Cavity Evolution Research Articles

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.

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.

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.

Analysis of influencing factors of the projectile entering the water through the ice hole

The ice hole, as a special constraint environment, affects the dynamics of water entry. In this paper, the finite volume method (FVM) is used to study the cavity dynamics of the projectile passing through the ice hole. The results show that the cavity evolution law of the projectile passing through the ice hole is different from that of the projectile entering the water freely, and the backflow of the splash crown occurs near the hole. As the Froude number (Fr) increases, the time when the splash crown flows back at the hole is earlier, and the time when the cavity pinch-off occurs at the tail of the projectile is also earlier. The fluctuation of force decreases, and the velocity variation becomes more stable as the Fr decreases. After the projectile with a large aspect ratio passes through the ice hole, the cavity collapse becomes more obvious, and the resistance increase. The constraint effect of the hole on the cavity strengthens as the hole thickness increases, and the cavity integrity after the pinch-off becomes worse. After the projectile passes through the hole, the resistance and force fluctuation increase as the hole thickness increases.

Experimental study on flow fields and bubble entrainments by the water droplet impact using 3D PIV tests

The phenomenon of a water droplet impact on a free surface is studied to understand the physics of free surface bubble entrainment. Particle image velocimetry (PIV) and high-speed image system are used to analyse the flow structure evolutions in the droplet impact cavity period and bubble entrainment cavity period, respectively. The photographic results show that the entrapped surface of the impact cavity remains intact and continuous, and individual bubble entrainment occurs during the secondary entrainment cavity evolution. The instantaneous distributions of velocity fields in the longitudinal and lateral directions are uniform and independent of the approaching impact velocity. For the entrainment cavity evolution, the transverse diffusion intensity is higher than that for the impact cavity period. An individual bubble forms during the transverse velocity penetration across the droplet impact area. Consequently, for the droplet impact on a water surface, the present study implies the presence of a mechanism by which the interior flow field plays a key role in the bubble entrainment.

Gas production performance of underground coal gasification with continuously moving injection: Effect of direction and speed

Underground coal gasification (UCG) is an in-situ gasification technology where the controlled retreating injection point (CRIP) method is usually adopted to optimize the cavity evolution and gas production stability. However, the reported studies only focus on the gasification behavior of the moving injection backward under fixed time intervals. In this work, the gas production performance and reaction region evolution were studied for UCG with continuously moving injection for different directions and different speeds. For moving backward injection, when a uniform moving speed is used, gas production is not satisfactory with a heating value of 1 MJ/m3. In order to optimize the gas production, the injection should move backward for a fixed distance at regular time intervals rather than continuously, and the distance should be long enough for a new distribution of combustion zone, gasification zone and pyrolysis zone. For moving forward injection, when the moving speed and oxygen flow rate match well, the maximum heating value is larger than 11 MJ/m3, and the gas production is stable. The moving forward method is recommended for low ash coal to avoid “ash cover pipe” problem, and the injection pipe should be manufactured with high temperature resistant material. The moving backward method is recommended for low volatile coal to avoid “gas product leakage” problem, and the retreatment distance should be specially designed to avoid re-ignition.

Dynamic characteristics of water entry under the effect of floating ice and an independent distance model of floating ice

An ice-constrained environment significantly affects the water entry of projectiles. In this paper, cavity evolution in the presence of floating ice is studied. The numerical method is validated by experiments. The effects of the layout of floating ice, Froude number (Fr), and distance between the floating ice and projectile (lg) on water entry dynamics are investigated. The dynamic model of the water entry under the effect of floating ice is established. The independent distance equation of the floating ice at different Fr is obtained. The results show that the floating ice affects the cavity surface closure and the deep pinch-off. In the presence of unilateral floating ice, the height variation of the splash crown on both sides of the projectile is different, and the height of the splash crown is lower than that under the effect of bilateral floating ice. The impact force of the projectile entering the water is the largest with bilateral floating ice, which is 20% higher than that without floating ice. As Fr increases, the splash crown completely seals on the free surface and the surface closure occurs. With the change of Fr, there are transition regions in the position and time of the cavity pinch-off plots. The effect of the floating ice on the splash crown weakens as lg increases, and the pressure on both sides of the projectile gradually reaches the pressure without floating ice. The fitted independent distance curve can predict the pressure on both sides of the projectile affected by the floating ice. This study provides a reference for the launch of projectiles and release of detectors in polar region with floating ices, and provides a new idea for water entry under environmental restrictions.

