For some vehicles travelling through water, it is advantageous to cover the vehicle in a supercavity for the sake of reducing the drag acting on it. The method of artificial ventilation is most effective for generatin...For some vehicles travelling through water, it is advantageous to cover the vehicle in a supercavity for the sake of reducing the drag acting on it. The method of artificial ventilation is most effective for generating and dominating the supercavity. This paper focuses on the numerical simulation of flow field around three dimensional body. The method is based on the multiphase computational fluid dynamics （CFD） model combined with the turbulence model and the full cavity model. The flow field of cavity is simulated by solving the compressible Navier-Stokes equations. The fundamental similarity parameters of ventilated supercavitaty flows that include cavitation number, Froude number, ventilation rate and drag coefficient are all investigated numerically in the case of steady flow and gravity field. We discuss the following simulations results in section 3： The variations of the cavitation number and the supercavity＇s relative diameter with ventilation rate （subtopic 3.1）; The drag coefficient versus the cavitation number （subtopic 3.2）; Deformation of supercavity axis caused by gravitational effect for three different fixed Froude numbers-2.8, 3.4, 4.2 （subtopic 3.3）. In subtopic 3.2, we give the comparison results of the drag reduction ratio among numerical simulation and experiment conducted in hydrodynamic tunnel and towing tank respectively. In subtopic 3.3, we summarize our discussion of gravitational effect on the axis deformation of supercavity as follows： In the case of smaller Froude number, the inclination of the cavity axis increases monotonously with increasing horizontal length, and reaches its maximal value at the end of supercavity; This deformation can be almost completely negligible when the Froude number Fr〉7. The comparisons with the experimental data in the hydrodynamic tunnel and the towing tank indicate that the present method is effective for predicting the flows around ventilated supercavity; that the numerical results is in good agreement with the experimental ones and that the maximal value of the drag reduction ratio can reach 88% compared with that the condition of fully-wetted.展开更多
This paper aims to explore cavity shape variation regularity in the acceleration phase of supercavitat- ing vehicle. According to the theory of Homogenous Equilibrium Flow,with Mixture Multiphase Model,by setting up t...This paper aims to explore cavity shape variation regularity in the acceleration phase of supercavitat- ing vehicle. According to the theory of Homogenous Equilibrium Flow,with Mixture Multiphase Model,by setting up the flow speed at the inlet boundary as a function of time,this study carried out the experiments for the supercavitation vehicle's numeral model and obtained the variation regularity of cavity shape,the viscous drag coefficient and the cavity hysteresis time when the supercavitating vehicle was in the phase of acceleration. Results show that when the vehicle is in the phase of acceleration,at the same cavitation number,the cavity size decrease with the increase of acceleration. With the decrease of cavity number,the effect of acceleration on cavity shape is smaller,but the viscous drag increases along with the increase of acceleration. On the condition when the velocity reaches equality uniform status,the cavity hysteresis decreases as the acceleration becomes smaller. On the condition of the same acceleration,the cavity hysteresis time decreases as the velocity increases.展开更多
In view of the supercavitation effect, a novel device named the rotational supercavitating evaporator (RSCE) has been designed for the desalination. In order to improve the blade shape of the rotational cavitator in t...In view of the supercavitation effect, a novel device named the rotational supercavitating evaporator (RSCE) has been designed for the desalination. In order to improve the blade shape of the rotational cavitator in the RSCE for the performance optimization, the blade shapes of different sizes are designed by utilizing the improved calculation method for the blade shape and the validated empirical formulae based on previous two-dimensional numerical simulations, from which the optimized blade shape with the wedge angle of 45° and the design speed of 5 000 r/min is selected. The estimation method for the desalination performance parameters is developed to validate the feasibility of the utilization of the results obtained by the two-dimensional numerical simulations in the design of the three-dimensional blade shape. Three-dimensional numerical simulations are then conducted for the supercavitating flows around the rotational cavitator with the optimized blade shape at different rotational speeds to obtain the morphological characteristics of the rotational natural supercavitation. The results show that the profile of the supercavity tail is concaved toward the inside of the supercavity due to the re-entrant jet. The empirical formulae for estimating the supercavity size with consideration of the rotation are obtained by fitting the data, with the exponents different from those obtained by the previous two-dimensional numerical simulations. The influences of the rotation on the morphological characteristics are analyzed from the perspectives of the tip and hub vortices and the interaction between the supercavity tail and the blade. Further numerical simulation of the supercavitating flow around the rotational cavitator made up by the blades with exit edge of uniform thickness illustrate that the morphological characteristics are also affected by the blade shape.展开更多
The present study focuses on simulating supercavitating projectile tail-slaps with an analytical method.A model of 3σ-normal distribution tail-slaps for a supercavitating projectile is established.Meanwhile,theσ-κe...The present study focuses on simulating supercavitating projectile tail-slaps with an analytical method.A model of 3σ-normal distribution tail-slaps for a supercavitating projectile is established.Meanwhile,theσ-κequation is derived,which is included in this model.Next,the supercavitating projectile tail-slaps are simulated by combining the proposed model and the Logvinovich supercavity section expansion equation.The results show that the number of tail-slaps depends on where the initial several tail-slaps are under the same initial condition.If the distances between the initial several tail-slap positions are large,the number of tail-slaps will considerably decrease,and vice versa.Furthermore,a series of simulations is employed to analyze the influence of the initial angular velocity and the centroid.Analysis of variance is used to evaluate simulation results.The evaluation results suggest that the projectile’s initial angular velocity and centroid have a major impact on the tail-slap number.The larger the value of initial angular velocity,the higher the probability of an increase in tail-slap number.Additionally,the closer the centroid is to the projectile head,the less likely a tail-slap number increase.This study offers important insights into supercavitating projectile tail-slap research.展开更多
