Process Of Water Entry Research Articles

Influence of ice sheet on the flow characteristics of vertical high-speed water-entry cavity of a projectile

Abstract The ice sheet will inevitably affect the dynamic cavity evolution of cross-media projectiles after breaking ice and entering water. Based on the Coupled Euler-Lagrange (CEL) algorithm, the process of vertical high-speed ice-breaking water entry of a cylindrical projectile at 100 m/s is studied. The results show that the continuous influx of external air during the ice-sheet condition delays the time for cavity closure. The velocity field distribution within the cavity becomes asymmetrical, displaying heightened nonlinear and turbulent characteristics. Furthermore, it is observed that the peak force experienced by the projectile in an ice-free environment is 69.55% of that during ice-breaking water entry. In the design of new cross-media weapons for ice-covered areas, the structural strength of the projectile head should be improved.

Interactional flow physics of freely falling sphere on stagnant water

Abstract The process of water impact and subsequent water entry of rigid objects is essential in engineering applications, including marine, offshore, and aerospace technologies. However, most studies have focused on the impact analysis of the object itself, with limited attention given to its interaction with water and the resulting flow dynamics. This work aims to address this gap by examining the effect of buoyancy on the hydrodynamics of a sphere in freefall, particularly its interaction with stagnant water. The investigation uses computational methodologies validated against experimental results to quantify flow and turbulence parameters, including pressure distribution, flow velocity, and other turbulence parameters for different buoyancy regimes. The study also explores the temporal evolution of the free surface profile to gain insight into the deformation and displacement of water resulting from the impact. The analysis reveals that buoyant spheres generate localized turbulent kinetic energy near the surface, while non-buoyant spheres induce higher, more dispersed turbulence. Pressure peaks at the bottom of the sphere, influenced by fall height but independent of density, while buoyancy affects the pressure distribution over time. Furthermore, buoyancy significantly influences the temporal evolution of pressure distribution and the formation of cavities compared to non-buoyant spheres, which exhibit more concentrated velocity streamlines. These results significantly affect designing and optimizing structures interacting with fluid environments, such as underwater vehicles and offshore platforms. Understanding the interplay between buoyancy and flow characteristics can enhance predictions of hydrodynamic behaviour, improving performance and safety in engineering applications.

Study on water entry impact characteristics and parameter influence analysis of the vehicle passing through the thin crushed ice zone in a polar environment

The problem of water entry in polar environments holds significant research value for polar resource exploration and military applications. This study uses computational fluid dynamics and the discrete element method (CFD-DEM) to investigate the complex process of a vehicle entering water through a thin crushed ice zone. Experimental data validates the efficiency of the coupling algorithm. The primary focus of this study is to investigate the influence of crushed ice shapes, arrangement patterns, coverage densities, and sizes within thin crushed ice zones on the process of water entry. A comprehensive analysis is carried out to assess how the characteristic parameters of different thin crushed ice zones affect the dynamic behavior of the vehicle during water entry and the movements of the crushed ice. Concurrently, the influence of different velocities of the vehicle on the water entry process through a specific thin crushed ice zone is analyzed. The results show that when the crushed ice is in a regular arrangement, it exhibits a circular immersion phenomenon and a cross-shaped passive movement trend. The arrangement patterns, coverage densities, and sizes of the crushed ice have different effects on the water entry process. Of these factors, the complexity of the influence of crushed ice sizes on the water entry process stands out. Meanwhile, the water entry velocity plays a crucial role in determining the relative significance of the hydrodynamic force and the collision force of crushed ice on the total resistance of the vehicle during water entry. These insights provide valuable guidance for the planning and execution of missions in polar regions.

ANALISIS KAPASITAS INFILTRASI LAHAN PERTANIAN DI SUB DAS KALISARI, MALANG

Watershed hydrological conditions can decrease due to changes in land use and inappropriate land management. One of the watershed functions is providing water availability for agricultural areas. Along with the increasing area of agricultural land, there is often the issue of water availability, which has implications for the low opportunity for plants to use it. Infiltration is the initial process of water entry into the soil, so the availability of water is greatly influenced by this process. Agricultural land in the Kalisari sub-watershed is dry land and is dominated by dry fields. The area of the Kalisari sub-watershed has an area of ±5,000 ha divided into 5 land uses, namely mahogany-coffee agroforestry, pine-coffee agroforestry, scrub, dry land, and paddy fields. Infiltration measurements were spread over 43 measurement points, with the observed parameters being texture, bulk density, porosity, permeability, aggregate stability, and soil organic matter. The infiltration rate for all land uses is very fast (>25 cm hour-1), ranging from 12.00 cm hour-1 to 74.37 cm hour-1. The infiltration rate for all land uses was not significantly different; this was in line with soil properties, which included texture, bulk density (0.61-1.02 g cm-3), porosity (51.02-68.06%), permeability (4.88–6.79 cm hour-1), aggregate stability (2.11–3.34 mm), and organic matter (1.61-4.06%). However, the infiltration rate at the study site had a significant relationship with clay (r = -0.77), sand (r = 0.64), silt (r = 0.52) and soil organic matter (r = 0.48).

