Blast Wave Theory Research Articles

Laser-induced radical generation and evolution to a self-sustaining flame

Images and emission spectra of sparks produced by laser-induced breakdown of methane and propane air mixtures were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified high-speed camera and a single/multi-fiber Cassegrain optics system coupled to an ICCD spectrometer. Emission spectra of OH ∗, CN ∗, CH ∗, and C ∗ 2 radicals were also collected using spectra boxes. The results provided information about the different stages of the laser-induced breakdown, with a specific focus on the transition from a flame kernel to a self-sustaining flame. The plasma shape and emission spectrum were very reproducible. The differences in the size of the flame kernel and the evolution of the radical emissions were analyzed for mixtures that fired or misfired. The impact of the level of radicals in the flame kernel was a critical parameter for the firing process, starting around 1 μs after the laser-induced breakdown. The transition from plasma cooling to the classical chemical reactions in the combustion zone was analyzed. Even though the flame kernel size was directly linked to the spark energy, this was not a key parameter toward evolution to a self-sustaining flame. The Taylor blast wave theory was used to plot the location of the shock based on the evolution of the flame kernel size. The location was calculated using a laser-supported detonation model. A very good correlation was observed with the hot gas ignition process. Our results allowed us to obtain information about the process leading to firing or misfiring for similar environments, resulting in a better understanding of the laser breakdown phenomena and the means of utilizing this technique in an industrial context.

On improvement to the Shock Propagation Model (SPM) applied to interplanetary shock transit time forecasting

This paper investigates methods to improve the predictions of Shock Arrival Time (SAT) of the original Shock Propagation Model (SPM). According to the classical blast wave theory adopted in the SPM, the shock propagating speed is determined by the total energy of the original explosion together with the background solar wind speed. Noting that there exists an intrinsic limit to the transit times computed by the SPM predictions for a specified ambient solar wind, we present a statistical analysis on the forecasting capability of the SPM using this intrinsic property. Two facts about SPM are found: (1) the error in shock energy estimation is not the only cause of the prediction errors and we should not expect that the accuracy of SPM to be improved drastically by an exact shock energy input; and (2) there are systematic differences in prediction results both for the strong shocks propagating into a slow ambient solar wind and for the weak shocks into a fast medium. Statistical analyses indicate the physical details of shock propagation and thus clearly point out directions of the future improvement of the SPM. A simple modification is presented here, which shows that there is room for improvement of SPM and thus that the original SPM is worthy of further development.

Acoustic-Gravity Waves from Bolide Sources

We have developed a new approach to modeling the acoustic-gravity wave (AGW) radiation from bolide sources. This first effort involves entry modeling of bolide sources that have available satellite data through procedures developed in ReVelle (Earth Moon Planets 95, 441–476, 2004a; in: A. Milani, G. Valsecchi, D. Vokrouhlicky (eds) NEO Fireball Diversity: Energetics-based Entry Modeling and Analysis Techniques, Near-earth Objects: Our Celestial Neighbors (IAU S236), 2007b). Results from the entry modeling are directly coupled to AGW production through line source blast wave theory for the initial wave amplitude and period at \(x=10\) (at 10 blast wave radii and perpendicular to the trajectory). The second effort involves the prediction of the formation and or dominance of the propagation of the atmospheric Lamb, edge-wave composite mode in a viscous fluid (Pierce, J. Acoust. Soc. Amer. 35, 1798–1807, 1963) as a function of the source energy, horizontal range and source altitude using the Lamb wave frequency that was deduced directly during the entry modeling and that is used as a surrogate for the source energy. We have also determined that Lamb wave production by bolides at close range decreases dramatically as either the source energy decreases or the source altitude increases. Finally using procedures in Gill (Atmospheric-Ocean Dynamics, 1982) and in Tolstoy (Wave Propagation, 1973), we have analyzed two simple dispersion relationships and have calculated the expected dispersion for the Lamb edge-wave mode and for the excited, propagating internal acoustic waves. Finally, we have used the above formalism to fully evaluate these techniques for four large bolides, namely: the Tunguska bolide of June 30, 1908; the Revelstoke bolide of March 31, 1965; the Crete bolide of June 6, 2002 and the Antarctic bolide of September 3, 2004. Due to page limitations, we will only present results in detail for the Revelstoke bolide.

