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 5800 $5800$ . The plate has an aspect ratio of 1.38 1.38 $1.38$ and the angle of attack varies from 4 Superscript ring 4 ∘ 4^∘ to 10 Superscript ring 10 ∘ 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.