Explosive Bubbles Research Articles

Analyzing asset of bubbles in the housing market with right-tailed unit root tests: the case of Turkey

Purpose- Housing markets are linked to macroeconomic and financial stability. The creation of different financial instruments on residence, the presentation of residence as an assurance, the impact of residence prices on saving and consumption through the wealth effect are some of the housing market and general economic interactions. Developments in the residential sector must be carefully monitored for financial stability. The residential sector has usually played an important role in the global economy and financial sector bubbles. It has been observed that the wealth effect created by the housing bubble surpasses the effect of the stock bubble and that the housing bubble explosion causes more economic devastation compared to the other assets bubbles. While housing markets are increasingly buried in financial markets, the connections between them have strengthened. Such a situation can cause financial crises as a way of extinguishment of a housing bubble. In the case of housing bubbles, bank balance sheets are generally more affected by real estate and it is more likely that the decline in real estate prices are transmitted to the other sectors of the economy with the credit channel. When the housing price bubble burst, the collapse in the financial system is also reflected in the real sector. In this study, with reference to the real housing price index and the real rent price index, it has been investigated whether there is the housing bubble in Turkey. Methodology- Sup ADF and Generalized SADF (GSADF) tests were used to determine the asset of bubbles and to determine when the housing bubbles had occured. The data set obtained in the study extends from January 2010 to November 2017. Findings- The results reveal empirical evidence on the absence of speculative bubbles in the Turkish housing market. Conclusion- There is no data with reference to housing price to say that there is the housing bubble in Turkey during the period under study. Turkey blocked the creation of the housing bubble in the period under review with corrections through its internal dynamics.

Infrasound explosion and coda signal investigated with joint analysis of video at Mount Erebus, Antarctica

Mount Erebus is a natural laboratory where lava lake strombolian activity can be directly observed from the crater rim, allowing for a unique opportunity to study infrasound generated during explosive bubble bursting. Erebus lava lake eruptions occur when pressurized bubble slugs distend the lava lake surface and then burst within a few tenths of seconds. Data from two infrasound stations and one video camera were used to study 13 explosions occurring in January of 2006. Infrasound recordings from the two stations at ~330 m from the source exhibit variable amplitudes (40–160 Pa), but similar shapes composed of high-amplitude ~1 s bipolar pulses followed by codas exhibiting 3 or 4 decaying oscillations and lasting ~3 s. Infrasound pulses and infrasound-inferred source volumes are compared to video imagery, which is used to quantify the expanding volumes of distending bubbles and subsequent erupted pyroclasts. Video-inferred volumes scale well with infrasound-inferred volumes during the first few tenths of a second of an eruption, assuming a monopole source. Afterwards, an inflection in infrasound waveform, corresponding to a more rapid increase in the pressure record, indicates the fragmentation of the bubble membrane. At this point, infrasound is still produced by expanding gases and pyroclasts, but infrasound-inferred volumes are smaller than the image processing suggests. This is due to the fact that the image processing tracks ballistics, which expand inertially and without exerting a significant volume change. After 0.5 to 1.5 s the volumetric expansion is no longer evident in the infrasound record, which is now comprised of a coda characterized by decaying ~1 Hz oscillations. Because the coda does not correspond to any motions evident in the video, we conclude that it represents an acoustic resonance of the void left in the lava lake. This resonance is modulated by echoes off crater topography giving rise to two peak frequencies that dominate the coda spectra and are different for the two infrasound recording sites. The two coda spectral peaks are well explained by site-dependent propagation effects. Deconvolving crater topography effects, which are different for the two stations, reveals a single frequency peak at 1.5 Hz and is attributed to resonance of the void left in the lava lake, which we model as an open cylindrical pipe or Helmholtz resonance.

