Vortex ring generation by a piston with non-uniform velocity

Vortex rings are involved in many industrial and natural flows. The ease with which they can be generated in laboratory make them a much-studied object of research. The most conventional technique involves sliding a piston inside a cylinder to expel a volume of fluid which rolls up at the cylinder exit in the form of a vortex ring that detaches and propagates by self-induction. Although this method is commonly used, questions remain concerning the dynamics and final characteristics of the generated vortex ring, particularly in the case of a piston moving with a non-uniform velocity. In this context, the aim of the present study is to analyze the influence of the piston velocity law on the vortex ring characteristics, and to identify the physical mechanisms involved. Direct numerical simulations of vortex ring generation at the outlet of an orifice are performed with OpenFOAM. The motion of the piston is explicitly simulated using a dynamic mesh technique. A parametric study is carried out in the case of a velocity law consisting of a succession of two constant velocities separated by an ascending or descending step. The results show that the sign and amplitude of the step control the dynamics of the generated vortex rings. It is found that their propagation speed scales with the space-averaged velocity of the piston, and that their circulation and velocity are optimal for a particular step amplitude.