Study on the effect of cavity oscillation on wedge water entry with a multiphase smoothed particle hydrodynamics model

Previous Smoothed Particle Hydrodynamics (SPH) study on water entry issues has primarily been conducted for the load analysis of impact phase rather than the cavity oscillation effect because the calculation and simulation of this complex physical process are more complicated and time consuming. In order to increase computational efficiency and accuracy, the multiphase δ+-SPH model is combined with Adaptive Particle Refinement technology to investigate the whole process of the wedge's water entry. The hydrodynamic phenomena in the stages before cavity closure for the four cases with different Froude numbers (Fn) are compared and analyzed. After the cavity is pinched off, the wedge exhibits kinematic oscillation. Our test shows that the adoption of sound speed has a significant influence on the oscillation period and peak value of closed cavities in weakly compressible SPH calculations. Then, a suitable sound speed adoption is selected to simulate the oscillatory phenomenon accurately. Comparing the pressure profile with the surface pressure and acceleration of the wedge at the same time, it can be concluded that the oscillation of the hydrodynamic load on the wedge is caused by the pressure oscillation in the closed cavity. Especially for the case of low Fn, the pressure peak on the wedge's surface in the oscillation stage is even greater than the pressure load in the impact stage. The peak pressure of closed cavity is positively correlated with Fn and negatively correlated with Euler number (Eu). Finally, by analyzing the influence of wedge width and impact velocity, it is found that the oscillation period of the closed cavity is related to the morphology of the cavity. The larger the aspect ratio of the closed cavity, the longer the oscillation period.

Experimental study on load characteristics of vehicle during high-speed water entry

During the process of high-speed water entry, the vehicle will bear large loads. Obtaining and analysing the time-frequency characteristics of loads is of great engineering significance for the protection of shell and equipment. The high-speed camera is used to capture the evolution of cavity and the accelerometer is used to collect the acceleration of model during water entry. The time-frequency characteristics of acceleration are obtained based on the short-time Fourier transform and discrete wavelet transform. The results indicate that the cavity exhibits an asymmetric shape and shrinks near free surface during the oblique high-speed water entry. When the cavity undergoes closure and collapse, the collapsing order of the upper and lower parts is opposite. Through the short time Fourier transform, it was found that the loads contain components of different frequency at different periods. When the model collides with free surface, the frequency contained in acceleration are more complex, reflecting the complexity of mechanical environment. The discrete wavelet transform is used to extract the components with more time-resolution and obtain the relative energy of each component. It is found that the relative energy of low-frequency components increases with the increase of water entry speed.

Study of the water entry and exit problems by coupling the APR and PST within SPH

In this manuscript, the adaptive particle refinement (APR) and particle shifting technique (PST) are coupled and applied to investigate the entire process of water entry and exit. The PST implementation for various types of particles is quantitatively discussed in detail in the coupled APR-PST approach. A comprehensive analysis of different APR-PST coupled schemes is conducted with crucial variables compared and analyzed for the first time. The stability, accuracy and robustness of the developed numerical model are verified by three benchmark tests: 2D wedge water entry, 2D cylinder water exit and 2D cylinder water entry and exit. The obtained results show that the current model can ensure the overall computational precision in the situation of local refinement, which indicates it is a viable way to solve the problems of water entry and exit.

Numerical analysis of water entry under ocean currents with smoothed particle hydrodynamics method

Water entry is a fluid–structure interaction process closely related to the ocean environment. Repeated water entries take place when ships are sailing in an ocean environment, e.g., ocean currents, which greatly affect the ship's safety and stability in navigation. In this paper, we adopt a smoothed particle hydrodynamics method to numerically study the water entry of a bow-flare ship body section under ocean currents. We simulate the process of water entry under different current velocities and analyze in detail the fluid field regarding the free surface evolution, the velocity and pressure distributions, and the body's forces and motions. It is revealed that the ocean current can induce multidirectional fluid impacts, and a stagnation point with zero velocity occurs at the upstream side. Asymmetric fluid fields including the evolution of the free surface and the velocity and pressure distributions around the body can also be found. In addition, discrepancies are caused in the formation time and the range of the high-pressure region. These fluid field changes greatly affect the ship body's dynamic responses. However, the effects of the ocean current are mainly reflected in the direction of the current flow and are relatively small in the direction perpendicular to the flow.

