Oblique Water Entry Research Articles

Investigation on the oblique water entry of the flared cavity

An experimental study of the oblique water entry of a flared cavity is reported in this paper. A numerical model is established that takes gas compressibility into account. This model effectively captures the additional flow phenomena that occur due to gas expansion and contraction within the cavity. There is a significant fluctuation phenomenon during the oblique water entry of the flared cavity, which is reflected not only in the movement of the water entry bubble but also in the slamming pressure. This fluctuation phenomenon is caused by the bottom cavity of the flared cavity, which adds the additional processes of external fluid inflow and internal fluid outflow compared with the closed-bottom structure, leading to additional flow due to conventional flow separation. When the external liquid flows into the cavity, the additional flow reduces the velocity potential of the fluid near the flow separation point, causing the bubble diameter near the flow separation point to contract. At the same time, the external liquid inflow causes the gas in the cavity to compress and the slamming pressure to increase. When the liquid flows out of the cavity, the additional flow increases the velocity potential of the fluid near the flow separation point, causing the bubble diameter near the flow separation point to expand. Meanwhile, the internal fluid outflow causes the gas in the cavity to expand and the slamming pressure to decrease.

CFD-DEM analysis of oblique water entry under a polar environment

Research on the water entry behavior of detectors passing through crushed ice zones in polar environments is essential for the exploration of polar oil resources. This study integrates computational fluid dynamics (CFD) and the discrete element method (DEM) with laboratory experiments to analyze projectiles' oblique water entry behavior through crushed ice zones. The investigation reveals that the unique polar environment, characterized by crushed ice accumulation, significantly influences the projectile's water entry behavior. Key parameters, such as water entry angle and the height of ice accumulation, are examined for their impact on the water entry process, flow field evolution, and projectile dynamics. The study identifies a complex multi-body coupling mechanism among the projectile, crushed ice particles, and fluid during oblique water entry. Crushed ice alters the cavity evolution and deflection patterns of projectiles, affecting the flow field dynamics. Variations in water entry angles modify the interaction patterns among the projectile, liquid level, and crushed ice, influencing cavity formation and fluid-ice coupling. Increasing crushed ice height enhances the coupling effects, leading to a greater degree of projectile deflection. These findings provide insights into optimizing detector design and launch strategies, contributing to more efficient oil exploration in polar environments.

Investigation of oblique water entry of high-speed supercavitating projectiles using transient fluid-structure interaction simulation

This study employs a fluid-structure interaction simulation to examine the structural deformation of supercavitating projectiles during high-speed oblique water entry. A cavitation flow field simulation model is established using the VOF multiphase and cavitation models, while the structural dynamics model of the projectile is implemented using the Johnson-Cook constitutive equation. The transient fluid-structure interaction simulation is conducted through data transfer between the two models. Numerical simulations are then conducted with the assumptions of a rigid body, linear elasticity, and nonlinearity, respectively. The results indicate that the high-speed water entry induced substantial hydrodynamic force and plastic strain accumulation in the projectile, leading to considerable bending deformation. It was found that the deformation significantly influenced the hydrodynamic forces, highlighting the importance of fluid-structure interaction modeling. Moreover, the attack angle directly impacts the resulting deformation mode. Specifically, under high-speed conditions, three distinct deformation modes are observed at different attack angles: minimal deformation at low angles, an L-shaped bending at higher angles, and an S-shaped bending when the angle exceeds the threshold. This study highlights the importance of fluid-structure interaction methods in analyzing the deformation behavior of supercavitating projectiles during high-speed oblique water entry. The results show that the bidirectional dynamic interaction between fluid and structure significantly influences the structural stability of the supercavitating projectiles. The relevant conclusions can provide a reference for the optimal design of supercavitating projectiles to ensure the structural stability under high-speed conditions.

Effect of fluid–structure interaction on the oblique water entry of the projectile under the influence of floating ice structure

The water entry of a projectile constrained by polar floating ice presents a unique cross-media challenge. This paper investigates the dynamics of oblique water entry for a projectile influenced by floating ice using the fluid–structure interaction (FSI) method. The validity of the numerical method has been confirmed through experimental validation. The water entry process of a projectile from the side of the floating ice is examined. The evolution of the cavity and the movement patterns of objects as the distance between the projectile and the floating ice decreases toward collision are investigated. The influence of water on the critical collision distance between the projectile and the floating ice during oblique water entry is analyzed. Additionally, the physical mechanism of floating ice deflection through collision is investigated based on the theory of cavity dynamics. Subsequently, the study focuses on the oblique water entry process of a projectile colliding with the upper surface of the floating ice. Different entry angles determine the collision mode between the projectile and the floating ice surface. This study also examines how varying entry angles influence cavity evolution and object movement patterns during oblique collisions. Different collision modes between the projectile and the floating ice lead to asymmetric cavity evolution and various modes of object deflection motion. Finally, changes in the flow field and vortex structure during oblique collisions are studied to examine the influence of the FSI process between the projectile and the floating ice on the flow field.

