| Abstract: | Three-dimensional, non-linear, non-hydrostatic simulations of rotating tidal flows interacting with aquaculture cages (represented as drag elements) at the geophysical scale are performed using an adaptive, finite volume fluid code “Gerris”. Exploiting the Gerris grid structure, sub-metre scale resolution can be obtained even for the farm scale experiments, enabling examination of the impact of the cage on the imposed tidal flows. Passive tracers are used to try to quantify these cage impacts, representing either feed or faecal matter (with specified fall speeds), or other biogeochemical markers such as dissolved oxygen. Using a relatively simple drag formulation, we show that the model is able to reproduce laboratory observations. The farm scale simulations can also be “tuned” in a similar fashion, for example by comparison with observations of total drag force on such structures, or with field measurements of flow retardation by cages. Single and multi-scale cage experiments are then examined to explore the potential impacts of perturbed horizontal and vertical flows on material redistribution through and within the cages. Even with the relatively smooth forcing and drag formulation the experiments reveal a surprising level of complexity in terms of the perturbed flows and their impact on transporting and diffusing passive material. |
