Post-shock Conditions Research Articles

Potential of a jet-induced shock wave to influence an upstream droplet cloud in compression-ignition engines using multiple injection strategies

High-pressure diesel-fuel sprays have been shown to emit shock wave under certain conditions, while the advanced injection strategy used in internal-combustion engines involve multiple injections taking place within a short time frame. Experimental study of double injection has shown in two instances that the first fuel-spray cloud could be accelerated before the arrival of the second fuel spray. Herein, it is hypothesized that a shock wave emitted from an injection interacts with the fuel-spray cloud of a preceding injection and is responsible of a push-away mechanism on the first droplet cloud reported on in the literature. In this context, the shock waves emitted by fuel-spray jets with a commercial 5-hole diesel injector injecting into a pressure vessel were characterized with schlieren visualizations and dynamic pressure measurements taken with single- and double-injection strategies. The experimental results confirm the shock presence based on schlieren measurements. The measured shock conditions show a different shock topology from most shock-tube experiments as the expansion wave closely followed the shock front, resulting in a thin shocked region and a short duration of the droplet exposition to the post-shock gas conditions. Experimental measurements were then used as initial conditions in a 1D multiphase simulation model allowing simulation of the shock-wave interaction with the droplet cloud under engine-related conditions. The model was used to conduct a parametric study on the droplet-cloud characteristics and showed that, as the cloud density increased, the shock intensity and droplet-induced velocity decreased. Finally, the model was used to illustrate that the push-away interaction mechanism could be explained by the shock wave–fuel-droplet-cloud interaction.

Investigating the Dynamic Tensile Response of Lean Duplex Stainless Steel and the Effects of Radial Waves Using the Recovered Plat-Impact Experiment

Lean Duplex Stainless Steel 2404 (LDX2404) is the newest commercial type of duplex stainless steels, recently introduced by Outokumpu®. This paper investigates the dynamic tensile failure (spall strength) of this new alloy under different peak stresses (4.5 GPa to 8.2 GPa) via the recovered plate-impact experiment using a single stage gas gun. The microstructural investigation for the as-received and soft recovered post-shock material conditions were accomplished using Electron Backscatter Diffraction (EBSD) and optical microscopy. The effect of the radial release waves and the effectiveness of using momentum trap rings on the spall strength and the spall damage during the impact experiment were studied. Results indicate that LDX2404 experienced spall damage for the entire range of tested peak stresses. The spall strength was not affected by the implementation of momentum trapping rings whereas; the damage content was highly influenced. The spall damage of LDX 2404 exists parallel to the phase boundary interfaces, which are normal to the impact direction. Grain orientation, based on Taylor factor values, was not controlling the voids nucleation and voids growth.

Contribution of early impact events to metal‐silicate separation, thermal annealing, and volatile redistribution: Evidence in the Pułtusk H chondrite

AbstractThree‐dimensional X‐ray tomographic reconstructions and petrologic studies reveal voluminous accumulations of metal in Pułtusk H chondrite. At the contact of these accumulations, the chondritic rock is enriched in troilite. The rock contains plagioclase‐rich bands, with textures suggesting crystallization from melt. Unusually large phosphates are associated with the plagioclase and consist of assemblages of merrillite, and fluorapatite and chlorapatite. The metal accumulations were formed by impact melting, rapid segregation of metal‐sulfide melt and the incorporation of this melt into the fractured crater basement. The impact most likely occurred in the early evolution of the H chondrite parent body, when post‐impact heat overlapped with radiogenic heat. This enabled slow cooling and separation of the metallic melt into metal‐rich and sulfide‐rich fractions. This led to recrystallization of chondritic rock in contact with the metal accumulations and the crystallization of shock melts. Phosphorus was liberated from the metal and subsumed by the silicate shock melt, owing to oxidative conditions upon slow cooling. The melt was also a host for volatiles. Upon further cooling, phosphorus reacted with silicates leading to the formation of merrillite, while volatiles partitioned into the residual halogen‐rich, dry fluid. In the late stages, the fluid altered merrillite to patchy Cl/F‐apatite. The above sequence of alterations demonstrates that impact during the early evolution of chondritic parent bodies might have contributed to local metal segregation and silicate melting. In addition, postshock conditions supported secondary processes: compositional/textural equilibration, redistribution of volatiles, and fluid alterations.

