Kurt Hansen

Current Loads on Net Panels With Rhombic Meshes

In this paper, hydrodynamic load models applicable to rhombic net structures are discussed and compared to experimental results with net panels in steady currents. Net panels with mesh sizes and mesh opening angles as is commonly used in fishing gear, were tested in the SINTEF operated flume tank in Hirtshals, Denmark. Drag and lift forces acting on the net panels were measured in two different water currents with attack angles ranging from 0 to 90 degrees.These measurement data were used as a benchmark to compare the performance of several established mathematical models of hydrodynamic loads on panels. Subsequently, an element-based approach with summation of lift and drag forces acting on all the individual twines of the element was performed, and the output from the simulations was compared with experimental data. The correspondence between results from the numerical simulations and experiments were in general quite acceptable for higher angles of attack, while the lack of a wake model overestimated the drag at lower angles. Lift was predicted with high accuracy for all panels.Copyright

The Physical Behaviour of Seine Ropes for Evaluating Demersal Seine Fishing

Demersal seining is an active bottom fishing method, which apply seine ropes and a seine net. The seine ropes and net are laid out on the fishing ground with the seine ropes encircling an aggregation of fish on the seabed. The area on the seabed encircled by the seine ropes is typical much larger than the swept area that will be covered by the seine net during the fishing process. Therefore, the catching performance of a demersal seine depends on the efficiency by which the seine ropes are able to herd the fish into and maintain them in the path of the net until they are overtaken by it in the late stages of the fishing process. This article describes flume tank experiments to validate a numerical model. This model simulates the physical behaviour of seine ropes during the fishing process. The experiments are conducted for varying physical properties of the ropes and for different layout patterns. The seine ropes are hauled back at different speeds. A motion tracking system, based on stereo vision, is applied to record the gradual change in the area encircled by the ropes. The experimental results from the flume tank are compared with data obtained using the simulation model. Finally, the validated simulation model is applied for predictions.Copyright