Roll-cell structure in a concave turbulent boundary layer

Flow visualization by laser-induced fluorescence and velocity profiles obtained by laser-doppler anemometry are used to study the structure of roll cells in a turbulent boundary layer on a concave surface. Vortex generators are used to lock the roll cells into stable spanwise locations, and the resulting flow is compared to the natural flow, in which the roll cells wander about, merge, separate, appear and disappear. In both cases the spanwise-average friction coefficient, C(f), is increased by about 40 percent relative to flat-plate values. A significant lag in the response of C(f) to the onset of curvature suggests that the large-scale eddy structure in the boundary layer must be modified before the full effect on skin friction is seen. Mean velocity profiles, which extend as far into the sublayer as Y(+) = 1.5, demonstrate that strong concave curvature produces a negative wake. This result confirms the visual evidence of vigorous mixing which accompanies larger than normal length scales across the boundary layer.