Bubble Pulsation Research Articles

INVESTIGATION OF WATERJET EFFLUX NOISE WITH NUMERICAL TECHNIQUES

The URN resulting from a waterjet propulsion (WJP) is to be accurately estimated for stealth considerations. Such WJP systems are designed to operate either with a submerged jet (wet transom) or with a free surface jet (dry transom). The URN due to the operation of WJP system can be broadly categorized as pump noise and nozzle efflux noise. Pump noise includes mainly the cavitation noise and the tonal mechanical/hydrodynamic noise, whereas the nozzle efflux noise is due to jet impact and bubble entrainment. The present study attempts to implement advanced numerical techniques to estimate URN due to the nozzle efflux of a WJP in both wet and dry transom modes. Large Eddy simulations (LES) are carried out to simulate the hydrodynamics of turbulent multiphase flow behind both submerged and free surface water jets. The impact of water jet on free surface and subsequent bubble generation rate is captured using multiphase volume of fluids (VOF) technique. The impact noise is estimated by directly recording the fluctuating acoustic pressure at certain field points during the simulation. A blob detection algorithm is implemented to compute the bubble generation rate. A mathematical model to compute the bubble pulsation noise from bubble generation rate is validated and implemented. The estimated URN levels in wet and dry mode are presented and compared. Key inferences are presented which could be useful in further investigation of URN from water jet of stealth vessels, as well as to investigate bubble noise from propeller cavitation, air lubrication etc.

Research on bubble pulsation and jet loading characteristics during a near-field underwater explosion

<p indent="0mm">To deeply study the bubble jet phenomenon occurring during the near-field underwater explosion, this study performed an underwater explosion test of small equivalent TNT exploding at the bottom of a clamped square plate. The load time–history curve of the bubble jet was successfully measured by the method employing an array wall pressure sensor. The experimental results show that the peak value of the bubble pulsation pressure at the bottom center of the clamped square plate is not significantly different from the peak value of the jet pressure. However, the duration of the jet pressure is 21.36 times that of the bubble pulsation pressure. The bubble pulsation pressure and its duration slightly attenuate, while the jet pressure and its duration attenuate faster with the increasing distance from the sensor to the center, indicating a quite concentrated range of jet load. Further, the numerical simulation of the whole process from the explosion to the bubble jet formation was performed via the Abaqus software. The bubble evolution process and the bubble jet load time-history curve obtained by the simulation are consistent with the experimental results. Finally, the distribution law of the specific impulse of the bubble jet, the evolution law of the bubble jet shape, and the change law of the head speed of the bubble jet during the near-field underwater explosion bubble pulsation process are obtained through numerical simulation. This work can provide a basis for the destructive strike of surface ship structures using bubble jets formed by a near-field underwater explosion.

Shock wave and bubble pulsation characteristics in a field generated by single underwater detonation

To promote the development and application of underwater detonation propulsion technology, we built a single underwater detonation experimental system and established the corresponding axisymmetric five-equation model to study the characteristics of the flow field generated by a single underwater detonation. The shock wave formed by the degeneration of the detonation wave in the detonation tube interacted with the water–gas interface. Moreover, the jetting of detonated gas was blocked by water, which sharply increased the gas pressure and yielded a transmitted wave entering the water and a reflected wave returning to the tube. The transmitted wave reached a peak pressure of 16.77 MPa at 1280 Hz. The extremely transient gas generated by detonation jetted into the water, forming bubbles with unique pulsation characteristics and completing the first pulsation cycle (28.4 ms) under the effects of the internal gas pressure and the inertia of water. In the contraction stage, the bubble changed into a complex linked annular bubble under the effects of gravity and a free surface. However, in the expansion stage, the bubble was less affected.

