On hydrodynamic stability of trawl board
Some constructive ways to self-regulation of the trawl board steady movement are analyzed. The self-regulation effect is provided by all effective forces tending to some geometrical point or area in which the resultant force of hydrodynamic pressure does not create any considerable tilting moment. Graphic-analytical method is proposed to define the area of steady pressure from the shape of conditional boundary layer. The method is applied for optimization of the trawl board shape using computer modeling by means of AutoCAD, 3Dmax, and MathCAD software. Several successful models of trawl boards with high hydrodynamic stability and modern propeller blades, which shapes were formed as a result of long historical development, are compared and analyzed by the proposed method. Trawl boards and propeller blades are similar by their geometrical parameters and hydrodynamic regime, they both are intended for a bearing strength producing and both have a slender section as aerodynamic profile. Asymmetry of this profile causes bending-torsion fluctuations in both cases. The main sectorial-linear coordinates defining the boundaries of steady pressure area for both devices, which depend on the shape of boundary layer only, can be found under assumption that surface friction is significant in very thin layer. The trawl board construction could be optimized by tuning its flaps, which center of pressure has to be independent on the attack angle. Stable location of the pressure center is proper to the round flaps with spherical aerodynamic profile of relative thickness = 26,6 % or with cylindrical profile of = 18,5 %. These ratios are the basement for modeling of dynamically stable trawl boards. Another decision of the problem of hydrodynamic stability could be seen in marine nature: the shells of some scallops, as Mizuhopecten yessoensis, have a beautiful wavy shape that allows them to maintain hydrodynamic stability by capturing of boundary layer as stringers do.