A comparative study of flow boiling HFE-7100 in silicon nanowire and plainwall microchannels

Extensive experimental investigations along with high speed visualizations have been performed to assess the flow boiling characteristics in Silicon Nanowire (SiNW) microchannels. Experiments have been also performed in Plainwall microchannels to compare their performances with SiNW configurations. HFE-7100 has been used as the working fluid and experiments are conducted in a forced convection loop at mass flux range of 400–1600 kg/m2s. Arrays of microchannel consist 5 (five) parallel straight microchannels with Width, Depth and Length dimension of 220 μm, 250 μm and 10 mm respectively. Flow boiling performances including heat transfer coefficient (HTC), pressure drop, two-phase flow instabilities and critical heat flux (CHF) have been studied in both the Silicon plainwall (smooth inner surface) and Silicon Nanowire (silicon nanostructured inner surface) microchannels. High speed flow visualizations have been performed at up to 70,000 frames per s (fps) to understand the difference in boiling mechanisms between Plainwall and SiNW. SiNW performs significantly enhanced HTC (up to 400% improvement), reduces flow boiling instabilities and pressure drop (up to 70% reduction) compared to Plainwall microchannels. However, little/insignificant effect of nanostructured surface has been observed on CHF. In addition, a major difference in two-phase flow regime development has been observed between the SiNW and Plainwall microchannels during flow visualization. Specifically, SiNWs introduce explosive bubble nucleation, reduce intermittent flow regimes (slug/churn), improve rewetting, maintain thin liquid film and thus, improve system performances.

Quantitative study on the influence of bubble explosion on evaporation characteristics of flash boiling spray using UV-LAS technique

Flash boiling is an effective way to enhance both atomization and evaporation characteristics of liquid fuel spray by introducing explosion of vapor bubbles, compared with the conventional high pressure injection. However, the evaporation characteristics and mechanism of flash boiling spray, especially, that associated with the explosion of vapor bubbles at high superheat degree, is still unknown. In this study, the vapor and liquid mass distributions in flash boiling sprays were quantified by use of ultraviolet/visible laser absorption/scattering (UV-LAS) imaging technique over a broad range of superheat degree. Based on the results of macro-structure characteristics of liquid/vapor phase spray and total evaporated vapor mass, influence of bubble explosion on detailed evaporation characteristics of flash boiling spray are investigated. And, by associating with our previous work on breakup mechanism of superheated droplet and liquid jet, the effect of two evaporation regimes, namely surface evaporation and flashing evaporation, on evaporation of flash boiling spray has been successfully identified for the first time. These results provide insightful information for understanding the evaporation mechanism of a superheated liquid jet and modeling a flash boiling spray.

Does Bitcoin bubble burst?

This paper explores when will occur and collapse in Bitcoin bubbles by applying generalized sup augmented Dickey–Fuller test method proposed by Phillips et al. (Testing for multiple bubbles: historical episodes of exuberance and collapse in the S&P 500. Singapore Management University, Working Paper, No. 04-2013, 2013). The results show that there are six explosive bubbles in China and five bubbles in U.S. market, mostly occur in the period of huge surges in Bitcoin price. This is consistent with the bubble model originated by Blanchard and Watson (Bubbles, rational expectations and financial markets. NBER Working Paper, No. 945 1982) that certain asset price is decomposed into fundamental and the bubble components. In particular, exogenous shocks, including foreign or domestic economic events lead to the origination of bubbles. Serious financial crisis may trigger long-term and large- scale bubbles, while relative not persistence (short-term) bubbles are caused by domestic particular components. It can be inferred that Bitcoin can be used as a hedge against market specific risk. Finally, Bitcoin bubbles would collapse due to the administrative intervention by economic authorities. Thereby, government should lead public expectation to keep the confidence to authority and reduce the speculation behavior to stabilize the asset price and financial market.

Fabrication, Visualization and Analysis of Fluorescein Sodium Encapsulated PLGA@CS Nanoparticles as Model for Photothermomechanical Drug Delivery Using Pulsed 532 nm Laser

