The streamwise development of Gortler vortices in a favorable pressure gradient

Measurements ave presented of the streamwise velocity variation within a laminar boundary layer on a concave surface of 4 m radius of curvature for which the free stream velocity gradient factor (nu/U-0(2))dU(0)/dx was approximately 1 x 10(-6). The velocity variation was consistent with the presence of counterrotating vortices resulting from the Gortler instability. The vortices exhibited exponential growth over the streamwise extent of the measurements to a disturbance amplitude of approximately 13 percent of the local free-stream velocity. The vortex growth rates were found to be less than those for a zero velocity gradient factor, indicating that a favorable pressure gradient stabilizes the pow with respect to the Gortler instability. Boundary layer profiles at local upwash and downwash positions are compared with the linear theory far which the mean pow was modeled using the Pohlhausen approximation to the solution of the boundary layer equations. The agreement between the measured and predicted profiles indicates that the lineal stability theory can provide a fair approximation to the small amplitude growth of the Gortler instability.