Comment on “three-dimensional analysis of laser induced plasmas in single and double pulse configuration”

Laser induced breakdown spectroscopy (LIBS) is an effective technique for real-time chemical analysis of samples in the laboratory and in the field. The performance of LIBS can be significantly improved by replacing the conventional LIBS configuration from single pulse laser to double pulse laser ablation. Corsi et al. showed that by firing two lasers with microsecond order delay can increase LIBS sensitivity [M. Corsi, G. Cristoforetti, M. Giuffrida, M. Hidalgo, S. Legnaioli, V. Palleschi, A. Salvetti, E. Tognoni, C. Vallebona, Three-dimensional analysis of laser induced plasmas in single and double pulse configuration, Spectrochimica Acta, Part B 59 (2004) 723–735] [1]. By studying plume evolution, they attribute this enhancement to the faster plume expansion in double pulse laser ablation. Blast wave theory was used in Corsi's paper to explain the higher expansion speed observed in double pulse laser ablation. However, it is questionable whether the blast wave theorem applies in laser ablation where the shockwave is driven by a vapor plume of mass. We introduce an alternative way to explain the faster plume expansion during double pulse laser through a more general thermodynamic relation.

An experimental and numerical study of laser-induced spark in air

The present work studied the expansion of a laser-induced spark in air. The experiments were carried out using the beam deflection technique. Numerical analysis used a simple one-dimensional spherical model and the governing equations were integrated numerically using the MacCormack predictor–corrector scheme. The present results indicated that the shock radius R is proportional to shock arrival time t 0.4 and the shock pressure is proportional to R −3 as functionally described by the blast wave theory. For the range of the spark energies from 15 to 50 mJ, the shock front reached a distance of about 2 mm within a few microseconds or less. During this period the shock-wave energy loss was from about 51% to 70%, the radiation energy losses accounted for were from 22% to 34%, and the energy of the remaining hot gas, was about 7–8% of the absorbed energy.

Fundamentals of high-energy spark ignition with lasers

An experimental study of laser-induced spark ignition of flammable, gaseous premixtures is reported, with theoretical interpretations. Ignition was in an explosion bomb, equipped with four variable-speed fans that facilitated the study of quiescent and isotropic turbulent conditions. Good optical access enabled the progress of plasma fronts, shock waves, igniting kernels, and propagating flames to be recorded with high-speed schlieren photography. A focused beam from a Q-switched Nd:YAG laser initiated electrical breakdown, with plasma energies between 85 and 200 mJ. Probabilities of breakdown were found for air and isooctane–air mixtures over ranges of pressures and temperatures. Blast-wave theory applied to shock-wave trajectories enabled initial plasma conditions to be inferred. This suggested electron temperatures of over 10 5 K and very high pressures. Calculated values of the absorption coefficient for the laser beam energy show these plasma properties to be commensurate with the observed energy and size. The ensuing rarefaction wave creates toroidal rings at the leading and trailing edges of the plasma. The former decays more rapidly and a third lobe of the kernel is generated that moves towards the laser. In flammable mixtures this enhances the flame spread. Laminar flame speeds are overdriven by this gasdynamic effect, as well as by the high energy of the plasma, to such an extent that the flame speed decays from elevated values as the flame stretch decreases, contrary to the increases that occur with normal flames with positive Markstein numbers. The extent to which turbulence narrows the ignition limits is found experimentally. For mixtures close to the lean flammability limit, strong gasdynamic flows induced by laser ignition can stretch the flames to extinction and narrow the ignition limits. If a flame becomes established, eventually the third lobe disappears as the initial gas dynamic effects decay and are overwhelmed by the imposed flow fields. Nevertheless, the overdrive effects persist for some time and overdriven flames were observed in regimes where normal flames would have quenched.

