destabilizing effect on a turbulent boundary layer that causes the formation of large-scale inflow and outflow regions. These structures are larger and more energetic than large eddies in a flat boundary layer, particularly in terms of velocity fluctuations normal to the wall. Flow visualization has suggested that the large-scale inflows and outflows have a strong influence on turbulence structure in the near-wall region. However, near-wall profiles of Reynolds-averaged quantities in the concave boundary layer do not indicate major structural changes. In this paper, the effects of concave curvature on near-wall structure are investigated in two flow cases: (i) the natural concave boundary layer, where the large-scale eddies do not have preferred spanwise locations and the layer remains nearly two-dimensional in the means; and (ii) a case in which vortex generators are used to induce a fixed array of longitudinal roll cells, allowing measurements to be made under stationary inflow and outflow regions. Burst frequencies obtained using an extension of the uv-quadrant method confirm the visual impression that inflows suppress the bursting process, while outflows enhance it. Reynolds-averaged measurements show that turbulence intensity and the uv correlation coefficient are also suppressed under the inflows. Based on these results, a conceptual model for the effects of large-scale eddies on near-wall flow and skin friction in a concave layer is proposed.