PLGA/CS nanoparticles containing fluorescein sodium as drug model were synthesized and characterized to investigate the feasibility of laser-induced drug delivery using pulse 532 nm. The main objective was to investigate the photothermally-induced mechanical force for transporting the nanoparticles. An argon laser was used to excite the fluorescence of the samples after irradiation. The preliminary results indicated that the drug nanoparticles encapsulated trapped by the cavitation bubbles can be transported by photothermomechanical effect. Different regions of interactions are defined and while in our case, the thermoelastic does not apply due to higher fluences, vaporization and laser-induced thermal breakdown (LITB) including the plasma formation and shock waves played an important and major role. Threshold fluences of 2.8, 18 and 102 Jcm-2 corresponding to 0.28, 1.8 and 10 GWcm-2 and 3.8, 30, and 171 MPa are determined for ablation, vaporization and LITB mechanisms respectively. The secondary microbubbles due to explosion of the primary transient cavitation bubbles played a key role in delivery process. Despite the dominant argon laser brightness, the laser-induced fluorescence spectroscopy (LIFS) demonstrated the fluorescence emission of the cavitation bubbles carrying due to the drug nanoparticles entrapped within the biogelatin after exposure to laser radiation, the irradiation, which confirms the possibility of transport of drug nanoparticles by laser cavitation. Finally, it is suggested that the nature of such photothermal and photo non-thermal mechanical effects is governed and influenced by determining and criticizing in terms of the type of nanomaterial as well as their synthesis process engineering and fabrication as they can be made case sensitive by selecting different types of materials for a specific application.

Behavior of an Explosive Bubble behind an Underwater Shock Wave

Behavior of an Explosive Bubble behind an Underwater Shock Wave

Collision dynamics of a single water droplet impinging on a high-temperature pool of oil

The article presents the dynamic process of a single water droplet impinging on a hot oil surface with various temperatures ranging from 205 to 260 $$^\circ \hbox {C}$$ . Distilled water is used to produce water droplets with different diameters. The impact behavior is recorded by using a high-speed digital camera with the speed of 2000 fps. The result shows that two typical phenomena, including crater–jet–bubble and vapor explosion, can be observed. The vapor explosion occurs when the oil temperature is higher than 210 $$^\circ \hbox {C}$$ . The oil temperature, the droplet size, and the Weber number are found to have significant influence on the vapor explosion time. The higher the oil pool temperature is, the earlier the vapor explosion occurs. Vapor explosion time increases with the droplet size, while decreases as the droplet Weber number increases. Moreover, the maximum heat absorption for a single water droplet immersing into the hot oil is calculated considering the changes of the droplet size. Both dimensionless maximum crater depth and maximum jet height increase with the pool temperature due to the surface tension, viscous force and decreasing density of the hot oil.

Microscopic investigation of near-field spray characteristics of 2-methylfuran, ethanol and isooctane under flash boiling conditions

Atomization of fuel sprays is a key factor in the control of combustion quality in direct-injection engines. In the present work, the near-field spray patterns of 2-methylfuran (MF), ethanol (ETH) and isooctane (ISO) under non-flash boiling and flash boiling conditions were investigated using an ultra-highspeed imaging technique. Fuel was injected from a single-hole solenoid injector into an optically accessible constant volume chamber at the injection pressure of 40 MPa. Various conditions were tested, ranging from non-flash boiling conditions (ambient) to flare-flash boiling conditions with fuel temperatures of 20 °C and 80 °C and different back pressures. High-speed imaging was performed using a long-distance microscope coupled with an ultra-highspeed camera (1 million fps). Results showed that under flash boiling conditions, near-nozzle spray patterns changed significantly and clear radial expansion was observed due to bubble formation and explosion. Among the three fuels, MF showed the most intense flash boiling behavior due to it having the highest vapor pressure. The effects of different non-dimensional numbers were also considered and it was found that saturation ratio and cavitation number were the two main governing factors for the near-nozzle spray behaviors. During the end of the injection process, the low effective pressure led to poorly atomized spray with a compact liquid column and large ligaments; this could result in poor air/fuel mixing and thus higher HC and particle emissions. Significant improvements were observed at Rs = 0.2 where flash boiling greatly promoted the spray atomization, even with low fuel velocity.

