Unsteady propulsion near a solid boundary

Abstract

Experimental and computational results are presented on an aerofoil undergoing pitch oscillations in ground effect, that is, close to a solid boundary. The time-averaged thrust is found to increase monotonically as the mean position of the aerofoil approaches the boundary while the propulsive efficiency stays relatively constant, showing that ground effect can enhance thrust at little extra cost for a pitching aerofoil. Vortices shed into the wake form pairs rather than vortex streets, so that in the mean a momentum jet is formed that angles away from the boundary. The time-averaged lift production is found to have two distinct regimes. When the pitching aerofoil is between 0.4 and 1 chord lengths from the ground, the lift force pulls the aerofoil towards the ground. In contrast, for wall proximities between 0.25 and 0.4 chord lengths, the lift force pushes the aerofoil away from the ground. Between these two regimes there is a stable equilibrium point where the time-averaged lift is zero and thrust is enhanced by approximately 40 %.

Self-propelled swimming of a flexible plunging foil near a solid wall

“…Interestingly, in inviscid simulations, the induced velocities on the vortex cores are caused by the images vortices, which are analogous to the secondary vortices observed in current viscous simulations. In studying the wall effect on the propulsion of a rigid pitching foil, the spinning of dipoles was also observed in experiments [10]. However, in that study, the vortex dipoles were found to spin in the clockwise direction, which is in contradiction with the result of current work.…”

“…By comparing the problem settings of these two studies, we think that there are several factors which can affect the strength of the positive and negative vortices, which in turn, causes the reversal of the dipoles' spinning direction. First, the foil is driven by a pitching motion in [10], which is different from the plunging motion considered here. Second, due to a much higher Re number in the experiments, the viscous effect in [10] is much smaller than that in current work.…”

“…First, the foil is driven by a pitching motion in [10], which is different from the plunging motion considered here. Second, due to a much higher Re number in the experiments, the viscous effect in [10] is much smaller than that in current work. Moreover, in the experiments of [10], the pitching foil is placed in a steady stream near a stationary wall and thus the vortex shedding process is also interfered by the vorticity generated in a fully developed boundary layer.…”

“…Recently, experiments were carried out by Quinn et al [10] to study rigid airfoils undergoing pitching oscillations near a solid surface, and it was demonstrated that thrust can be enhanced by the wall effect while the propulsive efficiency remained a constant. In another experiment conducted by Quinn et al [11], a flexible rectangular panel was actuated at the leading edge near the wall of a water channel.…”

Aquatic burst locomotion by hydroplaning and paddling in common eiders (Somateria mollissima)

“…Wing heights of this value and lower were observed in the present study, indicating probable use of ground effect during paddle-assisted flying. Furthermore, the flapping of the wings can enhance thrust and lift production due to ground effect (Molina and Zhang, 2011;Quinn et al, 2014).…”