This study on the hydrodynamics of fishing gear focuses on fish nets. A multi-scale concept has been introduced. By decomposing the fish net structure into 4 distinct scales of flow structure interaction (twine, node, mesh and substructure scales) the complexity is reduced with the potential to integrate new knowledge at each scale to form an overall picture of flow-gear interactions. Within the scope and time-frame of the project, experiments were carried out at the twine and mesh scales. Two sets of experiments were designed at twine scale. The first featured synchronous velocity and drag force measurements on various rigid cylinders consisting of circular cylinders and cylinders inspired by twisted twine. The second aimed to study the cylinder near-wakes in greater detail than previously using PIV. At mesh scale, experiments using rigid bi-plane grids were conducted with individual focus on investigating grid turbulence and flow-grid interactions respectively. The twisted cylinders did not affect mean drag, likely due to free-stream turbulence acting on the boundary layers and free shear layers possibly negating the effects of the twisted cylinder geometry. In the near-wake, the twisted cylinders deflected the free stream into the wake and secondary vortices were introduced, shed along the cylinder span, de-correlating the flow field and in one instance, destroying regular vortex shedding. At mesh scale, for low solidities, a limiting value for mesh length was found where the total drag was dominated by individual contribution of bars. The key to altering the turbulence properties of the flow was found to be alteration of the spectral energy in the largest turbulence scales. For turbulence generation, the spectral energy should be increased and for turbulence suppression, spectral energy should be decreased in the largest turbulence scales.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:668968 |
| Date | January 2015 |
| Creators | Gretland, Steffen Khoo |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=227581 |
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