To investigate the dynamic response of the cylindrical shell targets to supercavitating projectile transmedium penetration and the penetration mechanism,experiments and numerical simulations were conducted.Simulations...To investigate the dynamic response of the cylindrical shell targets to supercavitating projectile transmedium penetration and the penetration mechanism,experiments and numerical simulations were conducted.Simulations examined the effects of entry water velocity and impact angle on penetration behavior.The results indicate that,upon water entry,the supercavitating projectile transfers its kinetic energy to the surrounding water medium,causing a sudden rise in local pressure.This creates an approximately hemispherical pressure field in the water medium ahead of the nose of the projectile,where the pressure distribution and magnitude are positively correlated with the velocity of the projectile.As the pressure field approaches the cylindrical shell,the area around the impact point experiences pre-stress and deformation due to the hydrodynamic pressure,which is known as the hydrodynamic ram effect.The deformation of the cylindrical shell caused by the hydrodynamic ram effect increases with increasing velocity of the projectile and exhibits a non-linear relationship with the impact angle,first decreasing and then increasing as the impact angle rises.Additionally,the hydrodynamic ram effect leads to greater local deformation and higher peak stresses in the cylindrical shell,which reduces the penetration drag force faced by the projectile in water compared to air,indicating a lower ballistic limit for underwater targets.During the penetration process,as the impact angle increases,the supercavitating projectile undergoes repetitive bending deformation and even brittle fracture,while the failure mode of the target is characterized by ductile hole expansion.Furthermore,the critical penetration velocity required to perforate the cylindrical shell target increases with increasing impact angle.展开更多
With the application of supercavitation effect, a novel device named rotational supercavitating evaporator(RSCE) was recently designed for desalination. In order to improve the blade shape of rotational cavitator in R...With the application of supercavitation effect, a novel device named rotational supercavitating evaporator(RSCE) was recently designed for desalination. In order to improve the blade shape of rotational cavitator in RSCE for performance optimization and then design three-dimensional blades, numerical simulations are conducted on the supercavitating flows(with cavitation number ranging from 0.055 to 0.315) around two-dimensional planar symmetric wedge-shaped cavitators with different wedge angles varied from 10 to 180 degrees. Proper numerical method for simulating supercavitating flows around planar symmetric cavitator is established, and assessment of k-ε-v2 -f turbulence model in simulating cavitating flows is conducted. It shows that the size of computational domain would affect the simulation result. Empirical formulae for supercavity dimensions about cavitation number at different wedge angles are obtained, which are of significant importance in the subsequent design of three-dimensional blade. The characteristics of resistance at different wedge angles are discussed, which, together with the characteristics of supercavity dimensions, play important roles in the optimal design of RSCE.展开更多
The authors designed three different front profiles for supercavitating vehicles based on cavity theory and the Granville streamlined equation are designed. Experiments were done using these front profiles in the Nort...The authors designed three different front profiles for supercavitating vehicles based on cavity theory and the Granville streamlined equation are designed. Experiments were done using these front profiles in the Northwestern Polytechnical University high-speed water tunnel. The experiments indicated that the critical volume of gas required for supercavitation is affected by the axial distribution of the front-end＇s slope. The experimental data showed critical gas flow rates required for the three designs were less than rood-l, with the greatest decrease 24%. The experimental results also showed the supercavitation generation speeds of the models were faster than mod-1 by up to 32.4%. This verifies that the front profile of a supercaviting vehicle effects supercavity generation speed and critical gas flow rates. The smaller the changes in axial distribution of pressure, the higher the supercavity generation speed. The smaller the changes in curvature distribution of axial, the smaller the critical gas flow rates.展开更多
In order to speed underwater launch of minor-caliber weapons,a sealing device can be set in front of underwater muzzle to separate water,preventing the muzzle from water immersion.By establishing and simplifying the m...In order to speed underwater launch of minor-caliber weapons,a sealing device can be set in front of underwater muzzle to separate water,preventing the muzzle from water immersion.By establishing and simplifying the model of underwater weapon sealing device and unstructured mesh computing domain model based on computational fluid dynamics（CFD）,dynamic mesh and user defined function（UDF）,the N-S equation is solved and the numerical analysis and calculation of the complex two-phase flow inside the sealing device are carried out.The results show that the gas discharged from the sealing device is conducive to the formation of the projectile supercavity.When the projectile is launched at 5munder water,the shock wave before and after the projectile has impact on the box body up to 100 MPa,therefore the sealing device must be strong enough.The research results have the vital significance to the design of underwater weapon sealing device and the formation of the projectile supercavitation.展开更多
The fluid mechanics of water entry is studied through investigating the underwater acoustics and the supercavitation.Underwater acoustic signals in water entry are extensively measured at about 30 different positions ...The fluid mechanics of water entry is studied through investigating the underwater acoustics and the supercavitation.Underwater acoustic signals in water entry are extensively measured at about 30 different positions by using a PVDF needle hydrophone.From the measurements we obtain (1)the primary shock wave caused by the impact of the blunt body on free surface;(2)the vapor pressure inside the cavity;(3)the secondary shock wave caused by pulling away of the cavity from free surface;and so on.The supercavitation induced by the blunt body is observed by using a digital high-speed video camera as well as the single shot photography.The periodic and 3 dimensional motion of the supercavitation is revealed.The experiment is carried out at room temperature.展开更多