Shock wave induced primary thermal fragmentation of coal particles

Coal particles when subjected to shock waves can undergo rapid fragmentation, pyrolysis, and combustion, causing enhanced process intensity and efficiency. Particle fragmentation plays a crucial role in this process. Exposure of coal particles to a shock wave is modelled in the present work as combined convection and radiation at the surface and conduction in the interior. Local temperatures within a coal particle and the corresponding thermal stresses are computed to study particle failure. Particle fracture is modelled by a three-parameter Weibull probability to predict the failure location and time. Simulations indicate that pulverized coal of size up to 250 mu m subjected to a shock wave for varying operational, thermal, and physical parameters can experience initial failure within 150 mu s. Particles of size d ae 50 mu m or higher wave strengths (with Mach numbers M ae 5) mostly trigger exfoliation, while interior fragmentation dominates at smaller sizes (d ae 25 mu m). An initial fracture study reveals that pulverized coal with predominant sizes d ae 100 mu m and the coal rank from lignite to bituminous coal is potentially suitable for detonation combustion in waves at Mach numbers M = 3-6. Coal particles under continuous exposure to post-shock conditions undergo recursive exfoliation until the core is 20-40 mu m, after which an interior fragmentation phase is seen until the core is about 1-3 mu m. Much finer coal particles, of the order of internal fragmenting cores, are hardly fractured due to low thermal stresses caused by rapid uniform heating. The fracture model approach for studying shock-induced combustion is validated by a reasonable match of the computed ignition delay with experiments. The fragmentation history indicates a substantial increase in the particle surface area and temperature under shock exposure, as against conventional combustion, leading to an increased order of the burning rates at the onset of ignition, which can sustain through the entire burning phase.

Quantum and quasi-classical collisional dynamics of O2-Ar at high temperatures.

A hypersonic vehicle traveling at a high speed disrupts the distribution of internal states in the ambient flow and introduces a nonequilibrium distribution in the post-shock conditions. We investigate the vibrational relaxation in diatom-atom collisions in the range of temperatures between 1000 and 10 000 K by comparing results of extensive fully quantum-mechanical and quasi-classical simulations with available experimental data. The present paper simulates the interaction of molecular oxygen with argon as the first step in developing the aerothermodynamics models based on first principles. We devise a routine to standardize such calculations also for other scattering systems. Our results demonstrate very good agreement of vibrational relaxation time, derived from quantum-mechanical calculations with the experimental measurements conducted in shock tube facilities. At the same time, the quasi-classical simulations fail to accurately predict rates of vibrationally inelastic transitions at temperatures lower than 3000 K. This observation and the computational cost of adopted methods suggest that the next generation of high fidelity thermochemical models should be a combination of quantum and quasi-classical approaches.

Note: An improved solenoid driver valve for miniature shock tubes

A solenoid driver valve has been built to improve the operating performance of diaphragmless shock tubes, which are used for high pressure, high temperature chemical kinetics, and fluid mechanics studies. For shock tube driver application, the most important characteristics are those of sealing, strength, and quality of the generated shock waves and repeatability of opening characteristics and therefore subsequent post-shock conditions. The main features of the new driver valve are a face o-ring sealing design of the valve, the large internal volume, and through inserts near the solenoid core: adjustable opening characteristics of the valve.

Shock interaction with deformable particles using a constrained interface reinitialization scheme

In this paper, we present axisymmetric numerical simulations of shock propagation in nitromethane over an aluminum particle for post-shock pressures up to 10 GPa. We use the Mie-Gruneisen equation of state to describe both the medium and the particle. The numerical method is a finite-volume based solver on a Cartesian grid, that allows for multi-material interfaces and shocks, and uses a novel constrained reinitialization scheme to precisely preserve particle mass and volume. We compute the unsteady inviscid drag coefficient as a function of time, and show that when normalized by post-shock conditions, the maximum drag coefficient decreases with increasing post-shock pressure. We also compute the mass-averaged particle pressure and show that the observed oscillations inside the particle are on the particle-acoustic time scale. Finally, we present simplified point-particle models that can be used for macroscale simulations. In the Appendix, we extend the isothermal or isentropic assumption concerning the point-force models to non-ideal equations of state, thus justifying their use for the current problem.

