An experimental study has been made on the interaction of a vortex ring with a plane solid boundary which is inclined to the axis of the ring. Dye visualizations of the ring during the interaction revealed (i) the formation of bi-helical vortex lines around the circumferential axis of the ring, and (ii) that these vortex lines were constantly being displaced along the circumferential axis on either side of the plane of symmetry and towards the region of the ring furthest away from the wall. Key factors which may be responsible for this phenomenon have been identified and are discussed in this paper.

An experimental study of a vortex ring interacting with an inclined wall

It is well documented that a trailing vortex pair approaching the ground, and a vortex ring colliding head-on with a rigid plane, experience a reversal in axial velocity which is commonly referred to as “rebound”. One explanation of this phenomenon suggests that it is essentially an inviscid process due to the effect of the finite core-size, whereas another and more widely accepted explanation attributes it to the influence of a secondary vortex which is generated at the surface by viscous effects. The aim of this paper is to assess experimentally the validity of these competing explanations. To achieve this, flow visualization studies of the collision of a vortex ring with a wall are compared with those of the head-on collision of two identical rings. The head-on collision is designed to mimic the inviscid, free-slip case of a ring/wall interaction. This paper describes the experimental findings.

A note on the cause of rebound in the head-on collision of a vortex ring with a wall

An experimental study has been conducted to examine compressible vortex ring structure and propagation, and the interaction of vortex ring with generic-body configurations. The experiments used a circular 30mm internal diameter and 38mm outer diameter shock-tube. The driver and driven gas was air. A vortex ring was generated by the impulse of the shock wave emerging from the open end of the shock-tube. High-speed schlieren photography was employed. Both side view and 45˚ oblique photographs were taken. The interactions with generic shape objects, which were a circular cone, a sphere, a cube and a plane-wall, were studied. Three different driver pressures of 12, 8, and 4 bars were tested with driven gas at 1 bar; the shock wave Mach number inside the tube was 1.63, 1.52 and 1.25 respectively.

Compressible Vortex-Ring Interaction Studies with a Number of Generic Body Configurations

This article addresses the fascinating 150 years history of the classical Helmholtz paper that laid the foundation of the vortex dynamics. Among general theorems on vortex motion, this memoir contains the special section on circular vortex filaments and axisymmetric vortex rings, in particular. The objective of this article is both to clarify some purely mathematical questions connected with the Dyson model of coaxial vortex rings in inviscid incompressible fluid and to provide a historical overview of achievements in experimental, analytical, and numerical studies of vortex rings interactions. The model is illustrated by several examples both of regular and chaotic motion of several vortex rings in an unbounded fluid.

Coaxial axisymmetric vortex rings: 150 years after Helmholtz

Vortex–ring interactions with oblique boundaries were studied experimentally to determine the effects of plate angle on the generation of secondary vorticity, the evolution of the primary vorticity and secondary vorticity as they interact near the boundary, and the associated energy dissipation. Vortex rings were generated using a mechanical piston-cylinder vortex ring generator at jet Reynolds numbers 2,000–4,000 and stroke length to piston diameter ratios (L/D) in the range 0.75–2.0. The plate angle relative to the initial axis of the vortex ring ranged from 3 to 60°. Flow analysis was performed using planar laser-induced fluorescence (PLIF), digital particle image velocimetry (DPIV), and defocusing digital particle tracking velocimetry (DDPTV). Results showed the generation of secondary vorticity at the plate and its subsequent ejection into the fluid. The trajectories of the centers of circulation showed a maximum ejection angle of the secondary vorticity occurring for an angle of incidence of 10°. At lower incidence angles (<20°), the lower portion of the ring, which interacted with the plate first, played an important role in generation of the secondary vorticity and is a key reason for the maximum ejection angle for the secondary vorticity occurring at an incidence angle of 10°. Higher Reynolds number vortex rings resulted in more rapid destabilization of the flow. The three-dimensional DDPTV results showed an arc of secondary vorticity and secondary flow along the sides of the primary vortex ring as it collided with the boundary. Computation of the moments and products of kinetic energy and vorticity magnitude about the centroid of each vortex ring showed increasing asymmetry in the flow as the vortex interaction with the boundary evolved and more rapid dissipation of kinetic energy for higher incidence angles.

Experimental investigation of vortex rings impinging on inclined surfaces

Experimental studies were carried out on the flowfield produced by a vortex ring approaching a plane wall perpendicular to its axis of symmetry, using smoke visualization technique. The mechanism of rebound of a vortex ring from the wall is shown to be related to generation of a shear layer separated from the wall and its subsequent development. The effects of Reynolds numbers on the trajectory of the core center of a vortex ring were also examined. Growth of the separated shear layer and generation of the secondary vortex ring due to vorticity concentration in the separated shear layer were visualized in detail.

Toshimitsu Kohsaka

Papers

Flowfield Produced by a Vortex Ring Near a Plane Wall