Experimental investigation on the multiphase flow characteristics of oblique water entry of the hollow cylinders

The oblique water entry of the hollow cylinders with various inner diameters has been investigated experimentally. The multiphase flow characteristics of the cavity, through-hole jet and multiscale bubbles have been revealed and discussed. In the experimental setup, a high-speed photography technique is used to capture the process of water entry based on digital video images. The results show that the fluid flows to the inside and outside of the hollow cylinder along the flow separation position, the through-hole jet and cavity are formed. The deep closure mode of the cavity pinched off by line contact appears, the upper cavity moves upward and the secondary jet is formed, and the lower cavity presents the evolution process of stable transparent surface, fluctuating rippling surface, blurry broken surface and collapse. Three bubble clusters are formed due to the closure and pinching off of the cavity, the breaking up of the through-hole jet, and the blurry broken evolution of the lower cavity, and are shedding upward along the moving paths of the upper cavity, the tail and the head of the hollow cylinder, respectively. Moreover, a parametric study is conducted on the effects of inner diameters on the multiphase flow structure and dynamic characteristics.

Air-Backed Aluminum Shells Subjected to Underwater Penetration: Torpedo Interception Simulations

Underwater torpedoes have become a serious threat to ocean liners and warships, and the interception against attacking torpedoes is always the hotspot in marine engineering. To simulate the underwater torpedo interception by a high velocity projectile, this work numerically deals with the process of projectile water entry and sequent penetration into underwater aluminum shells, whereby conical and ogival nose projectiles are comparatively studied. With the arbitrary Lagrange–Euler (ALE) algorithm adopted to describe fluid medium, the projectile water entry model is developed and validated against the test data. Similarly, the penetration model validation is made by modeling a tungsten ball perforation on an aluminum plate. Covered by water fluid, the air-backed aluminum shell is utilized to simulate an underwater torpedo subjected to projectile impact. The numerical predictions of underwater penetration reveal that ogival nose projectiles have a superior performance in underwater motion and perforation while conical nose counterparts deteriorate the shell targets more severely. For 20 cm, 40 cm and 60 cm underwater depth scenarios, a numerical prediction suggests that the energy consumed by water is proportional to the water depth, meanwhile aluminum shell perforation absorbs almost the identical projectile kinetic energy. Such findings may shed some light on the nose shape optimization design of high velocity projectile intercepting underwater torpedoes.

Numerical investigations of a 2D bow wedge asymmetric free-falling into still water

The process of asymmetric water entry of a 2D bow is studied using the computational fluid dynamics (CFD) model with STAR-CCM+. The necessity of including the turbulence model in this investigation is discussed. The behaviors of bow slamming pressure, slamming pressure coefficients, slamming velocities with different deflection angles (0–10°) are discussed. The typical bow cross-section is simplified as a wedge with a deadrise angle of 45°. A numerical simulation was conducted and a good agreement was achieved between the numerical results and the experimental data. The results show that the turbulence model should be carefully considered in numerical simulation when the slamming occurs over time scales that are much shorter in duration than the time scales over which viscosity and gravity act on the flow. Additionally, with the increase in deflection angle, the slamming pressure coefficients increase on the upstream surface. And the slamming pressure coefficients decrease on the back stream surface. The jet area increases on the upstream surface and decreases on the back stream surface with the wedge asymmetric water entry.

Numerical Study on Dynamic Characteristics of Vehicle Entering Water at High Speed

Aiming at a design for buffering and load reduction configuration for a large-scale (diameter greater than 500 mm) vehicle entering water at high speed (greater than 100 m/s), a numerical model for a vehicle entering water at high speed was employed based on an arbitrary Lagrange-Euler (ALE) algorithm. Combined with modal analysis and shock response spectrum, the influence of the head cap on the dynamic characteristics of the structure was analyzed. The results showed that the peak value and pulse width of the impact load on the vehicle increased with the increase in the speed of water entry. The existence of the head cap increased the complexity of the forces on the vehicle during the process of water entry. The initial formation of the cavity was greatly affected by the head cap. The head cap and the vehicle separated in the later stage of the water entry. During the process of water entry, the shell of the vehicle was mainly compressed and bent and the head cap reduced the deformation. The relevant conclusions of this paper can provide some input for the design of a new buffering structure and vehicle shell.

