Water-exit Process Research Articles

ANALYSIS OF THE PROMOTING EFFECT OF ANNULAR GROOVE ON THE COALESCENCE OF VENTILATED CAVITY

The load distribution and the water-existing attitude of the vehicle will be affected by the natural cavitation on the shoulder of vehicles during the water-exit process. In engineering, active ventilation by opening vent at shoulder is often employed to improve the mechanical environment of vehicle surface so as to solve such problems. This article is aimed at solving the problem that the ventilated cavity circumferential coalescence of underwater vehicles is unsatisfactory. Using the VOF (volume of fluid) multi-phase flow model and dynamic grid technique of dynamic layering based on finite volume method, the mechanism of adding a small-scale annular groove at the downstream of the vent to promote the cavity coalescence was numerically investigated. And the promotion effect of the annular groove on cavity coalescence at different development stages and different working conditions was also studied. The results show that the cavity coalescence is greatly improved by the annular groove. Flow separation in boundary layer occurs when the incoming flow passes through the annular groove in the moving coordinate system because of flow expansion. The induced entrainment of annular groove, on the one hand, retards the axial development of cavity, and promotes the generation of a larger circumferential shear vortex along with expanding in the circumferential direction. On the other hand, part of the ventilation gas is sucked into the annular groove. The gas sucked into the groove is squeezed and broken contributing to the circumferential coalescence. The coalescence cavity in the groove overflows and leaks to promote the upward movement of the cavity coalescence boundary. In addition, under different working conditions, the shape and internal pressure of the cavity become more stable for the groove improving the internal flow state of the cavity.

Numerical Investigation on the Regime of Cavitation Shedding and Collapse During the Water-Exit of Submerged Projectile

Abstract Cavitation may develop on upward-launched submerged objects approaching sea surface with high speed. In this work, the cavitation shedding and collapse during the water-exit of an axisymmetric projectile is investigated using large eddy simulation (LES). High resolution is guaranteed by carefully fulfilling the requisites of y+<1, Δx+<100, and Δz+<40 to resolve at least 80% of the turbulent kinetic energy. The result indicates that the cavity in growth is always undeveloped as the ambient hydrostatic pressure keeps decreasing. The cavity is pushed by the water surface to shed downward and keep shrinking until its final collapse. The vapor inside cavity during the water-exit process is separated by a layer of water so as not to mix with the air. The front of the re-entrant jet barely catches up with the moving wall, and the cavity is pinched off by the joint effect of the jet front and water surface. It is also found that the angle of attack (AOA) generates inversely inclined liquid-vapor contact lines of the cavity leading edge and cavity closure. The advancing contact lines finally intersect on the pressure side to make the cavity break off, which can cause noticeable pressure impulse on the break-off spot. The pressure feature of the water-exit cavitation evolution is studied with intensively arranged monitor points on the wall, which can sense pressure peaks when the liquid-vapor contact lines sweep over them. The instantaneous high pressure induced by cavitation collapse is resolved.

Numerical Simulation of the water-exit process of the missile based on Moving Particle Semi-Implicit method

The phenomenon of a water mound can show up in the process of water-exit of the missile [1], and the a water mound will break and splash in a very short time, which will exert an extra force on the missile and therefore will affects the movement of the missile as well as the gesture when the missile comes out the water. Therefore, investigation into the water-exit process of the missile is of great significance to the launch of the underwater missiles. The traditional grid methods always fail to simulate the whole the water-exit process because of the large deformation of the mesh. The paper adopts a meshless method, called Moving Particle Semi-Implicit method (MPS), to study the water-exit process of the missile. Based on verifying it by the dam break simulation, MPS is applied to simulate the water mound during the water-exit process, which is aiming at providing some reference for the relevant theoretical research.

Numerical simulation study on flow field distribution and load characteristics of trans-media aerial underwater vehicle during its water-exit process

The oblique water-exit process of a trans-media aerial underwater vehicle is a highly non-linear, unsteady and dramatic process of flow field change. Based on the underwater configuration of a variable trans-media aerial underwater vehicle, the water-exit process of this configuration under typical operating conditions was simulated by CFD fluid simulation software, and the flow field characteristics and load distribution of the vehicle at different stages of the water-exit process were analyzed. The simulation results show that when the vehicle crosses the water-air interface obliquely, the phenomenon of liquid surface deformation and uplift, liquid surface attachment wrapping up the water-exit and wake dragging fracture will occur sequentially, which will lead to sudden change and continuous drastic change of the flow field around the vehicle, forming a backflow zone with different radius and direction on both sides. High-frequency positive and negative alternative oscillation of the body load of the vehicle wrapped by water produces pitching moment.

Motion law and density influence of a submersible aerial vehicle in the water-exit process

The water-exit process is a key step in connecting underwater navigation and air flight of a submersible aerial vehicle. In order to study motion law and density influence of the vehicle in the water-exit process, and taking into account the limitations of the experiment and numerical simulation in the study of the whole water-exit process, a theoretical modeling method is proposed. First, the vehicle’s shape is presented, and the water-exit dynamic model is established by mechanics and motion analysis. Then, numerical simulation and theoretical modeling are used to calculate the water-exit motion under same conditions, and the validity of the water-exit model is verified by the calculation results. Finally, on basis of the established model, the water-exit motion of the vehicle with three different densities is calculated under different initial water-exit angles, axial velocities and thrusts, and the water-exit motion law and vehicle’s density influence on the water-exit motion are obtained. The proposed method and obtained results can be used as effective guidance for the vehicle finishes the water-exit motion successfully.

