Three dimensional structure characterization and visualization in a turbulent boundary layer

For decades, researchers have worked towards developing a suitable model to represent turbulent boundary layers. Based on PIV experiments in the x − z planes of a turbulent boundary layer, Adrian et al. [1] proposed a model based on a packet of “hairpin vortices” as a primary feature in turbulence transport and production (In this paper, streamwise, spanwise and wall-normal directions are along x, y, and z axes respectively). Based on stereo PIV data in x − y planes, Ganapathisubramani et al. [2] concluded that these hairpin packets occupy a small percentage of the total area, yet contribute nearly 30% of the total Reynolds shear stress generated. Hence, the hairpin packets are a very important mechanism in turbulence production. With single plane data, however, we cannot completely characterize the intensity or orientation of the vortices responsible for generation of Reynolds shear stress. For example, swirl strength (λci, see Zhou et al. [3]) calculated from the imaginary part of the eigenvalue of a 2-D velocity gradient tensor, performs reasonably well at identifying vortex core locations but does not reveal their orientation. Hence, dual-plane stereo PIV experiments were performed and resulting datasets were used to retrieve the complete velocity gradient tensor. With the out-of-plane gradients provided by the dual-plane data, we can compute the complete swirl strength and the direction of the vorticity vector. We can also compute the instantaneous turbulence production term (uw ∂U ∂z ) and determine its relation to the vortex structures in the flow.