Wind tunnel experiments on wake-vortex decay in external turbulence

Abstract

Wake vortices behind aircraft may persist for long times and therefore represent a safety concern for following aircraft. It is well known from numerical studies that external turbulence promotes the decay of wake vortices, either by increased vorticity diffusion or by triggering 3D instabilities that lead to their destruction. So far, however, very few quantitative laboratory studies have been devoted to the issue of wake-vortex decay. In this study, we present an experimental investigation of the evolution of both single and double wing-tip vortices in a wind tunnel with and without a turbulence generating grid, where Re??=4 104 typically. Cross-sectional velocity fields were obtained by means of particle image velocimetry at several locations downstream of the airfoil. Special attention was given to resolve the vortex core, since this has often been a region of difficulty in most experimental investigations. The single vortex was found to exist of a small viscous core and an outer region with nonzero vorticity. The decay of the maximum azimuthal velocity is close to laminar, both with and without grid. The main effect of the grid turbulence is to increase the diffusion of vorticity in the outer region and to enhance the wandering of the vortex center. The wandering amplitude is inversely proportional to the strength of the vortex, indicating mutual interaction between the vortex and external turbulence. For the double wing-tip vortices a split wing configuration was used with three different tip spacings. The vorticity distribution of the double vortices is more compact than that of the single vortex but decays due to cross-diffusion. The cross-diffusion is without grid significantly larger than viscous diffusion and enhanced by grid turbulence. Grid turbulence is also found to promote the onset of the Crow instability. The instability causes the vortex centers to oscillate with an angle of approximately 33 degrees to the horizontal. The growth rate of the oscillation amplitude is approximately half the growth rate predicted by inviscid theory. The Crow instability is followed by the phenomenon of vortex linking, which results in a rapid destruction of the vortices. At very strong turbulence levels, core oscillations and decay of circulation are significantly present, though the Crow instability is no longer observed.

Date of Award	30 Jun 2008
Original language	English
Supervisor	A. Elsenaar (External coach) & Ruben R. Trieling (Supervisor 1)