A self-propelling clapping body

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 2 theta(o). 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 (mb) and initial separation angle (2 theta(o) = 45. and 60.) are varied. The body initially accelerates rapidly forward, then slowly retards 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 variation in theta(o) 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 cost of transport lies between 2 and 8 J kg(-1) m(-1).

Automated summary

We conducted an experimental study on the motion of a clapping body consisting of two flat plates. The results showed that the body initially accelerated rapidly forward and then slowly decelerated to almost zero velocity. The wake flow consisted of either a single axis-switching elliptical vortex loop or multiple vortex loops. The motion of the body was largely unaffected by the depth-to-length ratio but was influenced by the initial separation angle and the spring stiffness per unit depth.