The effect of curvature on boundary layers subjected to strong adverse pressure gradients

Two separate but complimentary experiments were conducted in turbulent boundary layers subjected to severe adverse pressure gradients: one developed over a flat surface, while the other had streamwise concave surface curvature. Measurements of surface pressures, as well as streamwise mean and fluctuating velocities,' established that gross characteristics of the boundary layer such as shape factor and skin friction coefficient were comparable. Detailed profile measurements normal to the wall, demonstrated that the most tangible evidence of the effect of curvature is that turbulence quantities do not scale with local boundary layer integral length scales, while on flat surfaces they do. On the flat surface, the measurement of streamwise cross-correlations showed that the spanwise wavelength of non-stationary three-dimensional structures did not increase. In contrast, curvature brought about an increase in the cross-stream wavelength. This behavior is consistent with the existence of a weak centrifugal or GOrtler-type instability mechanism of the type predicted by stability theory. The main conclusion was that concave surface curvature, whose effect is generally considered to be secondary to that of a strong adverse pressure gradient, plays a very significant role in the behavior of flow approaching separation.