Dynamic reliability analysis of supercavity vehicle with stochastic parameters under impact loads

With dynamic reliability problems of stochastic parameters,supercavity vehicle is subject to impact loads.The supercavity vehicle is modeled by using eight-node super-parametric shell elements.The tail impact loads of supercavity vehicle structures are simplified into two stationary random processes with a certain phase difference,and the random excitations are transformed into sinusoidal ones in terms of the pseudo excitation method.The stress response of stochastic structure can be obtained through combining Newmark method with pseudo excitation perturbation method,and then all required digital features for dynamic reliability of supercavity vehicle have be calculated.The expressions of the mean value and the variance of dynamic reliability of supercavity vehicle with stochastic parameters are educed on the basis of the Poisson formula of calculating dynamic reliability.Finally,the influence of the randomness of structural parameters on the dynamic reliability is analyzed.And the feasibility and availability of this method were validated by comparing with the Monte Carlo method.

Modeling and Simulation of Supercavity with Inertial Force in the Horizontal Curvilinear Motion

To make a curvilinear motion in the horizontal plane is one of the most contents for realizing the maneuverability of the supercavitating vehicle. It is significant to achieve the controllability and maneuverability of the vehicle in three dimensions both theoretically and practically on research. Models of angle of attack, gravity and inertial force effects on the supercavity in the horizontal curvilinear motion are established, respectively. The supercavity is simulated based on these models in combination with Logvinovich model and the unsteady gas-leakage rate model at the given ventilation rate, and the effect of the inertial force on it is analyzed numerically. Results show that the maximum deviation of the center line of the cross section of supercavity towards the outward normal direction of its trajectory increases as the cavitation number or curvature radius decrease and always occur in the tail because of the increase of inertial effects along the axis of supercavity from the cavitator when other models and flow parameters are constant for the given trajectory curvature. For the variable curvature, the supercavity sheds due to its instability caused by the time-varying angle of attack. The deviation increases along the length of supercavity if the curvature remains the same sign.

Numerical analysis of a ventilated supercavity under periodic motion of the cavitator

This paper presents a numerical simulation of the geometry and the pressure distribution of a ventilated supercavity at different cavitator amplitudes and periods of motion.The numerical method is validated by comparing with the results of a semi-empirical formula under specific conditions.It is shown that the simulation can capture the boundary fluctuations of the ventilated supercavity and its internal pressure variations in a cavitator motion cycle.The simulation results show that the supercavity boundary experiences wave-like deformations when the wavelength of the disturbance caused by the cavitator motion is comparable to the supercavity length.It is also shown that the supercavity closure changes in form between a re-entrant jet and a twin vortex owing to the variations of the pressure difference between the outside and the inside of the supercavity near the closure region.The maximum diameter of the ventilated supercavity exhibits periodic changes with a double peak in each cavitator motion cycle,caused by the corresponding changes of the difference between the internal and external pressures.With the increase of the amplitude of motion of the cavitator,the supercavity boundary has enhanced wave-like undulations,with an increased maximum diameter,and with fluctuations in the cavitation number.As the period of the cavitator motion increases,the wavelength of the disturbances caused by this motion becomes greater than the supercavity length,and so the wave-like undulations of the supercavity boundary and the maximum diameter of the supercavity gradually decrease,but the variations of the cavitation number increase.Moreover,with the increase of the periods,the delay effects on the variations of the characteristics of the supercavity geometry caused by cavitator motion gradually decrease,and they practically vanish for large periods.

SUPERCAVITY MOTION WITH INERTIAL FORCE IN THE VERTICAL PLANE

The curvilinear motion in a vertical plane is one of the most important features of the supercavitating vehicle. It is of great significance to study the controllability and the maneuverability of the supercavitating vehicle. Models are built for the effects of the angle of attack, the gravity and the inertial force in the curvilinear motion in the vertical p｜ane. Numerica~ simulations are carried out for the supercavity motion based on these models combined with the Logvinovich model. It is shown that the maximum deviation displacement in the outward normal direction of the trajectory with a constant curvature, which occurs in the tail of the supercavity, increases as the cavitation number or the curvature radius of the supercavity trajectory decreases under the condition that other model and flow parameters are kept constant. For a varied curvature, the supercavity shape changes evidently because of the change of the ambient pressure, but with the same trend as in constant curvature. The deviation displacement increases along the supercavity length gradually.