Asymmetric water entry of a wedged grillage structure investigated by CFD-FEM co-simulation

When a ship sails in ocean waves the relative movement between ship and wave is very complex. Slamming of bow or stern structures usually occurs in a manner of asymmetric oblique water entry. This implies that both the deflection angle θ and velocity angle α will likely grow to a large value at the same time. In this study, the hydrodynamics problem associated with the asymmetric and oblique water impact of a three-dimensional wedge is investigated using a partitioned CFD-FEM two-way coupled numerical tool by coupling the commercial software STAR-CCM+ and Abaqus within the framework of SIMULIA co-simulation engine. The hydrodynamic problem is solved by the CFD software using overset grid technique while the structural dynamic response is solved by the FEM software. Validation of the numerical results is conducted by using existing test results of symmetric water entry of the wedge first. Then the behaviours of movements, slamming pressure and structural response of the wedge structure during asymmetric water entry by varying the deflection angle θ or the velocity angle α or both are systematically and comparatively investigated. This research shows that the present numerical simulation well predicts both the fluid and structural dynamic characteristics such as fluid cavity and structural stress concentration of asymmetric impact problems, which has potential engineering application value in the field of ship design and slamming loads evaluation.

Analyzing cavity evolution and motion characteristics of asynchronous parallel oblique water-entry super-cavitating projectile

Based on the volume of fluid multiphase flow model and the overset mesh technique, a numerical method for an asynchronous parallel oblique water-entry super-cavitating projectile was established. Experimental studies of the oblique water-entry of a high-speed single-launch projectile were carried out to validate the viability of the numerical method. The paper performed the numerical simulations and analyses of cavity evolution and motion characteristics of the front and rear projectiles in different initial intervals and in two sequences of top-side water-entry projectile first and bottom-side water-entry projectile first. The results show that when the initial interval of the first launch projectile is 0.5 time the projectile length, the first launch projectile cannot produce a cavity to completely encapsulate the projectile due to the violent squeezing of the following launch projectile cavity, and its movement is seriously affected and eventually loses its trajectory stability. At the same time, the first launch projectile that enters water from top side is squeezed to a larger degree than the one from bottom side, and the wetting phenomenon occurs earlier and loses stability faster. As the initial interval increases, the influence of the following launch projectile cavity near the first launch projectile is weakened, and the first launch projectile in both water entry sequences move steadily. For the following launch projectile, due to the continuous influence of the first launch projectile cavity, its cavity is always asymmetrical, and its motion stability is affected. The following launch projectile deflects to the inner side and destabilizes when the initial interval is 0.5 times the projectile length. When the initial interval is 1 time the projectile length, it moves steadily. It deflects to the outer side and destabilizes when the initial interval is 2 and 3 times the projectile length. In addition, the motion characteristics of the following launch projectile are basically identical in two water-entry sequences.

Experimental and numerical investigations on the transient impact load of the oblique water entry of the concave-nose projectiles

During the landing and recycling on the sea surface of a spacecraft or rocket, a concave structure may impact on the water surface. In the present paper, we experimentally and numerically analyze the multiphase flows and transient impact load during the oblique water entry of the concave-nose projectile. The projectiles used in the experiments consist of a main body that embeds an acceleration measurement system, and a transparent concave-nose. Firstly, free surface evolutions and cavity dynamics are given for the concave-nose projectile, which shows significant changes compared with the projectile without concave-nose. A special jet inside the concave-nose during the formation of the air-entertainment cavity is observed. Subsequently, it is found that compared with the projectile without concave-nose in the early stage. The increase rate of the axial force of the concave-nose projectile is slower and the force curve has a steep drop stage, then they enter the uniform reduction stage. As the concave depth increases, a ‘double-peak’ phenomenon on the axial force curve appears. It is found that the force peak is transferred to a single peak with increasing the impact velocity. The force peak of the concave-nose projectile is greater than the projectile without concave-nose. Besides, with increasing the concave depth, the maximum axial force of the concave-nose projectile increases firstly, and then decreases. When the ratio of concave depth to inner diameter is greater than 2, the influence of concave depth on the peak value of axial force is small. Moreover, the pressure distributions of the fluid domain and the concave-nose are analyzed to reveal the mechanism of the variation of the force peak. The study is able to provide a basic reference for the analysis of the recycling of the rocket, aircraft, etc.

