The effects of the direction of current on the drag on fish cages are studied in the present paper. The drags on cages of different shapes, including cylindrical, tnmcated conical, cuboidal and hexagonal, are compared...The effects of the direction of current on the drag on fish cages are studied in the present paper. The drags on cages of different shapes, including cylindrical, tnmcated conical, cuboidal and hexagonal, are compared. The drag on the tnmcated conical net is smaller than that on other shapes of the same area. This net shape with a small apex angle is suggested for the design of fish cages.展开更多
To determine the type of surface roughness pattern that is suitable for adaptive suppression of the drag of an obstacle, we observed flow structures introduced by such obstacles. Several roughness patterns were tested...To determine the type of surface roughness pattern that is suitable for adaptive suppression of the drag of an obstacle, we observed flow structures introduced by such obstacles. Several roughness patterns were tested: geometric patterns, fractal patterns, reptile-skin patterns, and patterns of circular cylinders arranged in a lattice and in a zigzag manner. A suitable pattern for adaptive control of flow is one that generates longitudinal vortices with nonconstant distances. The preferred instability mode of a laminar boundary layer is expected to be selected automatically from fluctuations involving many frequencies and caused by fractal patterns. Snake- and reptile-skin patterns may have a similar ability as fractal patterns because they consist of multiscale patterns. The longitudinal vortices generated from peculiar positions and concave corners in patterns were observed. The distance between these vortices is not constant because the onset of vortices is at concave corners in fractal patterns. These vortices have differing strengths and easily cause nonlinear interactions, so they can disturb a laminar boundary layer with several higher-harmonic frequencies. The velocity profiles of the laminar boundary-layer flow over the fractal patterns were measured by using hydrogen bubbles. The results show the down-wash flow between the longitudinal vortices, which means that the vortices may effectively suppress the boundary layer separation in an adverse pressure gradient.展开更多
The flow patterns of the wake of an oblique honeycombed grid fin were visualized by hydrogen bubbles technique in a water channel. It was found that the hairpin vortices existed in the wake of surfaces in the neighbor...The flow patterns of the wake of an oblique honeycombed grid fin were visualized by hydrogen bubbles technique in a water channel. It was found that the hairpin vortices existed in the wake of surfaces in the neighborhood of each crossing of intersecting elements at moderate angles of attack. They shed periodically following the flow separation on surfaces and connected each other into chains. The main features of the flow patterns are presented and the flow mechanism concerned is preliminarily discussed.展开更多
基金This workis financially supported by the National Natural Science Foundation of China (No.10272118) ,the Key Scientific and Technological Program of China (No.2004BA526B03) and the Research Fund for the Doctoral Program of the Ministry of Education of China (No.20020558013)
文摘The effects of the direction of current on the drag on fish cages are studied in the present paper. The drags on cages of different shapes, including cylindrical, tnmcated conical, cuboidal and hexagonal, are compared. The drag on the tnmcated conical net is smaller than that on other shapes of the same area. This net shape with a small apex angle is suggested for the design of fish cages.
文摘To determine the type of surface roughness pattern that is suitable for adaptive suppression of the drag of an obstacle, we observed flow structures introduced by such obstacles. Several roughness patterns were tested: geometric patterns, fractal patterns, reptile-skin patterns, and patterns of circular cylinders arranged in a lattice and in a zigzag manner. A suitable pattern for adaptive control of flow is one that generates longitudinal vortices with nonconstant distances. The preferred instability mode of a laminar boundary layer is expected to be selected automatically from fluctuations involving many frequencies and caused by fractal patterns. Snake- and reptile-skin patterns may have a similar ability as fractal patterns because they consist of multiscale patterns. The longitudinal vortices generated from peculiar positions and concave corners in patterns were observed. The distance between these vortices is not constant because the onset of vortices is at concave corners in fractal patterns. These vortices have differing strengths and easily cause nonlinear interactions, so they can disturb a laminar boundary layer with several higher-harmonic frequencies. The velocity profiles of the laminar boundary-layer flow over the fractal patterns were measured by using hydrogen bubbles. The results show the down-wash flow between the longitudinal vortices, which means that the vortices may effectively suppress the boundary layer separation in an adverse pressure gradient.
文摘The flow patterns of the wake of an oblique honeycombed grid fin were visualized by hydrogen bubbles technique in a water channel. It was found that the hairpin vortices existed in the wake of surfaces in the neighborhood of each crossing of intersecting elements at moderate angles of attack. They shed periodically following the flow separation on surfaces and connected each other into chains. The main features of the flow patterns are presented and the flow mechanism concerned is preliminarily discussed.
