Application of Particle Image Velocimetry to Wall Turbulence Studies

Particle Image Velocimetry has been applied to the study of organized motions in the turbulent boundary layer over a flat plate, in absence of pressure gradients. Flow observations were conducted both in planes parallel to the wall (within the buffer layer) and in streamwise planes normal to the wall. Trains of "quasi-longitudinal vortices" (some of which highly inclined with respect to the wall) and packets of non-symmetric hairpin structures are postulated to co-exist in order to explain the detected swirling motions in both planes. Organized motions (low speed streaks, vortical structures) have been carefully analyzed together with their manifestations (VITA events, Q2 and Q4 motions); special attention is paid to the observation of the spatial relationships between these different phenomena.