Bubble Pulsation Period Research Articles

Experimental research into the dynamics of underwater explosion bubbles near mutually perpendicular walls

It is of great significance to characterize the dynamics of underwater explosive bubbles in close proximity to mutually perpendicular walls for ensuring the safety of important underwater structures. In this paper, a dynamic experiment on underwater explosion bubbles was carried out near constructed mutually perpendicular walls. High-speed cameras were utilized to capture high-resolution images, while pressure sensors recorded pressure–time history curves. The main focus was on studying the evolution process of bubble morphology and pulse characteristics. When the position of the charge's center relative to the horizontal wall remained fixed, decreasing the distance between the charge's center and the vertical wall resulted in a reduction in the equivalent maximum radius of bubbles and an increase in its pulsation period. Additionally, the asymmetric collapse of bubbles on a single wall transformed into asynchronous collapse on two walls, with most collapsed bubbles tending to migrate and expand toward the corner formed by mutually perpendicular walls. The resulting jet from the collapse of bubbles exhibited deflection toward the vertical wall, with an inclination angle increasing approximately proportionally with dimensionless distance ratio γh/γv. Moreover, it became more difficult for achieving effective focusing of bubble energy as the jet approached the corners formed by mutually perpendicular walls. The experiments also implied that reducing the dead weight of the vertical wall weakened its contact with the horizontal wall, causing an increase in the equivalent maximum radius of bubbles and jet inclination, as well as a decrease in the bubble pulsation period, under the same dimensionless distance γv.

Effects of computational domain in underwater explosion numerical simulation on bubble pulsation

To clarify the influence of the computational domain size on the underwater explosion bubble pulsation, a one-dimensional axisymmetric model of the underwater explosion was established. The computational model was verified using empirical formulas. Further research was conducted on the variation patterns of the bubble pulsation period and maximum radius with changes in the proportional computational domain radius. The results indicate that as the proportional computational domain radius increases, the maximum radius of the bubble and the bubble pulsation period gradually increase and stabilize. The proportional computational domain radius required to achieve stability is related to the static water pressure of the computational domain. Increasing static water pressure makes the required computational domain size for achieving high numerical accuracy smaller. When the proportional computational domain radius reaches around 1600, the calculation results are highly accurate under different static water pressures at different depths of explosion.

Effect of AP distribution on energy release characteristics and functional force of HMX/AP/Al explosives

AbstractIn order to understand the reaction kinetics of HMX/AP/Al ternary system, the different distribution of AP in HMX/AP/Al explosives was realized by two different preparation techniques. Detonation test results show that the detonation velocity, explosion heat and detonation pressure of HAP samples are higher than those of HAl samples, but the extent of improvement is not high, not more than 5 %. The results of scanning electron microscopy showed that AP in HAP samples was distributed on the surface of HMX crystal. AP were dispersed around HMX crystals in HAl samples. The experimental results of explosive fireball performance show that the fireball expansion speed of HAP samples is better than that of HAl samples, demonstrating a good fireball effect. Underwater test results show that the shock wave peak pressure and bubble pulsation period of HAP samples increase by 3.06 % and 7.95 % respectively, and shock wave energy and bubble energy increase by 9.8 % and 25.42 % compared with bubble energy. The experimental results show that HAP samples are superior to HAl samples in accelerating ability of Al flies. The dispersion of AP on the HMX crystal surface promotes the energy release of HMX/AP/Al explosives more.

A refined numerical investigation of a large equivalent shallow-depth underwater explosion

The large equivalent shallow-depth explosion problem is very significant in the field of naval architecture and ocean engineering, as such explosions can be used to attack and demolish ships and anti-ship missiles. In the current work, a refined numerical study of the flow-field characteristics of a large equivalent shallow-depth explosion is carried out using a self-developed Eulerian finite element solver. First, the numerical model is validated against theoretical results and a small equivalent explosion test in a tank. The numerical results are found to agree well with the theoretical and experimental results. In the next step, the cavitation cut-off effect is added to the underwater explosion model, and the cavitation phenomenon is quantitatively analyzed through the flow-field pressure. In addition, the dynamic characteristics of the bubble and water hump under various initial conditions for different stand-off parameters are analyzed. The effect of gravity on these physical processes is also discussed. The bubble pulsation period, taking into account the free surface effect, is then quantitatively studied and compared with Cole’s experimental formula for an underwater explosion. Overall, when the stand-off parameter γ > 2, the influence of the free surface on the empirical period of the bubble is not significant. Our investigation provides broad insights into shallow-depth underwater explosions from theoretical, experimental, and numerical perspectives.

