Vertical Water Entry Research Articles

Research on the process of vertical water entry experiment of truncated cone-shaped projectile

Research on the process of vertical water entry experiment of truncated cone-shaped projectile

Experimental study on synchronous high-speed vertical water entry of cylinders

This paper studied the synchronous parallel high-speed vertical water entry of cylinders through experimental methods. The study found that the double cavity exhibited favorable symmetry characteristics during the synchronous parallel vertical water entry of cylinders at the same speed. The outside of the double cavity develops freely, consistent with the single cavity. The lateral spacing deforms the inside contour of the double cavity. The diameter of the inside cavity increases as the lateral spacing increases and gradually approaches that of a single cavity. The cavity length increases with decreasing lateral spacing. In addition, the maximum diameter and length of the cavity increase with the increase in water entry speed under the same lateral spacing. This paper uses the deformation index β to present the double cavity contour prediction model for the water entry process. The model accurately predicts the cavity contour of a synchronous parallel vertical water entry. Additionally, it is deduced that the critical lateral spacing without mutual influence between cavities in this speed range is approximately 7–8D0. The “grass sprouting” splashing development rules were discovered during the synchronous parallel high-speed vertical water entry.

Performance study of different numerical methods for modeling the vertical water entry of a wedge

Performance study of different numerical methods for modeling the vertical water entry of a wedge

Ice model crevice effect on vertical water-entry of a sphere

The accelerated global warming in recent years, and consequent melting of Arctic ice, has highlighted the significance of Arctic shipping lanes, resources, and military strategy. The presence of ice greatly affects the hydrodynamic behavior of objects entering water. This study investigates the ice model crevice effect on the vertical water-entry of a sphere through an experimental technique and smoothed particle hydrodynamic simulations. The results illustrate the cavity evolution process and determine the sphere's trajectory, effectively constructing the three-dimensional shape of the cavity under different values of the crevice spacing in the ice model. Furthermore, the air flow characteristics in the cavity are systematically explored. This study provides insights into the hydrodynamics and fluid–structure interactions involved in ice–water interfaces, with potential applications in fields including marine engineering and offshore structures.

Vertical water entry of a cylinder considering wind and linearly sheared flow effect: A numerical investigation

Objects entering water is a complex multiphase flow event that exhibits nonlinear and transient characteristics. This study examines the impact cavities, multiphase flow characteristics, and motion behaviors of a cylinder during vertical water entry, considering different flow and entry velocities. A three-dimensional model was carried out using OpenFOAM® framework, taking into account the effects of wind and linearly sheared flow through newly customized initial and boundary conditions. The overset mesh technique was applied to capture the water entry trajectories of the moving cylinder. Numerical results for the cavity evolution and cylinder motion behaviors were validated against published laboratory tests. The cavity closure patterns were classified into four categories based on the evolution characteristics, which were found to be more complex than those observed under calm water and uniform current conditions. Furthermore, the rapid closure of the splash dome results in a unique cavity flow phenomenon, which creates a suction air channel. The velocities of the flow and water entry have a noticeable impact on the closure modes and time of the cavity. This, accordingly, affects the motion characteristics of the cylinder, as well as the evolution of the velocity field, pressure field, and vortex structures.

Experimental study on vertical water entry of the projectile with canard-wing

Flow control techniques play an important role during water entry. In this paper, the idea of water entry of the projectile with single canard-wing is proposed and applied to the water entry problem. The cavity evolution and motion characteristics of projectile with canard-wing were investigated through experiments, and the cavity length, trajectory, and attitude changes of projectile with canard-wing during water entry were quantified. The results show that, different from the water entry process of projectile without wing, the projectile with canard-wing has the typical characteristics of forming the attached cavity on the wing. Due to the influence of canard-wing, the trajectory deflection is always toward the side without the wing, and the initial moment of trajectory deflection is advanced with the increase in the impact velocity. The length of the fore-end cavity and the attached cavity on the wing increases as the impact velocity increases and the pinch-off depth of the fore-end cavity also increases. Moreover, the deviation of the trajectory and the attitude angle of the projectile with canard-wing increases as the impact velocity increases during water entry. The results can provide important support for the passive flow control during the water entry of the projectile and the development of the trans-media aircraft.

