An improved Morison hydrodynamics model for knotless nets based on CFD and metamodelling methods

Abstract

The hydrodynamic loadings on the knotless nets are studied using Improved Delayed Detached Eddy Simulations (IDDES) with Kriging metamodelling, then an improved Morison model for predicting hydrodynamic loadings on net twines is derived for the first time. The high-fidelity IDDES turbulence model is capable of simulating the separated flows from cylindrical objects. The 6 × 6 portions of smooth and fabric nets with roughness features are considered in flow models. The accuracy of the numerical method is validated through a typical cruciform case in fluid, wherein the pressure fields and turbulent intensity are compared with physical measurements. Initially, a comparative study of flowing over a circular cylinder, cruciform, and net panel is conducted, illustrating the necessity of the net twines utilisation to analyse the flow fields and hydrodynamic loadings accurately. Based on the review of existed Morison model for net structures, the design point cases varying the normal incoming velocities, diameters and lengths are simulated numerically, then Kriging metamodels of hydrodynamic loading coefficients are trained. It is addressed that the over-estimation of Reynolds number effects, the overlook of net solidity and tangential forces on the twine, or the constant-adoption in the previous models contribute to the deviations of hydrodynamic modelling. The polynomial-type, as well as power-type formulae of normal and tangential force coefficients, are fitted non-linearly using the metamodelling results. Finally, the applications of newly-proposed Morison models embedded in finite element modellings are presented to emphasise its improving abilities. The hydrodynamic loadings with new formulae are validated and compared with existed models through net panels, fish cage and trawl net cases.