A litre-scale turbulence facility for microorganism-flow interactions

Bacteria and phytoplankton are abundant in aquatic environments, forming the base of the food web and mediating elemental cycling at a global scale. Understanding the interactions these microorganisms have with their turbulent fluid environments is an active area of research, largely conducted in laboratory-based flow experiments. In this work, we provide an open-source design and rigorous flow characterization for a 1L, dual oscillating grid turbulence facility, the smallest volume facility to date which produces near-isotropic, homogeneous turbulence. We optimized the tank geometry (grid-to-grid and grid-to-wall spacing), the grid geometry (for both classical and fractal grids: effective mesh size, blockage ratio, and fractal grid parameters), and the grid forcing regimes (for both coupled, antiphase and decoupled, randomized forcing: frequency range, stroke range, and randomized forcing parameters) to minimize mean flows and to produce acceptably homogeneous and isotropic turbulence within the unique constraints of a litre-scale volume. We acquired particle image velocimetry (PIV) measurements for both classical and fractal grids across a wide range of grid forcing regimes. We discuss the resulting length- and timescales relevant to microorganism-flow interactions, from the integral to the Kolmogorov scales. Finally, we discuss how the range of turbulent kinetic energy (TKE) dissipation rates achieved across the operational space of the facility mimics oceanographic turbulence in a range of in situ conditions, from the nearshore to the open ocean. This facility meets a long-standing need in the oceanography community in which feasible experimental working volumes are constrained by labor-intensive culturing requirements for large volumes of aquatic bacteria and phytoplankton.