Experimental and numerical studies on vertical water entry of a cylinder under the influence of current

When a moving object penetrates the free surface, it is often accompanied by complex multiphase flow phenomena. In this work, the complicated impact cavity and flow characteristics of water entry by a cylinder are investigated both experimentally and numerically for different current velocities. A high-speed photography technique is employed to capture the cavity contours under both calm water and flowing water. Under the same working conditions, a three-dimensional simulation using a volume of fraction model is conducted to provide detailed flow field structures and motion behaviors for experimental results. The primary results are as follows: the effect of current causes the cavities to no longer develop symmetrically. The cavity on the upstream side is compressed close to the cylinder surface, while the cavity on the downstream side expands. The asymmetrical distribution of cavities causes different wetting states on both sides of the cylinder. The ensuing pressure difference prompts the cylinder to deflect which is quite different that without considering current. Vortices generated on the upstream side disappear faster, while those generated on the downstream side expand significantly along the flow direction.

Investigation and optimization of load characteristics of a multi-stage load-reduction structure for vehicles during high-speed vertical water entry

Vehicles are subjected to intense impact loads during water entry at high speed, so some appropriate measures are taken to reduce the impact response of the structure. In this paper, a multi-stage load-reduction structure composed of foam aluminum and cavitation structure is designed to improve the load characteristics of high-speed vehicles. Its three-dimensional numerical model is established based on the Coupled Euler-Lagrange (CEL) method. The accuracy of the numerical simulation was verified by comparison with various experimental results of the sphere and AUV head section, and irrelevant grids were obtained. The simulation of a vehicle with and without a multi-stage load-reduction structure was carried out to study the load characteristics under the influence of different parameters such as vertical velocity and relative density of foam aluminum. Researches were also carried out to select the optimal parameters. The results show that the impact acceleration and the optimal density increase synchronously with the initial velocity. The installation of the multi-stage structure transforms the single impact of direct water entry into three reduced acceleration peaks, and finally realizes the load reduction. When the velocity is 100, 200 or 300 m/s, the load reduction rates under the optimum density are up to 84.6%, 83.7% and 79.3%.

Numerical Study of High-Speed Vertical Water Entry of Hollow Projectiles with Different Aperture Sizes and Velocities

The hollow projectile is a new type of projectile that has complex water entry hydrodynamics characteristics and has attracted significant attention in recent years. As such, it is important to investigate the effects of different entry velocities and aperture diameters on the cavity morphology, cavitation, dynamics, and motion characteristics of hollow projectiles when entering water at high speeds. In this study, four stages of an open cavity, cavity stretching, cavity closure, and cavity contraction in the water entry processes of a hollow projectile at 50–200 m/s and four aperture diameter projectiles at 100 m/s were studied using the volume of fluid (VOF), realizable k-ε turbulence, and Schnerr-Sauer cavitation model. With an increase in the speed, the depth of the cavity closure increases, thereby advancing the closure time. The timing of the surface closure at 50 m/s is clearly different from that at 100–200 m/s. Cavitation is not obvious and is near the cavity wall at 50 m/s, although the entire cavity is almost filled with vapor at 100–200 m/s. The friction resistance has two step points when impacting the water surface and entering the water completely. As the velocity increases or the aperture ratio reduces, the splash is higher, the cavity volume is larger, the cavitation phenomenon is more obvious, the cavity closure time is delayed, and the frictional resistance of the projectile is greater. The results of this study can guide the production and application of hollow projectiles in the future.

