Experimental Investigation of Self-Propulsion in Pitching Airfoil

We experimentally investigate the self-propulsion characteristics of NACA0015 airfoil section by allowing the airfoil to propel in the longitudinal direction freely. We measure the self-propelling speed for different pitching amplitude and frequency for the airfoil pitching about three pitching points (from the leading edge to the mid-chord). The velocity increases monotonically with pitching frequency and amplitude. However, there is no further increase in the speed when the trailing edge excursion is more than half of the airfoil chord length. The distance traveled by the airfoil in terms of body length per oscillation is constant for all the frequencies but monotonically increases with amplitude and reaches a plateau. The wake exhibits different complex patterns across all the parameters, and it is characterized by a wide jet-like velocity profile with significant lateral momentum. Deflected vortex pairs are observed at lower speeds whereas, reverse BvK vortices are observed at moderate speeds. At higher speeds, many smaller vortices are shed, which coalesce into larger ones. These observations provide helpful insight into factors governing the speed and efficiency of self-propelling bodies.