Influence of the processing parameters on the formation and deposition of particles in UV pulsed laser ablation of Al 2O 3–TiC ceramics

In this work, the influence of the ambient gas (He, Ne, N 2, air, Ar and Kr) on the mechanisms of particle formation and deposition in laser micromachining of Al 2O 3–TiC ceramics using pulsed 193 and 248 nm radiation was investigated. Two kinds of particles redeposited around the processed area could be identified: ultrafine particles, of a few tens of nanometers (debris), which aggregate upon redeposition to form a continuous film; and larger particles, with a diameter of 0.5–8 μm (particulates), which remained individualised. The behaviour of debris follows blast wave theory predictions, indicating that this type of particles results from collisions behind the ablation plume front. The particulates are formed above a threshold laser fluence, which decreases with increasing ambient gas atomic/molecular weight and pressure. This threshold is lower for 193 than for 248 nm radiation, but shows the same dependence on the atmosphere for both wavelengths. Experimental evidence and theoretical analysis suggest that the particulates result from ejection of droplets from growing protuberances on the irradiated surface, due to the recoil pressure produced by the plume.

Direct initiation of detonation with a multi-step reaction scheme

The problem of direct initiation of detonation, where a powerful ignition source drives a blast wave into a gaseous combustible mixture to generate a Chapman–Jouguet (CJ) detonation, is investigated numerically by using a three-step chain-branching chemical kinetic model. The reaction scheme consists sequentially of a chain-initiation and a chain-branching step, followed by a temperature-independent chain termination. The three regimes of direct initiation i.e. subcritical, critical and supercritical, are numerically simulated for planar, cylindrical and spherical geometries using the present three-step chemical kinetic model. It is shown that the use of a more detailed reaction mechanism allows a well-defined value for the critical initiation energy to be determined. The numerical results demonstrate that detonation instability plays an important role in the initiation process. The effect of curvature for cylindrical and spherical geometries has been found to enhance the instability of the detonation wave and thus influence the initiation process. The results of these simulations are also used to provide further verification of some existing theories of direct initiation of detonation. It appears that these theories are satisfactory only for stable detonation waves and start to break down for highly unstable detonations because they are based on simple blast wave theory and do not include a parameter to model the detonation instability. This study suggests that a stability parameter, such as the ratio between the induction and reaction length, should be considered and a more complex chemistry should be included in future development of a more rigorous theory for direct initiation of detonation.

Assessment of the blast wave generated by a low-energy plasma igniter and spectroscopic measurements

Assessment of the output of plasma devices and comparison with thermodynamic calculations makes it possible to improve the efficiency of these devices. The experiment is based on two capacitor banks (1 and 20 kJ) and on a small plasma torch used in electrothermal-chemical (ETC) basic studies. The behavior of a plasma igniter is studied using spectroscopic measurements to obtain the electronic density from H/sub /spl alpha// line Stark width. The temperature is obtained from Boltzmann plots on copper and iron atomic lines. Under the local thermodynamic equilibrium assumption, the electron density (N/sub e/) depends on the electron temperature (T/sub e/) via Saha's law (N/sub e//spl prop/T/sub e//sup 3/4/). A check on this behavior permits validation of the thermodynamic equilibrium assumption, and the pressure of the emitting area is obtained by using Dalton's law. The flight time and the overpressure generated by the plasma igniter can be compared at the Sedov-Taylor blast wave theory. Finally, we give a comparison of the pressure of the emitting area found using Dalton's law and also by applying the Sedov-Taylor blast wave model.