Physical Simulation Test of Underground Coal Gasification Cavity Evolution in the Horizontal Segment of U-Shaped Well

A key point in the underground coal gasification process is the cavity evolution in the horizontal segment. The morphological evolution law of the gasification cavity has not been clarified, which is the bottleneck restricting the analysis of its controllability. In this paper, a physical simulation system for cavity generation was developed, and the cavity evolution in a targeted coal seam with overburden pressure was duplicated in the laboratory. A set of temperature field synchronous monitoring devices was developed to realize temperature sampling within a cavity and the surrounding rock. By analyzing the relationship between the overall temperature distribution pattern and the gasification agent injection condition, the morphological propagation law of the cavity is verified to be water drop-shaped, and influencing factors including the injection flow rate and the gasification agent component ratio are investigated. The axial length and volume of the cavity increase with an increasing injection flow rate. Higher oxygen content results in increased size in all dimensions. The research results provide theoretical support and reference for applying controlled cavity formation in the horizontal segment of U-shaped wells.

Grain boundary softening from stress assisted helium cavity coalescence in ultrafine-grained tungsten

The formation of helium cavities in coarse-grained materials produces hardening proportional to the number density and size of the cavities and due to the interaction of dislocations with intragranular helium defects. In nanostructured metals containing a high density of interfacial sinks, preferential cavity formation in the grain boundaries instead produces softening that is often attributed to enhanced interfacial plasticity. Employing two grades of ultrafine-grained tungsten, we explore this effect using targeted implantation studies to map cavity evolution as a function of the irradiation conditions and quantify its impact on the mechanical response through nanoindentation. Softening is reported at implantation temperatures above the threshold for preferential grain boundary cavity formation but at a sufficiently low fluence prior to the growth of intragranular cavities. Collective changes in the mean cavity size, density, and morphology beneath a residual impression on an implanted surface indicate that cavity coalescence accompanied the reduction in hardness. Complementary atomistic simulations demonstrate that, in tungsten grain structures exhibiting softening, grain boundary bubble coalescence is driven by stress concentrations that further act to localize strain in the grain boundaries through cooperative deformation processes involving local atomic shuffling and sliding, dislocation emission, and even the nucleation of unstable twinning events.

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.

Investigation of hydrodynamics of water impact and tail slamming of high-speed water entry with a novel immersed boundary method

High-speed water entry is a transient hydrodynamic process that is accompanied by strongly compressible flow, free surface splash, cavity evolution and other nonlinear hydrodynamic phenomena. To address these problems, a novel fluid–structure interaction (FSI) scheme based on the immersed boundary method is proposed which is suitable for strongly compressible multiphase flows. In this scheme, considering the multiphase interfaces at the immersed boundary, an improved immersed boundary method for effectively suppressing the non-physical force oscillation is proposed. Additionally, a quaternion-based six degrees of freedom motion system is used to describe rigid body motion, and the multiphase flow Eulerian finite element method is applied as the fluid solver. Using analytical solutions, experimental data and literature data, the accuracy and robustness of the FSI scheme are validated. Finally, the high-speed water entry of the slender body with different noses is investigated, and the hydrodynamic loads including the axial and normal drag forces and the bending moment are extensively discussed. The hydrodynamic load and motion trajectory are determined by the nose configuration. The tail slamming phenomenon is the primary focus, and it is revealed that its formation is primarily related to the pitch moment formed at the stage of crossing the free surface. Tail slamming also causes violent impact loads, especially bending moments, which may cause slender projectiles to break off. Finally, to combine the features of the flat and hemispherical noses, the water entry of the projectile with a truncated hemispherical nose is simulated and discussed.

Numerical study on flow separation and force evolution in liquid nitrogen cavitating flow

In this paper, a two-phase flow model and the improved thermal cavitation model are used to study the flow characteristics and the evolution of vorticity force for liquid nitrogen cavitation flow. The results are validated against the experimental data from NASA, which shows that the flow re-attachment occurs in the area where the boundary vorticity flux begins to decrease from its positive value. Alternatively, the flow separation occurs near the area with low boundary vorticity fluxes. Besides, the boundary vorticity flux changes typically from negative to positive around the cavity. The development of the cavity affects the surrounding vortex structure, which leads to the evolution of vorticity force and the fluctuations of lateral force and drag force. The results show that the drag force is magnified by the cavitation, and it first decreases and then increases in a typical cycle of cavity evolution. The negative resistance unit is mainly produced in the front of the cavity, and the positive resistance unit is also made in the rear of the cavity. When the attached cavity is shed, the positive drag elements decrease, and the drag force drops to its minimum. When the attached cavity is growing, the positive resistance elements behind the cavity increase, making the resistance force gradually increase. For the lateral force, the existence of the cavity produces lift elements pointing out of the surface outside the cavity, and the unsymmetrical distribution of the cavity leads to the fluctuation of the lateral force.

