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

The effects of turbulent motion on planktonic organisms have mainly been studied in the laboratory with devices capable of generating controlled turbulent conditions. Owing to technical and logistical difficulties, thorough assessments of hydrodynamics in such experiments are not routinely made. In this study, we examined the suitability of two widely used systems to generate isotropic, homogeneous, and stationary turbulence in laboratory containers: oscillating grid devices with large stroke length and relatively low frequencies of oscillation and orbital shaker tables. Turbulent kinetic energy dissipation rates were estimated from velocity measurements made with acoustic Doppler velocimeters. Both systems were shown to generate isotropic conditions in a relatively broad range of dissipation rates. Grid-stirred tanks produce homogeneous turbulence in both the horizontal and vertical dimensions, as long as stroke length is comparable to the height of the container. Turbulence in orbital shakers is not completely homogeneous, as it depends on the distance to the wall and to the surface. Empirical models are derived as a tool for the calculation of dissipation rates in the two systems within the ranges and conditions examined in this study