Visualizations of turbulent structures of wakes and boundary layers

Two turbulent-flow examples, a wake and a boundary layer, are used to illustrate the usefulness of visualizations for a better understanding of the structure of such flows. An experiment was conducted in a hydrodynamic tunnel to examine the mechanism of the counter-gradient turbulence which arises in the wake of two parallel rods of different diameters. The visualization shows that the flow pattern is strongly disymmetrical and that the counter-gradient stress is due to fluid entrainment by the large vortex generated from the larger rod. In addition, a photographic visualization of a turbulent boundary layer on one of the test-section walls is examined, and filament lines outside the boundary layer are shown to confirm the three-dimensional behavior of the intermittency region. In addition, the generation of hairpin vortices is observed near the wall. These observations support the accepted hypothesis concerning turbulence generation near a wall.