The study of vortex ring/wall interaction for artificial nose improvement

Abstract
Normal and oblique collisions of a vortex ring with a wall have been studied experimentally and numerically with the aim of analyzing the interaction of vortex structures with the solid boundary and to understand the effect of the impact angle alpha and the Reynolds number Re on the development of the azimuthal instabilities. The application of such analysis is relevant, for instance, in systems for artificial olfaction (Davide et al., 1994). In fact these devices consist of a box having a sensor array at its bottom wall, devoted to detect the presence of certain substances in the test mixture. The efficiency of the sensor depends on the concentration of the substance that reaches the surface; this requires that the fluid injected in the box organizes into structures that arrive at the wall as much coherent as possible. The vortex ring has been chosen as the advecting structure because its high degree of coherence in the laminar range, allows the mixture to be transported on the sensing surface. During the impingement the vortex ring generates an oppositely-signed vortical layer and the mutual induction between the primary ring and the vorticity induced at the wall is the leading process for the eventual break-down of the structure. The eigenvalues of the velocity gradient tensor have allowed to study the stabilizing effect of impact angle through the topological features of the field. Finally the vortex structure identification allowed to study the vortex dynamics in the vicinity of the wall in order to perform a measure of the stability of the field.