Master Equation Study of Vibrational and Rotational Relaxations of Oxygen

Transition rates in collisions for each vibrational and rovibrational state are generated by means of the quasi-classical trajectory method using potential energy surfaces of different fidelities. The first potential energy surface, obtained via the double many-body expansion method, is adopted to obtain vibrational transition rates assuming transrotational equilibrium. The trajectory simulation on a simpler potential surface, based on the two-body pairwise interaction, generates a complete set of rates for each internal state. Vibrational and rotational relaxations of oxygen in a heat bath of parent atoms are modeled by a system of master equations at translational temperatures between 1000 and 20,000 K. It is shown that the vibrational relaxation becomes less efficient at high temperatures, in contrast with the conventional equation for relaxation time proposed by Millikan and White (“Systematics of Vibrational Relaxation,” Journal of Chemical Physics, Vol. null, No. null, 1963, Paper 3209). Rotational and vibrational relaxation times are the same order of magnitude in the range of temperatures observed in hypersonic flows. The excitation of the vibrational mode in collisions under typical postshock conditions occurs earlier than in other molecular systems. The exchange channel plays an important role in the internal energy randomization.

Systematic validation of non-equilibrium thermochemical models using Bayesian inference

The validation process proposed by Babuška et al. [1] is applied to thermochemical models describing post-shock flow conditions. In this validation approach, experimental data is involved only in the calibration of the models, and the decision process is based on quantities of interest (QoIs) predicted on scenarios that are not necessarily amenable experimentally. Moreover, uncertainties present in the experimental data, as well as those resulting from an incomplete physical model description, are propagated to the QoIs. We investigate four commonly used thermochemical models: a one-temperature model (which assumes thermal equilibrium among all inner modes), and two-temperature models developed by Macheret et al. [2], Marrone and Treanor [3], and Park [4]. Up to 16 uncertain parameters are estimated using Bayesian updating based on the latest absolute volumetric radiance data collected at the Electric Arc Shock Tube (EAST) installed inside the NASA Ames Research Center. Following the solution of the inverse problems, the forward problems are solved in order to predict the radiative heat flux, QoI, and examine the validity of these models. Our results show that all four models are invalid, but for different reasons: the one-temperature model simply fails to reproduce the data while the two-temperature models exhibit unacceptably large uncertainties in the QoI predictions.

Shock interaction with one-dimensional array of particles in air

In this paper, we present axisymmetric numerical simulations of shock propagation in air over an aluminum particle for Mach numbers up to 10. The numerical method is a finite-volume based solver on a Cartesian grid that allows for multi-material interfaces and shocks. Validation of the solver is demonstrated by comparing to existing experimental data. We compute the unsteady inviscid drag coefficient as a function of time, and show that when normalized by post-shock conditions, the maximum drag coefficient decreases with Mach number. Furthermore, for supercritical Mach numbers, we show that the inviscid steady-state drag asymptotes to a non-zero value due to the presence of a bow shock formed just upstream of the particle. Using this information, we also present a simplified point-particle force model that can be used for mesoscale simulations. Finally, we investigate the dynamics of a shock propagating over a 1-D array of particles aligned in the flow direction. We show that the maximum drag coefficient increases as the shock travels deep into the array and then asymptotes to a final value, which can be as high as 50% more than that of the first particle, depending on Mach number and particle spacing.

Dynamic interactive events in gene regulation using E. coli dehydrogenase as a model.

