Effect of Gortler vortices on transitional boundary layers

Laser anemometry and hydrogen bubble visualization have been used to investigate favourable-pressure-gradient constant-curvature boundary layers on the outer walls of 90 deg bends in two water channels, with the ratio of initial boundary layer thickness to wall radius of curvature around 0.03. Data are presented on the mean and fluctuating velocity fields, identifying spanwise and streamwise variations in the phenomena leading to transition in the presence of a naturally-occurring Goertler vortex systems. In each case, non-linear vortex development, followed by unsteadiness, set in at a Goertler number between 6 and 9, the amplitude of the spanwise variation in streamwise mean velocity peaking at 0.3 times the potential wall velocity. With a Reynolds number and core-flow turbulence level sufficient to cause transition on a flat surface transitional phenomena were initially confined to the vortex upwash zones. Establishment of an apparently turbulent boundary layer, without significant spanwise variations in mean velocity, was accompanied by a fall in Goertler number to pre-transitional values.