Explosive Bubbles Research Articles

A micro-synthetic jet in a microchannel using bubble growth and collapse

A concept to power micro synthetic jets using bubble growth and collapse is introduced and studied over a range of operating frequencies (0.1–2.5 Hz) and heating powers (3–4.5 W). The microfluidic device has no moving parts and uses the interfacial layer between the vapor and liquid phases which substitutes the requirement for a physical flexible membrane. A micro heater inside a chamber (3.5 mm in radius and a height of 220 µm) was connected to a main microchannel through a nozzle with a 300 µm opening. Periodically powering the micro-heater triggered bubble explosion and implosion in the chamber, which in turn generated the synthetic jet. Sequential images of bubble nucleation, growth and collapse were captured using high-speed camera photography and a microscope. A momentum coefficient was used to characterize the synthetic jet. It was found that its average value exceeds unity for a large range of operating frequencies suggesting that this synthetic jet can improve the performance of a range of micro-system performance, such as micro-mixing in microfluidic devices.

Ultrasound-assisted bleaching: Mathematical and 3D computational fluid dynamics simulation of ultrasound parameters on microbubble formation and cavitation structures

Abstract The implementation of ultrasound-assisted bleaching (UAB) is gaining interest in commercial oil refining, as an alternative to the current bleaching procedure (BP), because this technique has several advantages compared to the conventional BP. The mathematical and 3D computational fluid dynamics (CFD) simulation of the influence of ultrasound power, frequency and temperature for predicting microbubble formation and heat transfer in the ultrasonic treatment of soybean oil was assessed in order to optimize the UAB process. The fluctuation frequency and wave speed showed opposite effects on system response to the imposed pressure pulse. As the bubble radius shrinks to zero (after bubble explosion), its internal pressure increases, showing a maximum pressure of 136, 180 and 188 kPa for fluid temperatures of 35, 50 and 65 °C, respectively. On the other hand, as the pressure applied was increased from 1.5 to 2.5 bar, the collapse radius increased more than 10 times for f = 25 kHz. In addition to the magnitude of the applied pressure, the oscillation frequency and temperature had a minor effect on the collapse radius. In general, the temperature showed a direct influence on bubble growth for an oscillation frequency of 55 kHz, although it was also observed that the bubble collapse radius tended to maximize at mid-range temperatures (around 50 °C). Finally, increasing the magnitude of cavitation pressure resulted in the enhancement of the collapse temperature.

A Bubble-Dragged Catalytic Polymer Microrocket.

We report the bubble dragged microrocket consisting of functionalized multilayer polymer covered asymmetrically by platinum nanoparticles. The microrocket is pushed back during bubble growth over a small step and dragged forward over a big step during bubble explosion. Each bubble explosion induced a shock wave of gas which propagates in water at ultrafast speed. The bubble dragged microrocket can move along an approximate straight line instead of a fluctuating circle which is the trajectory of a bubble-pushed microrocket in most cases, which makes it a promising candidate for drug delivery and simulating rod-shaped bacteria.

SIGN-BASED UNIT ROOT TESTS FOR EXPLOSIVE FINANCIAL BUBBLES IN THE PRESENCE OF DETERMINISTICALLY TIME-VARYING VOLATILITY

This article considers the problem of testing for an explosive bubble in financial data in the presence of time-varying volatility. We propose a sign-based variant of the Phillips, Shi, and Yu (2015, International Economic Review 56, 1043–1077) test. Unlike the original test, the sign-based test does not require bootstrap-type methods to control size in the presence of time-varying volatility. Under a locally explosive alternative, the sign-based test delivers higher power than the original test for many time-varying volatility and bubble specifications. However, since the original test can still outperform the sign-based one for some specifications, we also propose a union of rejections procedure that combines the original and sign-based tests, employing a wild bootstrap to control size. This is shown to capture most of the power available from the better performing of the two tests. We also show how a sign-based statistic can be used to date the bubble start and end points. An empirical illustration using Bitcoin price data is provided.