Different approaches in gene expression analysis always provide a snapshot view of cellular events. During the bacterial growth, the decisions are dynamically made with participation of various genes and their interactions with modulating factors. We have selected Escherichia coli dehydrogenases as a model to capture these interactions. We have treated the cells with hydrogen peroxide with very low level and asked the questions how cellular physiology has modulated itself to survive post-shock conditions. We hypothesized that while global regulators and associated gene network dictate the overall cellular intelligence, specific redox-sensitive classes of enzymes like dehydrogenase-mediated modulation could provide the option to cell for survival under peroxide after-effect. To understand the dynamic gene interaction, we used multidimensional scaling of genes and overlaid with minimum spanning tree to understand the clustering patterns under different conditions. Study shows that under peroxide after-effect, it is the interplay of ArcA (global regulator), with ldhA (involved in intermediary metabolism) and ndh (managing co-factor NADH), that emerges as modulating association. Knockout mutants of global regulators confirmed the promoter activity trend through gene expression change for dehydrogenases.

FORMATION OF MAGNETIZED PRESTELLAR CORES WITH AMBIPOLAR DIFFUSION AND TURBULENCE

We investigate the roles of magnetic fields and ambipolar diffusion during prestellar core formation in turbulent giant molecular clouds (GMCs), using three-dimensional numerical simulations. Our simulations focus on the shocked layer produced by a converging flow within a GMC, and survey varying ionization and angle between the upstream flow and magnetic field. We also include ideal magnetohydrodynamic (MHD) and hydrodynamic models. From our simulations, we identify hundreds of self-gravitating cores that form within 1 Myr, with masses M ~ 0.04 - 2.5 solar-mass and sizes L ~ 0.015 - 0.07 pc, consistent with observations of the peak of the core mass function (CMF). Median values are M = 0.47 solar-mass and L = 0.03 pc. Core masses and sizes do not depend on either the ionization or upstream magnetic field direction. In contrast, the mass-to-magnetic flux ratio does increase with lower ionization, from twice to four times the critical value. The higher mass-to-flux ratio for low ionization is the result of enhanced transient ambipolar diffusion when the shocked layer first forms. However, ambipolar diffusion is not necessary to form low-mass supercritical cores. For ideal MHD, we find similar masses to other cases. These masses are 1 - 2 orders of magnitude lower than the value that defines a magnetically supercritical sphere under post-shock ambient conditions. This discrepancy is the result of anisotropic contraction along field lines, which is clearly evident in both ideal MHD and diffusive simulations. We interpret our numerical findings using a simple scaling argument which suggests that gravitationally critical core masses will depend on the sound speed and mean turbulent pressure in a cloud, regardless of magnetic effects.

Unraveling the high-energy emission components of gamma-ray binaries

The high and very-high energy spectrum of gamma-ray binaries has become a challenge for all theoretical explanations since the detection of powerful, persistent GeV emission from LS 5039 and LS I +61 303 by Fermi/LAT. The spectral cutoff at a few GeV indicates that the GeV component and the fainter, hard TeV emission above 100 GeV are not directly related. We explore the possible origins of these two emission components in the framework of a young, non-accreting pulsar orbiting the massive star, and initiating the non-thermal emission through the interaction of the stellar and pulsar winds. The pulsar/stellar wind interaction in a compact orbit binary gives rise to two potential locations for particle acceleration: the shocks at the head-on collision of the winds and the termination shock caused by Coriolis forces on scales larger than the binary separation. We explore the suitability of these two locations to host the GeV and TeV emitters, respectively, through the study of their non-thermal emission along the orbit. We focus on the application of this model to LS 5039 given its well determined stellar wind with respect to other gamma-ray binaries. The application of the proposed model to LS 5039 indicates that these two potential emitter locations provide the necessary conditions for reproduction of the two-component high-energy gamma-ray spectrum of LS 5039. In addition, the ambient postshock conditions required at each of the locations are consistent with recent hydrodynamical simulations. The scenario based on the interaction of the stellar and pulsar winds is compatible with the GeV and TeV emission observed from gamma-ray binaries with unknown compact objects, such as LS 5039 and LS I +61 303.

