Some flow visualization experiments on the starting vortex

Abstract

A simple dye in water method has been used to visualize the growth of a two-dimensional starting flow vortex formed at a wedge-like sharp edge. Several cases were tested corresponding to different wedge angles and to different values of the time exponent in the velocity–time power law describing the starting flow. Photographic sequences showing the time-wise primary vortex growth are presented from which various secondary-flow details are identified. For the larger wedge angles these include a strong secondary vortex and in some cases a small separation bubble-like flow region immediately adjacent to the wedge apex. For a thin-wedge model the formation of what might be interpreted as small rotation centres along the outer turns of the primary-vortex shear layer is observed but these are not seen as a manifestation of an instability phenomenon in the fluid. Measurements of the trajectories of the primary-vortex centre are compared with the predictions of an inviscid similarity theory of the vortex growth. Although the appropriate Reynolds number in the present experiments was relatively low, comparison between theory and experiments is regarded as reasonable with differences being attributed to viscous effects absent in the similarity theory, and also to apparatus wall effects.