Reflected Shock Waves Research Articles

High-temperature absorption cross-sections and interference-immune sensing method for formaldehyde near 3.6-µm

Absorption cross-sections of formaldehyde (CH2O) were measured at elevated temperatures near 3.6 µm, a region of strong intensity due to the overlapping QQ5 branch of the v1 band and the PQ7 branch of the v2+v4 combination band. 1,3,5-trioxane was shock-heated in an argon bath gas to produce controlled mixtures of formaldehyde over a range of temperatures (900–1500 K) and pressures (0.4–4.8 atm). Spectrally-resolved absorption measurements of formaldehyde were obtained using a rapidly-tunable distributed-feedback interband cascade laser scanned at 40 kHz over 2778.1–2778.9 cm−1 behind reflected shock waves. The resulting absorption cross-sections were fit with functions reflecting the temperature and pressure dependence of the targeted features, enabling spectral reconstruction at any temperature and pressure within the range of the study. By exploiting the strong temperature and pressure sensitivity of the cross-sections, it is demonstrated that CH2O concentration, temperature, and pressure can be inferred from a measured absorbance spectrum. Cross-sections of common combustion species were also measured over the selected wavelength range to validate that the region is largely free of narrow-band interference. Further, the multi-parameter spectral fitting method is shown to correct for baseline interference and demonstrated in experiments involving transient gas properties.

The Effect of Explosions on the Protective Wall of a Containerized Hydrogen Fuel Cell System

With the development of hydrogen energy, containerized hydrogen fuel cell systems are being used in distributed energy-supply systems. Hydrogen pipelines and electronic equipment of fuel cell containers can trigger hydrogen-explosion accidents. In the present study, Computational Fluid Dynamics (CFD) software was used to calculate the affected areas of hydrogen fuel cell container-explosion accidents with and without protective walls. The protective effects were studied for protective walls at various distances and heights. The results show that strategically placing protective walls can effectively block the propagation of shock waves and flames. However, the protective wall has a limited effect on the reduction of overpressure and temperature behind the wall when the protective wall is insufficiently high. Reflected explosion shock waves and flames will cause damage to the area inside the wall when the protective wall is too close to the container. In this study, a protective wall that is 5 m away from the container and 3 m high can effectively protect the area behind the wall and prevent damage to the container due to the reflection of shock waves and flame. This paper presents a suitable protective wall setting scheme for hydrogen fuel cell containers.

Test of pressure load on surface of typical concrete structure caused by underwater explosion

To study the pressure load of the underwater explosion on the typical surface of a concrete structure, composition B were used to explode in water, and the shock wave pressure on the front, side, top and back of the concrete pile was measured. The theoretical peak pressure of shock wave at the same distance in infinite water was calculated. The results show that the peak pressure of reflected shock wave at the front of concrete pile is the largest, which is nearly twice the theoretical value of the peak pressure of reflected shock wave at the same distance in infinite water. When the shock wave continues to propagate on the side and top surface, the peak pressure on the side is higher than that on the top surface. The peak pressure of the shock wave diffused from the side and top surfaces on the back is smaller than that of the other surfaces.

Investigation of thermal decomposition of nitrobenzene: An energetic material

Oxidation and pyrolysis mechanisms of energetic materials shape their macro properties such as autoignition and detonation. The high reaction-enthalpy, microsecond order reaction time, and toxicity necessitate investigation of their detailed chemical kinetics. A shock tube combined with laser absorption diagnostic provides time-resolved, high-sensitivity, and homogeneous environment to study thermal degradation of energetic and hazardous materials. In this work, we investigated the thermal fragmentation of nitrobenzene (C6H5NO2), a prototypical energetic material and nitro-aromatic representative, behind reflected shock waves. We studied its pyrolysis and decomposition branching channels over temperatures of 1155–1434 K and pressures of 0.82–1.78 bar. Channel-specific branching ratio was determined by employing a two-color laser absorption diagnostic. Two decomposition pathways are found to be significant under our experimental conditions. The first channel (k1) produces phenyl radical (C6H5) and NO2 while the second channel (k2) generates phenoxy radical (C6H5O) and NO. We monitored the reaction progress by tracing channel-specific products, i.e., phenyl radical (C6H5) and NO, using ultraviolet–visible (UV–vis) and infrared (IR) absorption diagnostics. UV–vis light at 445 nm for phenyl radical detection came from the frequency doubling of 890 nm light, generated by a titanium–sapphire laser, which was pumped by a green (532 nm) Nd:YAG laser. IR light at 5.517 μm for NO detection was generated by an external-cavity quantum cascade laser. We extracted the rate coefficients of the two decomposition channels and fitted them in Arrhenius form (units of s−1):k1=8.49×1014exp(−30476KT)k2=2.31×1013exp(−27618KT)Rate coefficients of both channels show positive temperature dependence while we did not observe discernible pressure dependence over 0.82–1.78 bar. Our measurements agree with a previous theoretical work qualitatively that, under these high-temperature conditions, the decomposition channel producing phenyl radicals and NO2 (k1) prevails over the NO-producing channel. Quantitatively, however, the theoretical work overestimates the importance of k1 channel, particularly at low temperatures. Our rate of production analysis shows that most of the phenyl radicals are consumed through their self-recombination while N atoms are mainly converted to NO2. This work benefits high-fidelity chemical modeling of the relevant safety and ignition aspects of nitrogen-containing energetic/hazardous materials.