Frequency and Severity of Trauma in Fishes Subjected to Multiple‐Pass Depletion Electrofishing

AbstractThe incidence and severity of trauma associated with multiple‐pass electrofishing and the effects on short‐term (30‐d) survival and growth of Rainbow Trout Oncorhynchus mykiss, Brook Trout Salvelinus fontinalis, and five representative co‐inhabiting nontarget or bycatch species were examined. Fish were held in four rectangular fiberglass tanks (190 × 66 cm) equipped with electrodes, a gravel–cobble stream substrate, and continuous water flow. Fish were exposed to one, two, or three electroshocks (100‐V, 60‐Hz pulsed DC) spaced 1 h apart or were held as a control. The heterogeneous field produced a mean (±SD) voltage gradient of 0.23 ± 0.024 V/cm (range = 0.20–0.30 V/cm) with a duty cycle of 30% and a 5‐s exposure. Radiographs of 355 fish were examined for evidence of spinal injuries, and necropsies were performed on 303 fish to assess hemorrhagic trauma in soft tissue. Using linear regression, we demonstrated significant relationships between the number of electrical shocks and the frequency and severity of hemorrhagic and spinal trauma in each of the nontarget species (Potomac Sculpin Cottus girardi, Channel Catfish Ictalurus punctatus, Fathead Minnow Pimephales promelas, Green Sunfish Lepomis cyanellus, and Largemouth Bass Micropterus salmoides). Most of the injuries in these species were either minor or moderate. Rainbow Trout and Brook Trout generally sustained the highest incidence and severity of injuries, but those injuries were generally independent of the number of treatments. The 30‐d postshock survival for the trout species was greater than 94%; survival for the bycatch species ranged from 80% (Fathead Minnow) to 100% (Green Sunfish and Channel Catfish). There were no significant differences in 30‐d postshock condition factors despite observations of altered feeding behavior lasting several days to 1 week posttreatment in several of the study species.Received June 27, 2012; accepted November 20, 2012

Supermassive black hole formation by cold accretion shocks in the first galaxies

We propose a new scenario for supermassive star (SMS;>10^5Msun) formation in shocked regions of colliding cold accretion flows near the centers of first galaxies. Recent numerical simulations indicate that assembly of a typical first galaxy with virial temperature (~10^4K) proceeds via cold and dense flows penetrating deep to the center, where the supersonic streams collide each other to develop a hot and dense (~10^4K, ~10^3/cc) shocked gas. The post-shock layer first cools by efficient Ly alpha emission and contracts isobarically until 8000K. Whether the layer continues the isobaric contraction depends on the density at this moment: if the density is high enough for collisionally exciting H2 rovibrational levels (>10^4/cc), enhanced H2 collisional dissociation suppresses the gas to cool further. In this case, the layer fragments into massive (>10^5Msun) clouds, which collapse isothermally (~8000K) by the Ly alpha cooling without subsequent fragmentation. As an outcome, SMSs are expected to form and evolve eventually to seeds of supermassive black holes (SMBH). By calculating thermal evolution of the post-shock gas, we delimit the range of post-shock conditions for the SMS formation, which can be expressed as: T>6000K/(n/10^4/cc) for n<10^4/cc and T>5000-6000K for n>10^4/cc, depending somewhat on initial ionization degree. We found that metal enrichment does not affect the above condition for metallicity below 10^-3Zsun if metals are in the gas phase, while condensation of several percent of metals into dust decreases this critical value of metallicity by an order of magnitude. Unlike the previously proposed scenario for SMS formation, which postulates extremely strong ultraviolet radiation to quench H2 cooling, our scenario naturally explains the SMBH seed formation in the assembly process of the first galaxies, even without such a strong radiation.

Shock tube study of n-decane ignition at low pressures

Ignition delay times for n-decane/O2/Ar mixtures were measured behind reflected shock waves using endwall pressure and CH* emission measurements in a heated shock tube. The initial postshock conditions cover pressures of 0.09–0.26MPa, temperatures of 1 227–1 536 K, and oxygen mole fractions of 3.9%–20.7% with an equivalence ratio of 1.0. The correlation formula of ignition delay dependence on pressure, temperature, and oxygen mole fraction was obtained. The current data are in good agreement with available low-pressure experimental data, and they are then compared with the prediction of a kinetic mechanism. The current measurements extend the kinetic modeling targets for the n-decane combustion at low pressures.