Numerical Study on the Explosive Effect of RDX-based Aluminized Explosives on Stiffened Plates

The finite element software LS-DYNA was used to numerically investigate the damage of stiffened plates due to underwater explosive loads provided by RDX-based aluminized explosives. A finite element analysis model of fluid-solid coupling was established in order to study the elastoplastic dynamic buckling behavior of a stiffened plate structure. The effects of the RDX-based aluminized explosive energy release rate, shockwave energy, bubble energy and shockwave pressure on damage are discussed. A connection is established between aluminum content, the energy output of the underwater explosion and the response of the stiffened plate.

An Estimation of Explosive Bubbles in House Prices

본 연구는 주택가격에 있어 내재가치(intrinsic value)와 시장가격 간의 괴리로 정의되는 거품항을 정보착오모형(information error model)을 이용하여 측정하고 이 거품항에 대한 생존 분석(survival analysis)을 통해 폭발적 거품의 존재유무를 분석한다. 정보착오모형의 종속변수는 주택매매가격지수이고 설명변수로는 주택전세가격지수 또는 매매전세비율을 사용한다. 생존 분석은 거품의 생존기간이 시장기초요소와 무관함을 가정하는 외생적 거품모형과 시장기초요소에 의해 거품의 생존기간이 영향을 받는 것을 가정하는 내생적 거품모형을 대상으로 수행한다. 분석대상지역은 전국, 6대광역시, 서울강남, 서울강북의 4개 지역에 대해, 1987년 1월∼2017년 12월의 기간 및 6개 하부기간에 대해 분석을 수행하였다. 분석결과, 외생적 거품모형에서는 어떠한 시간적 범위 또는 공간적 범위에서도 폭발적 거품의 존재를 찾을 수 없었다. 내생적 거품 모형에서는 형태모수 α값이 대부분 1보다 크지만 2보다는 작은 것으로 나타났다. 외생적 거품 모형과 내생적 거품모형 모두에서 폭발적 거품의 존재를 확인할 수 없었다. 이는 주택가격에 안정적 거품은 존재하지만 폭발적 거품은 존재하지 않으며 내재가치 간의 괴리는 시간의 흐름에 따라 소멸됨을 시사한다.This study examines the existence of explosive bubbles in house prices. Even if there is a bubble in the house prices, as long as it is not explosive, we can not conclude that there exists a bubble only based on the fact that house prices soar or are too high because house prices can be restored to a proper level after a certain period of adjustment. Therefore, using Information Error model, we measure the bubble term, which is defined as the gap between the intrinsic value explained by the market fundamentals and the market price, and then check whether there is an explosive bubble by Survival analysis. Housing Sales Price Index as a dependent variable, and explanatory variables such as not only Jeonse Price Index but also Sales-Jeonse Prices ratio, which reflect properly the relation between sales and rental prices, are included for Information Error model. Survival analysis is performed by an exogenous as well as an endogenous bubble model. Survival times of bubbles are assumed to be independent of market fundamentals in exogenous bubbles model while dependent in endogenous bubbles model. Samples are the period from January 1987 to December 2017 and six sub sample periods. Regional ranges are for the four regions of the whole country, the six metropolitan cities, Seoul Gangnam, and Seoul Gangbuk. For the exogenous bubbles model, Not in the whole sample nor any sub-samples, the existence of explosive bubbles was found. For the endogenous bubbles model in which ‘interest rate’ are used as market fundamental variables, shape parameter α is estimated to be 1<α <2. Values of the shape parameter, α, with larger than 2 are not found in both of exogenous and endogenous bubbles model of Survival analysis, so we can not find any evidence supporting existence of explosive bubbles. It implies that there are no explosive but stable bubbles in any sub-sample, and the bubbles become extinct over time.

Transient response of stiffened composite submersible hull to underwater shock and bubble

This paper deals with the transient response of a glass-epoxy composite submersible hull subjected to underwater explosive shock and bubble. The physical process of the underwater explosive shock and the subsequent bubble evolution in the fluid is simulated by using the Multi-Material Arbitrary Lagrange Euler (ALE) formulation. The interaction of the composite submersible hull with the complex fluid is handled by the penalty contact algorithm. With the present approach, the investigation of the composite submersible hull subjected to underwater explosion can be accomplished, which accommodates both the shock and bubble phenomenon with the different time scales as it occurs in the actual underwater explosion event; and furthermore the transient responses (such as strains and internal energy densities) of the composite submersible hull to the underwater explosive shock and bubble can be predicted for different charge weights and charge distances. In addition, a comparison between the transient responses of the composite submersible hull and a steel submersible hull for the underwater shock and the subsequent bubble is made.

