Gust Response of Free-Falling Permeable Plates

This paper investigates the effect of discrete transverse gusts on the flight dynamics and descent velocity of two-dimensional free-falling permeable plates. Two-way coupled fluid-structure interaction simulations are carried out using an overset meshing strategy for a range of Galilei number (Ga) from 10 and 50, and of the nondimensional mass (m) from 0.5 to 2, and a fixed Darcy number (Da) of $10^{-4}$. The present results show that the plate falls steadily in quiescent flow at the lowest Ga and m values; whereas, fluttering and tumbling are observed for increased Ga and /or m. Transverse (horizontal) gusts temporarily decrease the terminal velocity of the plate in the transient regime. Consequently, a plate experiencing a transverse gust travels less vertical distance than a plate falling in quiescent flow over the same period of time. The gust effect increases with the gust acceleration, Ga and m. The underlying uplifting mechanism is not directly related to the permeability, and it is thus likely to occur also for impermeable bodies. The present findings might provide insights to interpret the effect of turbulence on the terminal velocity of free-falling bodies and inform the design of insect-scale flyers passively transported by the wind

The leading-edge vortex of yacht sails

In the present work we experimentally verify, for the first time, that a stable Leading-Edge Vortex (LEV) can be formed on an asymmetric spinnaker, which is a high-lift sail used by yachts to sail downwind. We tested a rigid sail in isolation in a water flume at a Reynolds number of ca. 104. The flow field was measured with Particle Image Velocimetry (PIV) over horizontal cross sections. We found that on the leeward side of the sail (the suction side), the flow separates at the leading edge reattaching further downstream and forming a stable LEV. The LEV grows in diameter from the root to the tip of the sail, where it merges with the tip vortex. We detected the LEV using the γ criterion, and we verified its stability over time. The lift contribution provided by the LEV was computed solving a complex potential model of each sail section. This analysis indicated that the LEV provides more than 10% of the total sail’s lift. These findings suggest that the maximum lift of low-aspect-ratio wings with a sharp leading edge, such as spinnakers, can be enhanced by promoting the formation of a stable LEV