Identifying the turbulent flow developing inside and around the bottom trawl by Electromagnetic Current Velocity Meter approach in the flume tank

The Knowledge of turbulent flow developing inside and around the bottom trawl net is of great importance not only for improving the hydrodynamic performance of the gear but also for the selectivity via the fish response, such as the herding response or escape behavior. The 3-D Electromagnetic Current Velocity Meter (ECVM) measurements were performed to investigate the effect of turbulent flow on the bottom trawl net performance and to analyze the turbulence intensity and velocity ratio inside and around different parts of the trawl net. Proper orthogonal decomposition (POD) method was applied in order to extract the phase averaged mean velocity field of turbulent flow from each available ECVM instantaneous velocity. The results demonstrated the existence of turbulence flow, consisting of turbulent boundary layer flow and the turbulence due to the trawl wake developing all inside and around the bottom trawl net. Increasing input streamwise velocity results in faster trawl movement and a significant turbulent flow. The maximum turbulence intensity inside and around trawl wing, square part, first belly, second belly, third belly, cod-end is 0.95 %, 1.34%, 3.40%, 4.10%, 4.25% and 3.80%, respectively. It was found that the mean velocity field in a turbulent flow inside and around trawl net cod-end recovered on the average was ~77.58% of the input streamwise velocity. It is ~12.92%, ~13.07%, ~11.40%, ~13.00% and ~0.45 % less than that inside and around trawl wing, square part, first belly, second belly, and third belly of the bottom trawl net, respectively. The turbulent flow behavior depends strongly on the structure oscillation, input streamwise velocity and, porosity of the net structure. It is necessary to take into account the velocity reduction inside and around a different part of the trawl net to improve the entire drag force determination, cod-end design, and further selectivity control of the fishing gear.