Analytical study of idealized two-dimensional cellular detonations

In this study, the idealized two-dimensional detonation cells were decomposed into the primary units referred to as sub-cells. Based on the theory of oblique shock waves, an analytical formula was derived to describe the relation between the Mach number ratio through triple-shock collision and the geometric properties of the cell. By applying a modified blast wave theory, an analytical model was developed to predict the propagation of detonation waves along the cell. The calculated results show that detonation wave is, first, strengthened at the beginning of the cell after triple-shock collision, and then decays till reaching the cell end. The analytical results were compared with experimental data and previous numerical results; the agreement between them appears to be good, in general.

THE EFFECT OF LASER BEAM ENERGY ON THE MECHANICAL EFFECT OF LSD WAVE OF A DEFINITE AREA

A mathematical model performed by computer for simulation the effect of laser beam enhanced by ( LSD ) on the mechanical effect of the wave explosion , which is due to interaction of laser beam with a tarried of definite area , The blast wave theory is adopted for this purpose with the implementation of shock relation , also the decay plasma pressure above a target surface calculated by Sedov law. This study obtained that the value of impulse which transferred to the target was increased with increasing laser beam value and decreased with increasing the laser area spot. This theoretical study can be useful for the experimental applications .

Converting spatial to pseudotemporal resolution in laser plasma analysis by simultaneous multifiber spectroscopy

Traditional chemical analysis based on laser plasma spectroscopy (LPS) requires time-gated detectors, to avoid the initial signal from the hot plasma. These detectors are expensive and often need to be cooled and protected against vapor condensation. We suggest a low-cost setup that may replace these gated detectors, while maintaining acceptable analytical performance. The proposed setup is a result of investigation of plasma-front propagation in LPS analysis. It is known that the LPS plasma propagation is similar to the shock wave propagation after a strong explosion in the atmosphere. We found that the propagation of the plasma fits well the Sedov blast wave theory, providing a good agreement between the theoretical and experimental figures. A proper observation geometry, which is perpendicular to the plasma expansion vector, enables converting spatial to temporal resolution. We take advantage of the fact that the plasma reaches a given distance above the analyzed surface at a certain time delay. Therefore, a single optical fiber, positioned at a well-defined geometry, can provide spectral information corresponding to a certain time delay. A multifiber imaging spectrometer provides information corresponding to a series of delay times, which is adequate for analysis of a variety of matrixes. It was found that the performance of the nongated detector observing a narrow solid angle is similar to that of a gated one observing the whole plasma. For one particular example, observing the plasma from a distance of 4.5 mm is equivalent to a delay of 4 micros and integration time of 2 mircos. The ratio of spectral lines of two elements was investigated using the spatially resolved (nongated) setup, and it was found that this mode is advantageous when internal calibration is applied. It was concluded that sensitive LPS analyses can be carried out by less expensive (nongated) detectors.

Ion time-of-flight study of laser ablation of silver in low pressure gases

The dynamics of ions from a laser-ablated silver target in low pressure background atmospheres have been investigated in a simple geometry using an electrical probe. A simple scattering picture for the first transmitted peak of the observed plume splitting has been used to calculate cross sections of the ablated silver ions in oxygen ( σ{O 2}=4.8×10 −16 cm 2) and in argon ( σ{Ar}=6.7×10 −16 cm 2). The dynamics of the blast wave is well described by blast wave theory.

Propagation of the shock wave generated from excimer laser heating of aluminum targets in comparison with ideal blast wave theory

Propagation of the shock wave generated during pulsed laser heating of aluminum targets was measured utilizing a probe beam deflection technique. The transit time of the laser-generated shock wave was compared with that predicted from the Sedov–Taylor solution for an ideal spherical blast wave. It was found that the most important parameters for the laser-generated shock wave to be consistent with the theoretically predicted propagation are the ambient pressure and the laser beam spot size. The prediction for laser energy conversion into the laser-induced vapor flow using the Sedov–Taylor solution overestimated the energy coupling efficiency, indicating a difference between a laser-induced gas-dynamic flow and an ideal blast wave.