Analysis of high-speed water entry in semi-sealed cylindrical shells: Cavity formation and self-disturbance characteristics

The high-speed water entry of a semi-sealed cylindrical shell is a complex unsteady process involving the multiphase flow. In order to study the influence of special structure of the semi-sealed cylindrical shell, the fluid-structure interaction method based on Star-CCM+ and ABAQUS is adopted. The results show that semi-sealed cylindrical shells with different structures all form nested cavities, among which the shell with a shorter length forms a nearly spherical cavity, while the shell with a smaller diameter forms a cavity that is close to a long strip shape. When the Mach number of the initial impact velocity is lower than 0.16, the top wall of the shell can be completely restored as the jet leaves the top wall. The von Mises stress at the top wall of the shell is higher than that at the side wall position. The deformation at the top of shells with larger diameter, shorter length, and smaller thickness is greater. The high-speed vertical water entry process of the semi-sealed cylindrical shell exhibits unique self disturbance characteristics, indicating that shells with thicker thickness and a longer length will exhibit obvious deflection after the self-jet phenomenon occurs.

Parallel water entry: Experimental investigations of hydrophobic/hydrophilic spheres

This study aims to experimentally investigate the vertical parallel water entry of two identical spheres (in geometry and material) with different surface wettability (hydrophilic or hydrophobic) pairings. The spheres simultaneously impact the water surface with velocities ranging from 1.71 to 4.32 m s−1. The corresponding ranges of the impact Froude, Weber, and Reynolds numbers are 3.87–9.75, 816–5167, and 38.5×103 to 96.8×103, respectively. The spheres' lateral distances vary from 1.0 to 5.0 times the diameter. A high-speed photography system and image processing technique analyze the event dynamics, focusing on air-entrainment cavity behavior (shapes, closure, shedding), water flow features (Worthington jets, splashes), and sphere kinetics. Results for hydrophobic/hydrophobic cases show that even at the maximum lateral distance, a slightly asymmetric cavity forms, but deep-seal pinching occurs at a single point, similar to a single water entry scenario. As the lateral distance decreases, the spheres significantly influence each other's behavior, leading to the formation of a highly asymmetric air cavity and an oblique Worthington jet. In the case of a hydrophobic/hydrophilic pairing, vortices generated behind the hydrophilic sphere influence the air cavity development of the hydrophobic sphere. This can cause a secondary pinch-off, especially at low lateral distances. This effect becomes more pronounced at higher impact velocities. Additionally, at higher impact velocities and minimum lateral distance (direct contact between the spheres), a smaller cavity detaches from the hydrophobic sphere's cavity, attaches to the hydrophilic sphere, and moves with it. These different regimes result in varying descent velocities for the spheres.

Ballistic Characteristics of High-Speed Projectiles Entering Water Vertically

The formation of supercavitation after a high-speed projectile enters water has a decisive impact on the underwater ballistic and penetration of the projectile. In this study, Ansysfluent19.0 simulation software is used to study water entry of a chosen projectile at velocities of 300, 400, 500, and 600 m/s. The underwater cavitation characteristics, trajectories, and flow-field characteristics are analyzed for a 5.8-mm caliber conical flat head projectile, as well as for t wo other projectiles of the same caliber and different head shapes — conical cone head and elliptical flat head — entering water vertically at the same velocities. The attenuation rate of water entry velocity increases with the increase of velocity. Within first 3ms, the velocity attenuation rate of the conical flat-head projectile with a water entry velocity of 600m / s is 55.6 %, while the velocity attenuation rate of the projectile with a water entry velocity of 300m / s is only 16.3 % within 3ms. Among the head shapes considered, the conical flat head projectile is the most stable for vertical water entry. The stability of an elliptical flat head projectile is worse, and the trajectory stability of a conical cone head projectile is still worse

Vertical water entry of a hydrophobic sphere into waves: Numerical computations and experiments