An experimental study was conducted to investigate the water entry phenomenon. A facility was designed to carry out the tests with the entry veloci- ties of around 352 m/s. Visualization, pressure ineasurement, veloci...An experimental study was conducted to investigate the water entry phenomenon. A facility was designed to carry out the tests with the entry veloci- ties of around 352 m/s. Visualization, pressure ineasurement, velocity measurement and underwater impact test were performed to investigate the hydroballistic behav- ior of the underwater moving body, the underwater flow field, the supercavitation, etc.. This study shows that the motion of a high-speed underwater body is strongly three-dimensional and chaotic. Furthermore, it is found that the distribution of the trajectory deflection of the underwater projectile depends on the depth of water. It is also found by measuring the deformation on a witness plate submerged in water, that the impact energy of an underwater projectile is reduced as it penetrates deeper into water.展开更多
Supercavitation in the diesel nozzle increases the instability of droplets in part due to the two-phase mixture, while the effect of cavitation bubbles on the instability of drops is still unclear. In order to investi...Supercavitation in the diesel nozzle increases the instability of droplets in part due to the two-phase mixture, while the effect of cavitation bubbles on the instability of drops is still unclear. In order to investigate the breakup of cavitation bubbles within the diesel droplet, a new mathematical model describing the disturbance growth rate of the diesel bubble instability is developed. The new mathematical model is applied to predict the effects of fluids viscosity on the stability of cavitation bubbles. The predicted values reveal that the comprehensive effect of fluids viscosity makes cavitation bubbles more stable. Compared with the viscosities of air and cavitation bubble, the diesel droplet＇s viscosity plays a dominant role on the stability of cavitation bubbles. Furthermore, based on the modified bubble breakup criterion, the effects of bubble growth speed, sound speed, droplet viscosity, droplet density, and bubble-droplet radius ratio on the breakup time and the breakup radius of cavitation bubbles are studied respectively. It is found that a bubble with large bubble-droplet radius ratio has the initial condition for breaking easily. For a given bubble-droplet radius ratio （0.2）, as the bubble growth speed increases （from 2 m/s to 60 m/s）, the bubble breakup time decreases（from 3.59 gs to 0.17 ps） rapidly. Both the greater diesel droplet viscosity and the greater diesel droplet density result in the increase of the breakup time. With increasing initial bubble-droplet radius ratio （from 0.2 to 0.8）, the bubble breakup radius decreases （from 8.86 trn to 6.23 tm）. There is a limited breakup radius for a bubble with a certain initial bubble-droplet radius ratio. The mathematical model and the modified bubble breakup criterion are helpful to improve the study on the breakup mechanism of the secondary diesel droplet under the condition of supercavitation.展开更多
To deal with the effect of compressible fluids on the supercavitating flow over the subsonic disk cavitator of a projectile, a finite volume method is formulated based on the ideal compressible potential theory. By us...To deal with the effect of compressible fluids on the supercavitating flow over the subsonic disk cavitator of a projectile, a finite volume method is formulated based on the ideal compressible potential theory. By using the continuity equation and Tait state equation as well as Riabouchinsky closure model, an“inverse problem”solution is presented for the supercavitating flow. According to the impenetrable condition on the surface of supercavity, a new iterative method for the supercavity shape is designed to deal with the effect of compressibility on the supercavity shape, pressure drag coefficient and density field. By this method, the very low cavitation number can be computed. The calculated results agree well with the experimental data and empirical formula. At the subsonic condition, the fluid compressibility will make supercavity length and radius increase. The supercavity expands, but remains spheroid. The effect on the first 1/3 part of supercavity is not obvious. The drag coefficient of projectile increases as the cavitation number or Mach number increases. With Mach number increasing, the compressibility is more and more significant. The compressibility must be considered as far as the accurate calculation of supercavitating flow is concerned.展开更多
The time-averaged velocity distributions in flows around a hydronautics hydrofoil were measured by using a digital particle image velocimeter(DPIV) system.The results show that the velocity distribution in the whole f...The time-averaged velocity distributions in flows around a hydronautics hydrofoil were measured by using a digital particle image velocimeter(DPIV) system.The results show that the velocity distribution in the whole flow field depends on the development of cavitation structures with the decreasing of cavitation number.The high-fluctuation region with lower velocity relates to the cavitation area.The lowest velocity distribution in the cavity core becomes more uniform,and its influence becomes smaller gradually as moving to downstream.The main-stream velocity distribution is even,then fluctuate and even at last.In the supercavitation stage,the fluid velocity in the cavitation region,corresponding to the front of the hydrofoil's suction surface,has a distribution close to the main stream,while the fluid velocity in other cavitation area is lower.展开更多
Using the supercavitation phenomenon is necessary to reach high velocities underwater. Supercavitation can be achieved in two ways: natural and artificial. In this article, the simulation of flows around a torpedo was...Using the supercavitation phenomenon is necessary to reach high velocities underwater. Supercavitation can be achieved in two ways: natural and artificial. In this article, the simulation of flows around a torpedo was studied naturally and artificially. The validity of simulation using theoretical and practical data in the natural and artificial phases was evaluated. Results showed that the simulations were consistent with the laboratory results. The results in different injection coefficient rates, injection angles, and cavitation numbers were studied. The obtained results showed the importance of cavitation number, injection rate coefficient, and injection angle in cavity shape. At the final level, determining the performance conditions using the Design of Experiment (DOE) method was emphasized, and the performance of cavitation number, injection rate coefficient, and injection angle in drag and lift coefficient was studied. The increase in injection angle in the low injection rate coefficient resulted in a diminished drag coefficient and that in the high injection rate coefficient resulted in an enhanced drag coefficient.展开更多
