Measurements of free surface turbulence

Free-surface turbulence is important to the transfer of heat and mass across an air-liquid interface. In the absence of wind shear and surface waves, turbulence generated below the water surface (as from a streambed) is one of the controlling factors in air-water mass transfer. Research has shown that the temporal fluctuations in the two-dimensional divergence of the surface velocity field are related to the liquid-film mass transfer coefficient. To aid in understanding the relationship between free-surface turbulence and mass transport, an oscillating grid chamber has been used to study turbulence at a water surface. A horizontal grid of square bars was oscillated vertically beneath the water surface. Turbulence generated by the grid propagates up towards the water surface, generating near-two dimensional turbulence right on the free surface. Particle Image Velocimetry (PIV) was used to measure the temporally varying flow pattern on the water surface for three different energy levels in the chamber. Data were recorded over eight different sub-regions of the water surface, and the velocity, vorticity, and two-dimensional divergence were calculated as functions of space and time for each region. The spatial variations in two-dimensional divergence are shown to be similar to those found in flume experiments; this suggests that that the oscillating grid chamber may be used as a direct analogue to open-channel flows for studying interfacial transport phenomena.