The water entry cavity evolution and its flow structures for a sphere interacting with periodic waves are investigated numerically and experimentally. The large eddy simulation is applied in the simulation to accurately capture the turbulent flow near the surface and within the cavity of the sphere. An overset mesh-based numerical wave tank is developed, integrating an overset mesh with a method for generating regular waves, to ensure high resolution simulation of velocity fields around the water entry cavity in waves. To validate the numerical model, a physical experiment system is developed, featuring a free-falling setup and an asynchronous pulse trigger system. This experimental setup allows for precise control of the vertical water entry of a sphere at a predetermined phase of a periodic wave. The computed cavity shape and the sphere motion are in good agreement with the experimental results. Notably, the hydrodynamic forces exerted on the sphere exhibit two distinct peaks at the moment of impact and the pinch-off of the cavity, respectively. The gas-phase force acting on the dry surface of the sphere, as the cavity forms and evolves, experiences significant fluctuations along the direction of the sphere's descent. These fluctuations are caused by the accelerating gas flow prior to the pinch-off of the cavity. The changes of the hydrodynamic forces on the sphere for the cases of different water entry phase locations of waves and Froude numbers are discussed.

Numerical simulation of subsonic and transonic water entry with compressibility effect considered

The purpose of this study is to analyze the subsonic and transonic water entry of the axisymmetric body with compressibility effect considered. The water-entry processes of axisymmetric body are numerically investigated, with Tait state equation and energy equation used to describe the compressibility of the liquid. Reasonable agreement has been obtained between the numerical results and the experimental measurements of trajectory, velocity and acceleration. The cavity evolution and dynamic characteristics of subsonic cases are investigated for vertical and oblique water-entry. The results show that the maximum drag coefficient of the oblique water entry is lower, and it is affected by a large moment which causes pitch angle and trajectory inclination angle to change significantly. In the oblique transonic conditions, the effect of compressibility of the liquid on the dynamic flow characteristics is analyzed. On this basis, the influence of entry angle on the hydrodynamic characteristics is investigated. It is concluded that the water-entry angle has a more significant influence on the trajectory characteristics.

Water entry of cups and disks

It is known that the water entry of a body with a recessed, cupped nose can suppress the splash and air cavity typically observed for solid body entry (Mathai, Govardhan & Arakeri, Appl. Phys. Lett., vol. 106, 2015, 064101). However, the interplay between the captive gas in the cup, the cavity and the splash is quite subtle and has not been thoroughly explored. Here we study the cavity and splash dynamics associated with the vertical water entry of cups and find a variety of regimes over a range of Weber numbers ( $We_D$ ) and dimensionless cup depths. Our parameter space spans a transition between slow-developing cavities with long closure times (low $We_D$ ) to fast-sealing cavities (high $We_D$ ). An important dynamic event is the evacuation of trapped gas from within the cup, which drives the ensuing cavity and splash behaviour. Through modelling, we predict the conditions for which the evacuating gas inflates a cavity that opens to the atmosphere versus inflating a submerged cavity that suppresses air entrainment from above the surface. We also compare our cup water entry findings to the impact phenomena observed for flat disks, which entrap gas on the front surface similar to cups. In doing so, we reveal the sensitivity of disk splash and cavity behaviour to impact angle, and show that disks share a common regime with cups, in which a thin splash quickly seals on the body. We deduce the mechanisms by which increasing cup depth delays the cavity seal time in this regime. These findings reveal that cups may in fact promote or suppress cavity growth, depending on the cup depth and impact conditions.

Cavitation dynamics of the semi-sealed cylindrical shell during high-speed water entry

This paper experimentally investigates the vertical high-speed water entry of a semi-sealed cylindrical shell, which has one end sealed and one end opened. The unsteady water-entry cavitating flow characteristics of the shell are analyzed, and the evolution of cavities and jet impacts with different structures is studied. The results show that a nested multi-cavity is generated due to the self-jet phenomenon during water entry. The jet causes the diameter of the secondary cavity to be much larger than that of the primary cavity, and the morphology of the secondary cavity is more atomized. Due to the irregular motion of the jet, the primary cavity undergoes neck-shrinking phenomenon and is compressed, and the neck-shrinking position moves up as the secondary cavity grows. After secondary impact, a small jet appears at the bottom of the shell, which ejects out from the shell and increases the size of the bottom cavity, leading to the formation of quaternary cavity. Moreover, as the inner wall length increases, the time of the primary jet is advanced, while the depth of secondary cavity shortens. With the increase in the thickness, cavity shape becomes more similar to traditional supercavity, and the maximum diameter of the primary cavity increases.