Impact characteristics and strength evaluation of a slender body obliquely entering water at high speed

Based on Coupled Eulerian-Lagrangian method, the impact characteristics and strength evaluation of high speed multi-angle oblique water entry of slender body were studied. In order to solve the complex fluid-structure coupling problem and large deformation problem, the Lagrangian grid is used to divide the slender body structure grid, and the Euler grid is used to divide the fluid grid, the effectiveness of the method is verified by comparing with the experimental results. Furthermore, by simulating the impact process of multi-angle water entry, the time-history curves of acceleration, stress and strain at the head position of the slender body were analyzed, and the strength evaluation was carried out to verify the structural safety combined with the residual strength formula. The results show that there is a positive correlation between the impact load and the water inlet angle, and the larger the water inlet angle, the greater the impact load; the residual strength factor of the slender body is positive at 15° and 20°, and the structure is safe, at 30°, the residual strength factor is -0.203, and the structure is unsafe.

Numerical study of wave effect on water entry of a three-dimensional symmetric wedge

Marine and aerospace structures are frequently subject to impact loads on wave surface. The primary aim of this paper is to obtain a fundamental understanding of wave effect on wedge entry problem. A three-dimensional (3D) full-scale symmetric wedge with three degrees of freedom (3DOF) is numerically modeled in the incompressible air–water two phase flows. The numerical implementation and regular wave surface are realized in Reynolds-averaged Navier–Stokes equations (RANS) framework (STAR-CCM+ 2020.1) by combining with volume of fluid (VOF) and velocity-inlet wavemaking methods. The accuracy of the present numerical model is validated by the experimental tests. Various case studies are then carried out to investigate the difference between calm water and wave crest entries, and to analyze the mechanisms of the wave position and wave height effects through several physical results, including acceleration, velocity, pressure, displacement, rotation and free surface. Results indicate that the process of water entry can be divided into early, vertical-down and bounce-up stages. Further, the wave effect on the water entry problem can be explained as that the incident wave particle velocities and the surface slope result in the variations of effective impacting velocity and entry type of the wedge, and thus remarkably influence the hydrodynamic loads and rotating motion of the wedge. The parametric study reveals that the wave height has no significant effect on the vertical force but positively affects the horizontal impact force and rotating motion of the wedge.

Experimental investigation on the multiphase flow characteristics of oblique water entry of semi-closed cylinder

The multiphase flow characteristics of the oblique water entry of semi-closed cylinders is investigated experimentally. The results from the experiment are used to analyze the flow mechanism and to validate the numerical simulations. In the experimental setup, a high-speed photography technique is used to capture the process of water entry based on digital video images. It can be observed that the cavity is formed in three (3) flow modes namely the fluctuation cavity, the transition cavity, and the blurry rippling cavity. The transparency of the cavity surface decreases, while the fragmentary and blurry rippling increases, with an increasing inclined angle of the cylinder. Owing to the instability of internal flow structure and the degree of cavity closure, the cavities of three (3) flow modes occur in sequential order from clear fluctuant cavity, rippling fluctuant cavity, to continuous blurry and fragmentary ripples. The cavity evolution is also found to affect the structure of multiphase flow field and the dynamic characteristics of semi-closed cylinder.

Numerical simulation of cavitation characteristics in high speed water entry of head-jetting underwater vehicle

Autonomous underwater vehicle will be subjected to a huge impact load during high speed water entry, which will damage the structure and the internal instruments of the vehicle. Therefore, it is of great significance to study the buffer mechanism of the vehicle during the process of water-entry. In this paper, a kind of head-jetting device with disk cavitation is used. The complex cavitation forms, under the three-phase coupling of gas, liquid and solid, in the water entry process of the vehicle on which the device is installed. In this paper, the numerical simulation of high-speed water entry of the vehicle equipped with head jet device is carried out. Through the analysis of water entry cavitation under typical working conditions, the following conclusions are obtained. After the installation of head jet device, the water entry cavity of the vehicle changes gradually from cone to spindle shape. The air jet, compared with that without jet, can promote the formation of water inlet supercavitation, decrease the interaction area between the vehicle and water, and reduce the impact load during water entry. At the same water entry depth, the diameter of cavitation increases with the amount of air jet. The water entry velocity has a great influence on the difference of cavitation shape. The water entry depth closure phenomenon, when the water entry velocity is less than 100 m/s, can be observed in the depth of 3.5 times of the projectile length. The water entry angle has a significant effect on the cavitation shape. The cavity shows obvious asymmetry when the vehicle slants into the water, and the diameter and length of the bubbles decrease with the increase of the water entry angle. The research content of this paper provides technical support for the engineering practice of high-speed water entry and load reduction, and the conclusions are of great significance in related fields.