Supercavitation dynamic characteristics around a hydrofoil are studied with both high-speed visualization and drag and lift measurements. In the supercavitation condition, the cavitation area covers the entire surface...Supercavitation dynamic characteristics around a hydrofoil are studied with both high-speed visualization and drag and lift measurements. In the supercavitation condition, the cavitation area covers the entire surface of the foil. There is a distinct interface between the main flow and the supercavitating flow regions. The cavitating region is filled with cavitation bubble and water mixture. Small fluctuations of the interface are observed, which indicates that even a relative smooth surface is formed in this state, the interface is unsteady. The minimum drag is to be obtained in the supercavitation condition.展开更多
Cavitation number and speed are capable of variation during the motion of supercavitating vehicle underwater, for example, under the condition of accelerated motion stage and external disturbance. The dynamic model an...Cavitation number and speed are capable of variation during the motion of supercavitating vehicle underwater, for example, under the condition of accelerated motion stage and external disturbance. The dynamic model and control challenge associated with the longitudinal motion of supereavitating vehicle with variable cavitation number and speed have been explored. Based on the principle of cavity expansion independence the properties of cavity and the influence on planning force of body were researched. Calculation formula of efficiency of the fin was presented. Nonlinear dynamics model of variable cavitation number and speed supercavitating vehicle was established. Stabilities of the open-loop systems of different situations were analyzed. The simulations results of open-loop systems show that it is necessary to design a control method to control a supereavitating vehicle. A gain schedule controller with guaranteed H∞ performance was designed to stabilize the dive-plane dynamics of supercavitating vehicle under changing conditions.展开更多
In order to understand the laws of motion for supercavitating vehicle better, simplified equations for longitudinal motion of supercavitating vehicle were derived. Then the corresponding simulation software for trajec...In order to understand the laws of motion for supercavitating vehicle better, simplified equations for longitudinal motion of supercavitating vehicle were derived. Then the corresponding simulation software for trajectory of supereavitating vehicle was programmed, by which the theoretical predicted trajectories of the supercavitating vehicle at different velocities were obtained. It was found that the predicted trajectories at low speed and without cavitation on the vehicle in theory agreed well with those in experiments, and the theoretical predicted trajectories at high speed and with supercavity on the vehicle correctly reflected the motion laws of the supercavitating vehicle. The influences of various parameters of eavitator and rudder on the underwater trajectory were compared and analyzed, which can provide a guide for the design of hydrodynamic distribution and gross parameters of the supereavitating weapons.展开更多
We present the results of theoretical and experimental studies of flow processes of extended axisymmetric cavitators during motion near a free surface taking into account a considerable effect of fluid weightiness. Th...We present the results of theoretical and experimental studies of flow processes of extended axisymmetric cavitators during motion near a free surface taking into account a considerable effect of fluid weightiness. The main objective of work is to study ways of reduction of fluid weightiness effect using lift at cavitator and other body （hull） elements within a cavity, in order to apply this way of drag reduction for a range of considerably lower motion speeds.展开更多
A global approximation based adaptive radial basis function（RBF） neural network control strategy is proposed for the trajectory tracking control of supercavitating vehicles（SV）.A nominal model is built firstly wit...A global approximation based adaptive radial basis function（RBF） neural network control strategy is proposed for the trajectory tracking control of supercavitating vehicles（SV）.A nominal model is built firstly with the unknown disturbance.Next, the control scheme is established consisting of a computed torque controller（CTC） for the practical vehicle and an RBF neural network controller to estimate model error between the practical vehicle and the nominal model. The network weights are adapted by employing a Lyapunov-based design. Then it is shown by the Lyapunov theory that the trajectory tracking errors asymptotically converge to a small neighborhood of zero. The control performance of the proposed controller is illustrated by simulation.展开更多
The control synthesis of the high-speed underwater vehicle faces many technical challenges due to its inherent structure and surrounding operational environment.In this paper,the dynamical behavior is firstly describe...The control synthesis of the high-speed underwater vehicle faces many technical challenges due to its inherent structure and surrounding operational environment.In this paper,the dynamical behavior is firstly described through a bifurcation analysis to give some insights for robust control synthesis.Then a novel adaptive fractional-order sliding mode controller(AFOSMC)is realized to effectively manipulate the supercavitating vehicle against payload changes,nonlinear planing force,and external disturbances.The fractional order(FO)calculus can offer more flexibility and more freedom for tuning active control synthesis than the integer-order counterpart.In addition,the adaptation law has been presented to directly handle the payload change effects.The stability of the controlled vehicle system is proven via Lyapunov stability theory.Next,the dynamic performance of the proposed controller is verified through extensive simulation results,which demonstrate the control accuracy with faster responses compared with existing integer-order controllers.Finally,the proposed fractional order controllers can provide higher performance than their integer order counterparts with control algorithms.展开更多
基金supported by National Defense Basic Research Foundation of China(No.K1800060604).
