Longitudinal vortices imbedded in turbulent boundary layers Part 2. Vortex pair with ‘common flow’ upwards

Abstract

Detailed mean flow and turbulence measurements have been made in a low-speed turbulent boundary layer in zero pressure gradient with an isolated, artificially generated vortex pair imbedded in it. The vortices, generated by two half-delta wings on the floor of the wind-tunnel settling chamber, rotate in opposite directions such that the ‘common flow’ between the vortices is away from the surface, and the vortex pair draws boundary-layer fluid upwards. The distance of the vortex cores above the surface grows downstream, and is roughly twice the local boundary-layer thickness. The cancellation of circulation by mixing of fluid from the two vortices is slow, and the vortices are identifiable down the full length of the test section. As in the case of the single vortex investigated in Part 1 of this series, large changes in structural parameters of the turbulence occur.