(Pro?)-Cyclicality of Collateral Haircuts and Systemic Illiquidity

Pro-cyclicality of collateral haircuts and margins has become a widely proclaimed behavior and is currently discussed not only by academic literature but also by regulatory authorities in Europe. Pro-cyclicality of haircuts is assumed to be a trigger of liquidity spirals due to its tightening effect of collateral portfolio values in times of market distress. However, empirical evidence on this topic is quite sparse and the discussions are primarily driven by insights derived from theoretical models. Nonetheless, oversight bodies are discussing macroprudential haircut add-ons in order to curb with the potential effects of pro-cyclicality in distressed periods. Based on a unique data set provided by a large European Central Counterparty we construct a measure of systemic illiquidity of bond collaterals and analyze the relationship between haircuts, the development of periods with explosive behavior and systemic illiquidity. We estimate the noise of bond yields to measure systemic illiquidity with and without considering haircuts. We then apply an explosive roots bubble detection technique to identify irrational periods of each of these two time series and to a combination of both. Finally, we propose a quantitative trigger and design for macroprudential haircut add-ons. Our results confirm that (1) bond collateral markets face irrational, i.e. bubble-like illiquidity during periods of systemic distress. The results indicate that (2) haircuts are not amplifying or increasing with systemic illiquidity. (3) The proposed haircut add-on mechanism exhibits desirable features to mitigate systemic illiquidity during lasting periods of distress.

Testing for explosive bubbles in the South African-US exchange rate using the sequential ADF procedures

This paper tests for the existence of speculative bubbles in the South African-US exchange rate using the sequential ADF procedures. In particular, the paper uses the SADF and GSADF right-tailed unit root tests to explore the existence of explosive bubbles in the South African-US exchange rate for the time period running from January1980 through July 2012. The results provide evidence in support of the existence of explosive bubbles in the nominal rand-dollar exchange rate, the real exchange rate of traded and non-traded goods. The explosive behavior exhibited by the South African rand-US dollar exchange rate can be interpreted as evidence of rational bubbles given that this behavior is driven by the fundamentals including relative prices of traded and non-traded goods.

Predicting Tissue Susceptibility to Mechanical Cavitation Damage in Therapeutic Ultrasound

Histotripsy is a developing focused ultrasound procedure that uses cavitation bubbles to mechanically homogenize soft tissue. To better understand the mechanics of tissue damage, a numerical model of single-bubble dynamics was used to calculate stress, strain and strain rate fields produced by a cavitation bubble exposed to a tensile histotripsy pulse. The explosive bubble growth and its subsequent collapse were found to depend on the properties of the surrounding material and on the histotripsy pulse. Stresses far greater than gigapascals were observed close to the bubble wall, but attenuated by four to six orders of magnitude within 50 μm from the bubble wall, with at least two orders of magnitude attenuation occurring within the first 10 μm from the bubble. Elastic stresses were found to dominate close to the bubble wall, whereas viscous stresses tended to persist farther into the surroundings. A non-dimensional parameter combining tissue, waveform and bubble properties was identified that dictates the dominant stress (viscous vs. elastic) as a function of distance from the bubble nucleus. In a cycle of bubble growth and collapse, characteristic times at which mechanical damage is likely to occur and dominant mechanisms acting at each time were identified.

Comparison of Si Refinement Efficiency of Electromagnetic Stirring and Ultrasonic Treatment for a Hypereutectic Al-Si Alloy

The effects of electromagnetic stirring, ultrasonic treatment (UT), and their combination on the Si particles refinement in a hypereutectic Al-Si alloy (A390) were examined. All three physical methods yielded a considerable change in the shape and size of the particles, with UT producing the finest size. The creation of holes inside the Si particles via the explosion of cavitation bubbles was determined to occur before fragmentation of primary Si. In addition, transmission electron microscopy analysis revealed twin formation on primary Si particles by application of an external field in the liquid. UT of the melt was sufficient to produce fine primary Si particles with average size of ~12 µm. The primary Si particles refinement and eutectic Si modification improved the mechanical properties.

Testing for Explosive Bubbles in the Presence of Autocorrelated Innovations

We analyze an empirically important issue with recursive right-tailed unit root tests for bubbles in asset prices. First, we show that serially correlated innovations, which is a feature that is present in most financial series used to test for bubbles, can lead to severe size distortions when using either fixed or automatic (based on information criteria) lag-length selection in the auxiliary regressions underlying the test. Second, we propose a sieve-bootstrap version of these tests and show that this results in tests which control size well across a number of simulation designs both with and without highly autocorrelated innovations. We also find that these improvements in size come at a relatively low cost for the power of the tests. Finally, we apply the bootstrap tests on the housing market of OECD countries, and generally find much weaker evidence of housing bubbles compared to existing evidence.