Proper orthogonal decomposition analysis of the cavitating flow around a hydrofoil with an insight on the kinetic characteristics

In this research, the cavitating flow around a NACA0015 (National Advisory Committee for Aeronautics) hydrofoil obtained by the large-eddy simulation method is analyzed using the proper orthogonal decomposition (POD) theory. Various fundamental mechanisms have been investigated thoroughly, including the reentrant jet behavior, pressure gradient mechanism, vortex dynamics, and dynamic properties of the hydrofoil. The influence of the vortex dynamics, pressure mechanism, and temporal/spatial evolution is revealed. The POD decomposition indicates that the first four dominant POD modes occupy 97.4% of the entire energy. Based on the vortex force field extracted from the first four single POD modes, it is found that the lift-and-drag characteristics in the cavitating flow are determined by the specific spatial distribution of mode vortex structures. In addition, the coupling of velocity pulsations and pressure fluctuations is carried out to obtain the POD modal pressure gradient field, which reveals that the pressure gradient has a close connection with the cavity evolution. Furthermore, the vortex force and pressure gradient are reconstructed using the first four modes, 17 modes, and 160 modes, which indicates that the low-order POD modes without the impact of small-scale structures and noise can clearly capture the fundamental aspects of the flow field.

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 behavior of ball penetrometer in soft-over-stiff soil deposits

Proper understanding of the ball penetration behavior under centrifuge condition is important for accurate soil property determination. In this study, large deformation finite element (LDFE) analyses are conducted to investigate the scale effect of ball penetrometer on its behavior in clay-over-sand deposit in centrifuge tests. The finite element model of this study is validated against previous theoretical solutions and available centrifuge test data with good agreement. Parametric studies are then carried out to explore the effects of potential factors, including the layer thickness ratio, the shaft-ball area ratio, the undrained shear strength of clay, and the relative density of sand. The results indicate that the soil failure mechanisms can be divided into three stages: i) shallow embedment stage, ii) cavity evolution stage, and iii) full-flow stage. The penetration resistance profiles in centrifuge tests can be better quantified, which can provide more accurate shear strength interpretation for the top clay.

Numerical study on the cavity dynamics for vertical water entries of twin spheres

In this study, a three dimensional (3D) numerical model of six-degrees-of-freedom (6DOF) is applied to simulate the water entries of twin spheres side-by-side at different lateral distances and time intervals. The turbulence structure is described using the shear-stress transport k-ω (SST k-ω) model, and the volume of fluid (VOF) method is used to track the complex air-liquid interface. The motion of spheres during water entry is simulated using an independent overset grid. The numerical model is verified by comparing the cavity evolution results from simulations and experiments. Numerical results reveal that the time interval between the twin water entries evidently affects cavity expansion and contraction behaviors in the radial direction. However, this influence is significantly weakened by increasing the lateral distance between the two spheres. In synchronous water entries, pressure is reduced on the midline of two cavities during surface closure, which is directly related to the cavity volume. The evolution of vortexes inside the two cavities is analyzed using a velocity vector field, which is affected by the lateral distance and time interval of water entries.

Thermodynamic determination of condensation behavior for the precursory elements of radioxenon following an underground nuclear explosion

