Oscillation and friction effects on mass exchange in a recirculating flow

Experiments were conducted to study oscillation and friction effects on the mass exchange between the recirculating flow in a square basin and the main flow in an open channel. During the experiments, a turbulent mixing layer was observed to form between the recirculating flow in the basin and the main flow. Dye was introduced into the turbulent mixing layer as a tracer to observe the motion of the eddies. The concentration of the dye was measured using a video imaging method. The data were analysed for an early period shortly after the dye entered the basin, and a late period when the dye in the basin had reached a quasi-steady state. Parameters such as retention time, accumulation time and arrival time were estimated using the time series of the concentration in the core of the basin and in the region outside of the core. The circulation time and circulation flow rate were computed using the area occupied by the dye. The rate of increase in the area was equated to the circulation flow rate. The mean and root-mean-square dye-concentration profiles were obtained for a number of cross sections along the mixing layer. The flow in the mixing layer was observed to oscillate due to feedback from the impingement of the mixing layer on the downstream corner of the basin. The oscillation frequencies were determined by spectral analysis of the concentration data at a location in the mixing layer near the point of the impingement.Tests were conducted for a range of Froude number varying from Fr = 0.09 to 0.47 and a range of bed-friction number from S = 0.023 to 0.27. Self-sustaining oscillations of the type known as "fluid-resonant feedback" were observed in the mixing layer. The onset of the oscillations in the mixing layer was detected to occur for a Froude number greater than 0.28. However, the mixing layer was not responsive to the feedback when the Froude number was less than 0.28. The dimensionless frequency of the oscillation was correlated with the Froude number. The Strouhal and Froude number relation analogously was consistent with the Rossiter's formula, which was developed for cavity oscillations in compressible flow. Beside the flow oscillations, the mixing layer and the recirculating flow in the basin were affected by the friction effect. Friction slow down the circulation in the basin. It changes the turbulence in the mixing layer. These effects of the friction were only significant when the water depth is sufficiently shallow. The dimensionless parameter for the friction effects was the bed-friction number. The demarcation of the flow between the deep and the shallow was found to occur at a bed-friction number S ≃ 0.09. The friction effect was negligible when the bed-friction number is less than the critical value of 0.09