Strong turbulence and free surface interaction in a grid-stirred tank

In this work the experimental analysis of the interaction between turbulence and free surface is detailed. The topic is interesting in many natural flows and industrial processes. Turbulence is generated by a vertically oscillating grid moving beneath the free surface, as widely reported in several experiments. Fluid velocity is measured through a hot film anemometer. This instrument is able to measure vertical velocity fluctuation to within 0.5 mm of the surface, from which profiles of r.m.s. velocity fluctuation, integral length scales and several turbulence estimators can be calculated. The free surface elevation is measured by using an ultrasonic sensor based on flight-time with a response time of 10 milliseconds and an overall error equal to 0.2 mm The present analyses follow previous analyses on different data regarding some experiment carried out in a flume, with free surface turbulence generated by a Crump weir. In the experiments in the flume a relevant evidence of free surface waves suggested a separation between potential and turbulence contributions. Free surface waves seem almost not present in the grid stirred tank and a pure turbulent flow is forecast. Aiming to detect the source effects of turbulence near the free surface, the correlation between free surface elevation and the underneath flow velocity have been studied, and the time lag between turbulence and free surface has been evaluated. As observed in the Crump weir generated turbulence, resonance is expected with turbulence excited by free surface growing at a specific frequency of the grid