The measurement of radioactive xenon isotopes (radioxenon) in the atmosphere is a tool used to detect underground nuclear explosions, provided that some radioxenon escaped containment and that fractionation leading to the alteration of the relative proportions of these isotopes, is accounted for. After the explosion, volatilization followed by melting of the surrounding rocks produces a magma where the more refractory radioactive species get dissolved while the more volatile ones contribute to the gas phase that might escape. Indium, tin, antimony, tellurium and iodine are the main fission products involved in the decay chains leading to radioxenon. In this study, condensation as a function of temperature for these precursors of radioxenon were determined using thermodynamic calculations for systems with complex chemical composition corresponding to major environments of known underground nuclear explosions and for a range of pressure values representative of the cavity evolution. Our results illustrate a large difference between the relevant condensation temperatures for the radioxenon precursors and the tabulated boiling temperatures of the pure compounds often used as indicators of their volatility. For some precursory elements such as tin, the often-considered Heaviside function represents an oversimplification of the concept of condensation temperature, as condensation occurs over a temperature range as large as 2000 K. This results from the speciation of the elements in the gas phase mainly driven by the formation of oxides. Condensation also strongly depends on pressure while it moderately depends on the bulk chemical composition of the system. This study shows the importance and complexity of the condensation process following underground nuclear explosions. It also shows how thermodynamic computations allow the prediction of the quantity and the relative proportions of radioactive xenon isotopes in the gas phase in the presence of magma, before their potential emission to the atmosphere. Better detection, discrimination and understanding of underground nuclear explosions should arise by taking into account the fractionation resulting from the condensation of the radionuclides producing radioxenon in nuclear cavities.

Numerical study on the water-entry characteristics of asynchronous parallel projectiles at an oblique impact angle

This study investigates the water-entry problems of asynchronous parallel projectiles with oblique entry speeds of 200 m/s for different water-entry intervals and two different water-entry sequences numerically. The experiment of the water-entry of the projectile with an oblique impact angle is conducted to verify the numerical method. The influences of the oblique asynchronous parallel water-entry on the evolutions of the cavitating flow and the movement characteristics under different working conditions are studied in detail. It can be found that for the minimum interval, the first launch projectile is not able to generate a complete cavity to encapsulate its body due to the intense compression of the cavity of the following launch projectile, and its movement is seriously affected and ultimately destabilized. Simultaneously, compared with the first launch projectile entering the water from the side close to the initial free surface, the cavity of the first launch projectile entering the water from the side away from the initial free surface is squeezed more, wetted earlier and destabilized faster. With the increase of the interval, the influence of the following launch projectile on the cavity evolution and movement stability of the first launch projectile weakens. Besides, for the following launch projectile, due to the continuous impact of the cavitating flow of the first launch projectile, the profile of the cavity of the following launch projectile presents asymmetry. For the minimum interval, the projectile deflects faster and towards the inner side of the projectile, while for the larger intervals, the projectile deflects to the outer side of the projectile and becomes unstable.

Multi-port real-time observation for ultrafast intracavitary evolution dynamics

Recent advances in real-time spectral measurements of a mode-locked fiber laser have found many intriguing phenomena and which have verified the soliton theory. However, most current results are based on laser single-port observation, and are rarely involved in the cavity evolution, which also has rich nonlinear dynamics according to the soliton theory. Here we present an approach for the intracavitary soliton evolution processes, where spectra from multi-ports are collected in time-division multiplexed sequence to realize synchronous real-time observation. The sinusoidal evolution of the spectral beating is observed clearly, agreeing with the reported prediction. Furthermore, the intracavitary spectral dynamics of the period-doubling bifurcation are also revealed. Our scheme observed the spectral expanding and shrinking alternately and periodically over two round trips, matching well with simulations. This work may open up possibilities for real-time observation of various intracavitary nonlinear dynamics in photonic systems.

Study on high-speed water entry of the projectile passing through an ice hole in a low-temperature environment based on a modified thermodynamic cavitation model

It is a complicated problem to study high-speed water entry of a projectile passing through an ice hole in a polar environment. This involves the constraint of the ice hole on the free surface and low-temperature cavitation during the water entry. In this paper, a numerical method involving a modified thermodynamic cavitation model is introduced to study the water entry process. The numerical method is validated by comparing the numerical results of cavity evolution with the experimental data. The cavity dynamics of the projectile passing through the overwater ice hole at high speed and different ambient temperatures are studied. The cavity evolution, flow field, and motion state of the projectile are analyzed. The results show that a nested cavity forms when the projectile passes through the ice hole at high speed. The drop in temperature accelerates the surface closure and deep pinch-off. The effect of the temperature on cavity evolution weakens as the Froude number (Fr) increases. Moreover, at high Fr, the temperature alters the appearance of the ripple on the cavity surface and the growth trend of the cavity size. The drop in temperature reduces the content of the vapor in the cavity and changes the flow characteristics. At a low temperature, the hydrodynamic drag of the projectile passing through the ice hole increases, and the pressure distribution on the surface of the projectile is different.