Analysis of blast waves induced by Q-switched Nd:YAG laser photodisruption of absorbing targets

An experimental investigation on photoacoustic effects originated in plasma-mediated laser photodisruption by a Q-switched Nd:YAG was done by means of a pump-and-probe set up providing time-resolved shadowgraphy of the shock wave produced onto the target surface. Estimations of coupled specific momentum and of the explosion energy were obtained by modelling the process on the basis of the variable energy blast wave theory. A comparison with the instantaneous (constant energy) explosion model is also discussed.

In Situ Plasma Characterization of Laser-Ablated BaxSr1-xTiO3

ABSTRACTHigh speed imaging and emission spectroscopy are used to characterize the plumes generated from the ablation of bulk barium strontium titanium oxide material in vacuum and 30–60 mTorr of O2 using a pulsed 248-nm laser with fluences ranging from 5–8 J/cm2. High speed imaging reveals that in vacuum the leading edge of the plume expands normal to the substrate surface with a velocity of 19.7 km/sec. The ratio of distance traveled to the plume's radius is ˜ 2 at the early stages of the expansion and 3.2 at the later stages near the substrate. Similar observations are observed when the plume expands in 30 mTorr of O2 for times less than 2.0 μsec. However, at times greater than 2.0 μsec the plume's expansion is retarded due to collisional momentum transfer and reactions with the background gas. This effect is more pronounced when the plume expands in 60 mTorr of O2. Blast wave theory is used to model these results. Time-resolved emission spectroscopy of Ba(I) lines in the 735–770 nm region reveals that the plume' electron temperatures, Te, in a vacuum expansion range from 18000±2000 K at 0.1 μsec to 15000±1500 K at 2.0 /μsec. In contrast, in 30 mTorr of O2 the values of Te range from 17000±2000 K at 0.1 μsec to 4000±500 K at 8.0 μsec. At 0.1 μsec the plume's electron density is estimated as 1.7±0.4 × 1017 cm−3 in both vacuum and 30 mTorr of O2. We also determine the Stark broadening of the Ba(I) line at 746 nm to be 0.5±0.1 Åat an electron temperature of 1 eV and an electron density of 1017 cm−3.

やや鈍い先端を有する平板周りの極超音速粘性流れ場における迎え角の影響

The effects of angle of attack α on viscous hypersonic flowfield over a flat plate with a slightly blunt leading edge were examined theoretically and experimentally. An analytical solution which includes the effects of the slight bluntness and of α was obtained by using blast wave theory. It was compared with the experimental shock shape. The change of drag coefficient CD for the blunt leading edge must be considered for small Reynolds number Red which is based on the characteristic dimension of the bluntness d because of the viscous effects there. Taking account of this, qualitative trends of the two were in good agreement. At small α, the shock shape is dominated by the effects of the bluntness. At large α, the strong effects of α make the shock shape straight downstream.

Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

Impulse transfer to the surface of aluminum and copper from a pulsed Nd: YAG laser

Impulse coupling coefficients in air from 1.06 μm, 10 ns, Nd: YAG pulsed-laser radiation to aluminum and copper targets are measured using the ballistic pendulum method in the laser power-density range from 2.0×108 W/cm2 to 4.0×109 W/cm2. A modified laser-supported detonation (LSD) wave and cylindrical blast wave theory incorporating the vaporization model is proposed to calculate the impulse coupling coefficients. It is found that the theoretical results obtained with the modified model agree well with the experimental data.

Laser beam deflection monitoring of Nd: YAG laser ablation: pulse shape and repetition rate effects

The laser beam deflection probe has been employed to study blast waves generated during ablation of metallic surfaces by sequences of 1.06 μm Nd:YAG laser pulses separated by less than 1μs. A fluence threshold has been found, below which the effects of individual pulses can be resolved by the laser probe. Above that, the deflection signal has a similar form as if the surface were irradiated with a single pulse. Analysis of the signals in terms of the spherical blast wave theory shows that a pulse sequence generates a weaker blast wave than a single pulse of equal total energy. On the other hand, the sequence yields a higher etch depth than the single pulse.