Explosive Leidenfrost droplets

Experiments show that Leidenfrost droplets made of water and surfactant undergo a violent explosion. This unexpected behavior is triggered by the formation of a shell during the evaporation. Shortly afterwards, the temperature increases above the boiling point, leading to bubble growth, shell stretching, and explosion.

Testing explosive bubbles with time-varying volatility

This article considers the problem of testing for an explosive bubble in financial data in the presence of time-varying volatility. We propose a weighted least squares-based variant of the Phillips et al.) test for explosive autoregressive behavior. We find that such an approach has appealing asymptotic power properties, with the potential to deliver substantially greater power than the established OLS-based approach for many volatility and bubble settings. Given that the OLS-based test can outperform the weighted least squares-based test for other volatility and bubble specifications, we also suggest a union of rejections procedure that succeeds in capturing the better power available from the two constituent tests for a given alternative. Our approach involves a nonparametric kernel-based volatility function estimator for computation of the weighted least squares-based statistic, together with the use of a wild bootstrap procedure applied jointly to both individual tests, delivering a powerful testing procedure that is asymptotically size-robust to a wide range of time-varying volatility specifications.

When will food price bubbles burst? A review

This paper examines whether there are multiple explosive bubble episodes in international food market by employing the Generalised Supremum Augmented Dickey-Fuller (GSADF) test. This method is particularly suitable for practical application of time series and provides an innovative consistent date-stamping strategy for the origination and termination of bubble episodes. Our results show that there are four explosive bubble episodes mostly accompanied by huge price volatilities during 1990–2017, which is largely in line with the asset pricing model (Gürkaynak 2008). The exuberance and collapse of bubble episodes can be explained by imbalance between supply and demand, depreciation of U.S. dollar, financial crisis and speculation. Our findings also provide supporting evidence for the Masters hypothesis that tremendous buying pressure from index investments contributes to substantial bubble episodes. The authorities should accurately identify bubble episode and monitor its evolving process, which is propitious to achieve the effective stabilisation of global food system. Particularly, restrictions on excessive speculative trading should be arranged under extreme market situations in order to forestall the explosion of multiple food bubbles.

Dynamics of expansion and collapse of explosive two-dimensional bubbles

An explosive gas mixture of hydrogen and oxygen is introduced in liquid water between two horizontal walls, forming a flat cylindrical bubble. Ignition and explosion of the bubble lead to a large depressurized cavity that finally implodes. We investigate the dynamics of the bubble collapse, which is qualitatively similar to the collapse of a spherical bubble. It exhibits a slightly weaker singularity than for spherical bubbles. We also analyse the explosion process. Starting with an initial radius $R_{0}$, the bubble reaches a maximal radius $R_{max}$ that depends on the gap thickness $h$ between the two walls: for a thinner gap, the condensation of water vapour is more efficient, the overpressure consecutive to the combustion is weaker, and its duration is shorter. This leads to a smaller maximal radius $R_{max}$. An indirect measurement of the transport coefficient of hot water vapour can be inferred from this observation.

GW29-e0867 Demonstration of Cavitation in the Coronary Arteries by Computational Fluid Dynamic

Coronary injuries are suspected to be caused by explosion of air bubbles. By hydraulics principle, when connecting pipes to pumps, the inlet should be larger than the outlet and the speed should be slow. If not, air bubbles would form causing major damage. During diastole with distal negative

GW29-e0862 NEW DISCOVERY: Gas Bubbles Rupture in the Coronary Arteries is Most Likely the Mechanism Breaking the Thin Cap of Plaques Causing Acute Coronary Syndrome

Coronary injuries are suspected to be caused by explosion of air bubbles. By hydraulics principle, when connecting pipes to pumps, the inlet should be larger than the outlet and the speed should be slow. If not, air bubbles would form causing major damage. During diastole with distal negative

Is hub-based pricing a better choice than oil indexation for natural gas? Evidence from a multiple bubble test

