Design and Underwater Numerical Analysis of Grid Fins Based on Negative Pressure Gradient Foils

Abstract

To improve the hydrodynamic characteristics of grid fins,the negative pressure gradient foil was designed and applied to the grid fins based on the theory of unseparated flow. The lift-drag characteristics and pressure distribution of that foil and NACA0015 under certain cavitation number and angle of attack were numerically investigated. The lift-drag characteristics,pressure distribution,and the cavity geometry of the grid fins based on negative pressure gradient foil with three kinds of blade spacing at different attack angles were investigated. The results indicate that the critical cavitation number corresponding to the negative pressure gradient foil is smaller than that of NACA0015 when the angle of attack is included, but the lift-drag coefficients of them are basically the same. When the number of blades increases of the grid fins,the lift will tend to a constant value,but the resistance will continue to increase under the small attack angle. In the case of a large attack angle,the increase in the number of blades will lead to a significant increase in both lift and drag. For grid fins with the same blade spacing,the greater the angle of attack,the greater the decrease rate of the length and thickness of the cavity from the top blade to the bottom blade of grid fins. For grid fins with different blade spacing,the greater the number of blades,the more intense the pressure interference of each blade,and the greater the pressure drag,leading to worse lift-drag characteristics. And the severe pressure interference will lead to a longer cavity geometry of grid fins. Therefore,when meeting the requirements of hydrodynamic characteristics,the number of blades should be reduced as far as possible in the design of grid fins based on negative pressure gradient foil.