Enhancing transverse mixing by using triangular vane in the straight channel

Abstract

The transport of pollutants in the large rivers especially in the lowest downstream reach of a river, in which the velocity and the intensity of flow turbulent become very low, is performed to a great extent by dispersion. Any measure to enhance the level of turbulence in the flow field will increase the mixing of pollutants. The main goal of the present study is to investigate how the transverse mixing is increased by using a triangular vane (TV). TV is a hydraulic structure used for bank protection and habitat restoration and is suitable for fish and other aquatic organisms. Experiments were carried out in a flume of 10m long and 0.8m wide. Two different conditions of with and no vane were tested under four different flow conditions. A salt solution was used as a tracer. An electronic concentration measuring equipment consisted of conductivity probes was used which can measure the concentration at 49 nodes in four cross-sections downstream simultaneously. The three-dimensional velocity components were also measured by using the electromagnetic velocity meter and the characteristics of three-dimensional flow patterns including turbulent kinetic energy (TKE) and the vertical distribution of the transverse velocity were analyzed. The result showed that the TV by deviating the velocity vectors to the transverse direction, by developing the secondary flows and by displacing the position of the rotating cells in different transverse sections, causes the transverse mixing to increase 3.2 times and to reduce the complete mixing length by as much as 76% compare to the case of no vane. The average values of the TKE found to be increased by as much as 3 times. Finally, a new dimensionless parameter has been proposed to correlate dimensionless transverse mixing to the flow parameters.