Water-exit Process Research Articles

Numerical study of vehicle motion during water exit under combined lifting force and wave action

During the retrieval process of the deep-sea mining vehicle (DSMV), the stability of the retrieval system is strongly influenced by the interaction between the vehicle body and the surrounding seawater due to the vehicle's complex shape and wave motion. Naturally, the negative side effects of significant changes in the vehicle's attitude and the water exit position can only increase retrieval's challenge. To investigate the characteristic of the flow field of the DSMV, this study employs the computational fluid dynamics method based on the Reynolds-averaged Navier–Stokes equations integrating the volume-of-fluid multiphase flow model with a fifth-order Stokes-wave model to explore the attitude and displacement changes of the vehicle during the water exit process in the ocean wave environment. The results indicate that the wave phase and lifting force are the major effect factors in the DSMV's water exit process. An appropriate lifting force under a specific wave phase can effectively reduce attitude changes and positional drift of the DSMV during water exit, thereby enhancing recovery efficiency and stability.

Hydrodynamic and kinematic characteristics of high-speed inclined water-exit process in wave environment

Aiming at the process of underwater vehicle crossing the water-air interface based on the computational fluid dynamics(CFD) method, integrating the volume of fluid(VOF) and dynamic fluid body interaction(DFBI) models, a numerical simulation method for the trans-media process in a wave environment was studied. The motion simulation method of underwater vehicle under the action of complex fluid environment was established. The simulation of the water-exit process of the underwater vehicle in a wave environment has been realized. The accuracy verification of the wave-making and trans-media processes has been completed. The hydrodynamic and kinematic characteristics of the vehicle during the water-exit process have been obtained. The influences of the wave phase and velocity on the force and motion characteristics of the underwater vehicle were obtained, which provides technical support for designing the unmanned vehicles.

Experimental study on cavitation characteristics and pressure load of actively ventilated double-vehicle configuration during water exit

The cavitation phenomenon is widely observed in the cross-medium movement of vehicles, seriously affecting their structural safety and motion stability. Attempts to control these effects include active ventilation, although existing research on active ventilation techniques has focused on single-vehicle configurations. In real situations, multiple vehicles may exit the water at the same time. However, the cavitation development process of the double vehicles during the water exit is completely different from that of the single vehicle, and the flow field around it is also quite different. Therefore, it is necessary to research the water-exit process of actively ventilated double-vehicle configurations. In this article, a double-vehicle water-exit device that injects high-pressure gas is designed, and the factors influencing changes in the cavitation bubbles are studied, such as the launch speed and environmental pressure. The water exit of a single vehicle under the same conditions is considered as the control case. According to our results, active ventilation effectively relieves the cavitation suppression phenomenon between the two vehicles, and reduces the pressure peak at which the cavitation bubbles collapse. Factors such as the launch speed, environmental pressure, and ventilation rate have a significant influence on the development of cavitation bubbles, especially their size. The influence of these factors on the pressure is mainly realized by changing the thickness of the cavity and the initial pressure.

Experimental study on the hydrodynamic of foldable-wing unmanned aerial-underwater vehicles during water-exit process

The Foldable-Wing Unmanned Aerial-Underwater Vehicle (F-UAUV) demands swift movement over water surfaces at significant pitch angles during the water-exit procedure, a phase highly influenced by strong, non-linear, time-varying hydrodynamic effects. To tackle these complexities, we devised a model experiment scheme mirroring the actual water-exit process. Crafted through structural drainage comparisons, high-fidelity models were developed, complemented by a test platform featuring a double track for accurate evaluations. Subsequent water-exit tests, conducted at various speeds and angles, unveiled a consistent increase in water-exit resistance with speed, peaking at the moment of exit. Concurrently, resultant torque escalated inversely with both speed and angle. Notably, the heightened resistance due to structural drainage exhibited a tendency to stabilize during the latter half of the water-exit process. Utilizing the foundational formula for resistance in a single medium, we proposed an enhanced formula for predicting maximum water-exit resistance, a tool instrumental in facilitating F-UAUV design and powertrain selection. This paper offers an exhaustive analysis of hydrodynamic influences during the water-exit process, serving as a pivotal reference for Unmanned Aerial-Underwater Vehicles development.