New insights into the pre-ignition behavior of methane behind reflected shock waves

Pre-ignition is an undesired combustion event known to restrict kinetic modeling validation. Previous methane oxidation studies reported premature ignition as part of ignition delay time measurements in shock tubes. In this context, the effect on the pre-ignition propensity and auto-ignition behavior of stoichiometric methane mixtures at different dilution levels of hbox {N}_2, Ar, He, and hbox {CO}_2 was studied at 10 bar and 25 bar and temperatures between 1080 K and 1350 K. In addition to conventional sidewall pressure and endwall light emission measurements, a high-speed imaging setup was utilized to visualize the ignition process. Relevant physicochemical parameters to describe and predict the pre-ignition phenomenon were used. The results suggest that dilution levels up to 80% of bath gas are not successful in mitigating early ignition occurrence and its effects at moderate pressures. Replacing hbox {N}_2 by He was found to suppress early ignition at 10 bar, attributed to an enhanced dissipation of temperature inhomogeneities in the test gas section. The present findings demonstrate that hbox {CO}_2 has potential for pre-ignition heat release mitigation, while Ar was confirmed to promote premature ignition. To the best of our knowledge, we present the first detailed study on pre-ignition mitigation for methane mixtures in shock tubes, where further insights into its ignition non-idealities are given.

An experimental and theoretical kinetic modeling study of the thermal decomposition of methyl-2-methyl butanoate behind shock waves

In order to investigate the combustion chemistry of a branched ester namely Methyl-2-methylbutanoate (M2MB), thermal decomposition of M2MB diluted in Argon was carried out behind reflected shock waves in the temperature range 959 -1392 K using a single pulse shock tube (SPST). The post shock mixtures were analyzed quantitatively as well as qualitatively using gas chromatography (GC) and gas chromatography coupled with mass spectrometry (GC–MS). Methane (CH4), Ethane (C2H6), Ethylene (C2H4), Methanol (CH3OH), Propene (C3H6) and Methyl acrylate (C4H6O2) were the major products observed during thermal decomposition. The various oxygenated compounds were found as minor products. The first order rate constant observed for the decomposition of M2MB is kexpt(959–1392 K) = (2.13 ± 0.457) × 109 exp [(19200 ± 561)/T] s−1 The temperature dependent kinetics of the CC and CO homolytic bond cleavages as well as of the H-migration for the unimolecular decomposition reactions were calculated using RRKM/ME over the temperature range of 959–1392 K. A reaction scheme involving 42 species and 61 elementary reactions, was proposed to simulate the reactant and product concentration over the studied temperature range. A comparison study of decomposition kinetics of branched chain ester and long chain ester is discussed in this work. The concentration profiles for reactant and all the products observed from simulations and experimental results were observed to be in very good agreement.

