Streak generation in wind tunnels

Computational results are presented which co-validate published experimental observations of steady streaks, deliberately generated by the steady wake of a wire placed upstream of a flat plate with a prescribed leading edge. The largest streak occurs when the wake is generated from a wire placed upstream of the wind tunnel contraction. Normal vorticity passing through the contraction leads to the creation of streamwise vorticity in the test-section via tilting and stretching. The computational results allow the original experiment to be reinterpreted as a receptivity experiment that demonstrates the boundary layer is more receptive to steady streamwise vorticity than normal vorticity. It also suggests an interesting mechanism for the generation of Klebanoff streaks in wind tunnels. The effect of shifting the attachment point at the leading edge on receptivity is also demonstrated. The streak growth is compared to the Optimal streak often used in computational studies. The modal growth of the streak generated by free-stream normal vorticity is found to have a streamwise location of peak energy close to the Optimal streak for wavelengths larger than the leading edge thickness. However, the location of the peak energy for the streamwise vorticity streak varies substantially with wavelength. Differences in wall-normal profiles are also noted.