A self propelling clapping body

Abstract

We report an experimental study of the motion of a clapping body consisting of two flat plates pivoted at the leading edge by a torsion spring. Clapping motion and forward propulsion of the body are initiated by the sudden release of the plates, initially held apart at an angle $2θ_0$. Results are presented for the clapping and forward motions, and for the wake flow field for 24 cases, where depth to length ratio ($d^* =$ 1.5, 1 and 0.5), spring stiffness per unit depth ($Kt$), body mass ($m_b$), and initial separation angle ($2θ_0 $= 45 and 60 deg) are varied. The body motion consists of a rapid forward acceleration to a maximum velocity followed by a slow retardation to nearly zero velocity. Whereas the acceleration phase involves a complex interaction between plate and fluid motions, the retardation phase is simply fluid dynamic drag slowing the body. The wake consists of either a single axis-switching elliptical vortex loop (for $d^*$ = 1 and 1.5) or multiple vortex loops (for $d^*$ = 0.5). The body motion is nearly independent of $d^*$ and most affected by variations in $θ_0$ and $Kt$. Using conservation of linear momentum and conversion of spring strain energy into kinetic energy in the fluid and body, we obtain a relation for the translation velocity of the body in terms of the various parameters. Approximately 80% of the initial stored energy is transferred to the fluid, only 20% to the body. The experimentally obtained $COT$ lies between 2 and 8.