Experimental study on wave-induced loads and nonlinear effects for pier-pile group foundations of sea-crossing bridges: Wave slamming and suction effect

Strong nonlinear effects, such as wave slamming, suction effects, and wave overtopping, may be induced during wave-bridge interaction. However, an accurate understanding of these nonlinear effects is still insufficient. This study comprehensively investigates the wave slamming and suction effects on pier-pile group foundations of sea-crossing bridges in a 1:50 scale laboratory experiment. Random and regular waves with different strengths are generated based on wave conditions at the bridge site. Five water depths with a 2 cm spacing are designed to simulate varying pile cap clearances. Various pile arrangements are also set up to explore the influence of piles. Results show that wave slamming is low-aeration and is triggered mainly on the cap bottom wall, whereas the wave action on the vertical wall of the pile cap is quasi-static. The suction effect is generally recorded in the quasi-static phase of pressures and depends on the water exit process and the wave crest height. Moreover, the pressure oscillation after the impact phase results from the flow separation and rotation at structure corners. The wave slamming enhances with the increasing wave frequency when the impact area is constant, otherwise, it depends on the impact area. On the cap bottom wall, the wave slamming on inter-pile areas is enhanced, but the suction effect is less affected by pile arrangements. The increase in pile cap clearances reduces vertical forces but has less effect on horizontal forces. Additionally, a sustained increase in the wave crest height may reduce the slamming force. An empirical method for predicting wave forces is finally proposed based on the water entry theory and experimental findings. This work enhances the physical understanding of nonlinear effects during wave-bridge interaction and aims to support the design of substructures for sea-crossing bridges.

Water-exit impact of deep-sea mining vehicles: Experimental and numerical investigations

The operation of the deployment and retrieval system of a deep-sea mining vehicle (DSMV) is closely related to its water exit process; rapid water exit processes under wave conditions tend to exert negative influences on the safety and reliability of the entire system. To evaluate the specific effect of the interaction between the DSMV and the surrounding seawater, computational fluid dynamics (CFD) was used to predict the hydrodynamic characteristics and water-exit impacts of the DSMV at different retrieval speeds. First, a stokes-wave-free surface based on an overset mesh method integrating the volume of fluid (VOF) method was developed to reproduce level 4 sea conditions. Subsequently, the water surface deformation and water resistance were recorded to verify the proposed numerical method. These numerical and experimental findings indicate that when a vehicle body passes through wavy water at a relatively high speed, its lifting cable experiences a water-esistance force that is several times its weight. Finally, a vehicle body with a small retrieval speed might undergo several wave cycles, resulting in its lifting cable becoming loose.

Experimental study on water-exit of cylinder

In this paper, the hydrodynamic characteristics of the cylinder water-exit are studied by high-speed camera experiments. The characteristics of cylindrical motion trajectory, velocity and acceleration are analyzed by image processing algorithm. The emission voltage (i.e. initial velocity) and emission depth are important influencing factors. Based on different emission voltage and depth, the hydrodynamic coefficient is compared and analyzed. The results show that the new research method has good adaptability to the experimental study of cylindrical water-exit. The emission voltage directly affects the initial velocity of the cylinder, which in turn affects the force changes during the water exit process. The increase in emission depth exacerbates the uncertainty of the moment when the cylinder exits the water, thereby affecting the changes in the wake and motion stability during the cylinder water exit process.

Experimental investigation of the dynamic evolution of cavity during the free water-exit of a high-pressure venting vehicle

The underwater vehicle is subjected to complex hydrodynamic loads during water-exit process, which seriously affect the structural strength and water-exit stability of the vehicle. This paper investigates the effect of high-pressure venting conditions on the dynamic evolution characteristics of the cavity during the completely free water-exit of a vehicle based on self-designed experiments. Some novel phenomena are found in the experiment, and the hypothesis of constant acceleration is established to explain the formation mechanism. The results reveal that the reentrant jet is the main factor influencing the pattern of cavity development and causes great differences between the partial and supercavity states. The balance between the initial cavity volume increase and the gas leakage rate results in the fact that increasing the launch Froude number in the partial cavity state does not cause significant changes in the cavity size. There is a stable interval between 0.0038 and 0.03 for the effect of gas entrainment coefficient on the development of the cavity. The influence of the launch Froude number on the cavity morphology varies within different gas entrainment coefficient ranges. Besides, six patterns of cavity closure are found for free water-exit conditions. The coupling closure pattern of reentrant jet and interaction vortex tube is the most stable during the water-exit process. The stable development intervals of cavity closure pattern and cavity morphology are roughly the same. The present study aims to provide a reference for the active venting flow control of the vehicle during water-exit.

