An Investigation into Ground Effect for an Underwater Biologically Inspired Flapping Foil
Chierico, Paul Stephen
Licht, Stephen C.
Dahl, Jason M.
Jouaneh, Musa K.
Zawia, Nasser H.
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The topic that was investigated was whether there is a lift and thrust benefit to flapping an underwater, biologically inspired three dimensional foil near the solid bottom surface (so that it was in ground effect). The experimental method used a dual canister device that allowed actuation in roll and pitch, with force sensors attached to the pitch shaft, in order to record the forces produced by the foil as it flapped. The dual canister was towed on a carriage at a constant speed in a large tow tank that had been configured to have a long run of constant, deep water depth, a very short transition period, and then another long run of constant, shallow water depth. Due to this configuration, in one run the foil was able to encounter freestream conditions and ground effect conditions. The results proved in all sixteen experimental cases for varying Strouhal number and maximum angle of attack that the mean lift coefficient near the bottom was larger than that in the freestream. A potentially useful data point, for which the mean thrust coefficient had a positive change from flapping near bottom as compared to the freestream, was found to have a change in maximum instantaneous lift force of 14%. This would give a large enough change in signal strength that it could be used as a parameter on a future underwater vehicle to control altitude above the ground. The benefit to flapping in ground effect was equivalent to a larger than 1° pitch bias difference at a zero mean lift coefficient. Additionally, there was a thrust benefit seen to flapping in ground effect, but only under certain kinematics. Though not as dramatic as the benefit in lift, there was still an 8% difference in the mean thrust coefficient observed between flapping near the bottom and flapping in the freestream, for the case where the largest change in mean thrust coefficient was observed. This could equate to a large savings in battery life, and hence a longer endurance for a vehicle taking advantage of the thrust benefit seen by flapping in ground effect. While this work remains preliminary in nature, it shows that much more useful work remains to be done to explore the benefit induced by flapping a foil close to a hard surface ground.
CIVINS (Civilian Institutions) Thesis document
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