Dual-plane PIV investigation of structural features in a turbulent boundary layer

Simultaneous dual-plane PIV experiments were performed in streamwise-spanwise planes in the log region of a turbulent boundary layer at a moderate Reynolds number (Reτ ∼ 1100). The acquired datasets were used to resolve all 9 velocity gradients from which the complete vorticity vector and other invariant quantities like 3-D swirl strength were computed. These derived quantities were used to analyze and interpret the structural characteristics and features of the boundary layer. Results of the vorticity vector and the 2-D swirl strength from the two neighbouring planes indicate the existence of hairpin shaped vortices inclined downstream along the streamwise direction. These vortices envelop low speed zones and generate Reynolds shear stress that enhances turbulence production. Plots of full 3D swirl strength indicate the existence of additional vortical structures in the middle of the low speed zones that may represent heads of smaller eddies intersecting the measurement plane. This concept is in accordance with the hierarchy of structure size in a hairpin packet proposed by Adrian et al.[2]. Computation of inclination angles of individual eddies using the vorticity vector suggests that most cores are inclined at 25◦ to the streamwise-spanwise plane with a resulting projected eddy inclination of 32◦.