Numerical study on the influence of floating ice on the water-exit hydrodynamic characteristics of a trans-media vehicle

In low-temperature combat conditions, the strong interference of floating ice on the water-exit process of trans-media weapons must be considered. The evolution of the cavity shape, hydrodynamic and ballistic characteristics during the water-exit process of a high-speed vehicle penetrating water (without floating ice) and ice-water mixture were studied in comparison based on computational fluid dynamics (CFD) and the discrete element method (DEM). The results revealed that the floating ice accelerates the collapse of the attached cavity, delays the collapse time of the tail cavity, and changes the vehicle’s wet degree. The floating ice causes the vehicle to be subjected to a severe fluctuating alternating load and lasts for a longer time, which changes the trajectory characteristics of the vehicle, reducing its motion stability and intensifying trajectory deviation. The findings can provide a reference for the design and development of cross-media weapons in low-temperature environments in winter.

Influence of cavitation state and launch angle on water-exit process of vehicle based on moving domain method

This paper, aiming at the water-exit process of the vehicle with cavitation, based on the moving domain method coupled with the SDOF (six degrees of freedom) solver, using VOF (volume of fluid) multiphase model and the Schnerr-Sauer cavitation model, carrying out numerical simulation research on multi-conditions. Comparing vertical launch vehicles with different water-exit Ca (cavitation number) cases, the sudden decrease in cavitation number is closely related to the cavitation state, and there is a maximum influent cavitation state of the cavitation collapse on the process of the water-exit. Comparing different initial launch angle cases, it is found that there is a most unstable launch angle, which makes the angular deflection of the vehicle the most severe during the water-exit process.

Numerical Analysis on Water-Exit Process of Submersible Aerial Vehicle under Different Launch Conditions

To study the influence of launch conditions and wave interference on the stability of submersible aerial vehicles at the water–air interface, a coupling model for water-exit motion of submersible aerial vehicles was established by using the RNG k-ε turbulence model and VOF method. The water-exit processes of submersible aerial vehicles under different initial inclination angles and velocities were numerically simulated and the effects of initial inclination angle and velocity on the water-exit motion of submersible aerial vehicles were obtained. Based on the response surface function theory, a mathematical model for the motion stability of submersible aerial vehicles at the water–air interface was established, so that the submersible aerial vehicle’s pitch angle and velocity at the end of vehicle’s water-exit process, corresponding to any initial inclination angle and velocity, can be solved. The deviation between the simulated calculation result and the established fitting function model result was 2.7%. The minimum water-exit velocity of submarine aerial vehicles should be greater than 10.8 m/s. The research provides technical support for the trans-media motion stability analysis and hydrodynamic performance design of the submersible aerial vehicle.

Experimental and Numerical Investigation of Cavity Structure Forced Water Exit from Calm Water at Constant Lifting Velocity

In marine engineering, the installation of structures inevitably involves the process of water exit. This paper studies the vertical force, the shape of the free surface, and the evolution of the water entrained in a cavity in the process of lifting a structure, so as to provide guidance for practical engineering operations. Using a 1:8 experimental model, this paper derives the governing equations based on the Reynolds-averaged Navier–Stokes approach and uses the volume of fluid method to capture the shape change of the free surface. The vertical forces obtained at different lifting speeds are found to be in good agreement with the results of previous model tests. The results show that the numerical simulation method and mesh generation described in this paper can simulate the changes in the physical quantities associated with the structure in the process of water exit. The vertical force on the structure increases nonlinearly as the lifting speed rises, and the maximum lifting speed is conservatively estimated to be 0.034 m/s using the Det Norske Veritas recommended method. The maximum vertical force occurs as the whole structure leaves the water. The water entrained in the structure is mainly located at the sides and bottom. The lifting velocity plays an important role in the water exit process. The water exit force first increases and then decreases to a stable value as the lifting velocity increases, while the maximum water exit force increases nonlinearly.