Experimental study on high-speed vertical water entry of the semi-sealed cylindrical shell

The semi-sealed cylindrical shell is a hollow cylinder, of which one end is opened and the other end is sealed. In this paper, the hydrodynamic characteristics of a semi-sealed cylindrical shell at high-speed are studied experimentally. The influence of the slenderness and Froude number on the expansion characteristics of the cavity is preliminarily studied, with special attention paid to the cavity evolution, splash and kinetics of the cylindrical shell under the influence of the internal jet. The results show that, with the semi-sealed cylindrical shell penetrating into water, it forms supercavity on the outer shell surface similar to a solid cylinder. Subsequently, with the internal jet poured out the shell, a foggy cavity is formed below. As the internal jet continuously pours out of the shell, a new water surface is formed below the shell and the shell will impact on the new water surface, thus causing new cavities generated. This process will repeat continuously as the shell moves downward. Moreover, the slenderness of the shell and Froude number have remarkable influence on the motion characteristics and cavity evolution. The findings in this paper can be used as a reference for future designs of cylindrical shell projectile for water entry.

Numerical investigation of the high-speed vertical water entry of a cylindrical shell

A semi-sealed cylindrical shell is a hollow cylinder in which one end is open and the other end is sealed. In order to systematically study the cavity evolution, the hydrodynamic characteristics, and corresponding structural response of the semi-sealed cylindrical shell during high-speed vertical water entry, a numerical simulation is carried out based on a Star-CCM+ and ABAQUS collaborative simulation method. The results show that a nested cavity is formed that presents three different morphologies as the semi-sealed cylindrical shell penetrates the water. Moreover, a jet is formed under the shell, which profoundly influences the hydrodynamic and structural characteristics. Compared with the completely sealed case, the velocity and displacement of the semi-sealed cylindrical shell are significantly changed upon water entry, and the deformation at the top wall is more prominent.

The effect of atmospheric density on splash of spheres during water entry

The dynamics of splash curtain by vertical water entry of a sphere under various atmospheric densities is investigated in the present study. To understand the mechanism of air density on the splash evolution, a simplified model is proposed to analyze the experimental results, revealing both the size of splash and surface seal time increase as the atmospheric density decreases. Considering the pressure drop caused by air flowing into the cavity and surface tension, the present model focuses on the rounded rim at the tip of splash, reducing the complex splash evolution to a ballistics problem. The results show that when pressure drop force is the dominant factor, the trajectory of the rounded rim is approximately a circular arc. As the atmospheric density decreases, the effect of pressure drop force is weakening and the surface tension gradually plays a role. This results in the increasing size of the splash curtain and longer surface seal time, transforming the trajectory into a spiral. Under the extreme conditions, the evolution of splash is such extended by surface tension that the surface seal may even disappear.

Numerical investigation on drag characteristics of the truncated hemispherical-nose projectile in vertical water entry

ABSTRACT In this study, the feasibility of the ALE algorithm in analyzing the drag of a rigid truncated hemispherical-nose projectile at the initial stage of vertical water entry is examined. The water entry process is simulated under the conditions of different dilated half-angles of the head plane (DHAHPs) and muzzle velocities (u 0). The drag coefficient is obtained by dimensionless processing of the drag. The maximum (Cd max) drag coefficients of the initial stage are fitted to obtain the generalised impact laws of water entry. The results show that the larger the DHAHP, the smaller is u 0 and the higher is Cd max. The bivariate fitting formula for Cd max of the truncated hemispherical-nose projectile has high prediction accuracy and wide applicability. The average (Cd ave) drag coefficients of the projectile changes significantly after DHAHP of approximately 40°. This change is related to the wet state of the head.