Transition Control Using Leading Edge Roughness for the Onera-D Wing

Abstract

Recent results by Saric et al. (1998) from experiments with an infinite swept wing document a passive transition control mechanism that originates from micron-sized roughness elements near the leading edge. In the experiments performed at Arizona State University (ASU), Saric et al. (1998) studied the natural transition case first. In that case, they determined the most amplified spanwise wavelength as λ_X2=12mm and observed transition at x₁/c ≃ 0.71. Second, they forced disturbances of equal, or larger wavelength than the most amplified one by placing roughness elements at spanwise distances of 12mm, 18mm and 36mm near the leading edge. When forcing the 12mm disturbances, they found that transition moved forward to x₁/c ≃ 0.52. Forcing the 18mm disturbances, they observed that neither the 12mm, nor the 36mm modes were amplified in the downstream direction. Third and most interestingly, they were able to delay transition to x₁/c ≃ 0.80 by forcing disturbances with a wavelength smaller than the most amplified one. In that case, they placed the roughness elements at a spanwise distance of λ_X2=8mm. The transition delay was attributed to the strong initial growth of the 8mm disturbances that suppressed the 12mm disturbances. The early saturation and decay of the 8mm disturbances then gave rise to the growth of longer wavelength disturbances that eventually lead to transition. However, this successful transition delay could be accomplished for the Reynolds number of Re=2,400,000 only. Increasing the Reynolds number slightly moved the transition location forward.

First author curently at the German Aerospace Center in Göttingen.