Pitching airfoils produce more thrust near solid boundaries

Airplanes gliding near the ground experience experience a boost in lift. We discovered that this boost extends to unsteady lift forces, such as those governing thrust production in fish. Fish and fish-inspired vehicles can therefore produce up to 40% more thrust near a solid boundary with the same swimming motions. The boundary also breaks the symmetry of the fish’s wake, causing its momentum jet to angle away from the ground. (This work is from PI Quinn’s research prior to the SFS Lab and is archived here for reference.)


Authors: Daniel Quinn, Keith Moored, Peter Dewey, Alexander Smits

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%.

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