Numerical Simulation Research on Characteristics of Underwater Explosive Bubble Jet in Offshore Water

Underwater explosions have always been a hot topic in the field of ship protection. When explosives explode in offshore waters, the influence of seabed and structural boundaries on shock wave propagation and bubble pulsation will become more complicated. In this paper, a numerical simulation study of the underwater explosion between a deformable seabed and a rigid boundary is carried out. Firstly, the ABAQUS software was used to establish a numerical model by using the CEL method. The seabed was regarded as a heavier fluid, and the density ratio of the seabed and water was used to describe the characteristics of the seabed. The validity of the model was verified by comparison with experiments. Then, a series of numerical simulations were carried out by adjust the position of the explosive, the thickness of water medium layer, and the density of the seabed. The results show that: when the position of the explosive is close to the seabed and the rigid boundary, the bubble pulsation period is longer. The water jet and the pulsating pressure of the bubbles have a strong impact on the structure when the explosive is located near to 1 times the theoretical maximum radius of the bubble. As the depth of the water decreases, it can be observed that the bubbles transform from “ellipsoid” to “nipple-like”, and finally tear into upper and lower halves. When the thickness of water medium layer is 1 times the theoretical maximum radius of the bubble, the incident pressure waveforms of the bubble pulsation and the water jet near the structure are chaotic, which is caused by the “tear” phenomenon of the bubble. As the density of the seabed increases, the depth of the intrusion of the bubbles into the seabed becomes smaller and the shape of the bubbles becomes flatter. The research results of this paper can provide reference for the protection design of ships.

Effect of Al/O ratio on underwater explosion energy characteristics of CL‐20‐based aluminized explosives

AbstractTo study the underwater explosion energy characteristics of CL‐20‐based aluminized explosives with different Al/O ratio, underwater detonation parameters of explosives, including the shock wave peak pressure, the decay time, the bubble pulsation period, the shock wave energy and the bubble energy at 1 m, were measured by a series of underwater explosion experiments. The calculation model of CL‐20‐based aluminized explosive in the pool was established by using Ansys LS‐DYNA software and was calculated the shock wave energy and bubble pulsation period. The results showed that Al/O ratio significantly changed the energy output structure of CL‐20‐based aluminized explosives. With increasing Al/O ratio, the proportion of shock wave energy in the total energy decreased, while the proportion of bubble energy increased. When Al/O ratio was 0.35, the shock wave peak pressure and shock wave energy of the explosive were the largest, and when the ratio was 0.6, the bubble period and total energy reach their maximum; when the ratio was 0.94, the bubble energy and the bubble pulsation reached the maximum. The experimental values are in good agreement with the simulation results, and the model can be used to estimate the underwater detonation parameters of CL‐20‐based aluminized explosives with different Al/O ratio.

Experiments and simulation of block motion in underwater bench blasting

The blasting mechanism underlying drilling and blasting of underwater rocks, as an important component of the engineering blasting technology, has not been systematically studied. Laboratory model experiments are expensive and take a long time, while field tests fail to obtain timeous breakage and accumulation effects of underwater blasting, and may even be impossible. Considering this, a model experiment of underwater concrete bench blasting was designed, and the motion of blasted blocks was observed and evaluated with a high-speed camera. Then, numerical simulation was conducted based on Fluent and an engineering discrete element method coupling program complied using the application programming interface. Results show that the blocks form a bulge in the underwater blasting experiment under action of blast waves and expansion in the first period of bubble pulsation. Then, some blocks shrink in the first period of bubble pulsation. As the charge increases, the blast load exerts larger disturbance on the block group, resulting in significant motion of blasted blocks along the vertical direction. At the same time, the horizontal displacement of blasted blocks in the throwing direction increases.

Investigation of the Effect of Nozzle on Underwater Detonation Shock Wave and Bubble Pulsation

The subject of a gas jet generated by underwater detonation is an important issue in the field of underwater propulsion. The experimental system of underwater detonation is established, which utilizes a high-speed camera to record the morphological changes in bubbles and various pressure sensors to measure the flow field pressure. The effect of nozzles and the pressure of the flow field are analyzed thoroughly. The comparison of the bubble and field pressure shows that the shrinking nozzle increases the peak pressure of the transmitted shock wave generated by underwater detonation compared with that of the straight nozzle. Simultaneously, the water–air mixing phenomenon caused by the gas jet is enhanced. Under the influence of the reflected shock wave and the converging angle of the nozzle, the pulsation process of the bubble is inhibited enormously, which results in the bubble energy being substantially below that of the straight nozzle. The bubble pulsation period is 24.2 ms when the shrinking nozzle is installed, and the pressure of the bubble pulsation is quite small, only 9.8 kPa. On the contrary, the expansion angle increases the velocity of the gas jet, suppressing the water–gas mixing phenomenon while enhancing the bubble pulsation process. The bubble pulsation period is 33.0 ms when the expanding nozzle is equipped, which is larger than the 31.2 ms of the straight nozzle and the bubble pulsation pressure is higher, at 26.1 kPa. Although the bubble energy is increased when the expanding nozzle is installed, thus generating a higher pulsation pressure, the peak pressure and direction of the shock wave are changed in the water.

