Experimental investigation of leading-edge roughness effects on stationary crossflow instability of a swept wing

Wind tunnel experiments were conducted to evaluate surface pressure distribution over a semi span swept wing with a sweep angle of 33°. The wing section has a laminar flow airfoil similar to that of the NACA 6-series. The tests were conducted at speeds ranging from 50 to 70 m/s with and without surface roughness. Surface static pressure was measured on the wing upper surface at three different chordwise rows located at the inboard, middle, and outboard stations. The differences between pressure distributions on the three sections of the wing were studied and the experimental results showed that roughness elements do not influence the pressure distribution significantly, except at the inboard station. On the other hand, spectral analysis of the pressure–time signals acquired from the pressure orifices over the wing upper surface showed that roughness had significantly affected the zero frequency amplitude. In this study, the zero frequency amplitude and its variations with roughness elements was investigated at three different chordwise positions; inboard, middle, and outboard stations. Results showed that the 3-D roughness elements amplified zero frequency amplitude over the wing surface. Zero frequency distribution at the inboard station, closer to the wing root, in comparison with the middle station, was reduced after an initial amplification along the chord. Moreover, the effect of roughness on the zero frequency instability at the first section was negligible due to the narrow instability amplification region. On the other hand, at the outboard station, closer to the wing tip, the instabilities were amplified over a larger region, with respect to the middle station.