Influence of rigid wall on the nonlinear pulsation of nearby bubble

This paper mainly focuses on the nonlinear pulsation of a bubble near the rigid wall. Dynamics of near-wall bubble and free bubble are discussed and compared in details. Investigation reveals as the driving acoustic pressure amplitude increases, nonlinear pulsation of bubble becomes intense gradually. Besides, decreasing the viscosity of host liquid is advantageous for the nonlinear pulsation of bubble. Bifurcation diagrams of bubble radius show acoustic reflection of the rigid wall makes the initial bifurcation appear at low driving acoustic amplitude and on bubble with small ambient radius, and makes the bifurcation still exist for bubble in high-viscosity liquids. That indicates the rigid wall will produce enhancement on the nonlinearity of nearby bubble. As the bubble approaches the wall, the enhancement becomes strong. Moreover, research on the influence of driving frequency shows the rigid wall makes the frequency band corresponding to chaos around the resonant frequency of free bubble shift downward.

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.

Pulsating bubbles dynamics near a concave surface

Bubble dynamics near different boundaries have always been a hot topic in ocean engineering and many other fields. In the present study, we investigate the dynamics of high-pressure pulsating bubbles near a concave surface via experimental and numerical methods. Pulsating cavitation bubbles are generated by the electric discharge technique, and the violent expansion, contraction, and jet formation are observed through high-speed photography. Additionally, a multi-vortex ring model implemented in the boundary integral method is established to simulate the bubble motion near a curved surface. The precise non-spherical bubble shapes during bubble pulsation and detailed information on the jet velocity and wall pressure are obtained. The numerical results are found to be in good agreement with the experimental results, indicating the validity of the numerical model. Finally, a quantitative study is provided on the influence of two parameters: the distance parameter γ=L/Rm (L is the distance between the initial bubble center and the lowest point of the concave surface and Rm is the maximum bubble radius) and the concave curvature ξ=Rm/Rs (Rs is the radius of concave boundary).

Experimental investigation on the steam condensation dynamic characteristics at low steam mass flux

An experimental study has been performed to investigate the dynamic characteristics of steam direct contact condensation at low steam mass flux. Visible steam bubble parameters can be identified through the image processing algorithm. The maximum dimensionless diameter of steam bubble is calculated and the analysis model of identifying condensation regime is developed to predict transition boundaries of steam chugging and oscillatory bubble regimes. The pressure oscillation waveforms in pipe and in pool are distinguished. In chugging regime, the pressure pulse wave transforms to continuous wave in pool as water temperature increasing, however, in oscillatory bubble regime, continuous wave becomes pulse wave because of increased bubble pulsation volume. The oscillation frequencies are obtained from four different methods of bubble dynamic parameters, pressure pulse periods, pressure frequency spectrums in pipe and in pool to reveal the oscillation mechanism. The low-frequency component in chugging regime increases with water temperature increasing, while the intermediate-frequency component in chugging regime and the frequency in bubble regime decreases.

Experimental studies of bubble dynamics under a broken horizontal plate

This paper presents an experimental study on the pulsation characteristics of bubbles under a damaged boundary. Bubble phenomena in the fluid domain, such as bubble shape, pulsation period, water jet, and motion, are observed, and the influence of distance parameters on these phenomena is analyzed in detail with the help of high-speed photography technology. We also discuss the relationship between bubble pulsation, the water spike form, and characteristic parameters under different boundary conditions, such as different hole shapes. It is found that the broken boundary weakens the effect of the wall on bubble motion, while the shape and area of the hole exert noticeable effects on the water spike. In addition, the rising part of the free surface consisting of three parts, the water spike, water skirt I, and water skirt II, along with their evolutionary forms at the gas–liquid interface, are experimentally studied.