Estimation on the Underwater Explosion Equivalent Based on the Threshold Monitoring Technique

Underwater nuclear explosions can be monitored in near real-time by the hydroacoustic network of the International Monitoring System (IMS) established by the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which could also be used to monitor underground and atmospheric nuclear explosions. The equivalent is an important parameter for the nuclear explosions’ monitoring. The traditional equivalent estimation method is to calculate the bubble pulsation period, which is difficult to obtain satisfactory results under the current conditions. In this paper, based on the passive sonar equation and the conversion process of acoustic energy parameters in the hydroacoustic station, the threshold monitoring technique used for underwater explosion equivalent estimation was studied, which was not limited to the measurement conditions and calculation results of the bubble pulsation period. Through the analysis of practical monitoring data, estimation on the underwater explosion equivalent based on the threshold monitoring technique was verified to be able to reach the accuracy upper boundary of current methods and expand the measurement range to further ocean space, along with the real-time monitoring capability of IMS hydroacoustic stations which could be estimated by this method.

Experimental and numerical investigations of near-field underwater explosions

Near-field underwater explosion (UNDEX) phenomena were investigated by experiments and numerical simulations. The UNDEX experiments were performed in a water tank using a ship-like model. One kilogram of TNT, one of the most widely used military high explosives, was used for the experiments. Numerical simulations were performed under the same conditions as in the experiments using the commercial software LS-DYNA. Underwater pressures, accelerations, velocities, and strains by shock waves were measured at multiple locations. Further, the bubble pulsation period and the whipping deformations of the ship-like model were explored. The experimental results are presented and examined through comparison with the results obtained from widely used empirical equations and numerical simulations.

Study on the influence of rigid wall surface on the bubble characteristics of underwater explosion

In order to investigate the influence of rigid wall surface on the bubble characteristics of underwater explosion, the underwater explosion experiment under the boundary conditions of free surface and rigid wall surface was carried out on 2.5g cylindrical charge TNT in a 2×2×2m tank. The time history curves of shock wave and bubble pulsation were obtained by underwater pressure sensor, and the bubble pulsation process was observed by high-speed photography. The experimental results show that compared with the free surface underwater explosion, the shock wave peak pressure and the bubble pulse peak pressure in rigid wall surface underwater explosion are increased, and a large cavitation area appeared at the junction of rigid wall and water surface. After the bubbles contacted the rigid wall surface, the bubble morphology changed significantly, the first bubble pulsation period and the maximum radius of bubble expansion became larger. After the first bubble pulsation, the bubble partially collapsed and split into two parts, bubbles continued to pulsate in the direction of the rigid wall and the bottom of the water. Finally, combined with the experimental data of rigid wall surface at different explosion depths, the relationship between the first bubble pulsation period, the maximum bubble expansion radius and the explosion depth of 2.5gTNT under rigid wall conditions is given.

Thrust production from in-water gas charge combustion near a flat wall

The paper presents experimental results on study of production of force pulses and bubble pulsation near a flat thrust-producing wall with the diameter of 100 mm designed for combustion of stoichiometric propane-oxygen gas mixture. In combustion of the same gas charges for production of thrust, the design of combustion at a flat wall has qualitative and quantitative benefits vs. case of combustion in a cylindrical duct. The force amplitude becomes higher due to the larger contact area between the bubble and thrust wall. The period of bubble pulsation is shorter for this case as compared to bubbles generated in a cylindrical duct; the pulsation period is described by the Rayleigh-Willis law. The shorter bubble pulsation period ensures better conditions for increase in the average thrust generated in pulse-in-cycle mode of combustion of gas charges in water medium.

