Experimental Study On Wall Pressure And Damage Of Different Structures To Near-field Underwater Explosion

To improve both anti-damage and survival ability of the warship,accurately predicting the damage of submarines and surface vessels subjected to underwater weapon is the most essential technology.In this paper,we focus on the accurate damage prediction of warship attacked by near-field underwater weapon.Experimental,numerical and theoretical methods are used to study the wall pressure and the damage prediction caused by the underwater explosion.Three bubble-generating methods were applied in current study,including high-voltage underwater discharges(400 V,Rmax～25 mm),minor-charge underwater explosions(0.07 g,Rmax～80 mm)and small-charge underwater explosions(5 g,Rmax～250 mm).Aim at different research objects,the experiments based on specified bubble-generating method and a nearby structure were carried out.The structural forms used in this paper are mainly rigid wall,single-layer plate,single-layer cylindrical shell,double-layer plate with a circular opening,double-layer stiffened plate and double-layer cylindrical shell.The equation of state near the fluid and structure interaction(FSI)surface will change under the impact of near-field underwater explosion shock wave.Therefore,by taking the acoustic impedance into account,with the combination of the compressibility of the fluid medium and structure,the coupling mechanical model between the strong shock wave of underwater explosion and the simple plate structure is established,and the wall pressure under the under shock wave are given.On this basis,the concept of the general acoustic impedance is introduced,the process of stress wave propagation in double-bottom structure and coating plate of warship are solved by wave motion theory.The influence of cavitation effect is also taken into account by using bilinear criterion to obtain more accurate wall pressure prediction results.The analytical method based on wave motion theory proposed in this paper are verified the numerical method,which can provide accurate shock wave load input and fluid-structure interaction model for local damage research of ship structures.Secondly,for wall pressure induced by bubble dynamics,a series of minor-charge underwater explosion experiments near a rigid wall were carried out in this paper.Combined with the boundary element method(BEM),an engineering prediction method of the wall pressure caused by an underwater explosion bubble was given.In the experiment,the bubble morphological characteristics near the rigid wall were captured by high-speed camera,in which the formation of the bubble ring,splitting and collapsing of bubble were mainly investigated.At the same time,regular features of bubble loads and bubble impulsive were obtained by using pressure transducer to investigate the wall pressure characteristics caused by bubble.Based on BEM,the numerical model to calculate the wall pressure caused by an underwater explosion bubble was proposed and then space distribution of the wall pressure was obtained.Compared with the experimental results,it is shown that the engineering prediction method proposed in this paper can be used as an effective means to calculate the time and space distribution of the wall pressure caused by an underwater explosion bubble,providing bubble load input for structural damage prediction.To investigate the load acted on the inner structure of warship caused by the gas bubble near the hole,systemically experimental research on the bubble dynamics near the double-layer plate with a circular opening were carried out in a water tank.The radius of the opening,the vertical distance of two layers,and the distance between the bubble center and opening center are the main variables to study the bubble dynamics and wall pressure on the center of the inner plate.The interesting phenomenon including the counter jet,the splitting,detaching and collapsing of bubble were captured between two plates.The pressure during the time history with different distance parameters was recorded.By analyzing the pressure peaks and the impulse induced by different phases of bubble motion,the mechanism of the bubble load caused by bubble splitting,detaching and collapsing between two plates can be obtained,which offer an effective means to reveal the distribution of bubble load in the double structure in latter research.After fully revealing the load characteristics on the structure surface,in order to observe the dynamic damage process of the structure caused by shock wave and bubble load,a systematic theoretical and experimental study was carried out on the local damage of single layer plate under near-field underwater explosion.The experimental results showed that the near-field underwater explosion would blow a hole on the surface of the structure.When the hole was generated with the air-backed condition,the underwater explosion bubbles near the hole would penetrate through the hole and generate multiple high-speed jets.On the one hand,high-speed camera is adopted to capture the bubble morphological characteristics and some special phenomena(detonation gas,water jet,etc.)near complex boundary conditions.On the other hand,the dynamic response of the inner structure was measured by the strain gauge,and the loading characteristics were analyzed according to the dynamic response.The experimental results showed that the shock wave and detonation gas will enter the cabin when the outer structure was blow a hole and the jet generated by the later passing through the hole will damage the inner structure.Finally,considering the effects of local loading effect and the additional water mass,based on the rigid-plastic assumption and the basic shape function of the structure,a simplified method for evaluating the damage of simple plate,single-layer and double-layer curved shell subjected to underwater near-field or contact explosion was presented.Then,small-charge underwater explosion experiments on different structural models were carried out,which finally shows that the method in this paper can be used as an effective means to calculate and evaluate the local damage of the different ship structures.The experimental results also show that some special dynamic behaviors of bubble may occur in the water between two layers of the double-layer curved shell,such as "induced cavitation in the water between two layers" and "inrush bubble".The analysis shows that the scale distribution of the "induced cavitation in the water between two layers" is almost 3/4 of the largest radius of the "outboard bubble",and its non-symmetrical collapse could cause damage of inner structure."Inrush bubble "can accelerate the formation of counter jet of "outboard bubble" and reduce the damage effect of bubble load on the inner structure.