Oil indexation and hub-based pricing are two competing pricing mechanisms in the international natural gas markets. The debates over whether hub-based pricing is preferable to oil indexation have become intense among academics and practitioners, for example, whether and when East Asia should adopt hub pricing. This paper contributes empirically to the debate using a multiple bubble test. Adopting the generalized sup augmented Dickey-Fuller test proposed by Phillips et al. (2015), we show that more explosive bubbles exist in Japan and European gas prices than in the US prices. The argument is that hub-based pricing mechanism can better reflect fundamental values in the gas markets and thus is less subject to speculations. Given the recent trend of financialization in energy markets, gas prices are more likely to deviate from fundamental values when they are not clear to investors. Although oil indexation is simple and has been an effective tool over the past few decades, our results suggest that hub pricing is associated with less extreme price movements in the market and thus is a better choice for both policy makers and practitioners.

TCT-307 Mechanism Triggering Acute Coronary Syndrome by Rupturing Plaque Based on the Cavitation Phenomenon (Bubble Formation and Explosion) at the Distal Left Main Bifurcation by Computational Fluid Dynamic Study

As in hydraulics practice, coronary injuries are suspected to be caused by explosion of air bubbles, Cavitation Phenomenon. During diastole with distal negative suctioning, if the coronary dynamic pressure decreases below the vapor pressure (VP) most likely of nitrogen in the blood, bubbles will

Shock wave generation by collapse of an explosive bubble in water

Shock wave generation by collapse of a single explosive bubble exposed to an underwater shock wave in water has been studied experimentally and numerically. The explosive bubble was made of a stoichiometric ethylene-oxygen mixture and its initial volume-equivalent radius was varied from 1.0 to 2.9 mm. The underwater shock wave, whose strength ranged from 7 to 19 MPa, was driven by gasous detonation in a vertical detonation tube. It was found that after passage of the underwater shock wave, the bubble starts to shrink, resulting in explosion just before reaching the minimum radius. In the bubble rebound phase a shock wave is generated whose strength is about twice as compared to that for the air bubble. The maximum pressure of the shock wave nondimensionalized by the ambient pressure behind the underwater shock wave was found to be inversely proportional to the non-dimensional distance. The conversion rate of the bubble energy to the shock energy was estimated to be 0.10, regardless of chemical heat release. Simple numerical analysis of bubble dynamics including compressibility of water was performed. The calculated bubble motion and the shock propagation agree with the experimental results, although the calculated shock strength is overestimated mainly because of non-sphericity of the initial shape of the bubble floating up in water. There is also a good agreement between the experimental and the calculated results in regard to the ratio of the shock pressure for the explosive bubble to that for the air bubble.

A study of bubble collapse pressure pulse waves from small scale underwater explosions near the water surface

Explosive detonators have been used to study the effect of proximity of the water surface on the pressure wave generated by the collapse of an underwater explosive bubble. Transducers at four different depths recorded the pressure pulse, and a low resolution high speed video camera was used to observe the overall dynamics of the bubble. The detonators were fired at selected depths (z) within the range of 6.0–161.3 cm, corresponding to dimensionless depth (γ) values of approximately 0.3–9.0 where γ = z/Rmax, and Rmax is the maximum radius of the bubble during its first pulsation. Analysis of the recorded pressure waves was complicated by the superposition of the emitted wave with its reflection from the water surface. Despite this difficulty there was clear evidence of a dependency of the peak pressure (Pmax) and impulse (I) of the emitted wave on γ. The pressure pulse generated by the deeper events (γ > 3.4) did not appear to be strongly influenced by the water surface, exhibiting only a weak dependence on γ. However, Pmax and I decreased markedly for shallower detonations in the range γ ≈ 1.6–2.8. Imaging results showed that this decrease was associated with migration of the bubble away from the surface during its collapse. A markedly different behaviour was observed when γ was decreased further to 1.1. It was found that although the bubble was still repelled from the surface and I continued to decrease, Pmax increased dramatically. Details of the pulse and imaging results suggested a more complex bubble collapse behaviour at this depth. For shallower values of γ (<0.6) the bubble did not collapse and appeared to breach the water surface and ingest additional gas from the atmosphere.