Experimental study of the methane/hydrogen/ammonia and ethylene/ammonia oxidation: Multi-parameter measurements using a shock tube combined with laser absorption spectroscopy

Ammonia (NH3) is a promising hydrogen carrier and carbon-free fuel, which can be synthesized using renewable energy and has attracted increasing attention from both academic and industrial communities. This study selected six absorption lines and developed a laser absorption diagnostics system that can simultaneously measure the temperature, CO, CO2, H2O, NH3, and NO time-histories during the oxidation of CH4/H2/NH3 and C2H4/NH3 mixtures (equivalence ratio ϕ ≈ 1.2) behind reflected shock waves. Since the fuels were highly diluted to reduce the nonideal effects and heat release behind reflected shock waves, the maximum NO mole fraction was only ∼0.2% and led to a small absorption (< 2.0%). Therefore. the wavelength modulation spectroscopy (WMS-2f/1f) was used for the NO time-history measurements while the other species were measured using fixed-wavelength absorption spectroscopy. A total of thirteen kinetics models were used to interpret the measurements. The main differences between the model predictions and measured data lie in two aspects: the mixture reactivities and the NO production. Generally speaking, for both CH4/H2/NH3 and C2H4/NH3 mixtures, the Glarborg model performs the best in predicting the temperature and species time-histories but the differences still exist. The high-quality experimental data support further analyses and improvements. Detailed kinetics analyses based on the Glarborg 2018 model were performed. It indicates that H + O2 = O + OH is the most sensitive and promoting reaction for both mixtures, followed by the H-abstraction reactions of H2 and C2H4. Conversely, the H-abstraction reactions of CH4 and NH3 impede oxidation due to competition with promoting reactions for reactants (H, OH, or O). Moreover, subsequent reactions involving the produced CH3 or C2H3 radicals also have significant effects on oxidation. The reactions affecting the NO plateau were also selected as the basis to optimize the predicted NO plateau. Although updating the rate constants of some key reactions improved model performance for both mixtures, discrepancies still exist, particularly for the C2H4/NH3 mixture.

Self-ignition and flow characteristics of pressurized hydrogen in Y-shaped tubes

The geometrical diversion of combustible gas molecules notably affects their self-ignition in high-pressure tubes. This study investigates the propagation of shock waves and flame induced by a sudden pressure release of hydrogen in various Y-shaped tubes. The impact of the initial burst pressure, downstream tube length, and bifurcation angle (60°, 120°) were analyzed by recording the pressure signals, photoelectric curves, and jet fire evolution. The results showed that the gas diversion caused by the Y-shaped structure significantly reduces the maximum overpressure of the hydrogen jet, the intensity, and the velocity of the leading shock wave. Also, it was indicated that increasing the Y-shaped tube's length downstream of the bifurcation point could enhance the strength of leading and reflected shock waves after the Y-bifurcation, making self-ignition more likely to happen. Applying a 60°-bifurcation angle could create stronger leading and reflected shock waves during the propagation downstream of the bifurcation point, while a 120°-bifurcation angle could make a more substantial reflected shock wave propagating upstream. It leads to the re-ignition downstream of the bifurcation in the 60°-bifurcation tube after the flame quenching and stronger far-field jet fire outside the tube.

Dynamic compression of a polymer stamper using high-impulse underwater shock waves to imprint a sub-micrometer structure on a metal plate surface

Press-forming technology is excellent for mass production. However, for micrometer-order forming, manufacturing press tools is unprofitable. Because laser-induced shock waves cannot rapidly accelerate heavier workpieces into molds, the workpieces are limited to thin metal foils. In this study, an explosion-derived high-impulse shock wave was applied to dynamically compress a polymer stamper into a metal plate. The stamper was pressed for a few microseconds, resulting in a well-imprinted submicron structure on the aluminum plate surface. Numerical simulation clarified the imprinting mechanism, which involved local compression and restoration of the stamper profile that occurred when the reflected shock wave generated at the boundary between the stamper and the workpiece reached the free surface of the stamper. It was discovered that the shock wave must continue to act for a significantly longer time than that required for the reflected shock wave to reach the free surface of the stamper. Therefore, explosion-derived shock waves with a long pressure duration are effective for imprinting. This method has the potential to be developed into a practical technique for imparting functional microsurfaces to structural components.

Three-dimensional numerical study on the interaction between RDC and NGV

In this paper, the three-dimensional numerical method is used to study the change of the internal flow field distribution characteristics of a RDC with a deflector installed at the outlet, using the premixed gas of kerosene and air as reactants. It is found that the total pressure in the front section of the combustor has been improved. Different configurations of deflectors were designed. It was found that the flow field in the combustion chamber could not be significantly changed by changing the number of blades, but the maximum average total pressure and the total pressure loss in the mixing section could be effectively improved by adjusting the angle of incidence. Because the propagation direction of rotating detonation wave and the deflector blade have directionality, the study found that when the two directions are opposite, the reflected shock wave formed at the inlet of the deflector changes greatly, the working environment of the fixed blade is worse, the uniformity of gas after passing the deflector decreases, the flow field along the axial average total pressure loss increases significantly, and the peak value of the total pressure shifts significantly downstream. The study of the interaction between NGV and RDC provides a basis for the design of the rotaing detonation turbine engine.

