Linking flow structures and aerodynamic performance for low-aspect-ratio plates at low Reynolds numbers

This study presents an experimental investigation of the flow over a trapezoidal plate and its wake at a chord-based Reynolds number of 5800. The plate has an aspect ratio of 1.38 and the angle of attack varies from 4◦ to 10◦. Volumetric flow fields are acquired through stereoscopic particle image velocimetry and aerodynamic forces are estimated via the wake-integral approach. A key novel finding is achieved: the swallow-tailed separation bubble, characterised by a distinct concavity, enables the plate to achieve its maximum lift-to-drag ratio. This favourable performance arises from the formation of a counter-rotating vortex pair in the vicinity of the bubble concavity. This vortex pair suppresses the velocity deficit in the wake, thereby contributing to drag reduction. Overall, the swallow-tailed separation bubble structure substantially improves aerodynamic efficiency, highlighting the practical potential of spanwise fluid transport mechanism, discussed previously by Zhu et al. (J. Fluid Mech., vol. 965, 2023, p. A12), for developing effective physics-based flow control strategies.