Flow Visualization and Its Digital Image Processing in a Wall-Bounded Turbulent Shear Flow

A digital image processing has been applied to a quantitative analysis of the visual data obtained by the hydrogen-bubble technique to investigate the streamwise vortical structures, which are suggested as a primary turbulence mechanism in the near-wall region of wall turbulence. A good agreement of the statistical quantities of velocity fluctuations between the present result and those obtained by others verifies the present method as a new turbulence measuring technique. The streamwise vorticity fluctuations, of which measurements are extremely difficult, are calculated from the instantaneous spatial distributions of velocity fluctuations. The fact that the rms vorticity fluctuation has its minimum in the vicinity of the wall is first revealed experimentally, while it has been predicted by previous numerical works. In addition, the double layer structure of the instantaneous vorticity field near the wall is reconfirmed.