Mechanisms Of Improving The Water Exit Characteristics And Robustness Of Underwater Launched Vehicles With Pressure-equalizing Exhaust

The water-exit load,trajectory and attitude of underwater vehicles are the key parameters of the underwater launch technique.Due to the impact of the complex factors in the process of passing through the water layer,it has become one of the technical difficulties to be solved urgently of the underwater launch technique that how to ensure the water-exit parameters of the vehicle meet the requirements.As a special technique of ventilated cavitation,pressure-equalizing exhaust can generate an air boundary layer covering the partial exterior of the vehicle,and effectively improve the water-exit load,trajectory and attitude of the vehicle,even the robustness of each parameter.Recently this technique has been proposed and received widespread attention.Taken the water-exit process of the vehicle as the research object,the investigation of the mechanism of improving the water-exit characteristics and robustness of the vehicle with the pressure-equalizing exhaust is performed.Firstly,the evolution mechanism of the pressure-equalizing exhaust air film is numerically studied.The characteristics and relationships of the inner composition,pressure distribution,vortex structures and other parameters in the developing process of the air film are analyzed.Besides,the flow patterns before and after the circumferential merge of the air films are established.The results show that the pressure-equalizing exhaust is an air leakage process of the air chamber caused by the decrease of ambient pressure and the scouring effect of the incoming flow.The development of the formed air film is motivated by the continual air injection,the constant pushing of incoming water and the persistent decrease of ambient pressure.According to the inner composition and vortex structures,the air film can be divided into the air phase region impacted by the counter-rotating vortex pair,the mixture phase region dominated by the re-entrant jet and the film tail leakage region.Within the circumferential merging process from banding air bunch into lamellar air film,the closure mode of the air film tail changes from the saddle-point-double-spiral-node mode into the reattachment-point-double-spiral-node mode.The physical mechanism of the air film in improving the water-exit characteristics of the vehicle is that the approximate pressure-equalizing region composed by the air phase region and mixture phase region can improve the load characteristics of the vehicle.In addition,the mechanisms of the influence of exhaust structure parameters on exhaust characteristics are systematically discussed.The results show that the change of the exhaust structure parameters can significantly affect the exhaust process and the developing process of the air film.For single-row hole cases,the increase of hole-to-hole spacing promotes the fast axial extension of the air film by the significant growth of single-hole mass flow rate.But the circumferential merging of the adjacent air films is delayed.The decrease of the exhaust angle increases the axial exhaust area and brings about the pre-declination of the air jet,which promotes the fast exhaust of the chamber and the fast axial growth of the air film.Exhaust with double-row holes can achieve faster developing speed of the air film by the twice exhaust area of the single-row hole cases and the combined exhaust mode of hole1 air film covering hole2.The increase of row-to-row spacing of double-row hole cases can increase the combined exhaust efficiency.However,the high exhaust efficiency of the combined exhaust cases weakens the effect of hole-to-hole spacing and exhaust angle on the exhaust process.On the basis of the above study,based on methods of one-way coupling simulation for one-degree-of-freedom motion and two-way coupling simulation for three-degree-of-freedom motion,the characteristics of water-exit load and multi-degree-of-freedom motion of the vehicle are investigated considering the impact of the platform velocity.The results show that the horizontal motion of the vehicle caused by the platform velocity results in the decrease of the exhaust flow rates on the front side of the vehicle and the gathering of the air films to the back side.The vehicle experiences horizontal motion,vertical deceleration motion and pitching motion in the motion plane under the impact of platform velocity,and exits from the water with large angular velocity and attitude bearing high hydrodynamic load.The implementation of the pressure-equalizing exhaust can effectively reduce the pressure difference between the front side and back side of the vehicle by the formed air film boundary layer,and realize the reduction of the hydrodynamic load and the inhibition of the pitching rotation of the vehicle.By comparing the cases with different exhaust structures,exhaust with double-row holes can improve the water-exit characteristics of the vehicle more timely and more efficiently compared to the single-row hole case.Finally,by introducing the uncertainty quantification theory,the statistical properties of the pressure-equalizing exhaust under stochastic launch parameters and the sensitivities of some air film parameters to these stochastic launch parameters are studied.Based on these results,the statistical properties of the water-exit motion,load and flow field of a vehicle with three-degree-of-freedom motion are investigated considering the stochastic characteristics of the platform velocity.The mechanism of the pressure-equalizing exhaust on improving the water-exit robustness of the vehicle is discussed.The results show that the air film can maintain good “pressure equalization” and “pressure robustness” under stochastic launch parameters.Under the stochastic platform velocity,the uncertainty bars of the horizontal velocities of the vehicle with or without an exhaust both exhibit a “contracting-expanding” pattern with an “hourglass” shape along the water-exit trajectory.By contrast,the uncertainty bars of the horizontal displacements present “expanding-contracting-expanding” mode with a “spindle” shape,which progresses a half cycle later.The uncertainty bars of the vehicle attitude persist increasing along with the continuous growth of its mean value.Pressure-equalizing exhaust can reduce the mean values and uncertainty bars of the pressure difference between the front side and back side of the vehicle by the good pressure “equalization” and “robustness” of the air film.Thus,the sensitivities of the lateral load and pitching moment of the vehicle to the stochastic platform velocity decrease and the improvement of the water-exit characteristics and robustness of the vehicle is achieved.