Investigation into the Water Exit Behavior of a Cavity

Launching-type ship lifts are commonly used in navigational mountain rivers to realize river channelization and communicate different water systems. However, the complicated water–gas–solid coupling process incurred during the water exit of cavities beneath a ship chamber can strongly affect the stability of the chamber and even affect the ship lift operation. In this study, the water exit behavior of a generalized cavity model was investigated using an experimental–numerical approach. Both the air pressure and flow patterns during the water exit process were analyzed. The results demonstrate three different types of air pressure process in cavity exits. Based on the results, a series of relationships are proposed to predict the maximum negative pressure incurred in the water exit process. Moreover, a method was developed to determine the optimum ported area of the cavity regarding the absence of additional hydrodynamic loads. Furthermore, a classification system to typify the flow patterns manifesting in the cavity is proposed. It was found that the transition from a slug flow to a drop flow could be determined as a transition coefficient K equal to 1.

A study on the performance of the cavitating flow structure and load characteristics of the vehicle launched underwater

This paper analyzes cavitating flow structure and load characteristics of vehicles launched underwater for different cavitation numbers and different angles of attack. The improved delayed detached-eddy simulation model and volume of fluid, as well as overlapping mesh technique, are adopted. Additionally, a verification of the underwater launch simulation method and cavitation model is presented. Cavitating flow structure, wall vortex structures, and load characteristics are studied with a focus on the evolution mechanism of the cavitation flow field during the water-exit process. The results show that the attached cavitation rapidly collapses from top to bottom under the combined effect of large–medium density difference and reentry jet. Due to the presence of attachment cavitation, the development of the wall vortex structure represented by the hairpin vortex is inhibited. Considering the compressibility of the vapor phase, the peak of the synchronous collapse pressure is much larger than the collapse pressure with incompressibility. The pressure appears to be characterized by short widths and high peaks during the collapse of the water-exit. As the vehicle exits the water with a certain angle of attack, the range and peak of the cavitation collapse pressure rapidly reduce. In particular, the pressure side cavitation shedding and collapse behavior at the initial moment may lead to a larger pressure peak.

Experimental study on the cavitation flow and motion characteristics of the vehicle launched underwater

In this paper, an experimental system was independently designed to investigate the cavitation flow mechanism and motion characteristic of the vehicle launched underwater. Conducted experiments based on high-speed photography, and the influence of cavity morphological evolution and kinematic characteristics with the different launch and transverse velocities were analyzed. The results show that cavity shedding phenomenon occurs on different scales. Subsequently, a wide range of pressure impulse on the surface of the vehicle is caused by the collapse of large-scale cavity groups, resulting in an obvious sputtering phenomenon on its surface during the water-exit stage. The more the launch speed increases, the longer the initial cavity length is. Moreover, the moment of completely shedding the cavity delays during the water exit process; Furthermore, with the increase of transverse speed, the attitude angle generally shows an increasing trend, but its growth rate slowed down during the large-scale shedding stage of the cavity.

Research on the characteristics of cavitation flow and pressure load during vertical water exit of different head-shaped vehicles

It is a complex process for a vehicle to emerge from the water. During this period, the cavitation flow state and the load on the vehicle change sharply, which has a great impact on the movement of the vehicle. It is necessary to study the cavitation flow state and loading characteristics during the process of water-exit. At present, the research on this aspect at home and abroad mostly focuses on the use of commercial software or self-written programs to simulate the pressure distribution on the surface of the extended vehicle and the flow field at the head of the vehicle during the process of water exit. However, there is a lack of relevant experimental data on the changes of pressure and cavitation flow regimes during the water-exit process of different head-shaped vehicles. The research on water plumes mostly focuses on the interaction between the explosion bubble and the free surface, but there is little research on the water plumes generated when different head-shaped vehicles exit the water. Based on this, this paper designs a set of surface pressure load measurement system for the water-exit process of the vehicle under the decompression environment, and designs and processes the vehicle with different head types such as cone head, hemispheric head, and swelling conical head. The flow characteristics and pressure loads of different head-shaped vehicles in the initial stage, development stage and collapse stage of cavitation were studied. The study found that the shape of the vacuoles in the initial stage, the development stage and the collapse stage of different head-shaped vehicles is quite different, and the change process of the pressure is also different; In addition, the water plumes generated when the vehicle is out of water can be divided into three categories according to the collapse method, and the collapse time of each type are also different.