文摘For some vehicles travelling through water, it is advantageous to cover the vehicle in a supercavity for the sake of reducing the drag acting on it. The method of artificial ventilation is most effective for generating and dominating the supercavity. This paper focuses on the numerical simulation of flow field around three dimensional body. The method is based on the multiphase computational fluid dynamics （CFD） model combined with the turbulence model and the full cavity model. The flow field of cavity is simulated by solving the compressible Navier-Stokes equations. The fundamental similarity parameters of ventilated supercavitaty flows that include cavitation number, Froude number, ventilation rate and drag coefficient are all investigated numerically in the case of steady flow and gravity field. We discuss the following simulations results in section 3： The variations of the cavitation number and the supercavity＇s relative diameter with ventilation rate （subtopic 3.1）; The drag coefficient versus the cavitation number （subtopic 3.2）; Deformation of supercavity axis caused by gravitational effect for three different fixed Froude numbers-2.8, 3.4, 4.2 （subtopic 3.3）. In subtopic 3.2, we give the comparison results of the drag reduction ratio among numerical simulation and experiment conducted in hydrodynamic tunnel and towing tank respectively. In subtopic 3.3, we summarize our discussion of gravitational effect on the axis deformation of supercavity as follows： In the case of smaller Froude number, the inclination of the cavity axis increases monotonously with increasing horizontal length, and reaches its maximal value at the end of supercavity; This deformation can be almost completely negligible when the Froude number Fr〉7. The comparisons with the experimental data in the hydrodynamic tunnel and the towing tank indicate that the present method is effective for predicting the flows around ventilated supercavity; that the numerical results is in good agreement with the experimental ones and that the maximal value of the drag reduction ratio can reach 88% compared with that the condition of fully-wetted.
基金Sponsored by the National Natural Science Foundation (Grant No.51149003)
文摘This paper aims to explore cavity shape variation regularity in the acceleration phase of supercavitat- ing vehicle. According to the theory of Homogenous Equilibrium Flow,with Mixture Multiphase Model,by setting up the flow speed at the inlet boundary as a function of time,this study carried out the experiments for the supercavitation vehicle's numeral model and obtained the variation regularity of cavity shape,the viscous drag coefficient and the cavity hysteresis time when the supercavitating vehicle was in the phase of acceleration. Results show that when the vehicle is in the phase of acceleration,at the same cavitation number,the cavity size decrease with the increase of acceleration. With the decrease of cavity number,the effect of acceleration on cavity shape is smaller,but the viscous drag increases along with the increase of acceleration. On the condition when the velocity reaches equality uniform status,the cavity hysteresis decreases as the acceleration becomes smaller. On the condition of the same acceleration,the cavity hysteresis time decreases as the velocity increases.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51806051,51706050,51276046 and 51576051).
文摘In view of the supercavitation effect, a novel device named the rotational supercavitating evaporator (RSCE) has been designed for the desalination. In order to improve the blade shape of the rotational cavitator in the RSCE for the performance optimization, the blade shapes of different sizes are designed by utilizing the improved calculation method for the blade shape and the validated empirical formulae based on previous two-dimensional numerical simulations, from which the optimized blade shape with the wedge angle of 45° and the design speed of 5 000 r/min is selected. The estimation method for the desalination performance parameters is developed to validate the feasibility of the utilization of the results obtained by the two-dimensional numerical simulations in the design of the three-dimensional blade shape. Three-dimensional numerical simulations are then conducted for the supercavitating flows around the rotational cavitator with the optimized blade shape at different rotational speeds to obtain the morphological characteristics of the rotational natural supercavitation. The results show that the profile of the supercavity tail is concaved toward the inside of the supercavity due to the re-entrant jet. The empirical formulae for estimating the supercavity size with consideration of the rotation are obtained by fitting the data, with the exponents different from those obtained by the previous two-dimensional numerical simulations. The influences of the rotation on the morphological characteristics are analyzed from the perspectives of the tip and hub vortices and the interaction between the supercavity tail and the blade. Further numerical simulation of the supercavitating flow around the rotational cavitator made up by the blades with exit edge of uniform thickness illustrate that the morphological characteristics are also affected by the blade shape.
基金Supported by the National Natural Science Foundation of China(Grant No.62101590).
