Aquaculture Net Drag Force and Added Mass

Abstract

The hydrodynamic loads on plane net samples of differing mesh geometry are measured in steady and oscillating flows. The steady loads on plane nets are also numerically simulated. The net is modeled as an inter-connected system of lumped masses and springs. The loads are computed for each twine segment and applied to the lumped masses at the segment ends. The equations of motion are formulated for the coupled dynamics of the masses and solved numerically. Drag data from the experiments is compared with analytical and numerical models and existing empirical formulae. Results for steady flows indicate that drag coefficients for nets and cylinders, as a function of the Reynolds number, have identical trends with consistent offsets. It is concluded that the drag coefficient for nets is equivalent to the drag coefficient for cylinders (and spheres for knotted nets) modified by a function of net solidity. For unsteady flows, the drag and added mass are extracted from the total wave force by applying a vector approach. It is shown that drag and added mass coefficients are not well quantified by conventional non-dimensional parameters (i.e. Keulegan–Carpenter and Reynolds numbers). The unsteady drag coefficient is presented as a function of wave particle velocity, wave period and net porosity. It is proposed that the added mass coefficient be expressed by an assumption of an effective thickness—conceptually the width of water affected by the net, which is a function of wave frequency and net solidity.