Influence of vehicle length on the aerothermodynamic environment of the Hyperloop

Hyperloop has received much attention as a futuristic mode of transportation because of its energy efficiency and high speed, among other advantages. The length of the capsule vehicle has essentially little influence on the distribution of the aerodynamic environment, but makes significant differences in the evolution of waves and in the fluctuations of pressure and temperature in the aerodynamic environment. This study employs fluid simulation to learn the aerothermodynamic effects of a capsule vehicle traveling in an evacuated tube of Hyperloop. Based on a validation of CFD simulations via wind-tunnel test measurements, a meshing strategy was designed to ensure that errors in the computational results were minimized. An axisymmetric model of the capsule vehicle was used to reduce computational costs. The results show that different capsule vehicle lengths affect the formation of pressure and temperature flow fields, boundary-layer thickness of the capsule-vehicle surface, and interference of shock waves and expansion waves in Hyperloop. As the vehicle length increased, the pressures at the vehicle head and tail gradually decreased, whereas the middle of the vehicle did not undergo this decrease in pressure. Inside the tube, the high temperature and pressure upstream of the vehicle head decreased slightly as the vehicle length increased. The temperature and pressure around the vehicle tended to increase, whereas at the tail of the vehicle, the pressure tended to decrease, and the temperature to increase. For short, medium and long vehicles, the shock waves were 1.06, 1.120, and 1.243 m, respectively, from the top of the tail, from which it is inferred that the flow separation point moved forward as the length of the vehicle increased. The wake widened as the length of the capsule vehicle increased. The boundary layer at the vehicle tail gradually thickened along the contour. Before separation, the maximum difference in boundary-layer thickness was able to reach 50.3 %, after separation, the difference decreased to a minimum of 8.8 %. The interaction (IE1) and reflection (RE2) of the expansion wave led to pressure decreases of 5.86 % and 4.07 %, respectively, whereas the interaction of shock wave (IS1) and reflection of shock wave (RS1) led to pressure decreases of 4.15 % and 3.67 %, respectively. The interaction had a greater effect on the Hyperloop flow field than that of the reflection.

Gamma-valerolactone (GVL) as a biofuel: Investigation of GVL thermal decomposition and GVL + OH reaction

Recent works have highlighted the potential of lignocellulosic-derived γ-valerolactone (GVL) as a promising biofuel or a fuel additive. Its detailed chemistry is, however, not yet well-understood. In this work, we present the first high-temperature measurements of GVL + OH reaction behind reflected shock waves over 990–1294 K and pressures ∼ 1.2 bar. Around 990 K, our rate coefficients are ∼3 times faster than the average room-temperature determination. Our measured OH + GVL rate coefficients may be given as (unit: cm3molecule−1s−1):k1=4.87×10−10e(−4440T)We also conducted the first direct measurements of GVL thermal decomposition over 1214–1427 K and pressures ∼ 1.4 bar. GVL decomposition rate coefficients exhibit a similar slope to its acyclic counterpart, ethyl propionate, but GVL decomposes ∼80–120 times slower than ethyl propionate. Our determined GVL thermal decomposition rate coefficients may be expressed as (unit of s−1):k2=2.13×1012e(−28548T)Our experimental work represents the first quantitative measurements of the reactions of the lactone family. Both reactions of the lactone family studied here proceeded slower than the normal-chain counterparts which affirms that ring constraints must be considered in rate coefficient analogies between normal-chain and cyclic molecules.

Investigation of shock wave propagation and water cavitation in a water-filled double plate subjected to underwater blast

