The effects of stable stratification on turbulent diffusion and the decay of grid turbulence

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Abstract
Experiments are described in which a grid is towed horizontally along a large tank filled first with water and then with a stably stratified saline solution. The decay rates of the r.m.s. turbulent velocity components (w’, v’) perpendicular to the mean motion are measured by a ‘Taylor’ diffusion probe and are found to be unaffected by the stable stratification over distances measured from 5 to 47 mesh lengths (M) downstream, and over a range of Froude number U/NM of ∞ and 8·5 to 0·5, U being the velocity and N the buoyancy frequency. The Reynolds number Mw’/ν of the turbulence was about 103, where v is the kinematic viscosity. The vertical velocity fluctuations produced near the grid were reduced by the stratification by up to 30% when U/MN ≈ 0·5. Large-scale internal wave motion was not evident from the observations within about 50 mesh lengths of the grid.The turbulent diffusion from a point source located 4·7 mesh lengths downstream was studied. σy, σz, the horizontal and vertical plume widths, were measured by a rake of probes. σy was found to be largely unaffected by the stratification and grew like t½, while σz was found in all cases to reach an asymptotic limit σz∞ where 0·5 [les ] σz∞N/w’s [les ] 2, w’s being the r.m.s. velocity fluctuations at the source; the time taken for σz to reach its maximum was about 2N−1. These results are largely in agreement with the theoretical models of Csanady (1964) and Pearson, Puttock & Hunt (1983).

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