Experiments on the inhibition of mixing in stably stratified decaying turbulence using laser Doppler anemometry and laser-induced fluorescence

Abstract

The decay of velocity and density fluctuations in buoyancy influenced turbulence generated by a biplanar grid, towed horizontally through a stably stratified fluid with uniform optical index of refraction was measured for three different constant density gradients. Experiments were performed in a closed rectangular test section. Fluid velocities were measured with a laser Doppler anemometer. Density was measured by adding a fluorescent dye (Rhodamine 6G) to the working fluid in such a way that the light fluoresced by the dye when excited by a laser was proportional to the local density of the solution. Simultaneous single-point measurements of the vertical velocity, the horizontal velocity in the direction of grid motion, and the density were obtained at a fixed point in the center of the test section. Statistical moments representing the state of the turbulent flow field at different periods of the decay were computed by ensemble averaging data from 100 tows with similar initial conditions. Density profiles of the quiescent fluid, before and after the grid was towed through the test section, were measured, and estimates of the total mixing accomplished by a single sweep of the grid through the fluid were calculated. The measured internal Richardson number Rii={− (1)/(2) [gρ′2/(∂ρ̄/∂z)]} ×(1/ (1)/(2) ρ̄〈w2〉) indicates that, immediately behind the grid, the turbulence was unaffected by the stable stratifications. At a time depending on the strength of the mean density gradient, the buoyancy forces acting on the turbulent eddies become as large as the inertial forces and have significant effects on the final stages of the decay of the turbulent flow field. Measurements of the convective mass flux indicate that the vertical transport of mass due to turbulence was significantly reduced by buoyancy forces. This was confirmed by integral estimates of the mixing, which indicate that the ability of the turbulent eddies to homogenize the stratified fluid decreases significantly with increasing density gradients. These results are compared with earlier measurements obtained in both stratified air and water facilities.

Biogenic inputs to ocean mixing

“…The circulating currents of the oceans have to work against the vertical stratification in order to transport fluid. This circulation is maintained by external mixing processes whose identification and estimates of power input describe the ocean mixing problem (Linden, 1979;Barrett and Van Atta, 1990;Fernando, 1991;Ivey and Imberger, 1991;Ledwell et al, 1993). The importance of the ocean mixing problem to marine organisms is evident from our observations about biomass being distributed throughout the ocean's depths.…”

The dynamics of variable-density turbulence

“…A number of laboratory experiments have been performed to quantify the effect of buoyancy forces on mixing (see, e.g., Lienhard and Van Atta (1988) [54], Yoon and Warhaft (1990) [go], and Itsweire, et a1 (1986) [40]). Barrett and Van Atta (1991) [2] studied the decay of velocity and density fluctuations in buoyancy influenced turbulence generated by a grid, towed horizontally through a stably stratified fluid. They showed that immediately behind the grid (i.e., at an early time in the flow), the turbulence was unaffected by buoyancy forces.…”

Turbulence within inertia‐buoyancy balanced axisymmetric intrusions

“…For such flows the vertical buoyancy flux may be dominated by large-scale restratification, and at the same time, smallscale turbulence can be actively destratifying [Lienhard and Van Atta, 1990]. As found in a number of studies, including the grid-generated turbulence experiments by Stillinger et al [ 1983], ltsweire et al [ 1986], and Barrett and Van Atta [1991], internal wave breaking experiments by Taylor [1993] (who also reanalyzed the work of Ivey and Nokes [1989]), and theoretically by Gibson [1980], the complete collapse of active turbulence into internal wave oscillations, with zero net mass flux, typically occurs when the rate of dissipation of turbulent kinetic energy:…”

“…In the experiments performed by Stillinger et al [1983], Itsweire et al [1986], Lienhard and Van Atta [ 1990], and Barrett and Van Atta [1991], where turbulence was generated by grids in zero mean shear, linearly stratified fluid columns, the complete collapse of turbulent mixing occurred at a time of <(19/2•r)Nt', where t' is the time since the initiation of turbulent activity. The turbulent overturns initially began to decay at a period • (2/2•r)Nf, which was also found in the differential mixing experiments of Browand et al [1987].…”