Effects of Anisotropy on the Microstructural Characteristics and Mechanical Behavior of Shock-Loaded of AZ31 Alloy

Effects of anisotropy on the microstructural characteristics and mechanical behavior of shock loaded of AZ31 magnesium alloy have been investigated. Using electron backscatter diffraction, tension twinning was observed in both shock loading directions along the normal (ND) and rolling directions (RD). Compression tests were carried out along ND and RD in both as-received and post-shock conditions. It indicated that the RD samples show a more notable hardening behavior compared to the as-received conditions. Moreover, it is postulated here that detwinningresults in a drop of strain-hardening rate for the ND samples under post shock reload conditions and tension twinning formed during the shock wave loading process leads to a significant moving left of the peak strain hardening rate for the RD samples under post shock reload conditions.

Development of Shock Tube for Ground Testing Reentry Aerothermodynamics

A free-piston double-diaphragm shock tube with 70×70 mm cross-section at test section is newly developed to investigate thermochemical nonequilibrium phenomena behind a shock wave. This paper presents the performance of the shock tube and the measurement system newly developed. Experimental investigations to clarify its characteristics are conducted for various operational parameters, such as rupture pressure of the first diaphragm and initial pressures in the low-pressure tube, the high-pressure tube, and the compression tube. Based on the characteristics experiment, a performance map of the shock tube is obtained by changing the operation parameters. The result shows that the shock tube can generate the post-shock condition corresponding to super-orbital reentry flight conditions. The newly developed measurement system enables us to obtain a spatial distribution of spectra behind a shock wave with high spatial and time resolution. A description of the measurement system and typical examples of the measured spectra are presented.

Maximum intensity of rarefaction shock waves for dense gases

Modern thermodynamic models indicate that fluids consisting of complex molecules may display non-classical gasdynamic phenomena such as rarefaction shock waves (RSWs) in the vapour phase. Since the thermodynamic region in which non-classical phenomena are physically admissible is finite in terms of pressure, density and temperature intervals, the intensity of RSWs is expected to exhibit a maximum for any given fluid. The identification of the operating conditions leading to the RSW with maximum intensity is of paramount importance for the experimental verification of the existence of non-classical phenomena in the vapour phase and for technical applications taking advantage of the peculiarities of the non-classical regime. This study investigates the conditions resulting in an RSW with maximum intensity in terms of pressure jump, wave Mach number and shock strength. The upstream state of the RSW with maximum pressure drop is found to be located along the double-sonic locus formed by the thermodynamic states associated with an RSW having both pre- and post-shock sonic conditions. Correspondingly, the maximum-Mach thermodynamic and maximum-strength loci locate the pre-shock states from which the RSW with the maximum wave Mach number and shock strength can originate. The qualitative results obtained with the simple van der Waals model are confirmed with the more complex Stryjek–Vera–Peng–Robinson, Martin–Hou and Span–Wagner equations of state for selected siloxane and perfluorocarbon fluids. Among siloxanes, which are arguably the best fluids for experiments aimed at the generation and measurement of an RSW, the state-of-the-art Span–Wagner multi-parameter equation of state predicts a maximum wave Mach number close to 1.026 for D6 (dodecamethylcyclohexasiloxane, [O-Si-(CH3)2]6). Such value is well within the capacity of the measurement system of a newly built experimental set-up aimed at the first-ever demonstration of the existence of RSWs in dense vapours.

Shock Tube Study of Methylcyclohexane Ignition over a Wide Range of Pressure and Temperature

Ignition delay times were measured for gas-phase methylcyclohexane (MCH)/O2/argon and MCH/air mixtures behind reflected shock waves. Initial postshock conditions covered temperatures of 795−1560 K, pressures of 1−50 atm, fuel concentrations of 0.25−2%, and equivalence ratios (ϕ) of 0.5−2.0. Ignition delay times were measured using side-wall pressure and CH* and OH* emission measurements. Current measurements complement past high-pressure rapid compression machine results, are in good agreement with past low-pressure shock tube data, and significantly extend the pressure range of available shock tube ignition time data. Detailed comparisons of experimental data with predictions of available MCH mechanisms are presented, and comparisons of shock tube MCH ignition delay times to those of other important jet fuel surrogates and cyclo-alkanes are discussed.