Turbulence Measurements Near the Free Surface in Stirred Grid Experiments

Abstract

Turbulence measurements were made in a grid-stirred tank using a rotating split hot-film anemometer probe, which is able to measure both vertical and horizontal velocity fluctuations to within 0.4 mm of the surface. The shapes of spectra, profiles of velocity fluctuations, and profiles of integral length scales within the surface-influenced layer (roughly one integral length scale from the surface) are predicted to a good first approximation by the irrotational source theory of Hunt and Graham (1978). The magnitudes (as opposed to the shapes) of the turbulent velocity and integral length scale profiles scale according to the values that would otherwise occur in the vicinity of the surface-influenced layer were the surface not present, and are adequately predicted by the bulk relations of Hopfinger and Toly (1976) and Thompson and Turner (1975), with no apparent dependence on turbulent Reynolds number. This simple picture of the surface-influenced layer should help guide the development of better theoretical models describing the hydrodynamics and dissolved gas transport in the much thinner dissolved gas boundary layer at the surface.