Calibration of Numerical Simulation Methods for Underwater Explosion with Centrifugal Tests

The centrifugal underwater explosion tests and corresponding numerical simulations were carried out to study the laws of shock wave and bubble pulsation. A semiempirical method to determine JWL state equation parameters was given. The influence on numerical results caused by factors such as state equation of water, boundary condition, and mesh size was analyzed by comparing with the centrifugal underwater explosion test results. The results show that the similarity criterion is also suitable in numerical simulation; the shock wave peak pressure calculated by polynomial state equation is smaller than that of shock state equation. However, the maximum bubble radius and the pulsation period calculated by the two equations are almost the same. The maximum bubble radius is mainly affected by the boundary simulating the test model box, and the pulsation period is mainly affected by the artificial cutoff boundary. With the increase of standoff distance of measuring point, the mesh size required for the numerical calculation decreases; the size of the two‐dimensional model is recommended to take 1/30 ∼ 1/10 explosion radius. In three‐dimensional models, when mesh size is 2 times larger than explosion radius, the bubble motion change in the second pulsation period is not obvious. When mesh size is smaller than 1 time explosive radius, the full period of bubble pulsation can be well simulated, but calculation errors increase slowly and computation time greatly increases, so the three‐dimensional mesh size is suggested to take the charge radius. Shock wave peak pressure is basically unaffected by gravity. As the gravity increases, the bubble maximum radius and the first pulsation period both decrease.

Simulations of the dynamics and interaction between a floating structure and a near-field explosion bubble

The simulation of the interaction between a surface ship with a near-field underwater explosion bubble is performed using the three-dimensional (3D) boundary integral method for simulating the physical process of the bubble coupled with the finite element method for calculating the structural response in conjunction with the doubly asymptotic approximation for handling the fluid-structure interaction problem. An indirect procedure is proposed solely for investigating the effect attribute to the free surface elevation on the bubble dynamics and the structural response through a comparison of two different 3D boundary integral numerical methods. The present model is validated by comparing the numerical simulation results with the experimental data. For a near-field explosion, the results indicate significant effects due to the free surface elevation on the bubble dynamics and transient response of the ship hull, including modification of the bubble pulsation period and the pressure distribution on the wet surface, which is the key initiator of the destructive action. The transient structural response characteristics including the global and local response of the hull ship subjected to an underwater explosion bubble are investigated in detail and the related mechanism behind the phenomena are also analyzed.

Efficiency analysis of bubble pulsation formed by high-voltage discharge in nonfree field water

Bubble pulsation is known to occur as a result of high-voltage discharge in water. Due to advantages such as long duration and significant bubble pulsation production, this technique is widely used in industrial production. Bubble pulsation is directly reflected by the pulsation efficiency, which is mainly embodied in the transformation efficiency from the high-voltage discharge energy to the bubble expansion energy, as well as the residual rate of bubble pulsation energy. In this study, high-voltage discharge was used in water to form pulsating bubbles. The vertical displacement of water was restricted by the experimental equipment, so horizontal displacement was mainly generated. Therefore, any influence caused by the ascending motion of the bubbles could be effectively ignored. Consequently, the bubble pulsation process could be measured at stable hydraulic pressure. The pressure, bubble pulsation period and pulsation times generated by bubble expansion were measured in our experiments. The transient voltage and current were also measured. The relationship between bubble pulsation efficiency with discharge voltage and hydrostatic pressure was analyzed by combining the experimental results with results from simulations of bubble pulsation expansion.

Calculations of pulsation period of a toroidal bubble during diaphragm discharge in electrolyte

Numerical calculations of pulsation period of a toroidal bubble on the edge of a round diaphragm current concentrator during electric discharge in electrolyte were performed for the self-oscillations mode. It was shown both theoretically and experimentally that the dependence of the period of the pulsations on the applied voltage has a minimum. This minimum corresponds to the conditions of the change in the leading mechanism of the bubble growth. The derived formula for the bubble pulsation period is in a good agreement with experimental data on discharges with the round diaphragms of the radii from 0.025 to 0.75mm.

Dynamics of bubble generated by low energy pulsed electric discharge in water

Results of investigations of bubble formation and dynamics for discharge in water are presented. Experiments were carried out in discharge chamber with axisymmetric electrode system “wire to wire”. Interelectrode gap was varied from 1 to 10 mm. Energy in a pulse was <1 J. Velocity of bubble expantion and collapse is about several hundreds meter per second at early stage of discharge. Bubble pulsation period is 0.5 – 1 ms. Increasing of energy released in the discharge gap will increase bubble pulsation period. Little bubble was formed by reducing energy input into discharge. But the main stage of discharge always followed by bubble formation. Specific erosion is measured for different energy in pulse and matched up with bubble collapse.

Early Stage of Pulsed Discharge in Water

The bubble radius at the early stage of discharge in water is investigatedusing high-speed photography. Some simulation results on the bubble radius arepresented, which are in agreement with the experimental results, with a maximumdifference of about 10%. The reasons why the peak pressure of the firstshock wave is only related to the energy released in the bubble during thefirst half period are addressed. The energy released in the bubble after thefirst half period increases the bubble pulsation period, but it produces nomore than 10% under the peak pressure of the second shock wave.