DYE VISUALIZATION AND P. I. V. ANALYSIS OF STREAMWISE VORTICITY STRUCTURE IN A SMOOTH-BED OPEN CHANNEL FLOW

Abstract

In a smooth-bed open channel flow of water at low Reynolds number, with flow depth 10 cm and width 50cm, the near bed vortex structure was visualized by injecting fluorescein dye from a slit on the channel bottom, and illuminating the flow by a light sheet tilted upstream of a cross-stream plane by 30°. Viewing from downstream reveals mushroom-shaped patterns of dye, inferred to be counter-rotating pairs of streamwise vortices. Newly appearing mushroom patterns are strongly inclined with respect to the vertical, and seem to alternate between oblique left and oblique right. Analysis of flow in a crossstream plane by PIV tends to confirm many of these data and subjective impressions. Both instantaneous vorticity distributions and two-point correlations of vorticity indicate preferred directions in the cross-stream plane at about 45° from the vertical.