An Investigation into Multivariate Variance Ratio Statistics and their Application to Stock Market Predictability*

We propose several multivariate variance ratio statistics. We derive the asymptotic distribution of the statistics and scalar functions thereof under the null hypothesis that returns are unpredictable after a constant mean adjustment (i.e., under the weak form Efficient Market Hypothesis). We do not impose the no leverage assumption of Lo and MacKinlay (1988) but our asymptotic standard errors are relatively simple and in particular do not require the selection of a bandwidth parameter. We extend the framework to allow for a time varying risk premium through common systematic factors. We show the limiting behaviour of the statistic under a multivariate fads model and under a moderately explosive bubble process: these alternative hypotheses give opposite predictions with regards to the long run value of the statistics. We apply the methodology to five weekly size-sorted CRSP portfolio returns from 1962 to 2013 in three subperiods. We find evidence of a reduction of linear predictability in the most recent period, for small and medium cap stocks. The main findings are not substantially affected by allowing for a common factor time varying risk premium.

A new flash boiling model for single droplet

A series of new flash boiling sub-models inside a droplet were proposed in this study. Compared to the previous flash boiling model, the proposed model includes the classical homogeneous nucleation sub-model, the Rayleigh bubble growth sub-model, and a new bubble explosion sub-model. Both the energy and momentum balances inside a droplet are considered. By coupling with the variation of the fuel thermo-physical properties with temperature, the criterion of the bubble nucleation, growth, and explosion criteria are well predicted. The model is validated by the experimental data of superheated water. The computational results indicate that the flash boiling process can be divided into three stages, i.e., the initial stage controlled by the surface tension of the fuel, the rapid growing stage controlled by the heat diffusion in the droplet, and the final explosion stage. Based on the present model, the boiling explosion time is quantitatively provided at wide ranges of fuel temperature and ambient pressure which have the vital significance in predicting the secondary breakup of the droplet.

Quantitative observation on characteristics and breakup of single superheated droplet

Flash-boiling atomization is an effective way to enhance fuel jet breakup by introducing explosion of vapor bubbles and thus improve the evaporation of fuel spray, compared with the conventional high pressure injection. However, the break-up mechanism of a superheated jet, especially, that associated with the explosion of vapor bubbles inside the droplets, is still unknown. In this study, a superheated droplet generator was developed for observing the droplet morphology variation and the breakup process resulting from the vapor bubbles inside a superheated droplet by microscopic imaging. It was found that the droplet morphology is mainly influenced by droplet temperature, but micro bubbles formation and the breakup of the superheated droplet are dominated by superheat degree, and the superheat degree of 25°C is an important critical point at which the droplet breakup occurs resulted from the ever-increasing void fraction exceeding a value of approximately 50% and the breakup mode shifts from aerodynamic mode to thermodynamic mode. The surface tension of superheated droplet was also evaluated by the droplet morphology, and the results show that the maximum reduction in surface tension reaches 70% as superheat degree increases to approximately 25°C, and this explains the sharp decrease in SMD for a flash boiling spray when the superheat degree approaches this level. These results provide insightful information for understanding the breakup mechanism of superheated droplets and liquid jet and its modeling.