Research on characteristics of deep-sea implosion based on Eulerian finite element method

Marine structures operating in the deep-sea, such as submersibles and photomultiplier tubes, bear enormous hydrostatic pressure of deep-sea, which may cause structural damage and implosion. And a pressure wave will emit from the collapse of the cavity and pose a threat to nearby structures. On the basis of the Eulerian finite element method, the numerical model of multiphase flow for deep-sea implosion is established in this paper. The numerical method is verified by comparing the results with those from Keller equation and literature. The physical process under the spherical simplification is firstly analyzed in our work. The basic phenomena are studied, and the pressure changes under different distances and Mach numbers are given. Subsequently, the basic physical process and pressure distribution under cylindrical simplification are introduced. Then, the bubble evolutions under different Mach numbers and initial aspect ratios are given. In the early stage of bubble movement, a rarefaction wave will radiate to the surrounding. The multi-peak phenomenon will occur in the cylindrical model because of the bubble sides breakage and the bubble pulsation. Finally, asymmetric three-dimensional cylindrical bubble implosion near the solid wall with different standoff distances are present in this paper, where the typical oblique jet is observed and analyzed.

The dynamics of cavitation bubbles in a sealed vessel

A model of cavitation bubbles is derived in liquid confined in an elastic sealed vessel driven by ultrasound. In this model, an assumption that the pressure acting on the sealed vessel due to bubble pulsations is proportional to total volume change of bubbles is made. Numerical simulations are carried out for a single bubble and for bubbles. The results show that the pulsation of a single bubble can be suppressed to a large extent in sealed vessel, and that of two matched bubbles with same ambient radius can be further suppressed. However, when two mismatched bubbles have the same ambient radii, an interesting breathing phenomenon takes place, where one bubble pulsates inversely with the other one. Due to this breathing phenomenon the suppression effect becomes weak, so the maximum radii of two mismatched bubbles can be larger than that of a single bubble or that of two matched bubbles in sealed vessel. Besides that, for two mismatched bubbles with different ambient radii, the small one in sealed vessel under some certain parameters can pulsate as strong as or even stronger than that of a single bubble in an open vessel.

Theoretical and experimental study of bubble dynamics in underwater explosions

The development of analytical theory and experimental methods for understanding the correlation between the explosive properties and bubble dynamic characteristics in underwater explosions has important engineering application value for underwater weapons and ships. Based on the assumption of an instantaneous explosive detonation, we introduced the Jones–Wilkins–Lee equation of state to describe the high-pressure state in an explosion bubble and established the initial conditions for the bubble dynamics calculations. Considering the high-Mach-number flow and high pressure at the initial boundary of the explosion bubble, the Lezzi–Prosperetti equation with second-order Mach accuracy was used. Thus, an analytical model and a calculation method of the explosion bubble dynamics for an explosive detonation were established. This direct link between the detonation parameters and the bubble features is significant for the subtle design, selection, and optimization of explosives' properties. A micro-equivalent explosive bubble pulsation experiment was carried out in a water tank using a customized experimental system, which can offer nearly boundary-free condition to mitigate the reflective wave effects on bubbles. Three types of explosives were used in the experiment: the Research Department explosive (RDX), the Pentaerythritol tetranitrate (PETN), and the Hexanitrohexaazaisowurtzitane (CL20). Finally, the experimental results and the practicability of the experimental system were analyzed. The influence of the explosive type on the dynamic characteristics of the explosion bubbles and the differences between the theoretical and experimental results were compared. The results showed that the proposed explosion bubble dynamics model and calculation method have high accuracy and practicability. The proposed model can be used for explosives with known detonation parameters and equation of state parameters. The detonation parameters, velocity, and pressure are linked to the bubble features pulsation period and the maximum radius directly. The designed experimental system, which is capable of simulating an infinite water for the explosion of micro-equivalent explosives, was stable and easy to use. The work is significant for the subtle design, selection, and optimization of explosives' properties.

Dynamics of twin bubbles formed by ultrasonic cavitation in a liquid

Based on potential flow and perturbation theory, a theoretical model is derived to describe the pulsation, translation, and deformation of twin bubbles in an ultrasound field. The amplitudes of radial oscillation, translation, and deformation of twin bubbles are found to depend on initial translation velocities. The radii, translation, and deformation of twin bubbles also exhibit periodic behavior. As the initial translation velocities increase, the periods of two bubbles’ oscillations reduce, and the instable area in the phase space of R01-R02 enlarges because of the interaction between bubbles.

