Improved methods for generating and estimating turbulence in tanks suitable for fish larvae experiments

Abstract

An experimental design has been developed for the purpose of testing the response of larval fish feeding to turbulence. The main purpose of the design is to reproduce natural turbulence within the range of scales relevant to the feeding processes. The design is based on grid-generated turbulence in a circular tank with a diameter of 1.5 m and a water depth of 0.90 m. The spatial distribution of the turbulence was measured. The data analysis of our experiment indicate that we successfully simulated natural turbulence covering six orders of magnitude for the energy dissipation rate (4*10-11 - 2*10-4 W/kg) and over three orders of spatial scales from the Kolmogorov microscale (0.3 mm -13 mm) to the size of the tank. It is concluded that the tank is well designed for studies of zooplankton - larval fish interactions, as the turbulence is generated on a range of scales similar to the separation distance between the zooplankton particles found in nature. The short and relatively slow movement of the grid in addition to the relatively large volume of water should make it possible for the larvae to avoid extreme shears, and minimises mechanical impact on larvae behaviour compared to other known turbulence generating systems. A method is developed to calculate the turbulent energy dissipation rate in a zero mean velocity field, assuming the turbulent cascade follows a -5/3 law in the inertial subrange. This method is also expanded for use on noisy time series. Comparisons with five other methods have been conducted. The comparisons show that Taylors frozen turbulence hypothesis can be used even when the ratio between the turbulent and the mean velocity is approximately 1.