文摘The present study focuses on simulating supercavitating projectile tail-slaps with an analytical method.A model of 3σ-normal distribution tail-slaps for a supercavitating projectile is established.Meanwhile,theσ-κequation is derived,which is included in this model.Next,the supercavitating projectile tail-slaps are simulated by combining the proposed model and the Logvinovich supercavity section expansion equation.The results show that the number of tail-slaps depends on where the initial several tail-slaps are under the same initial condition.If the distances between the initial several tail-slap positions are large,the number of tail-slaps will considerably decrease,and vice versa.Furthermore,a series of simulations is employed to analyze the influence of the initial angular velocity and the centroid.Analysis of variance is used to evaluate simulation results.The evaluation results suggest that the projectile’s initial angular velocity and centroid have a major impact on the tail-slap number.The larger the value of initial angular velocity,the higher the probability of an increase in tail-slap number.Additionally,the closer the centroid is to the projectile head,the less likely a tail-slap number increase.This study offers important insights into supercavitating projectile tail-slap research.
基金funded by National Natural Science Foundation of China(Grant Nos.12102202,12372361)the Fundamental Research Funds for the Central Universities(Grant No.30924010833).
文摘To investigate the dynamic response of the cylindrical shell targets to supercavitating projectile transmedium penetration and the penetration mechanism,experiments and numerical simulations were conducted.Simulations examined the effects of entry water velocity and impact angle on penetration behavior.The results indicate that,upon water entry,the supercavitating projectile transfers its kinetic energy to the surrounding water medium,causing a sudden rise in local pressure.This creates an approximately hemispherical pressure field in the water medium ahead of the nose of the projectile,where the pressure distribution and magnitude are positively correlated with the velocity of the projectile.As the pressure field approaches the cylindrical shell,the area around the impact point experiences pre-stress and deformation due to the hydrodynamic pressure,which is known as the hydrodynamic ram effect.The deformation of the cylindrical shell caused by the hydrodynamic ram effect increases with increasing velocity of the projectile and exhibits a non-linear relationship with the impact angle,first decreasing and then increasing as the impact angle rises.Additionally,the hydrodynamic ram effect leads to greater local deformation and higher peak stresses in the cylindrical shell,which reduces the penetration drag force faced by the projectile in water compared to air,indicating a lower ballistic limit for underwater targets.During the penetration process,as the impact angle increases,the supercavitating projectile undergoes repetitive bending deformation and even brittle fracture,while the failure mode of the target is characterized by ductile hole expansion.Furthermore,the critical penetration velocity required to perforate the cylindrical shell target increases with increasing impact angle.
基金supported by the National Natural Science Foundation of China(Grant No.51276046)Specialized Research Fund for the Doctoral Program of Higher Education of China(Grant No.20112302110020)
文摘With the application of supercavitation effect, a novel device named rotational supercavitating evaporator(RSCE) was recently designed for desalination. In order to improve the blade shape of rotational cavitator in RSCE for performance optimization and then design three-dimensional blades, numerical simulations are conducted on the supercavitating flows(with cavitation number ranging from 0.055 to 0.315) around two-dimensional planar symmetric wedge-shaped cavitators with different wedge angles varied from 10 to 180 degrees. Proper numerical method for simulating supercavitating flows around planar symmetric cavitator is established, and assessment of k-ε-v2 -f turbulence model in simulating cavitating flows is conducted. It shows that the size of computational domain would affect the simulation result. Empirical formulae for supercavity dimensions about cavitation number at different wedge angles are obtained, which are of significant importance in the subsequent design of three-dimensional blade. The characteristics of resistance at different wedge angles are discussed, which, together with the characteristics of supercavity dimensions, play important roles in the optimal design of RSCE.
文摘The authors designed three different front profiles for supercavitating vehicles based on cavity theory and the Granville streamlined equation are designed. Experiments were done using these front profiles in the Northwestern Polytechnical University high-speed water tunnel. The experiments indicated that the critical volume of gas required for supercavitation is affected by the axial distribution of the front-end＇s slope. The experimental data showed critical gas flow rates required for the three designs were less than rood-l, with the greatest decrease 24%. The experimental results also showed the supercavitation generation speeds of the models were faster than mod-1 by up to 32.4%. This verifies that the front profile of a supercaviting vehicle effects supercavity generation speed and critical gas flow rates. The smaller the changes in axial distribution of pressure, the higher the supercavity generation speed. The smaller the changes in curvature distribution of axial, the smaller the critical gas flow rates.
基金National Natural Science Foundation of China(No.51175481)
文摘In order to speed underwater launch of minor-caliber weapons,a sealing device can be set in front of underwater muzzle to separate water,preventing the muzzle from water immersion.By establishing and simplifying the model of underwater weapon sealing device and unstructured mesh computing domain model based on computational fluid dynamics（CFD）,dynamic mesh and user defined function（UDF）,the N-S equation is solved and the numerical analysis and calculation of the complex two-phase flow inside the sealing device are carried out.The results show that the gas discharged from the sealing device is conducive to the formation of the projectile supercavity.When the projectile is launched at 5munder water,the shock wave before and after the projectile has impact on the box body up to 100 MPa,therefore the sealing device must be strong enough.The research results have the vital significance to the design of underwater weapon sealing device and the formation of the projectile supercavitation.
