Shallow wake in open channel flow---A look into the vertical variability

The vertical variability of a typical wake behind a bluff body in an open channel flow has been investigated. The focus of the study was to explore the variability of the flow structures in terms of mean velocity profiles, turbulent parameters and coherent structures in the vertical direction. A sharp-edged bluff body was chosen to minimize the effect of Reynolds number and ensure fixed flow separation points in the vertical direction. Velocity measurement was performed by Particle Image Velocimetry at three vertical locations: near-bed, mid-depth and near the free surface. In the streamwise direction, three different fields-of-view are taken to cover a distance 10 times the width of the body. At all locations, 2000 image pairs of 2048x2048 pixel resolution were acquired at a sampling frequency of 1 Hz. Proper orthogonal decomposition (POD) was used as a tool to educe information of coherent structures in the flow. A robust closed-streamline based coherent structure identification algorithm was developed to systematically detect the presence of the coherent structure in the flow. This step was followed by a statistical study of the location, size and strength of the coherent structures.;The results show that the bed and the free surface as well as the approaching vertically sheared flow, have a significant effect on the structure of the wake. The bed was found to restrict the transverse growth of the wake, whereas the free surface enhances the turbulent energy redistribution at the free surface. The size, shape and the development of the recirculation zone behind the bluff body also indicates vertical variability. Analysis based on signed swirling strength indicates rapid dissipation of vorticity at the near-bed region, compared to other vertical locations. The maximum size of the detected coherent vi structures was found to be largest at the mid-depth location, and smallest at the near-bed location. Also, the strength of the structures was found to be smaller at the near-bed location, compared to other vertical locations. Indirect evidence of the merging of the coherent structures was also demonstrated in the present study.