QUANTIFICATION OF AQUACULTURE FARM DRAG BASED ON THE MOMENTUM SINK METHOD

The demand of aquaculture will increase in the future, which presents an economic opportunity for farm expansion. In order to ensure sustainable growth for both the industry and the ecosystem, an important consideration is the flow response from larger farms. This work aims to quantify the friction caused by oyster farms using a computational fluid dynamic (CFD) method. A Large Eddy Simulation (LES) model predicted the flow response of a case study oyster farm in the Damariscotta River. The simulation provided the flow reduction and turbulence induced by the farm. A drag coefficient was derived based on the momentum sink induced by the oyster farm and will be used in the future to predict the impact of larger farms through theoretical and regional scale numerical model techniques. The outcome of these combined efforts will provide guidance for sustainable farm expansion in the future.

A three dimensional CFD model was built to study the momentum sink induced by the farm. Long-lines within the farm were idealized as continuous porous media. The domain size was 100 m x 5 m x 3.2 m, with 4 long lines. The flow velocity was set to 0.2 m/s, a typical current velocity in the Damariscotta River.

Near the surface, the flow reduced by 30% and accelerated 25% near the bottom, as shown in FIGURE 1. Results also indicated strong turbulence and vertical mixing caused by flow interaction with the farm. The momentum sink is assessed by subtracting Reynolds Stress before and after the farm. Drag coefficients from directly simulated Reynolds Stress compared favorably with a parameterized Reynolds Stress, which will be used in future field observations. As shown in TABLE 1, the cross sectional-averaged drag coefficient nearly doubled due to the presence of the farm, which highlights the frictional impact of the farm on the local velocity field.