Flow development over inclined flat plates in ground effect and relation to aerodynamic loads

Abstract

The aerodynamics of finite-span inclined flat plates in ground effect is experimentally investigated at a chord-based Reynolds number of 50 000 for aspect ratios of 1 and 2. The minimum ground height is varied between 0.1 and 1.0 chord lengths, and lift and drag forces are measured using a force balance for angles of attack between −90° and 90°. Planar, two- and three-component particle image velocimetry is used to perform streamwise and cross-plane measurements at the midspan and one chord length downstream of the trailing edge, respectively. Ground effect is significant at ground clearances below 0.5 chord lengths, most notably near the stall angle, where it leads to significant changes to flow development. At sufficiently low free flight pre-stall angles, the increase in edge velocity at low gap ratios caused greater suction, generating higher lift with a minimal increase in drag for both orientations. Closer to the free flight stall angle, a decrease in aerodynamic loading is observed for negative orientations due to earlier onset of stall with a decreasing gap ratio. The exception was the higher aspect ratio plate at negative orientations, where the loading was largely invariant to changes in gap ratio for all angles tested. At positive orientations, the increase in average static pressure along the pressure surface in ground proximity led to an overall increase in loading prior to deep-stall conditions for both aspect ratios. The ground effect was minimal at post-stall angles of attack. The results may be used to guide the design of photovoltaic supports at relevant latitudes.