The Temporal Coherence of Prograde and Retrograde Spanwise Vortices in Zero-Pressure Gradient Turbulent Boundary Layers

Spatial and temporal statistics associated with spanwise aligned vortical structures are extracted from high repetition rate particle image velocimetry (HR-PIV) experimental measurements of a zero-pressure gradient turbulent boundary layer. Measurements were performed in the LTRAC water tunnel with a momentum thickness-based Reynolds number of \(\text{ Re }_\theta = 2{,}250\). Streamwise wall-normal planes of the field were recorded at rate of \(\varDelta t = 0.008\delta /U_\infty \), spanning a streamwise domain of \(3.2\delta \). This enables a single structure to be sampled approximately 400 times for a duration of 3.2\(\delta /U_\infty \) as it convects downstream. A model Oseen vortex is fit to each local peak in swirling strength, in order to detect and classify the radius, centroid velocity, circulation, and centroid location of each spanwise vortex. Attempts to track the evolution of these vortices show that on average these Oseen vortices only appear to remain temporally coherent for a time of 0.02\(\delta /U_\infty \).