Water Entry Angle Research Articles

Numerical Study on the Water Entry Impact Forces of an Air-Launched Underwater Glider under Wave Conditions

In this study, the water entry of an air-launched underwater glider under wave conditions is numerically simulated by the computational fluid dynamics method. The numerical model is validated by the comparison of nondimensional water entry impact force with published experimental and numerical results. And the influence of water entry points, water entry angles, and water entry attack angles on impact force is studied, which provides guidance for the design of air-launched underwater gliders. The results show that the water entry point has a great influence on the peak value of vertical impact force. In the present study, the peak value of the maximum vertical impact force at different water entry points is almost twice the minimum peak value with the same water entry velocity. The water entry angle at the same water entry point has great influence on the peak value of horizontal impact force, which is mainly related to the horizontal component of the impact velocity. The greater the horizontal component of impact velocity, the greater the peak value of impact force. In addition, the attack angle hardly affects the water entry impact force of the glider with hemispherical head.

Experimental study on water-entry cavity and trajectory of vehicle with asymmetric nose shape

Different nose shape can cause a significant influence on the water entry trajectory and cavity. In order to study the evolution characteristics of cavity and trajectory of asymmetric nose in the process of water entry, based on the high-speed photography technology, the low-speed water entry comparative experiments were carried out for three different cut-angle nose types of 35°, 40° and 45° under different water entry angles. Through digital image processing technology, the contour of the water entry cavity and motion attitude information of the vehicle at different times are extracted, and the law of influence of the cut angle and entry angle on the cavity and trajectory is obtained. The experimental results show that: the cut angle side of the asymmetric nose can effectively promote the expansion of the cavity, thus forming the asymmetric water entry cavity; the expansion scale of the cavity is positively related to the initial horizontal expansion speed of the cavity and the area of the section of the head; the water entry trajectory of the asymmetric head will deflect to the cut angle side; the smaller the water entry angle is, the shorter the trajectory leveling time is, and the deeper the leveling depth is However, the change of attitude angle is almost independent of the entry angle.

Experimental research on influence of wave environment on high-speed water entry load and trajectory characteristics

Wave environment is a common fact happened when objects enter water under real conditions, which changes the morphology of the free interface when entering water. However, the influence of wave environment on the flow when entering water and on the evolution of object motion parameters after entering water is not clear. In this research, a rocking plate wave maker is used to generate a wave environment in the test pool, and a truncated cone rotating body equipped with an internal test system is used as the test model. The compressed air cannon is used to provide launch power, and the launch control system uniformly controls the start timing process of the wave maker and the launcher. The wave maker and the launcher will control the relative position of the object's entry point and the wave surface in collaboration. Various condition tests were carried out, including enter in static water, different wave surface positions and different wave heights. Quantitative trajectory parameters including the acceleration, and attitude angle of the model in the water-entry process were obtained, and the evolution of the flow field was photographed. The experimental results show that under high-speed conditions, the influence of waves on the water-entry process is mainly reflected in the change of the water entry angle relative to the water surface at the water-entry moment. The wave surface encountering negative inclination can effectively reduce the impact load of water entry, but it is more likely to induce the extreme situation such as "jumping".

Mechanical characterization of flat faced deformable AUV during water entry impact considering the hydroelastic effects

In this paper, the dynamic behavior of flat faced deformable autonomous underwater vehicle (AUV) subjected to hydroelastic water entry impact is numerically investigated by using Arbitrary Lagrangian Eulerian (ALE) method. The experiments on the flat faced AUV head section of 200 mm diameter are carried out in a water tank to validate the numerical model by comparing the impact accelerations and underwater cavity evolution by using accelerometers and high speed cameras. Then, the effect of hydroelasticity is numerically investigated by comparing the impact characteristics of rigid and deformable AUV with different rigidities. It is revealed that the hydroelastic effect increases with the decrease in the rigidity of the structure. Mechanical behavior of deformable AUV of various rigidities under the initial conditions of same velocity, same kinetic energy and same mass is examined. The influence of various initial conditions on the hydrodynamic load, impact pressure and effective strain for deformable AUV are discussed. The influence of the water entry angle and impact velocity on the semi-rigid and deformable AUVs are also studied. It is found that the effective strain increases exponentially with the increase in impact velocity and entry angle for deformable AUV, however, semi-rigid AUV exhibit linear strain behavior. Numerical conclusions drawn from this study are vital in selecting the rigidity of AUV and highlighted the influence of hydroelasticity for flat faced AUVs. Additionally, the results obtained in this study can be beneficial for the structural design of high speed air launched AUVs.

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.

