Fluid dynamic effects of small projections based on dorsal ridge tubercles of the finless porpoise

The finless porpoise has characteristic tubercles on its dorsal ridge. These tubercles are supposed to have biological and mechanical functions. In the former, specific nerve endings and fibers exist in the tubercles indicating its function as a sensory organ. In the latter, tubercles provide fluid dynamic functions such as underwater drag reduction or inhibition of wave or spray drag while breathing at the sea surface. This research focuses on the latter function aiming at the application to the drag reduction in moving vehicles. Small projection models, or tubercle models were fabricated with resin and tested in the wind tunnel to investigate the effect on the lift and the drag characteristics of an airplane model with the tubercle models attached on the wing-fuselage connection. In consequence, it was shown that the tubercles effectively delayed the wing stall at high angles of attack without largely degrading the wing performance. The oil flow experiments also clarified that this delay of wing stall was caused by the inhibition of flow separation on the wing surface. The tubercles then had a positive influence on the flow around the wing or fuselage effectively inhibiting the flow separation and thus may be a useful device to reduce drag, noise, or vibration of moving vehicles.