Water-filled double hulls are a prevalent design for underwater warships. Studying the propagation of underwater shock wave in such structures is essential to comprehend their shock resistance. The aim of this paper is to investigate the propagation of underwater shock wave and water cavitation in a water-filled double plate. To achieve this objective, the paper establishes a Eulerian compressible fluid model that combines the isentropic single fluid model and Newton's second law. The accuracy of the numerical solver is confirmed through validation tests. The reflection and transmission characteristics of shock wave, the dynamic response of the plate, and the cavitation in external and gap water are analyzed. It also examines the effects of the supporting springs of outer and inner plates and the area density of the outer plate on the shock wave propagation and cavitation evolution. The findings indicate that the transmitted wave in the gap water is influenced by the area density and supporting spring of the outer plate. A thicker outer plate can reduce the peak value of the transmitted wave, and a stiffer supporting spring can decrease the transmitted impulse. On the other hand, if the outer plate is freestanding or softly supported, it has minimal effects on the response of the inner plate. Moreover, the water cavitation is determined by the supporting spring of the inner plate in the case of a softly supported outer plate, and the cavitation initially develops in the gap water near the inner plate before expanding towards the external water. When the supporting spring of the outer plate is stiff, it significantly reduces the occurrence of cavitation in the external water, while the cavitation in the gap water is still determined by the support spring of the inner plate. These research findings are valuable for analyzing the shock resistance of typical double-hull submarines.

New Parent Flowfield for Streamline-Traced Intakes

The prespecified flowfield is essential in the inverse design method for generating inward-turning streamline-traced intakes, i.e., the parent flowfield. The internal conical flow C (ICFC) flowfield shows superiority over other parent flowfields in terms of performance and length. A drawback to the ICFC flowfield is the existence of the expansion zone, which creates a second reflected shock and decreases the flow uniformity. Hence, this paper proposes a new basic flowfield called the internal conical flow M (ICFM) to overcome the shortcoming of the ICFC flowfield. For the proposed basic flowfield, a new methodology has been devised for connecting the M-flow and truncated Busemann flowfields. This methodology merges the singular line of the M-flow with the truncated ray of Busemann flowfield in order to reduce the flow inclination angle difference effectively. The concept of the basic flowfield is reviewed, and the new ICFM basic flowfield is calculated using the Taylor–Maccoll equations. Complete details of the new merging procedure and calculation of the ICFM flowfield are presented. The characteristics of the new basic flowfield with design conditions of Mach 4.0, 5.0, 6.0, and 7.0 are examined at different outflow conditions. A comparison with the ICFC flowfield indicates that the new ICFM basic flowfield demonstrates better inviscid performance and a shorter length. Besides, the expansion region is significantly reduced, and the second reflected shock wave is eradicated.

Formation and Evolution of Cellular Structures in Wedge-Induced Oblique Detonations

Numerical simulation of an oblique detonation induced by a wedge is performed to investigate the formation and evolution of the oblique cellular detonation structure and the quantitative comparison of the cellular structure of a normal and an oblique detonation. The compressible reactive Euler equations are solved using the seventh-order WENO scheme on an adaptive mesh based on the open source program Adaptive Mesh Refinement in Object-oriented C++ (AMROC). The numerical results show that there are two sets of transverse waves, the left-running transverse waves (LRTW) and the right-running transverse waves (RRTW), which form the oblique cellular detonation structure. Although both sets of transverse waves are convected downstream, they propagate at almost the same relative velocity in the opposite direction. The LRTWs start in the transition zone because of the detonation instability. However, the RRTWs form due to the interaction between the deformed detonation front and the reflected shock wave from the wedge. For the same degree of overdrive, numerical simulations reveal that the characteristic cell size of an oblique detonation is almost the same as that of a normal detonation.

Atmospheric-pressure shock-tube measurements of high-temperature propane laminar flame speed across multiple equivalence ratios

Laminar flame speeds of premixed propane in 21%O2-79% Ar (so-called ”airgon”) were experimentally measured in a shock tube at atmospheric pressure (1 atm±2%) for a range of equivalence ratios (0.5, 0.75, 1, and 1.2) and unburned-gas temperatures (644–1,044 K). Simultaneous side-wall schlieren and end-wall CH* emission imaging was utilized to track the propagation of outwardly expanding laminar flames initiated by laser-induced spark ignition behind reflected shock waves. The linear curvature model with an area-averaged formulation was used to extrapolate laminar flame speeds to zero-stretch. The high-temperature laminar flame speed results were compared with 1D simulations performed using two detailed kinetic models, NUIG 1.3 and Aramco 3.0, and two skeletal models, USC Mech II and San Diego 2016. The experimental results agree with the NUIG 1.3 model within 3.5%, while the predictions of the models disagree by up to 16%. Sensitivity analyses conducted using the models revealed six chemical reactions that strongly impact the prediction of laminar flame speed. In order for the present data to be compatible with practical combustion systems using air as the oxidizer, the propane/airgon measurements were scaled to obtain laminar flame speeds of propane/air using ratios calculated from 1D simulations. The scaled experimental measurements showed good agreement with literature propane/air results at 650 K. Empirical correlations of laminar flame speed as a function of unburned-gas temperature are then reported for the studied equivalence ratios.