基金The project supported by the "BaiRen Plan" of Chinese Academy of Sciences
文摘The fluid mechanics of water entry is studied through investigating the underwater acoustics and the supercavitation.Underwater acoustic signals in water entry are extensively measured at about 30 different positions by using a PVDF needle hydrophone.From the measurements we obtain (1)the primary shock wave caused by the impact of the blunt body on free surface;(2)the vapor pressure inside the cavity;(3)the secondary shock wave caused by pulling away of the cavity from free surface;and so on.The supercavitation induced by the blunt body is observed by using a digital high-speed video camera as well as the single shot photography.The periodic and 3 dimensional motion of the supercavitation is revealed.The experiment is carried out at room temperature.
基金The project supported by Japan Society for the Promotion of Science under a Grant-in-Aid for Scientific Research (C) (Grant No. 12650162)
文摘An experimental study was conducted to investigate the water entry phenomenon. A facility was designed to carry out the tests with the entry veloci- ties of around 352 m/s. Visualization, pressure ineasurement, velocity measurement and underwater impact test were performed to investigate the hydroballistic behav- ior of the underwater moving body, the underwater flow field, the supercavitation, etc.. This study shows that the motion of a high-speed underwater body is strongly three-dimensional and chaotic. Furthermore, it is found that the distribution of the trajectory deflection of the underwater projectile depends on the depth of water. It is also found by measuring the deformation on a witness plate submerged in water, that the impact energy of an underwater projectile is reduced as it penetrates deeper into water.
基金supported by National Natural Science Foundation of China(Grant No.51276011)Beijing Municipal Natural Science Foundation of China(Grant No.3132016)+1 种基金National Hi-tech Research and Development Program of China(863 Program,Grant No.2013AA065303)Opening Foundation of State Key Laboratory of Engines of China(Grant No.K2013-3)
文摘Supercavitation in the diesel nozzle increases the instability of droplets in part due to the two-phase mixture, while the effect of cavitation bubbles on the instability of drops is still unclear. In order to investigate the breakup of cavitation bubbles within the diesel droplet, a new mathematical model describing the disturbance growth rate of the diesel bubble instability is developed. The new mathematical model is applied to predict the effects of fluids viscosity on the stability of cavitation bubbles. The predicted values reveal that the comprehensive effect of fluids viscosity makes cavitation bubbles more stable. Compared with the viscosities of air and cavitation bubble, the diesel droplet＇s viscosity plays a dominant role on the stability of cavitation bubbles. Furthermore, based on the modified bubble breakup criterion, the effects of bubble growth speed, sound speed, droplet viscosity, droplet density, and bubble-droplet radius ratio on the breakup time and the breakup radius of cavitation bubbles are studied respectively. It is found that a bubble with large bubble-droplet radius ratio has the initial condition for breaking easily. For a given bubble-droplet radius ratio （0.2）, as the bubble growth speed increases （from 2 m/s to 60 m/s）, the bubble breakup time decreases（from 3.59 gs to 0.17 ps） rapidly. Both the greater diesel droplet viscosity and the greater diesel droplet density result in the increase of the breakup time. With increasing initial bubble-droplet radius ratio （from 0.2 to 0.8）, the bubble breakup radius decreases （from 8.86 trn to 6.23 tm）. There is a limited breakup radius for a bubble with a certain initial bubble-droplet radius ratio. The mathematical model and the modified bubble breakup criterion are helpful to improve the study on the breakup mechanism of the secondary diesel droplet under the condition of supercavitation.
基金Foundation item： Supported by the National Natural Science Foundation of China （Grant No. 51309230）, and China Postdoctoral Science Foundation （Nos. 2014T70992 and 2013 M542531）We would like to thank Dr. Tao Miao for closely following our work and making several useful suggestions.
文摘To deal with the effect of compressible fluids on the supercavitating flow over the subsonic disk cavitator of a projectile, a finite volume method is formulated based on the ideal compressible potential theory. By using the continuity equation and Tait state equation as well as Riabouchinsky closure model, an“inverse problem”solution is presented for the supercavitating flow. According to the impenetrable condition on the surface of supercavity, a new iterative method for the supercavity shape is designed to deal with the effect of compressibility on the supercavity shape, pressure drag coefficient and density field. By this method, the very low cavitation number can be computed. The calculated results agree well with the experimental data and empirical formula. At the subsonic condition, the fluid compressibility will make supercavity length and radius increase. The supercavity expands, but remains spheroid. The effect on the first 1/3 part of supercavity is not obvious. The drag coefficient of projectile increases as the cavitation number or Mach number increases. With Mach number increasing, the compressibility is more and more significant. The compressibility must be considered as far as the accurate calculation of supercavitating flow is concerned.
基金Sponsored by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0720)the National Natural Science Foundation of China (50679001)Chinese Universities Scientific Fund(09QG12)
文摘The time-averaged velocity distributions in flows around a hydronautics hydrofoil were measured by using a digital particle image velocimeter(DPIV) system.The results show that the velocity distribution in the whole flow field depends on the development of cavitation structures with the decreasing of cavitation number.The high-fluctuation region with lower velocity relates to the cavitation area.The lowest velocity distribution in the cavity core becomes more uniform,and its influence becomes smaller gradually as moving to downstream.The main-stream velocity distribution is even,then fluctuate and even at last.In the supercavitation stage,the fluid velocity in the cavitation region,corresponding to the front of the hydrofoil's suction surface,has a distribution close to the main stream,while the fluid velocity in other cavitation area is lower.