Cavity flow characteristics of a curved hull section impacting a free surface with inclined postures

The water entry of structures is a complex gas–liquid flow. This paper studies the asymmetrical flow characteristics of a curved hull section entering water through numerical and experimental methods. The free-falling test from drop heights of 250–900 mm and inclination angles from 0° to 20° is carried out. Compared to a smooth hull section (cutting the bottom appendage), the experimental results observe some special asymmetrical flow phenomena (i.e., flow separation, jet impact, bubble flows, and bubble expansion). The physical mechanisms behind these flows are explained through combing the free surface flow and pressure distribution obtained by the numerical method. The effects of the inclination angle and impact velocities on these flow phenomena are further discussed, and they increase the degree of flow separation, bubble volume, and fragmentation. The load characteristics before and after cavity formation are analyzed based on a volume-of-fluid method. The high pressure caused by bubble closure can produce an instantaneous impulse pressure that even be 34% larger than the conventional impact pressure and is worth noting. This study clarifies some complex asymmetrical impact flow characteristics of curved hull sections and thus reveals the evolution mechanism of gas–liquid flows for complex geometries during oblique water entry.

Hydrodynamics and stability of oblique water entry in waves

Waves, a constant presence in the ocean, significantly influence the stability of a vehicle's water entry. In the present work, the effects of waves on the water entry of a vehicle equipped with a cavitator, utilizing the Computational Fluid Dynamics (CFD) method. The Stokes V wave theory serves as the basis for wave simulations, while the moving computational domain method is used to realize the motion of the vehicle. The water entry process is most influenced by waves at the peak point of wave inclination. This research delves into the impacts and underlying mechanisms of wave height, entry angle, and speed on the water-entry process, particularly focusing on the range of wave influence. Re-entry may occur when the entry angle is smaller than the maximum wave inclination. Under such circumstances, the vehicle's motion could become more erratic, potentially leading to re-entry failure. Consequently, two critical angles are defined: one where re-entry is likely to occur, and another where re-entry is inevitable. The influences of wave parameters and entry angle on the possibility of re-entry are investigated. The proposed critical angles and their calculation method lay the groundwork for future research aimed at ensuring safe water entry for vehicles amid waves.

Oblique water entry of a curved foil with varying speed

The hydrodynamic problem of a curved foil entering into water obliquely with varying speed is investigated through the boundary element method in time domain, and fully nonlinear boundary conditions on the deforming free surface are adopted. The process of foil entry begins with a single point at the lower edge; posing numerical challenges due to the extremely small wetted area, we utilize the stretched co-ordinate system method to address this. An auxiliary method is adopted to solve for pressure distribution. The whole process of the attached flow forming along the curved body and then detaching from the top edge is considered. We engage in extensive discussions on the effects of curvature, gravity, and acceleration, exploring their physical significance and potential applications, particularly within the context of surface-piercing propellers.

Experimental Investigation into the Tail-Slapping Motion of a Projectile with an Oblique Water-Entry Speed

In this study, the tail-slapping behavior of an oblique water-entry projectile is investigated through high-speed photography technology. The experimental images and data are captured, extracted and processed using a digital image processing method. The experimental repeatability is verified. By examining the formation, development and collapse process of the projectile’s cavity, this study investigates the impact of the tail-slapping motion on the cavity’s evolution. Furthermore, it examines the distinctive characteristics of both the tail-slapping cavity and the original cavity at varying initial water-entry speeds. By analyzing the formation, development and collapse process of the cavity of the projectile, the influence of the tail-slapping motion on the cavity evolution is explored. Furthermore, it examines the evolution characteristics of both the tail-slapping cavity and the original cavity under different initial water-entry speeds. The results indicate that a tail-slapping cavity is formed during the reciprocating motion of the projectile. The tail-slapping cavity fits closely with the original cavity and is finally pulled off from the surface of the original cavity to collapse. In addition, as the initial water-entry speed increases, both the maximum cross-section size of the tail-slapping cavity and the length of the original cavity gradually increase. With the increase in the number of tail-slapping motions, the speed attenuation amplitude of the projectile increases during each tail-slapping motion, the time interval between two tail-slapping motions is gradually shortened, the energy loss of the projectile correspondingly enlarges, and the speed storage capacity of the projectile decreases.

