PhD Defence at DTU Mechanical Engineering

PhD Defence 27th March: Modelling of floating fish cage dynamics with computational fluid dynamics

Tuesday 14 Mar 17

Hao Chen from DTU Mechanical Engineering defends his PhD, "Modelling of floating fish cage dynamics with computational fluid dynamics", Monday, 27th March, at 14:00. The defence takes place in Building 341, Auditorium 22 at the Technical University of Denmark in Kgs. Lyngby. Principal supervisor is Professor Erik Damgaard Christensen. Examiners are Professor Harry Bingham, DTU Mechanical Engineering, Professor Trygve Kristiansen, NTNU and Reader, Director Dr. Jun Zang, University of Bath.

The present thesis studied the responses of floating fish cages in current and wave conditions. The numerical model was developed based on the computational fluid dynamic (CFD) approach. The modeling framework is OpenFOAM, an open source CFD toolbox. The volume of fluid (VOF) method was applied to track the free surface. A floating fish cage usually contains a net cage, a floater, a sinker and mooring lines. The focus of the present work is on modeling the floater and the net cage, the sinker and the mooring lines were not explicitly modeled. Only the constant forces were added to the relative equations for the motion/deformation of the floater and the net cage.

A net cage contains a large number of twines and knots. Therefore, a detailed modeling of the geometry of the net cage is not possible yet. In the present work, it was modeled as a sheet of porous media with very thin thickness. Volume averaged Navier-Stokes equations were applied as the governing equations for the porous media flow. Due to the volume averaging process, a resistance term appears in the governing equations, representing the viscous force of the net cage on the fluid flow. An analytical expression was derived to relate the quadratic force coefficients with the physical parameters of the net cage, e.g. the length of the mesh bar, the solidity ratio, the drag force coefficient for the twins etc. The derivation indeed was based on the transformation from Morison type load model. The proposed expressions were validated against model test results for current and wave interaction with fixed plane net panels and circular net cages.

To model the flexibility of the net cage in response to the current and waves, the porous media model was further coupled with a lumped mass structural model. A new coupling scheme was implemented in the numerical model. The coupling scheme was based on the static mesh, therefore the mesh does not need to conform the deformed geometry of the net cage. In general, for a fish cage in steady current flow, the net cage is the main part to stand the drag force on it. However, when modeling the floating fish cage in wave conditions, the motion of the floater is the main contributor to the forces on the net cage.

A six degree of freedom motion solver was applied to solve the motion equations of the floater with different motion integration methods. This floater model was successfully coupled with the above
described model for the net cage, hereby is capable to model the responses of the Whole floating fish cage system. This integrated numerical model was validated against the experimental.

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