Real‐Time Monitoring for Explosive Financial Bubbles

We propose new methods for the real‐time detection of explosive bubbles in financial time series. Most extant methods are constructed for a fixed sample of data and, as such, are appropriate only when applied as one‐shot tests. Sequential application of these tests, declaring the presence of a bubble as soon as one of these statistics exceeds the one‐shot critical value, would yield a detection procedure with an unknown false‐positive rate likely to be far in excess of the nominal level. Our approach sequentially applies the one‐shot tests of Astill et al. (), comparing sub‐sample statistics calculated in real time during the monitoring period with the corresponding sub‐sample statistics obtained from a prior training period. We propose two procedures: one based on comparing the real‐time monitoring period statistics with the maximum statistic over the training period, and another that compares the number of consecutive exceedances of a threshold value in the monitoring and training periods, the threshold value obtained from the training period. Both allow the practitioner to determine the false‐positive rate for any given monitoring horizon, or to ensure that this rate does not exceed a specified level by setting a maximum monitoring horizon. Monte Carlo simulations suggest that the finite‐sample false‐positive rates lie close to their theoretical counterparts, even in the presence of time‐varying volatility and serial correlation in the shocks. The procedures are shown to perform well in the presence of a bubble in the monitoring period, offering the possibility of rapid detection of an emerging bubble in a real‐time setting. An empirical application to monthly stock market index data is considered.

Giant and explosive plasmonic bubbles by delayed nucleation

When illuminated by a laser, plasmonic nanoparticles immersed in water can very quickly and strongly heat up, leading to the nucleation of so-called plasmonic vapor bubbles. While the long-time behavior of such bubbles has been well-studied, here, using ultrahigh-speed imaging, we reveal the nucleation and early life phase of these bubbles. After some delay time from the beginning of the illumination, a giant bubble explosively grows, and collapses again within 200 μs (bubble life phase 1). The maximal bubble volume [Formula: see text] remarkably increases with decreasing laser power, leading to less total dumped energy E. This dumped energy shows a universal linear scaling relation with [Formula: see text], irrespective of the gas concentration of the surrounding water. This finding supports that the initial giant bubble is a pure vapor bubble. In contrast, the delay time does depend on the gas concentration of the water, as gas pockets in the water facilitate an earlier vapor bubble nucleation, which leads to smaller delay times and lower bubble nucleation temperatures. After the collapse of the initial giant bubbles, first, much smaller oscillating bubbles form out of the remaining gas nuclei (bubble life phase 2). Subsequently, the known vaporization dominated growth phase takes over, and the bubble stabilizes (life phase 3). In the final life phase 4, the bubble slowly grows by gas expelling due to heating of the surrounding. Our findings on the explosive growth and collapse during the early life phase of a plasmonic vapor bubble have strong bearings on possible applications of such bubbles.

Study on immersion waterjet assisted laser micromachining process

The heat affected zone (HAZ), recast layer, and taper angle are the issues remained to be addressed in laser micromachining. In immersion waterjet assisted laser micromachining (IWALM) the waterjet flushes the ablated materials, plasma, and micro bubbles away from the machining area at laser pulse interval. A higher machining resolution could be obtained by reducing the laser beam through focusing system. This paper studied the mechanisms of IWALM and the effects of microbubbles explosion on the workpiece surface roughness around the entrance of machining area. With the developed experimental setup, the influences of the laser power, waterjet flow speed, and workpiece-nozzle gap were studied on the machining efficiency, taper angle, while machining microhole with diameter 0.5 mm by scanning the laser beam within waterjet diameter. In particular experimental conditions, the machining efficiency of IWALM was larger than that of laser micromachining in the air, while the workpiece thickness was smaller than 0.5 mm. Microholes with reduced HAZ and free of recast layer were fabricated by IWALM. The taper angle of the microholes processed by IWALM was 376% smaller than that by Laser micromachining in the air. Furthermore, the mechanisms of the formation of micro-pits during IWALM around the machining area were discussed, considering the explosions of micro bubbles near the workpiece surface due to the pressure difference. Experimental results indicated that a higher waterjet flow speed and smaller nozzle-workpiece gap, laser power could improve the surface quality in close to the machining area.