Vortex formation in front of a piston moving through a cylinder

This paper contains the details of an experimental study of the vortex form ed in front of a piston as it moves through a cylinder. The mechanism for t he formation of this vortex is the removal of the boundary layer forming on the cylinder wall in front of the advancing piston. The trajectory of the vortex core and the vorticity distribution on the developing vortex have be en measured for a range of piston velocities. Velocity field measurements i ndicate that the vortex is essentially an inviscid structure at the Reynold s numbers considered, with viscous effects limited to the immediate corner region. Inviscid flow is defined in this paper as being a region of the flo w where inertial forces are significantly larger than viscous forces. Flow visualization and vorticity measurements show that the vortex is composed m ainly of material from the boundary layer forming over the cylinder wall. T he characteristic dimension of the vortex appears to scale in a self-simila r fashion, while it is small in relation to the apparatus length scale. Thi s scaling rate of t(0.85+0.7m), where the piston speed is described as a po wer law At-m, is somewhat faster than the t(3/4) scaling predicted by Tabac zynski et al. (1970) and considerably faster than a viscous scaling rate of t(1/2). The reason for the structure scaling more rapidly than predicted i s the self-induced effect of the secondary vorticity that is generated on t he piston face. The vorticity distribution shows a distinct spiral structur e that is smoothed by the action of viscosity. The strength of the separate d vortex also appears to scale in a self-similar fashion as t(2m+1). This r ate is the same as suggested from a simple model of the flow that approxima tes the vorticity being ejected from the corner as being equivalent to the flux of vorticity over a flat plate started from rest. However, the strengt h of the vorticity on the separated structure is 25% of that suggested by t his model, sometimes referred to as the 'slug' model. Results show that sig nificant secondary vorticity is generated on the piston face, forming in re sponse to the separating primary vortex. This secondary vorticity grows at the same rate as the primary vorticity and is wrapped around the outside of the primary structure and causes it to advect away from the piston surface .