Platinum Thin Catalyst Layer for Proton Exchange Membrane Fuel Cell

As one of the renewable energy, polymer electrolyte membrane fuel cell (PEMFC) is a promising candidate because of its good performance efficiency and environment-friendly technique. However, high cost and poor durability of Pt catalys are main obstacles to a commercialization of PEMFC. As a solution for the problems, some researchers have focused on thin film catalyst layer instead of Pt/C catalyst for reducing the amount of platinum use and for enhancing performance at high current density. For example, NSTF (nanostructured thin film) catalyst layer and inverse opal structured catalyst layer were studied for improving mass-transfer and enhancing durability and stability of catalyst.[1-2] In this study, we used ultrasound to form a thin catalyst layer. By using ultrasound, we synthesized Pt nanoparticle and coated it on MPL (microporous layer), simultaneously. It has being revealed that ultrasound is an effective way to create metal nanoparticle because of ultrasound’s cavitation effect.[3] Bang et al explained about cavitation effect. Bubble explosion provides huge pressure and energy for a chemical reaction if the bubble size reached the critical point under ultrasound irradiation.[4] In addition, ultrasound has a micro jet effect, which has ability to clean a substrate and coat nanoparticles on it.[5] In our study, ultrasound applied to form a catalyst layer and the we adopted it for fabricating MEA (Membrane Electrode Assembly) of single cell. The MEAs were compared with MEA fabricated by a conventional method with Pt/C catalyst. To verify a formation of nanoparticle via a cavitation effect of ultrasound, we used UV-visible spectroscopy and observed spectrum change nearby 450 nm which is known for Pt4+ after ultrasound’s irradiation.[6] To coat Pt nanoparticles on the carbon paper, we prepared carbon papers fixed at an acrylic frame in platinum precursor solution with reducing agent. Ultrasound was irradiated at various conditions. For example, time and power of irradiation, concentrations of the precursor and the reducing agent were changed to find an optimum condition for a sonochemical coating. As shown in Figure 1, images of the carbon papers acquired by FE-SEM (Field Emission Scanning Electron Microscope) and EDS (Energy Dispersive Spectroscopy) were used for checking platinum distribution. Cathode’s catalyst layer of MEAs was prepared by sonochemical method and anode’s catalyst layer was formed by spraying Pt/C on carbon paper. The MEAs were fabricated using hot-pressing without ionomer. Figure 2 shows I-V curve of the MEAs made at the 12 W of ultrasound power for 120 min and 90 min. To figure out experiment condition effect, we are still carrying out single tests and perform a characterization of MEA to elucidate the difference between MEAs. All the results investigated in the study will be presented with in-depth discussion. Figure 1

Micro-foundation using percolation theory of the finite time singular behavior of the crash hazard rate in a class of rational expectation bubbles

We present a plausible micro-founded model for the previously postulated power law finite time singular form of the crash hazard rate in the Johansen–Ledoit–Sornette (JLS) model of rational expectation bubbles. The model is based on a percolation picture of the network of traders and the concept that clusters of connected traders share the same opinion. The key ingredient is the notion that a shift of position from buyer to seller of a sufficiently large group of traders can trigger a crash. This provides a formula to estimate the crash hazard rate by summation over percolation clusters above a minimum size of a power [Formula: see text] (with [Formula: see text]) of the cluster sizes [Formula: see text], similarly to a generalized percolation susceptibility. The power [Formula: see text] of cluster sizes emerges from the super-linear dependence of group activity as a function of group size, previously documented in the literature. The crash hazard rate exhibits explosive finite time singular behaviors when the control parameter (fraction of occupied sites, or density of traders in the network) approaches the percolation threshold [Formula: see text]. Realistic dynamics are generated by modeling the density of traders on the percolation network by an Ornstein–Uhlenbeck process, whose memory controls the spontaneous excursion of the control parameter close to the critical region of bubble formation. Our numerical simulations recover the main stylized properties of the JLS model with intermittent explosive super-exponential bubbles interrupted by crashes.

Microscopic characterization of isooctane spray in the near field under flash boiling condition

The isooctane spray characteristics were experimentally investigated under flash boiling condition which represents the part load operating condition for modern gasoline engine. Various tests were carried out with back pressure ranging from 0.2bar to 1bar and ambient temperature varying between 20°C and 100°C. A long distance microscope together with an ultrahigh speed camera was employed to capture the spray development in the near field to study the primary breakup characteristics. The study was performed by using a diesel common rail injection system so that the influence of hydraulic force can be investigated. It was found that flash boiling led to dramatic radial propagation due to the explosion and collapse of the vapor bubbles, significantly boosting the atomization. The strength of vapor bubble explosion in the near field tended to be strongly affected by the flow regime in the nozzle. Besides, higher injection pressure led to larger cone angle during the initial injection stage but smaller cone angle during the quasi-steady stage due to the dominance of the hydraulic force.