Study on high-speed water entry process of non-rotating vehicles

The non-rotating trans-media vehicle can better adapt to the hypersonic flight conditions, but new problems in the hydrodynamic characteristics and motion stability arise during the water entry process. With the VOF multiphase model, Realizable k-ε turbulence model, and Schnerr-Sauer cavitation model, this paper establishes a simulation method of coupling the movement of non-roating body into water with multiphase flow fields based on overset grid technology, and the accuracy of the established model is verified. The simulation calculation of the oblique water entry process of the vehicle with different ratio of short axis to long axis in cross section is carried out. And the results show that although the ratio of short axis to long axis in cross section does not change the cavitation morphology, it affects the hydrodynamic and kinematic characteristics of the vehicle by influencing the relative position of the cavitation. As the ratio of short axis to long axis decreases, the motion stability of the vehicle increases gradually. However, if the ratio of short axis to long axis is too small, the drag reduction effect will be weakened, and the structure will suffer a bigger load.

Experimental investigation on the impact force of the oblique water entry of a slender projectile with spring buffer

On water impact, a projectile experiences impact forces that can threaten its structural safety and the stability of its trajectory. In the engineering applications, the underwater vehicle may need to entry and exit of water several times. The elastic buffer can be used repeatedly, which meet the requirement of the scene. To mitigate the effects of a strong impact, a projectile with a spring buffer connecting its main body and disk is designed in this work. The impact force of the main body during water entry is studied experimentally. The experimental equipment and the force measurement system are newly developed by the authors. The main focus is the cavity evolution and the axial force of the main body. The cavity of the projectile with a buffer is slightly elongated and thickened compared with that without a buffer. A special necking phenomenon of the cavity is observed as the spring buffer is compacted. The effects of the spring stiffness, the disk mass, and the impact velocity on the axial force and impulse are extensively discussed. Compared with the case without a buffer, the axial force of the main body is only mitigated within a small range of stiffness values. When the stiffness increases, the axial force begins to fluctuate with a larger peak and a narrow pulse. The peak force is nearly twice that in the case without a buffer. However, the impulse is always smaller, although some fluctuations occur in the larger stiffness cases. Additionally, an increased disk mass can also mitigate the force peak and the impulse of the main body, but fluctuations also occur in cases with larger masses. The coefficients of the axial force and the impulse of the main body decrease with increasing impact velocities. The result indicates that the elastic buffer only in a narrow stiffness can effectively reduce the impact load of the water entry projectile. This study can provide references for the load reduction design of the projectile with an elastic buffer during water entry or exit.

Experimental study on ballistic stability of truncated cone projectile in high-speed oblique water entry

In this paper, the high-speed photography technique was used to study the high-speed oblique water entry of truncated cone projectiles, and the effects of nose shape, projectile velocity and entry angle on the cavity and ballistic characteristics of oblique water entry were discussed. The experimental results show that the influence of nose shape on the oblique water entry cavity and ballistic characteristics is related to the nose wetted area. For the same type of projectile, with the increase of projectile velocity, the cavity size becomes larger at the same penetration displacement and the surface closure time gets longer. Moreover, with the increase of entry angle, the surface closure time decreases, the velocity attenuation gets faster, at the same penetration displacement the cavity opening length becomes shorter and the cavity asymmetry weakens. Additionally, the results indicate that the trajectory stability of the truncated cone projectile varies under different water entry velocities and attitude angles.

Asymmetric flow action on hydrodynamics and structural dynamics for high-speed oblique water entry of the semi-sealed cylindrical shell

To study the hydrodynamics and structural characteristics, numerical simulation of the oblique water-entry process of the semi-sealed and completely sealed cylindrical shell is carried out based on a Star-CCM+ and ABAQUS collaborative simulation method described in this paper. The results show that when the internal fluid pours out of the shell, the jet formed below the shell is asymmetrically distributed, resulting in a significant change in the attitude angle of the shell. This, in turn, causes the upstream surface of the shell to hit the side wall of the cavity, further changing in the attitude angle. The force and stress of the shell with an water entry angle θ0 = 90° is found to be greater than that of the shells with other oblique water entry angles. Additionally, the trajectories of the shells at different water entry angles move upward after reaching the inflection point, and the range of upward movement increases gradually with the decrease of the water entry angle. In summary, the asymmetric flow caused by an hollow structure has a profound impact on the high-speed oblique water entry of the semi-sealed cylindrical shell, causing changes in its trajectory, cavity evolution, and structural characteristics.

