Experimental Study on Hydrodynamic Characteristics of Three-Stranded Polyethylene Rope

Abstract

Three-stranded polyethylene ropes and twines are widely used as one of the main materials for fishery facilities. The shape and performance of the net fishing facilities in water are determined by the hydrodynamic properties of these ropes and twines. In the present work, experiment investigation was made in the flume tank in Tokyo University of Marine Science and Technology. Five kinds of three-stranded polyethylene rope with different sizes of diameter were tested. Drag and lift forces, together with sideways force acting on ropes under various trial conditions were measured by underwater load cells which were independently developed by the authors to record three axial force components. The drag, lift and sideways forces per unit rope length, under various attack angles at several flow velocities were obtained through data analysis. Then the drag coefficient, the lift coefficient and the sideways force coefficient were calculated within the range of Reynolds numberRd= 2.5×103~3.0×104. The following conclusions may be made from the experimental results: (1) At all the three testing attack angles i.e. 45o, 60o and 90o, the drag per unit rope length is proportional to the square of flow speed; (2) As the attack angle equal to 90o, the drag coefficient can be expressed in a power function of Reynolds number as given below: It can be inferred that the drag coefficient is approximately equal to 1.4 in the automatic model region, i.e. 1.2×104<Rd< 3.0×104. (3) As the attack angle equal to 45o or 60o, the lift force per unit rope length is proportional to the 2.5thpower of flow speed and the lift coefficient has a similar trend as the drag coefficient. But the lift coefficient makes smaller fluctuation compared to the drag coefficient when tending to a constant value in the automatic model region, i.e. 1.2×104<Rd<3.0×104.