Research On Drag Reduction Characteristics Of Underwater Ventilated Jet

In order to reduce energy consumption and improve the maneuverability of underwater vehicles,underwater ventilated jet drag reduction technology is one of the most promising and cutting-edge issues,and has received extensive attention from researchers.At present,the ventilating jet drag reduction technology on the wall of the vehicle is one of the important methods to improve the mobility of the underwater vehicle.By injecting gas into the wall of the underwater vehicle,a thin gas phase dielectric film is formed on the wall of the vehicle,which changes the wall surface.While the fluid is flowing,it effectively reduces the frictional resistance of the high-speed vehicle in the water.Based on the method of numerical simulation,this paper mainly studies the influence law of nozzle shape and jet velocity on the flow field and drag reduction effect of underwater ventilating jet,and discusses the drag reduction effect of underwater ventilating jet applied to underwater rotating body projectile.The main work and research results are as follows:Numerical research is carried out based on the drag reduction characteristics of the underwater flat vented jet with a single circular hole,and the drag reduction characteristics of the gas jet injected into the flat wall are discussed.The results show that after the gas is injected,it is affected by the incoming flow and develops downstream from the wall.With the development of time,the length of gas coverage on the wall increases linearly with time.After 0.02s,the growth rate slows down and gradually stabilizes.When the gas is stably covered on the wall,the resistance coefficient of the plate is reduced by 16.73%compared with the unventilated condition.The study also found that the injection of gas increases the wall velocity gradient and reduces the viscosity coefficient of the wall fluid.In the air film coverage area,the decrease of the fluid viscosity coefficient is much greater than the increase of the velocity gradient.Therefore,in the air film core area(?_l>80%),the fluid viscosity coefficient plays a decisive role in the drag reduction rate,and the drag reduction effect reaches 80%Above;and outside the core area of??the air film,the contribution of the tangential velocity gradient to the frictional resistance coefficient is enhanced.By numerically studying the effect of jet velocity on the drag reduction characteristics of underwater ventilated jets,it is obtained that the flat plate drag coefficient decreases with the increase of jet velocity.However,due to the increase of the jet velocity,the inertial impulse of the gas jet along the normal direction is enhanced,and the adhesion strength of the gas on the wall surface is reduced,resulting in fluctuations in the gas content of the wall surface in the gas film coverage area.Therefore,at high jet speeds,the drag coefficient exhibits strong pulsation during the decline process.When the incoming flow velocity is 10m/s and the jet velocity is 7m/s,the drag reduction rate of the plate begins to turn.By numerically studying the drag reduction characteristics of the air flow into the plate from different nozzle holes,the drag reduction rate from high to low is square(17.52%),diamond(17.01%),triangle(16.96%),and ellipse(16.87%),Round(16.73%).Studies have shown that the difference in drag reduction rate caused by air injection from different nozzle holes into the wall of the plate is mainly due to the difference in the tangential velocity gradient of the downstream wall.In general,the flat nozzle has the best drag reduction effect,followed by the pointed nozzle,and the curved nozzle the worst.Numerical research on the drag reduction characteristics of the ventilated jet of the underwater rotating body projectile shows that the drag reduction mechanism of the ventilated jet to the underwater moving projectile is consistent with the air film drag reduction mechanism of the flat structure in Chapter 3.However,due to the blocking effect of the projectile on the water flow,piezoresistance will be formed in the warhead.In addition,the injection of gas changes the flow structure of the boundary layer on the wall of the projectile,increases the pressure of the projectile,and makes the drag coefficient of the projectile rise sharply at the beginning of the gas injection.With the development of gas along the wall of the projectile,the pressure drop of the projectile and the viscosity coefficient of the wall fluid are reduced,resulting in a drop in the drag coefficient of the projectile,and the maximum drag reduction rate can reach 30.2%.Subsequently,by numerically studying the influence of nozzle shape on the drag reduction characteristics of underwater rotating body projectiles,the maximum drag reduction rate is obtained from high to low as square holes(82.21%),triangular holes(76.87%),and diamond holes.(76.21%),round holes(69.45%)and elliptical holes(66.68%);this result is similar to the case of a flat plate.The flat nozzle has the best drag reduction effect,followed by pointed ends,and curved heads the worst,but At this time,the drag reduction rate of the triangular hole is slightly higher than that of the diamond hole,and the round hole is also slightly higher than that of the elliptical hole.