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.

Numerical research of high-speed oblique water-entry of projectile based on rigid body and fluid-structure interaction model

When a projectile enters the water at high speed, the projectile can suffer a huge impact of water. If a deformation occurs in the structure of the projectile during water-entry, the numerical method based on rigid body model will be limited, while the accuracy and precision of the results can be affected. The two numerical methods of the rigid body and fluid-structure interaction (FSI) model have been built to get the results of the oblique water-entry of the projectile in this paper. Moreover, the numerical results for the given projectile under the same water-entry conditions (initial angle of incidence of θ = 15°, initial angle of attack of α = 5°, initial speed of V = 200m/s ) have been contrasted. The results show that if a small deformation occurs in the body of the projectile during the water-entry, the numerical results of the FSI and the rigid body are different. Further, the small deformation within the limit of flexibility can change the motion attitude and the trajectory of projectile during the water-entry, meanwhile the change can cause the bending of the ballistic and the super-cavity. For the projectile with the structural response, the change in the distribution of the stress concentration area can affect the drag force suffered by projectile.

Oblique water entry of an inclined finite plate with gravity effect

The hydrodynamic problem of an inclined finite plate entering into water obliquely is investigated through the velocity potential flow theory in the time domain, together with the fully nonlinear boundary conditions on the deforming free surface. A boundary element method is adopted. A stretched coordinate system method is used for the varying computational domain, which starts from a single point at the lower edge of the plate. The whole process of the flow attached on the plate and flow detached from its upper edge is considered, which may involve self-similar flow, transient flow, and steady flow. The gravity effect is also considered. Studies are further conducted for oblique water entry and varying speed entry. Extensive results are provided. Their physical implications and potential applications are discussed.

Analysis of fluid dynamic behavior and impact load on oblique water entry of a two-dimensional seaplane based on VOF method

Seaplane landing is a typical water entry impact phenomenon. However, our understanding on the development of jet flow and air cavity, and corresponding load characteristic is still limited. In this paper, the impact problem of oblique water entry of a two-dimensional seaplane is studied to clarify the relationship between theses complex flows and loads. A gas-liquid two-phase model is established to model these complex fluid flows, where the air-water interface is captured based on a VOF algorithm. The accuracy is validated by the published experimental data. Some hydrodynamic impact characteristics related to seaplane model under different oblique water entry postures, including air-water free surface and local impact pressure are discussed. Complicated flows (air cavity and jet flow impact) are the internal cause to some unusual load characteristics such as multiple pressure peaks, negative pressure, simultaneity of impact load on the airfoil, and great difference of load at different positions on the cabin. The evolution of jet flow and air cavity is further discussed through combing free surface and pressure distribution. Results clarified the mechanism of complex flows under the influence of bottom cabin and inclined postures, which will be helpful to further understand the impact phenomenon of seaplane landing.

Numerical Study on the Water Entry of a Freely Falling Unmanned Aerial-Underwater Vehicle

The unmanned aerial–underwater vehicle (UAUV) is a new type of vehicle that can fly in the air and cruise in water and is expected to cross the free water surface several times to perform continuous uninterrupted observation and sampling. To analyze the hydrodynamic and motion characteristics of the vehicle, the whole water-entry process of a multi-degree-of-freedom UAUV with various velocity and pitch angle was investigated through a Reynolds-averaged Navier–Stokes method. The computational domain was meshed by trimmer cells. The relative movement between the vehicle and fluid domain was simulated using moving reference frame overset mesh to delineate the interaction region around vehicle body. To reduce the computational cost, adaptive mesh refinement and adaptive time-stepping strategy were used to capture the slamming pressure accurately with reasonable computational effort. First, convergence study is considered. Simulations of the vehicle with various initial velocities and different pitch angles were performed. The variable physical properties were analyzed, and detailed results through the time-varying force and velocity were shown. Initial velocity and pitch angle are found to significantly influence hydrodynamic behavior, including the time-varying force, while thickness ratio has a great impact on added mass and pressure. The results show that higher entry velocity results in greater peak vertical force. The transverse hydrodynamic characteristics for oblique water entry of the vehicle with varies pith angle are quite different.