Numerical research on the high-speed water entry trajectories of AUVs with asymmetric nose shapes

During the water entry process, autonomous underwater vehicles (AUVs) take a long time to reach a horizontal attitude. To reduce the time and distance required to adjust the attitude, an AUV with an asymmetric nose shape is proposed in this paper. A numerical model based on the volume of fluid (VOF) method is established to describe the cavity and trajectory characteristics of an AUV with a high-speed water entry. Since the velocity of the AUV impacting the water surface is greater than 0.3 Ma, the Tait equation is used to describe the compressibility effects in water, and the air phase is modelled as an ideal gas. The simulation results, such as cavitation shape and velocity, are compared with the experimental data from an earlier study. Good comparative results reveal the accuracy and performance of the employed numerical method. Subsequently, the computational fluid dynamics (CFD) code Fluent is used to simulate and analyse the variations in the trajectory characteristics with different water entry velocities, water entry angles, nose shapes and rudder deflection angles. The results obtained in this study can provide a good guideline for the trajectory control and nose shape design of an AUV.

The Numerical Simulation of the Vertical Water Entry Process of High Speed Projectile with Small Angle of Attack

Based on the numerical simulation method, the high speed vertical water entry process of projectile moving at 1000m/s with small angle of attack is studied. By analyzing the cavity configuration and hydrodynamic characteristics of the process, the influences of the small entry angle on the water entry stability process are obtained. The simulation results indicate that the projectile’s ballistic can achieve relative stable state when the water entry angle is less than 0.5°, otherwise the water entry cavity can be pierced by tail fin. The pierced size tend to increase with the increased water entry angle as well as the projectile’s ballistic become unstable. In addition, the rotation of the projectile owing to pitching moment has little effect on cavity configuration and water entry stability.

Stability of Oblique Water Entry of Cylindrical Projectiles

Water entry is an interesting subject but many of its physical aspects have remained unknown so far. Using computational fluid dynamics (CFD), this study investigates the dynamic stability of cylindrical projectiles in the oblique water entry at shallow angles in the presence of three phases of air, water and water vapor. The three-dimensional and transient numerical model has been verified using the former experimental results in the literature. In this study, the effects of projectile length-to-diameter ratio (L/D) and water entry angle on the projectile stability within cavity were investigated. Accordingly, the water entry of six projectiles was simulated with aspect ratios of 2 to 6 at three water entry angles of 6, 9 and 12 degrees with respect to the free surface with an initial velocity of 280 m/s. At each of the aforementioned angles, the critical L/D, where the projectile avoids tumbling inside the cavity at a larger value, was determined. This study showed that in the oblique water entry of a cylindrical projectile at the angles of 6, 9 and 12 degrees, the projectile tumbled within the cavity with a L/D of less than 5, 4 and 3.5, respectively. The simulation results showed that increasing the L/D as well as the water entry angle relative to the free surface resulted in the improvement of the cylindrical projectile motion stability, which is in agreement with the experimental results. By analyzing the details of each simulation, it was found that the projectile stability within the cavity is correlated with the magnitude of the angular momentum which is generated in the projectile by the impact of the cavitator on the free surface and it was shown that the projectile with a specific L/D can withstand destabilizing angular momentum to a certain extent. Considering the fact that the atmospheric ballistics of gyroscopically stabilized projectiles lead to a limit for increasing L/D, this study showed that, for aluminum cylindrical projectiles in which air stability is achieved via the gyroscopic effect, the minimum water entry angle is 6° to attain the gyroscopic stability of the projectile in the air and stable motion inside the cavity. This fact is very important from a practical point of view.

Three-dimensional simulation of ship bow slamming

The bow flare slamming load was studied by using the software Ls-dyna. A coupling finite element model including air, water and 3d bow was established. Flare slamming pressure was picked up from the finite element model in order to discuss the relation between flare slamming pressure and the velocity as well as the distribution rule of slamming pressure in different velocity and different water entry angle along the length and height of the ship.

Study on the oblique water entry impact performance of AUV under different launch conditions based on coupled FEM-ALE method

In this study, the hydrodynamic response of an autonomous underwater vehicle (AUV) subjected to oblique water entry impact is investigated by employing an explicit FEM technique with arbitrary Lagrangian–Eulerian formulation. The predictive ability of the employed numerical model is validated by correlating the cavity evolution of AUV and peak impact accelerations with the experimental outcomes of the previous research work. The influence of advection algorithms on the water entry phenomenon is studied. It is found that the first order donor cell advection scheme is more suitable for low speed impact problems as it is computationally more efficient. However, for high speed water entry simulations, the van Leer advection scheme gives more accurate results. Moreover, using different water entry angles, the effect of the angle of attack (AOA) and length of AUV and its head shape on the impact performances are investigated and discussed. The results show that the radial impact load is more likely to be influenced by AOA and the effect of AOA on the radial impact load is more significant for a small water entry angle, i.e., 30°, as compared to a large entry angle, i.e., 60°. It is also observed that AUV of a smaller length shows the ricochet behavior at a water entry angle of 30°. It is seen that the head shape has a substantial effect on the axial and radial impact loads and velocity attenuation of a vehicle under water. The conclusions drawn in this research work will be beneficial for the selection of appropriate initial launch conditions and for the designing of an AUV structure.