Experimental study on the influence of the angle of attack on cavity evolution and surface load in the water entry of a cylinder

The characteristics of cavity evolution and surface load in water entry by a constrained-posture cylinder at different angles of attack are experimentally investigated. High-speed photography is used to capture flow behaviors, and a measurement system is established to measure the pressure on the surface of the cylinder and its acceleration during the process of water entry. The experimental results show that when the cylinder enters the water at a certain angle of attack, the cavity shows an obvious asymmetry. Comparison of angles of attack shows that the cavity evolution and load characteristics during the water entry process are closely related to the angle of attack of the cylinder. Increasing the angle of attack can short the period if cavity pinch-off but prolong the time of surface seal. Meanwhile, the peak value of impact pressure shows a gradual downward trend with the increased angle of attack, and the relationship between the duration of high load and the angle of attack is approximately linear. Moreover, the characteristics of the flow field play an important role in the surface load of the cylinder, an obvious and short-lived negative relative pressure will appear on the surface of the model inside the cavity at the initial stage of water entry, the part of cylinder that is not affected by the cavity and directly interacts with water has a continuous positive pressure trend. In addition, in can be found that there is a significant pressure fluctuation when the cavity separation line passes over the surface.

Numerical and experimental investigation on the oblique water entry of cylinder.

The study of water entry cavity and the analysis of load characteristics are hot topics in water entry research. The coupled Euler-Lagrange method is used to carry out simulation research on the water entry process of a cylinder. Aiming at the water vapor mixing phenomenon caused by structure slamming on the water at the initial time of water entry, the slamming load is further studied by correcting the sound speed in the water. The differences between the calculated results obtained by adjusting the number of units in the numerical simulation prove the convergence of the numerical method. Water entry experiments of a cylinder were carried out, and the results are in good agreement with the simulation data. The motion state simulation and analysis are carried out for the process of water entry with different initial speeds and angles. The changes in the structure's positions, air cavities, and slamming loads are obtained. The rule of slamming pressure with the water entry angle and the relationship between pressure and acceleration are determined.

Numerical investigation of the water entry of a hydrophobic sphere with spin

The process of water entry is a complex and attractive field of research, since from more than a century ago there are wide applications in industry, nature science, bionics, and naval technology. The objective of this work is to investigate the cavity dynamics during water entry of a spinning sphere by numerical simulations. The algorithm of these simulations uses the Boundary Data Immersion Method (BDIM) to model the solid/fluid interactions, and the interface between the liquid and the gas is tracked by the volume-of-fluid (VOF) method. The uniform Cartesian-grid is used, and the effect of surface wettability is considered. The governing meta-equations for the full domain ensure the stable and accurate prediction on the pressure value on the solid boundary. Numerical results are validated with experiments, and good agreement between numerical and experimental results has been obtained for the transient cavity formation and the motion of the sphere. To further investigate the influence of the spin rate on the splash and cavity evolution and hydrodynamic characteristics, five different spin rates with a constant impact velocity are considered. Results reveal that the spin rate has significant influence on the cavity and splash asymmetry as well as the moment of pinch-off, while the depth of pinch-off is almost constant. The vortex structures show that both effects of rotation and shear play a role on the evolution of splash and cavity. The hydrodynamic forces fluctuate dramatically both at the moment of impact and the pinch off of the cavity. They gradually tend to be constant as the sphere descent with an open cavity.

Numerical simulations for water entry of hydrophobic objects

Based on the boundary data immersion method (BDIM) and the volume of fluid (VOF) method, the water entry process of hydrophobic objects is numerically simulated. The water crown, the cavity and the flow pattern are analyzed. In the present study, the cavity shape caused by the water entry and trajectories of objects are simulated by the numerical method. Simulation results are compared with experimental data, and the comparisons show that the present numerical approach has the ability to predict the complex process of water entry as well as the effects of the density of an object and entry velocity. Numerical results indicate that the depth of pinch-off increases as reducing the characteristic length of the cavity; the pinch-off time increases for the sphere with a larger density; the depth of cylinder, where occurs the cavity shape inversion behavior, is not affected by the entry velocity of the cylinder.