Trailing Vortices in the Far-Wake of a Delta Wing

In this research we study the trailing vortices behind a 30° apex delta wing at 15° angle of attack. The delta wing is towed in an XY-Towing Tank, generating a spatially developing vortex pair with streamwise (axial) flow in the vortex cores. PIV using a transverse light sheet is used to find the transverse velocity field, and to characterize the vortices using the superposition of two Lamb-Oseen vortices. The axial flow is captured using a longitudinal light sheet; to ensure we capture the vortex core, the light sheet is set such that it is slightly oblique to the length of the vortex. This technique was first developed by Miller & Williamson (1995) and allows the streamwise velocity profiles to be captured over 20 chord-lengths downstream. When the vortex pair is in ground effect, the boundary layer that forms between the vortices and the wall separates, generating secondary vorticity and causing the vortex pair to ‘rebound’. The maximum axial velocity and vortex core size are analyzed in and out of ground effect, and a wake-like deficit in axial velocity (flow upstream) is observed. We also trigger the long-wavelength instability (Crow 1970) in the vortices by introducing perturbations in the towing velocity, and compare the measured growth rate of the instability to theory.