超音速超空泡射弹的空泡流型和流体动力特性研究

A supercavitating projectile is launched underwater with supersonic speed,and then,the speed decreases to transonic and subsonic conditions orderly because of the drag coming from surrounding water.The flow regime and hydrodynamic characteristics are significantly influenced by the flying speed,the influence laws in supersonic,transonic,and subsonic regions are totally different.These issues aren’t well studied.A numerical model consisting of VOF model,moving frame method and state equation of liquid is established to calculate the compressible supercavitation flow field,and validated by comparing with a published result.The influences of water compressibility and Mach number on supercavity shape and hydrodynamic characteristics are quantitatively summarized.The results show that the flying speed of supercavitating projectiles exerts significant influences on the flow regime,supercavity shape and hydrodynamic characteristics for the transonic and supersonic conditions.With the decrease of flying speed,the drag coefficient decreases gradually,and the dimensions of the supercavity near supercavitating projectiles significantly increases in the high-speed conditions.An underwater bow shock is numerically observed before the disk cavitator in supersonic condition.However,no obvious changes are found for the incompressible water cases with different speeds.For supersonic conditions,the supercavity near supercavitating projectiles of compressible water is smaller than that of incompressible water,the drag coefficient is larger,and the relative difference significantly increases with the flying speed.For the case of Ma 1.214,the relative difference of supercavity diameter at the tail section 3.98%,and the difference of the drag coefficient is 23.90%.

周期性阵风流对通气超空泡尾流结构影响的数值研究

A computational model is established to investigate the effects of a periodic gust flow on the wake structure of ventilated supercavities.The effectiveness of the computational model is validated by comparing with available experimental data.Benefited from this numerical model,the vertical velocity characteristics in the entire flow field can be easily monitored and analyzed under the action of a gust generator;further,the unsteady evolution of the flow parameters of the closed region of the supercavity can be captured in any location.To avoid the adverse effects of mounting struts in the experiments and to obtain more realistic results,the wake structure of a ventilated supercavity without mounting struts is investigated.Unsteady changes in the wake morphology and vorticity distribution pattern of the ventilated supercavity are determined.The results demonstrate that the periodic swing of the gust generator can generate a gust flow and,therefore,generate a periodic variation of the ventilated cavitation numberσ.At the peakσ,a re-entrant jet closure appears in the wake of the ventilated supercavity.At the valleyσ,a twin-vortex closure appears in the wake of the ventilated supercavity.For the forward facing model,the twin vortex appears as a pair of centrally rolled-up vortices,due to the closure of vortex is affected by the structure.For the backward facing model,however,the twin vortex appears alternately as a pair of centrally rolled-up vortices and a pair of centrally rolled-down vortices,against the periodic gust flow.

SUPERCAV——一种高速超空泡反潜战导弹

介绍了美国海军SUPERCAV超空泡验证器的概念、研制背景、技术验证器的发展现状、技术途径,今后的发展方向及工作重点。

THE COMPARATIVE STUDY OF VENTILATED SUPER CAVITY SHAPE IN WATER TUNNEL AND INFINITE FLOW FIELD

In order to understand the difference of ventilated supercavity in water tunnel and infinite flow field, 3-D numerical simulations are carried out to obtain the ventilated supercavity in above mentioned conditions based on RANS equations, using the finite volume method and SST turbulence model in the framework of the two fluid multiphase flow model. The numerical method adopted in this article for the infinite flow field and water tunnel experiments is validated by comparing results with those of empirical formulas and experimental data. On this basis the difference between water tunnel experiments and infinite flow field is studied, including the influence of the route loss and the blocking effect in the water tunnel. Finally, some suggestions are made for water tunnel experiments.

Hydrodynamics and modeling of a ventilated supercavitating body in transition phase

Compared to other underwater vehicles, supercavitating vehicles can attain a high speed because they eliminate drag by creating a large cavity, thus establishing the so-called ＂supercavitating condition.＂ Such a cavity is difficult to develop under normal conditions, hence, ventilation is used to attain the supercavitating condition in the initial phase of flight. In this paper, we focus on the hydrodynamic characteristics of a ventilated supercavitating vehicle. First, dynamic modeling of the supercavitating vehicle is performed to calculate the hydrodynamic force/moment acting on the vehicle for a given size of cavity. We then define the relationship between the ventilation rate and the cavitation number based on an air entrainment model of the ventilated cavity. Numerical simulations were performed to analyze the physical feasibility and characteristics of the modeling. The results show that the cavity length/radius increases with the ventilation rate, proving that ventilation can be used to attain the supercavitating condition.

尾部射流作用下通气超空泡的瞬态动力学分析

通气空化可应用于水下航行器以实现高减阻率.在推进系统产生的高速尾喷流的影响下,通气超空泡可能会发生变形、波动和不稳定,本文重点针对尾射流作用下通气超空泡瞬态演化动力学机制开展研究,实验在具有高除气率的开放式水洞系统中进行,详细记录了不同尾射流流量下气液界面的演变过程.采用可压缩多相流模型,结合剪切应力湍流模型和界面追踪方法,对流场进行了深入研究.随着射流速度从亚音速增加到声速,流场呈现三种不同的模式,分别为透明空泡(TC)、透明空泡-射流(TCJ)和变形空泡-射流(DC-J)模式.在TC模式下观察到-.种新的泄气机制,该机制下双涡管泄气与界面震荡泄气耦合.进一步对内部流场结构变化和局部压力脉动规律进行了详细讨论,得到流型随无量纲射流动量比和动能比的变化规律,所得结果可为通气超空泡的控制提供有价值的见解.