Fixed-time sliding mode attitude tracking control for a submarine-launched missile with multiple disturbances

This paper studies a novel adaptive fixed-time sliding mode attitude tracking control for a submarine-launched missile, which is affected by sea winds, sea waves, ocean currents and other disturbances during the water-exit process. Firstly, the nonlinear water-exit dynamic model of the submarine-launched missile is established, and then it is transformed into a simple second-order attitude tracking system. Subsequently, a novel non-singular fixed-time fast terminal sliding mode surface (NFFTSMS) with fixed-time convergence is presented, and the pre-established settling time is also developed. Moreover, a novel adaptive non-singular fixed-time fast terminal sliding mode control (ANFFTSMC) is presented by employing a fixed-time disturbance observer, a fixed-time differentiator and the proposed NFFTSMS. Closed-loop stability of the proposed controller is proved by utilizing the Lyapunov methodology. Finally, numerical simulations including two typical launch trajectories of the missile are carried out to demonstrate the strong robustness of the proposed control scheme.

Water-Exit Process Modeling and Added-Mass Calculation of the Submarine-Launched Missile

Abstract In the process that the submarine-launched missile exits the water, there is the complex fluid solid coupling phenomenon. Therefore, it is difficult to establish the accurate water-exit dynamic model. In the paper, according to the characteristics of the water-exit motion, based on the traditional method of added mass, considering the added mass changing rate, the water-exit dynamic model is established. And with help of the CFX fluid simulation software, a new calculation method of the added mass that is suit for submarine-launched missile is proposed, which can effectively solve the problem of fluid solid coupling in modeling process. Then by the new calculation method, the change law of the added mass in water-exit process of the missile is obtained. In simulated analysis, for the water-exit process of the missile, by comparing the results of the numerical simulation and the calculation of theoretical model, the effectiveness of the new added mass calculation method and the accuracy of the water-exit dynamic model that considers the added mass changing rate are verified.

Research on water-exit and take-off process for Morphing Unmanned Submersible Aerial Vehicle

This paper presents a theoretic implementation method of Morphing Unmanned Submersible Aerial Vehicle (MUSAV), which can both submerge in the water and fly in the air. Two different shapes are put forward so that the vehicle can suit both submergence and flight, considering the tremendous differences between hydrodynamic configuration and aerodynamic configuration of a vehicle. The transition of the two shapes can be achieved by using morphing technology. The water-to-air process, including water-exit, morphing, take-off and steady flight, is analyzed. The hydrodynamic and aerodynamic model of the vehicle exiting the water surface obliquely and then taking off into the air has been founded. The control strategy after morphing is analyzed and the control method is given. Numerical method is used to validate the motion model of the water-exit process. Results of simulations show the validity of the proposed model and the feasibility of MUSAV in theory.

Water exit model and takeoff control for a morphing cross-media vehicle

The water exit of a morphing cross-media vehicle is a nonlinear, multi-coupled, physical process with strong disturbance. It severely affects the subsequent flight of the vehicle after leaving the water. Because of the short duration, it requires very fast response time from the controller. To overcome the shortcomings of existing research on the water exit process of morphing cross-media vehicles, a possible solution of uncontrolled water exit and controlled takeoff is proposed. A water exit dynamic model is established after properly simplifying the complexity and unpredictability of the underwater environment, in consideration of changing variables, such as buoyancy, wetted area, and added mass. A numerical simulation is conducted to prove the validity of the model. Furthermore, an improved sliding-mode controller is designed to ensure that the cross-media vehicle reaches the target conditions after leaving the water and to prevent the vehicle falling back into the water. Simulation results demonstrate the reasonability and feasibility of the proposed solution. These conclusions can provide valuable reference information for solving control problems in morphing cross-media vehicles.

A combined wind and wave energy-converter concept in survival mode: Numerical and experimental study in regular waves with a focus on water entry and exit

A combined wind and wave energy converter concept, named STC concept was proposed. Model tests were performed in terms of operational and survival modes. Water entry and exit phenomena as well as green water on deck were observed during the survivability model tests. In this paper, a nonlinear numerical model based on a blended station-keeping potential-flow solver with a local impact solution for bottom slamming events and an approximated model for the water shipped on the deck is proposed to simulate these nonlinear phenomena. Physical investigation of the water entry and exit process was firstly carried out and uncertainty analysis of the model test results were performed. Numerical comparisons between the nonlinear solver and model test results are then performed in terms of mean, wave frequency and double wave frequency motion response components. The slamming and green water involved in the water entry process are specially investigated, in terms of the physical evolution and the effects on the dynamic motion responses. The validation work on the occurrence of slamming and water on deck as well as the slamming pressure are performed.

The Simulation Research of Damping Characteristics for the Underwater Vehicle Moving under or Exceeding Water

The vehicle may vibrate when it moves under or exceeding water. This vibration will interact with the flow filed around to form a fluid-structure interactiondynamics system. In this paper, the numerical simulation of a cylinder in the process of moving under or exceeding water with ABAQUS and FLUENT is presented. Simultaneously, the first three damping ratio of the system are obtained by damping identification. For different density, viscosity and velocity of water, model separately to get the impact of each factor. Besides, the continuous process of exceeding water has been discretized to obtaindifferent oscillating curvesat different positions with respect to the water,and then the change of the damping ratio is presented. Thus the damping characteristics of the system in the water-exit process have been obtained approximately.

Numerical simulation of water-exit of a cylinder with cavities

The water-exit process of a cylindrical body was simulated in this paper. In the simulation, cavities were formed on the nose and at the tail of the body when the body approaches to the free surface, and then collapsed during the passage of body through the water surface. The interaction of free surface with cavity were simulated, and the high pressure due to collapse of cavities were captured.