Mixing across an interface due to turbulence generated by an oscillating grid

Abstract

Many experimenters have used oscillating grids to produce turbulence for various laboratory purposes, especially in studies of mixing, but there have been few direct measurements of the properties of the turbulence itself. In the present paper we report experiments which attempt to relate the turbulent velocity and length scales to the external parameters, the frequency and amplitude, for three forms of grid oscillated in a tank of water. Turbulent velocities have been measured in the absence of a mean flow by using a hot film moved through the fluid to provide its own mean velocity. The output is stored and analysed in a small computer, which rapidly evaluates velocity and length scale statistics from an ensemble of records. The spatial variation of these quantities with distance from the stirrer is of special interest. It agrees with results suggested by an inertial-decay theory, and with previous measurements made by Bouvard & Dumas (1967) using a different form of stirrer. A particular purpose of the work has been to ‘calibrate’ the entrainment experiments of Turner (1968), by providing absolute scales of velocity and length in the fluid near a mixing interface, for the same grid as was used in the earlier experiments. Evidence is presented which suggests that other forms of grid may not be calibrated simply by extrapolating these results.

Effect of background turbulence on an axisymmetric turbulent jet

“…These values are comparable with those obtained by Variano et al (2004) and Variano and Cowen (2008). The small mean flow measured herein is also significantly lower than that generally found in quasi-isotropic turbulence generated by oscillating grids (up to 60%) (Fernando and De Silva, 1993;Thompson and Turner, 1975;McDougall, 1979).…”

“…(Thompson and Turner (1975) and McDougall (1979) et al (1996), which suffered from large mean flows. More recently, eight fans (Birouk et al, 2003) or synthetic jets (Hwang and Eaton, 2004) located at the corners of a cubic chamber and pointing towards its center have been employed.…”

Evaluation of oscillating grids and orbital shakers as means to generate isotropic and homogeneous small‐scale turbulence in laboratory enclosures commonly used in plankton studies

“…Couette cylinders are used as viscosimeters and for the study of particle aggregation under shear flow (e.g., Serra et al 1997). Oscillating grids, a favorite of fluid dynamics experiments, have been extensively used in the study of sediment dynamics and mixing in stratified fluids (e.g., Rouse and Dodu 1955, Thompson and Turner 1975, Hopfinger and Toly 1976. To try to meet ecologically realistic conditions, these devices are often used in biological experiments using settings outside the ranges defined in their original application without a previous examination of hydrodynamics.…”

The length scale equation in turbulence models

“…Thus these experiments provide a particularly useful and interesting data set for evaluating the modeling of diffusion terms in second moment closure models as suggested by Sonin (1983). Although it was feared that earlier such experiments (for example, Thompson and Turner, 1975) might have been contaminated by unwanted secondary circulations due to unavoidable asymmetries in the grid, it appears that these fears were greatly exaggerated. More recent experiments Hopfinger, 1986, see also Hopfinger, 1987), with considerable attention given to eliminating secondary circulations, have in general confirmed earlier results.…”

“…Turbulence generated by an oscillating grid has been studied extensively in the laboratory both in neutrally and stably stratified fluids by turbulence researchers (Thompson and Turner, 1975;Hopfinger and Toly, 1976;Turner, 1981;E and Hopfinger, 1986;Nokes, 1988;Hannoun et al, 1988). These experiments provide considerable empirical knowledge on the behavior of turbulence in a particularly unique situation, when the only source of turbulence in regions far away from the grid is the turbulent diffusion.…”