Numerical investigations of trajectory characteristics of a high-speed water-entry projectile

A detailed analysis of the characteristics and stability of the trajectory of a high-speed projectile during water entry is investigated numerically. The Zwart–Gerber–Belamri cavitation model and the shear stress transport k–ω turbulence model based on the Reynolds-averaged Navier–Stokes method are employed. The numerical method is validated by comparison of its results with the experimental images of cavity shape and with measurements of the penetration distance. Using this numerical method, the influences of rotational speed, initial velocity, and water-entry angle of the projectile on the stability and characteristics of its trajectory are investigated. The variations in projectile trajectory, total drag, and velocity reduction are analyzed. In addition, the cavity characteristics at water entry are presented and analyzed. The results show that the rotation of the projectile has an adverse influence on the stability at water entry, although the magnitude of the rotational speed has little effect on the reduction in projectile velocity. The initial velocity of the projectile has a direct effect on the trajectory stability. It is also found that in practice, there should be a critical water-entry angle for the stability of a projectile.

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 evaluation of the hydrodynamic impact characteristics of the air launched AUV upon water entry

In this paper, water entry process of air launched AUV is investigated by employing fully coupled finite element method and arbitrary Lagrange–Euler formulation (FEM-ALE) and using penalty coupling technique. Numerical model is established to describe the hydrodynamic characteristics and flow patterns of a high-speed water entry AUV. The effectiveness and accuracy of the numerical simulation are verified quantitatively by the experiments of the earlier study. Selection of suitable advection method and mesh convergence study is carried out during experimental validation process. It is found that appropriate mesh size of impact domain is crucial for numerical simulations and second-order Van Leer advection method is more appropriate for high speed water entry problems. Subsequently, the arbitrary Lagrange–Euler (ALE) algorithm is used to describe the variation laws of the impact load characteristics with water entry velocities, water entry angles and different AUV masses. Dimensionless impact coefficient of AUV at different velocities calculated using ALE method is compared with SPH results. This reveals that ALE method can also simulate the water entry process accurately with less computational cost. This research work can provide beneficial reference information for structure design of AUV and for selection of the water entry parameters.

Improvement of cavity shape modeling in water-entry of circular cylinders by considering the cavity memory effect

Water entry is an important field in many natural, industrial and military practical applications. Several experimental and numerical studies have been conducted in order to investigate the mechanism of cavity dynamics and vortex evolution through various water entry angles of different objects. The purpose of the present research is to develop a new model to predict the cavity shape of circular cylinders at water entry stages. The proposed approach for developing the model is based on the available empirical models for the cavity radius in steady motion which is improved with consideration of the cavity memory effect for the transient stage based on Logvinovich's independence principle. In addition to the memory effect of the cavity, the effect of gravity on the axis deformation of the cavity and cavitator angle of attack are considered in developing the model. Three different cases of water entry have been considered in this paper and it is shown that the results of the proposed model are in good agreement with respect to the experimental data. The new proposed model can also predict the necking phenomenon as well as the pinch-off of the cavity with good accuracy and also the funnel shape splashes after pinching off can be observed in the results of the model.

Numerical study on the cavity characteristics and impact loads of AUV water entry

The high-speed water entry process of an autonomous underwater vehicle (AUV) has a strong impact nonlinearity, and a cavity formed by air and water will often be generated as part of the entry process. The shape of the water-entry cavity plays an important role in the load characteristics and stability of the water-entry trajectory. In this paper, a numerical model for describing the cavity and impact load characteristics of a high-speed water-entry AUV is established. The simulation results such as cavity shape and impact load are compared with experimental data. The good agreement between the numerical results and those of the experiments reveals the accuracy and capability of the numerical algorithm. Subsequently, the arbitrary Lagrange-Euler (ALE) numerical algorithm is used to simulate and analyse the variation laws of the cavity characteristics and impact loads with different head shapes, water-entry velocities, water-entry angles and angles of attack. The results obtained in this study can provide a good reference for the trajectory control and structural design of the AUV.

