Comparison of moving and stationary clapping bodies

Abstract

We report differences in the flow dynamics and in the body kinematics between a clapping body that is allowed to propel forward freely (Dynamic) and one that is constrained from moving forward (Stationary). The experiments were done in quiescent water. The body consists of two interconnected plates hinged at one end with a 'torsion' spring. Initially, a thread loop holds the plates apart at an interplate angle of 60 degrees. Cutting of the thread initiates the clapping motion, and if allowed, the body propels forward a certain distance. Experiments have been performed for three values of $d^*$(= depth/length): 1.5, 1.0, and 0.5. In both cases, vortex loops initially develop along the three edges of each plate, which reconnect by the end of the clapping motion resulting in the formation of an elliptical vortex loop in the wake for $d^*$ = 1.5, 1.0 bodies, and multiple connected rings for $d^*$ = 0.5 bodies. Three main and unexpected differences are observed between the 'Dynamic' and 'Stationary' bodies. In the dynamic case, the clapping action is faster compared to the stationary case, with the maximum angular plate velocity being twice as high. The mean thrust coefficient, $\overline{C_T}$, based on plate tip velocity, is higher for the stationary. The value of $\overline{C_T}$ and the circulation in the starting vortices is almost independent of $d^*$ for the dynamic case, whereas it increases with $d^*$ for the stationary case. The core separation of starting vortices closely matches in both stationary and dynamic cases although circulation varies.