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.

了解底部拖网内部和周围发生湍流的知识不仅对于提高渔具的水动力性能非常重要，而且对于通过鱼类响应（例如放牧响应或逃逸行为）的选择性也非常重要。进行 3-D 电磁流速度计 (ECVM) 测量以研究湍流对底拖网性能的影响，并分析拖网不同部分内部和周围的湍流强度和速度比。为了从每个可用的 ECVM 瞬时速度中提取湍流的相位平均平均速度场，应用了适当的正交分解 (POD) 方法。结果证明湍流的存在，包括湍流边界层流和由于拖网尾流在底部拖网内部和周围形成的湍流。增加输入流向速度会导致更快的拖网运动和显着的湍流。拖网翼、方形部分、第一腹、第二腹、第三腹、鳕鱼尾内部和周围的最大湍流强度分别为0.95%、1.34%、3.40%、4.10%、4.25%和3.80%。结果发现，平均恢复的拖网鳕鱼尾内部和周围湍流中的平均速度场约为输入流向速度的 77.58%。分别比底拖网的拖网翼、方形部分、第一腹、第二腹和第三腹的内部和周围少~12.92%、~13.07%、~11.40%、~13.00%和~0.45%。湍流行为在很大程度上取决于结构振荡、输入流向速度和网状结构的孔隙度。有必要考虑拖网不同部分内部和周围的速度降低，以改进整个拖曳力确定、鳕鱼末端设计以及渔具的进一步选择性控制。