Numerical Investigations on the Water Entry of Cylindrical Projectiles with Different Initial Conditions

In this paper, coupled with Reynolds-averaged Navier–Stokes equations and ballistic equations, the numerical simulations of high-speed water entry of projectiles under different conditions have been conducted. The water-gas flow was modeled by the mixture multiphase model. The numerical results indicated that the simulations agree well with analytical solutions by two cavity models, which validates the model applied. Then the effects of variations of project length, entry angle and velocity on the entry process of projectiles were further investigated. The results show that, for small water entry angles, the cavity wall interacts with the projectile, affects the trajectory of the projectile, and even ricochets for projectiles with small length (5D). On the other hand, the projectile vibrates during the whole process of water entry; the vibration amplitude decreases with the increase of projectile length and entry angle; however, it is the contrary for the vibration period. Furthermore, after the initial impact period, the influence of these parameters on the drag coefficient is not obvious.

Design and load reduction performance analysis of mitigator of AUV during high speed water entry

The Autonomous Underwater Vehicle (AUV) will suffer severe impact load during the water entry, and its shell structure will undergo elastoplastic deformation, even breakage or damage, which seriously threatens the safety and reliability of the AUV. Therefore, it is necessary to carry out research on the buffering technology of the impact load of the vehicle into the water to ensure that the AUV can enter the water safely and reliably. In this paper, a mitigator is designed to reduce the impact load of the AUV entering the water. Numerical simulations based on the explicit finite element method with an Arbitrary-Lagrangian Eulerian (ALE) solver are carried out to study the mitigation characteristic of the designed mitigator. The accuracy of the model was verified by the experimental results of the water-entry of the small sphere, and the mesh independence was verified to determine the mesh size. Then, the effects of initial conditions such as buffer length, buffer material density, entry velocity and entry angle on the cushioning effect of the buffer head cap were studied. When the vertical water entry velocity is 50 m/s, the impact load reduction ratio is 42.2%; while the water entry angle is 60° and the water entry velocity is 50 m/s, the axial load reduction ratio is 28.8%, and the normal load reduction ratio is 27.8%. These conclusions will be useful for the designing of the crashworthy structural and the analysis of underwater weapon impact.

Experimental study on water entry of cylindrical projectiles with different nose shapes

Water entry experiments of projectiles with different nose shapes were performed under different entry angles and velocities using high-speed photography technology. The cavity flow characteristics of the near water surface, including splash jet, splash crown, surface seal of cavity, pull away, deep seal of cavity and cavity collapses, were systematically investigated using a high-speed camera. The emphasis of the study is paid on the effect of nose shape, water entry angle and velocity on the evolution of the air entraining cavity. The experimental results demonstrate that the nose shape of projectile has a significant influence on the jet flow, the cavity diameter and trajectory stability in the case of certain other conditions. On the other hand, the splash scale, cavity diameter increase gradually with the increasing of the water entry velocity, as well as the cavitation closed in advance. Furthermore, the water entry angle of the projectile plays an important role in the cavity evolution and the close type.

Numerical Simulation on the Cavitation Flow of High Speed Oblique Water Entry of Revolution Body

To explore the effects of water entry angle on the cavitation flow field of high-speed revolution body, based on the finite volume method, VOF (Volume of Fluid) multiphase model, Schnerr-Sauer cavity model, SST k-ω turbulence model, and dynamic mesh method, numerical simulation for modeling the oblique water entry of revolution body at high speed is performed. The evolution laws of cavity shape, motion characteristics, and hydrodynamic characteristics of revolution body at different water entry angles are analyzed. The results show that the numerical calculation method can effectively simulate the change of cavity shape during the water entry of the revolution body. With the increase of water entry angle, the uplift of liquid level decreases in the positive direction of the open cavity and increases in the negative direction. The angle of water entry has little effect on the velocity of the revolution body. The larger the angle of water entry, the greater the peak pressure and the faster the pressure decay at the moment of water entry.

Large eddy simulation and experimental investigation on the cavity dynamics and vortex evolution for oblique water entry of a cylinder

Water entry problem is an important topic, and the physical understanding of the mechanism of the cavity dynamics and vortex evolution have still not been clear up to now. In the present study, the complicated unsteady characteristics for the oblique water entry problem of a cylinder were investigated both numerically and experimentally. In the numerical study, a three-dimensional simulation with a six-degree-of-freedom model was carried out, where the large eddy simulation (LES) model and the volume of fluid (VOF) technique were used for turbulence computation and air-liquid interface tracking, respectively. In order to provide the validation data for the numerical calculation, the corresponding experiment was carried out, in which a high-speed camera was used to record the cavity evolution. The cavity evolution mechanism and flow pattern at a low Froude number are analyzed and it is found that the multi-scale vortex structures around the cylinder can be successfully captured by LES method. Moreover, a numerical parametric study is conducted on the effects of water entry angle on the dynamic characteristics numerically.