Separation mechanics of non-slender delta wings during streamwise gusts

Abstract

The dynamic-stall behavior on a generalized non-slender delta wing subjected to streamwise gusts has been investigated experimentally. A 45∘ sweep, NACA0012 delta wing serves as the canonical shape. In lieu of varying inflow conditions, the model is accelerated in the streamwise direction from a chord-based Reynolds number of 300,000 to 450,000 through a ramp motion at incidence angles near maximum steady-state lift. Force augmentation both during and after the gust is shown to be dependent on the initial structure of separated leading edge shear layers, with strong sensitivity at an angle of attack of 30∘. It is shown that lift enhancement stems from initially separated flow near the wing apex reattaching during the gust. Surface pressure measurements show that the reattachment is driven by a strong favorable pressure gradient that is maintained after the gust ends. The equations governing near-wall vortex dynamics state that vorticity flux from the wall increases with favorable pressure gradients, indirectly predicting an increase in circulation near the wing surface during a streamwise gust. However, circulation obtained through Particle Image Velocimetry (PIV) on the z∕c=0.1, 0.3, and 0.5 spanwise sections does not support this prediction, rather showing that the circulation in the wake (near the wing surface) remains nearly constant throughout the gust. Overall, strong three-dimensional gradients dictate flow structure before and after the gust. Spanwise variation of flow structure is especially pronounced for the 30∘ angle of attack case, where each planar section exhibits unique separation behavior in response to the gust.