Experimental and theoretical analysis of detonation products state on bubble dynamics and energy distribution in underwater explosion

The initial states and pressure of detonation products in a bubble have a great impact on bubble pulsation in underwater explosions; particularly, the initial accelerated expansion of a bubble can determine the energy distribution. The energy output and distribution of explosives were obtained on the basis of the underwater explosion experiment in this paper. To study the process of bubble pulsation and energy output, we proposed a gas equation of state (EOS) combining the pvk form, Jones–Wilkins–Lee (JWL) EOS, and the initial states of a bubble to take the effects of the initial bubble pressure and detonation products state transformation into account; furthermore, the bubble radius, velocity, and acceleration vs time were obtained through the Rayleigh–Plesset equation under our experimental condition. The differences of bubble behaviors were compared by adopting the JWL EOS and a polytropic EOS with k = 3. The results showed that the initial bubble pressure and detonation products state transformation influence the accelerating expansion and the subsequent bubble oscillation, respectively. Subsequently, comparisons of the energy output and distribution for different gas EOSs showed that the initial shock wave energy for the JWL EOS was underestimated in accelerating expansion, and the bubble energy was overestimated using the polytropic EOS for k = 3; the obtained energy output and distribution had a better agreement with experimental data when adopting the improved gas EOS. In addition, the energy distribution was determined before the detonation products turned to the explosion gas state in initial expansion based on the relationship of the accelerating expansion characteristics and the initial shock wave energy generation. The research has a great significance to reveal the mechanism of bubble pulsation in underwater explosions.

Effect of Computational Parameters on Output Properties of Underwater Explosion by Intelligent Numerical Simulation

The computational parameters are of great influence on underwater explosion load. A one-dimensional wedge model is established to analyze the influence of boundary condition (BC), water domain and mesh density on the numerical simulation results. The results show that flowout BC is rigid boundary and transmit BC is not suitable for simulating the collapses phase of bubble pulsation. According to propagation distance of shock wave and its reflected wave, a simple method to calculate appropriate water domain is proposed. A positive correlation between mesh density (λ) and calculated peak pressure of shock wave (P m) is found. When λ tends to infinity, simulated Pm in near field is quite reliable, but the values in relatively far field are lower than empirical results.

Sagging damage characteristics of hull girder with trapezoidal cross-section subjected to near-field underwater explosion

To investigate the overall damage characteristics and failure modes of a warship subjected to an underwater non-contact near-field explosion, a hull girder with a trapezoidal cross-section was designed, manufactured, and tested. The design criteria and parameters were determined according to the similarity criterion. Dynamic responses of the girder freely floating on water were obtained under varying conditions, including stand-off distance, charge mass, and position of attack. Damage morphologies of the girder model were obtained. Based on our analysis, basic conditions for sagging damage of the hull girder are proposed. The aim of this study was to determine an efficient method of attack resulting in the most severe damage to the ship hull. The experimental results show that the girder mainly exhibits a first-order response when the first wet frequency of the girder is close to the frequency of the explosion bubble pulsation. The largest deformation was observed when the underwater explosion occurred directly below the midspan of the girder compared to other explosions of the same intensity at different attack positions. When the ratio of stand-off to maximum bubble radius (λ) satisfies 0.7 ≤ λ＜2, the bubble mainly causes sagging damage instead of hogging. As λ decreases (1≤ λ＜2), the sagging damage increases under the same charge mass. However, as λ decreases further (0.7≤ λ＜1), the sagging deformation decreases. This is likely due to the impact of the liquid jet formed by the collapsing bubble, which causes the girder deformation to shift from sagging back to hogging deformation. The initial shock wave excites the high-frequency response of the girder structure but contributes very little to the overall velocity and displacement. However, bubble pulsation typically causes a low-frequency response, which will affect the velocity and displacement of the girder. The low-pressure region of the flow field formed by bubble pulsation and resonant coupling between the girder and the bubble are the predominant causes of damage to the overall girder structure.