Experimental study on flow characteristics of cavitation bubbles in underwater vertical movement of double vehicles

The cavitation generated during the water-exit process of the vehicle will have a great influence on the water exit attitude and stability of the vehicle. At present, the research on the cavitation flow mainly focuses on the single vehicle, and there is little research on the cavitation flow during the water-exit process of the double vehicles. However, in practical applications, it often happens that the double vehicles emerge from the water at different timings. Therefore, it is necessary to study the flow characteristics of the cavitation of the vehicles. In this paper, a set of underwater vertical launching experiment device of double vehicles under decompression condition is designed. Based on this device, different time sequence water-out experiments of the vehicles are carried out, and multiple high-speed cameras are used to record the whole process of the vehicles from different angles. The effect of launch velocity, environmental pressure, and lateral and vertical distances on cavitation morphological changes and sizes are explored. Studies have shown that there will be a certain degree of interference between the two vehicles during the water-exit process. When the launch speed increases and the environmental pressure decreases, the size of the cavitation when the double vehicles exit the water will increase greatly, and gradually move closer to the single vehicle; When the lateral and vertical distances increase, the interference between the vehicles will be weakened.

Experimental investigation of the influence of floating ices constraint on the cavity dynamics of an axisymmetric body during the water exit process

The process of water exit in an ice-constrained environment involves complicated gas-liquid-solid multiphase turbulence flows. This paper considers the effects of the floating ices during the water exit process of a cone-shaped axisymmetric test body with an acceleration sensor and a pressure sensor. The cavity evolution, motion features and dynamics loads are addressed based on the simultaneous measurement system. The results show that the water exit process is divided into three stages, and the evolution patterns of the flow field and motion characteristics of the vehicle are closely related to the free water surface. The cavity evolution, movement characteristics and pressure loads during the water exit process of the vehicle are related to the existence of crushed ice. The existence of crushed ice has an effect on cavity dynamics. Especially, the blocking and restraining effects are mainly concentrated in the process of the vehicle crossing the free water surface.

Experimental Research on Cavitation Evolution and Movement Characteristics of the Projectile during Vertical Launching

Aiming at the problem of unsteady cavitation during a projectile’s vertical water-exit process, scaled model experiments were carried out based on the self-designed underwater launch platform and high-speed cameras, which focus on changes in cavitation shape and projectile posture. In this paper, the general process of the cavitation evolution and projectile’s movement is described; the relationship between the re-entry jet, local cavitation number and cavitation stability is discussed. Meanwhile, the effect of head forms and launch speeds on the cavitation evolution and movement characteristics is analyzed, including 60° cone, 90° cone and hemispherical head with velocity of 16.8 m/s, 18.5 m/s and 20.0 m/s, whose launch cavitation number is 0.714, 0.589 and 0.504. The results show that the attached cavities fall off from the bottom up under the influence of the end-re-entry jet and the shedding frequency declines when the launch cavitation number decreases. The cavitation growth of 60° cone is easily disturbed by the air mass near the launcher, the cavitation development of 90° cone is characterized by small-scale and high-frequency growth and shedding, while the hemispherical head is not prone to cavitation. Moreover, increasing the speed can improve the stability of cavitation development and the projectile’s movement.

Experimental study on the effects of brash ice on the water-exit dynamics of an underwater vehicle

To investigate the impact of brash ice on the dynamics of the water exit of an axisymmetric vehicle, an experimental study was conducted on the water exit of an axisymmetric vehicle in brash ice using a water tank, launching system, control system, data acquisition system, and high-speed camera system. The brash ice zone was modeled using actual observational data from the Antarctic marginal ice zone off Wilkes Land in late winter. Brash ice was created from polypropylene material, and the influence of brash ice on the dynamics of the water-exit of the vehicle is analyzed. The results show that the collapse of bubbles during water-exit of a vehicle in the presence of brash ice creates a state of eruption, which splashes upward from the shoulder to the center of the vehicle, and the underwater cavities undergo shrinkage and shedding. The effects of brash ice on the magnitude of the force and moment on the vehicle during the process are diverse and operate in different directions. The overall deflection angle is greater than that under ice-free conditions. As water-exit speed increases, the splash time becomes earlier, and the extent of the splash becomes more dispersive. In addition, pressure falls as the vehicle approaches the water surface, and the extent and magnitude of pressure fluctuation during the whole water-exit process becomes more severe.