Viscous effects in Mach reflection of shock waves and passage to the inviscid limit

The influence of viscosity on the Mach reflection of shock waves in a steady flow of a monatomic gas is studied by solving the Navier–Stokes equations numerically. Based on the nested block grid refinement technique, the flow near the shock wave intersection is simulated, and its behaviour with increasing Reynolds number is studied. The computations are performed for the interaction of both strong (free-stream Mach number$M_\infty = 4$) and weak ($M_\infty = 1.7$) shock waves. In the strong reflection of shock waves at all Reynolds numbers in the examined range, it is found that there exists a small-size zone behind the shock wave intersection where the flow parameters differ from those predicted by the Rankine–Hugoniot relations and hence deviate from the predictions of the inviscid three-shock theory. The structure of this zone is self-similar: in coordinates normalised to the mean free path of molecules in the free stream. The structure is identical at all Reynolds numbers considered in the study. As the Reynolds number increases, the size of this zone in physical coordinates decreases, but the maximum difference between the viscous and inviscid solutions in this zone remains constant, reaching approximately$10\,\%$for pressure. In the weak reflection of shock waves, the flow structure behind the shock wave intersection is not self-similar, i.e. the flow fields at different Reynolds numbers do not coincide in the normalised coordinates, but converge, as the Reynolds number increases, to the parameters predicted by the inviscid three-shock theory.

Effect of CO&lt;sub&gt;2&lt;/sub&gt; Additives on the Non-Catalytic Conversion of Natural Gas into Syngas and Hydrogen

Abstract—A kinetic analysis of the non-catalytic carbon dioxide reforming of CH4 has been carried out in the temperature range of 1500–1800 K under conditions of variable temperature behind the reflected shock wave. The stages of conversion of methane into synthesis gas, the characteristic time intervals corresponding to these stages, and the most important elementary reactions have been established. At the first stage, as a result of thermal pyrolysis, methane molecules are sequentially converted into ethane, ethylene, and then acetylene, the most stable hydrocarbon in this temperature range. At the second stage, acetylene is normally converted into CO and H2, being accompanied by the formation soot particles in the case of rich mixtures. The conversion of CO2 proceeds at the second and third stages, when CH4 conversion is almost complete. It occurs as a result of the interaction of CO2 molecules with H● atoms arising in the reacting system and leads to the formation of CO molecules and OH● radicals. Acetylene is predominantly consumed in the reaction with OH radicals. A high concentration of acetylene during methane reforming promotes the intensive formation of soot nuclei, for which acetylene makes the highest contribution to the rate of their surface growth. At the same time, acetylene itself is not a precursor of soot particle nuclei, which are mainly formed from \({{{\text{C}}}_{{\text{3}}}}{\text{H}}_{3}^{\centerdot }\) radicals.

Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy.

Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultra-high-speed shadowgraph imaging, hydrophone measurements, three-dimensional passive cavitation mapping (3D-PCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance (SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated "pear-shaped" bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laser-induced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A non-circular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and self-intensified collapse of an inverted "triangle-shaped" or "horseshoe-shaped" bubble. Third, high-speed shadowgraph imaging and 3D-PCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a "smiling-face" shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pear-shaped bubble are decisive for the solid damage.

REFINED DATA ON O2 DISSOCIATION RATE MEASURED BYO-ARAS BEHIND SHOCKWAVES

The formation of atomic oxygen in high-temperature gas flows affects both the thermodynamic state of the gas and the kinetics of proceeding chemical processes. During hypersonic and space flights, there is still a significant lack of understanding of the phenomena of flow around high-speed vehicles. One of the main reactions occurring in the bow shock zone is the dissociation of molecular oxygen into O atoms. Experimental studies of the kinetics of O2 dissociation were carried out by various methods; however, the O2 dissociation rate constants in modern combustion mechanisms still differ by orders of magnitude. Therefore, the clarification of these values is a very urgent task. In this work, precision measurements of the rate constant of oxygen dissociation performed by the ARAS (atomic resonance absorption spectroscopy) method in the temperature range of 2600-5000 K behind the reflected shock waves were carried out which made it possible to noticeably refine the previous data. The best fit to the present experimental data is given by the expression k = 1.34 ± 0.4 • 1014 exp(-(53620 ± 2620)/T) cm6 /(mole-s).