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.

Interaction of two out-of-phase underwater explosion bubbles

This study presents an experimental investigation of the dynamic properties of underwater explosion (UNDEX) bubble pairs produced with a range of phase differences Δθ, defined as 2π(t1−t2)/Tosc, where ti (i = 1,2) represents the bubble inception moment and Tosc is the experimentally obtained first period of a single UNDEX bubble. Each bubble was generated by a spherical hexogen explosive charge detonated in a cubical tank and observed via high-speed photography. The phase difference was adjusted by setting different delays between the two detonations, with an accuracy of 1.0 ms. Experiments were conducted with both horizontally and vertically positioned bubble pairs and with single bubbles as well. UNDEX bubble pairs are subject to a larger buoyancy effect than cavitation or spark-generated bubble pairs. The resultant bubble behavior in the bubble–bubble interaction is more complex and is yet to be understood. In our experiments, various bubble parameters, including bubble pulsation periods, bubble elongation ratios, and collapse-induced shock wave pressures bubble, were measured and studied. Dependence of the bubble dynamics on Δθ was found, demonstrating the significant influence of Δθ on the morphology and shock wave pressure of bubble pairs. The findings suggest a method of strengthening or weakening the damage potential of an UNDEX bubble pair based on the proper adjustment of the delay between two detonations. It may also lead to a better understanding of the dynamics of interacting bubbles with buoyancy effects.

An underwater discharge shockwave separation method based on minimum-phase cepstrum

Underwater discharge sound sources are widely utilized. The acoustic signals generated by underwater discharge consist of shock waves and bubble pulsations. The shock wave is a major component of underwater discharge sound sources and is worthy of further study. The minimum-phase cepstrum method is selected to separate the shock wave, and a customized experiment is designed to determine the optimal parameters of this method. The mixed shock wave bubble signal and single shock wave signal are simulated and fitted and are transmitted and received in the anechoic pool, respectively. Different parameters are used to process the signals, and the optimal parameters are selected by comparing the separation results. To verify the validity of the method, the shock waves generated with the same electrode and circuit parameters in the anechoic pool and non-anechoic pool are separated and compared. Experimental results show that this shock wave separation method exhibits significant performance. This method of shock wave separation is of service to the research of underwater discharge acoustic sources.

Effect of nonlinear translations on the pulsation of cavitation bubbles

The pulsations and translations of cavitation bubbles obey combined ordinary differential equations, and their nonlinearities are studied by the bifurcation diagram and the phase diagram in a strong ultrasonic field. Bubble pulsation can change regularly or irregularly with changing driving pressure in the time domain. The bifurcation diagrams of the pulsation versus driving pressure show that the pulsations and translations of bubbles have nonlinear characteristics, and the nonlinear translations of bubbles can disorder the pulsations for certain parameters. Disorder of the pulsation can also be caused by nonlinear pulsation itself. In addition, the phase diagrams also show that the nonlinear translations make a large contribution to the pulsations. The same result can also be obtained when the ambient radii of two bubbles are different.

Response of seated human subject to ship structure motion caused by the combined effect of shock wave and bubble pulsation

Underwater explosion (UNDEX) can cause severe damage to hull structure, equipment and human. In this paper, the effect of UNDEX load, including shock wave and bubble pulsation, on seated human response was investigated. The incident pressure of non-contact UNDEX was calculated. A lumped parameter interaction model of the ship structure (single-deck and multi-deck) and seated human was created based on the Taylor’s theory and its veracity was verified. The results indicated that the pelvis, which is in direct contact with the structure, is the most vulnerable part of seated human when suffered impact. The shock wave and bubble pulsation had equal destructive potential to upper torso, viscera and head. The low pass filtering feature of multi-deck configuration may magnify the human response caused by the upper deck motion. The energy carried by the low and high frequency component was the dominant factors to human injury and the broadband protection to human shock isolation design is essential.