文摘Using the supercavitation phenomenon is necessary to reach high velocities underwater. Supercavitation can be achieved in two ways: natural and artificial. In this article, the simulation of flows around a torpedo was studied naturally and artificially. The validity of simulation using theoretical and practical data in the natural and artificial phases was evaluated. Results showed that the simulations were consistent with the laboratory results. The results in different injection coefficient rates, injection angles, and cavitation numbers were studied. The obtained results showed the importance of cavitation number, injection rate coefficient, and injection angle in cavity shape. At the final level, determining the performance conditions using the Design of Experiment (DOE) method was emphasized, and the performance of cavitation number, injection rate coefficient, and injection angle in drag and lift coefficient was studied. The increase in injection angle in the low injection rate coefficient resulted in a diminished drag coefficient and that in the high injection rate coefficient resulted in an enhanced drag coefficient.
文摘Supercavitation dynamic characteristics around a hydrofoil are studied with both high-speed visualization and drag and lift measurements. In the supercavitation condition, the cavitation area covers the entire surface of the foil. There is a distinct interface between the main flow and the supercavitating flow regions. The cavitating region is filled with cavitation bubble and water mixture. Small fluctuations of the interface are observed, which indicates that even a relative smooth surface is formed in this state, the interface is unsteady. The minimum drag is to be obtained in the supercavitation condition.
基金Sponsored by the National Natural Science Foundation of China (Grant No. 10832007)the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 200802130003)
文摘Cavitation number and speed are capable of variation during the motion of supercavitating vehicle underwater, for example, under the condition of accelerated motion stage and external disturbance. The dynamic model and control challenge associated with the longitudinal motion of supereavitating vehicle with variable cavitation number and speed have been explored. Based on the principle of cavity expansion independence the properties of cavity and the influence on planning force of body were researched. Calculation formula of efficiency of the fin was presented. Nonlinear dynamics model of variable cavitation number and speed supercavitating vehicle was established. Stabilities of the open-loop systems of different situations were analyzed. The simulations results of open-loop systems show that it is necessary to design a control method to control a supereavitating vehicle. A gain schedule controller with guaranteed H∞ performance was designed to stabilize the dive-plane dynamics of supercavitating vehicle under changing conditions.
文摘In order to understand the laws of motion for supercavitating vehicle better, simplified equations for longitudinal motion of supercavitating vehicle were derived. Then the corresponding simulation software for trajectory of supereavitating vehicle was programmed, by which the theoretical predicted trajectories of the supercavitating vehicle at different velocities were obtained. It was found that the predicted trajectories at low speed and without cavitation on the vehicle in theory agreed well with those in experiments, and the theoretical predicted trajectories at high speed and with supercavity on the vehicle correctly reflected the motion laws of the supercavitating vehicle. The influences of various parameters of eavitator and rudder on the underwater trajectory were compared and analyzed, which can provide a guide for the design of hydrodynamic distribution and gross parameters of the supereavitating weapons.
文摘We present the results of theoretical and experimental studies of flow processes of extended axisymmetric cavitators during motion near a free surface taking into account a considerable effect of fluid weightiness. The main objective of work is to study ways of reduction of fluid weightiness effect using lift at cavitator and other body （hull） elements within a cavity, in order to apply this way of drag reduction for a range of considerably lower motion speeds.
基金supported by the National Natural Science Foundation of China(5167920161473233)
文摘A global approximation based adaptive radial basis function（RBF） neural network control strategy is proposed for the trajectory tracking control of supercavitating vehicles（SV）.A nominal model is built firstly with the unknown disturbance.Next, the control scheme is established consisting of a computed torque controller（CTC） for the practical vehicle and an RBF neural network controller to estimate model error between the practical vehicle and the nominal model. The network weights are adapted by employing a Lyapunov-based design. Then it is shown by the Lyapunov theory that the trajectory tracking errors asymptotically converge to a small neighborhood of zero. The control performance of the proposed controller is illustrated by simulation.
文摘The control synthesis of the high-speed underwater vehicle faces many technical challenges due to its inherent structure and surrounding operational environment.In this paper,the dynamical behavior is firstly described through a bifurcation analysis to give some insights for robust control synthesis.Then a novel adaptive fractional-order sliding mode controller(AFOSMC)is realized to effectively manipulate the supercavitating vehicle against payload changes,nonlinear planing force,and external disturbances.The fractional order(FO)calculus can offer more flexibility and more freedom for tuning active control synthesis than the integer-order counterpart.In addition,the adaptation law has been presented to directly handle the payload change effects.The stability of the controlled vehicle system is proven via Lyapunov stability theory.Next,the dynamic performance of the proposed controller is verified through extensive simulation results,which demonstrate the control accuracy with faster responses compared with existing integer-order controllers.Finally,the proposed fractional order controllers can provide higher performance than their integer order counterparts with control algorithms.
