Concentration Profiles in Shallow Turbulent Wakes

Abstract

The present study deals with the noninvasive measurement of concentration in the intermediate shallow turbulent wake region using a video-imaging technique. The flow depths considered in the present study are small compared to the width of the channel and the generated wakes are categorized as shallow. On the basis of the observed behavior, the waves are classified as deep-shallow wakes and shallow-shallow wakes. The topology of the dye concentration distribution in the near and intermediate wake region indicates that the vortex structure tends to be preserved when the flow depth is relatively high and the dominant eddy structures are similar to that noticed in conventional two-dimensional wakes. In shallow-shallow wakes, the conventional Karman vortex street appears to be annihilated or intermittent. The lateral concentration distribution at several axial stations covering the first thirty body widths are considered for analysis. The instantaneous concentrations are observed to be several times higher than the corresponding mean values. Attempts are also made to determine the paths traversed by the vortex cores and the vortex core convection velocity. The axial variation of the wake half-width with depth of flow is also examined. A model is developed to predict the spread of the wake with downstream distance from the test body. A friction length scale is introduced in the model to account for the influence of depth and bed friction on the development of the wake.