Showing papers on "Shock tube published in 1984"

Hydrodynamic aspects of caldera‐forming eruptions: Numerical models

Abstract: Comparison of results from a two-dimensional numerical eruption simulation (KACHINA) to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera-forming ash flow eruptions may be dominated by blast wave phenomena. Field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera-related ash flow sheets, on top of a preceding Plinian fall deposit (ground surge), and separating individual ash flow units. We model the eruption of the Tshirege member of the Bandelier Tuff (1.1 Ma B.P.) from the Valles caldera, New Mexico. In the model a magma chamber at 100 MPa (1 kbar) and 800°C is volatile rich, with an average H2O abundance above saturation greater than 8.7 wt % increasing to nearly 100 wt % near the very top of the chamber. Using a shock tube analogy, decompression of the chamber through a wide-open dikelike vent 0.1 km wide and 1 to 5 km long forms a shock wave of 3 MPa (≃3O atm) with a velocity greater than 1.0 km s−1. Steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7-km depth from the ground surface to a reflective (density) boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s−1. The velocity of the ash front behind the shock wave is 300 to 500 m s−1. The shock tube model serves as a basis to evaluate the consistency of the KACHINA code results which are similar to a one-dimensional problem along the symmetry axis. The results of the KACHINA simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly Prandtl-Meyer expansion in generating compressive wave fronts following the initial shock. The rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock. Furthermore, these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash. The blast wave shocks ambient air to higher pressures and temperatures, the magnitudes of which depend strongly on the initial chamber overpressure, distance, and direction from the vent. In consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of ≃0.2 to 0.7 MPa (2–7 atm) and temperatures of ≃200° to 300°C for distances to 10 km from the Bandelier vent(s).

Shocks generated in a confined gas due to rapid heat addition at the boundary. II: Strong shock waves

Abstract: An inert compressible gas, confined between infinite parallel planar walls, is in an equilibrium state initially. Subsequently energy is added at the boundary during a period that is short compared to the acoustic time of the slot $t'\_a$ (the wall spacing divided by the equilibrium sound speed), but larger than the mean time between molecular collisions. Conductive heating of a thin layer of gas adjacent to the wall induces a gas motion arising from thermal expansion. The small local Mach number at the layer edge has the effect of a piston on the gas beyond. A linear acoustic wave field is then generated in a thicker layer adjacent to the walls. Eventually nonlinear accumulation effects occur on a timescale that is longer than the initial heating time but short compared with $t'\_a$. A weak shock then appears at some well defined distance from the boundary. If the heating rate at the wall is maintained over the longer timescale, then a high temperature zone of conductively heated expanding gas develops. The low Mach number edge speed of this layer acts like a contact surface in a shock tube and supports the evolution of the weak shock propagating further from the boundary. One-dimensional, unsteady solutions to the complete Navier-Stokes equations for an inert gas are obtained by using perturbation methods based on the asymptotic limit $t'\_a$/$t'\_c \rightarrow$ 0, where $t'_c$, the conduction time of the region, is the ratio of the square of the wall spacing to the thermal diffusivity in the initial state. The shock strength is shown to be related directly to the duration of the initial boundary heating.

Collapse of multiple gas bubbles by a shock wave and induced impulsive pressure

Abstract: The problem of bubble‐bubble interaction is studied experimentally. The motions of multiple gas bubbles attached to a solid wall by a shock wave are observed by using a high speed camera, and the induced impulsive pressures are measured. On the basis of these results, the effects of number and configuration of gas bubbles on the collapsing process and the impulsive pressure are clarified. The maximum impulsive pressure quickly decreases with reducing the interval beteween bubbles due to significant interaction. The direction of a liquid jet formed within a bubble is determined as a resultant of effects such as shock direction, shock strength, and interactions between bubbles and between a bubble and a solid wall.

The shock wave ignition of dusts

Abstract: Dust explosions pose a serious hazard in many industries. The detonability and flam inability of dust/oxidizer mixtures depend on the ignition delay of the dust particles when suddenly exposed to a high temperature environment. Consequently, the ignition delay time of dust particles behind a shock wave in the Mach number range of 4.0-5.0 has been measured using a photomultiplier tube to determine the onset of ignition. The dusts investigated included Pittsburgh Seam Coal, graphite, diamond, oats, and RDX. The experimental arrangement, consisting of a shock tube and two different dust injection devices, is described in detail, and experimental results for dusts ranging in particle size from 2 to 74 /*m are presented. In the Mach number range considered, ignition delay times varied from 2 to 100 /*s. A detailed analytical model based on a solution of the heat conduction equations for the particle interior coupled with a solution of the particle equation of motion has been developed. Heterogeneous reactions occurring on the particle surface and in the pores within the particle are used to model the chemistry. The results were in reasonable agreement with most of the data. Approximate analyses based on a comparison of characteristi c thermal and chemical times were also developed. A key conclusion is that the ignition delay is determined mainly by the heat-up time of the particle surface.

Rapid-tuning frequency-doubled ring dye laser for high resolution absorption spectroscopy in shock-heated gases

Abstract: This letter describes a ring dye laser system and its initial application to fully resolved absorption line shape measurements of the OH molecule during the brief test time available following shock waves generated in a shock tube. The operating wavelength has been extended into the UV through intercavity second harmonic generation.(AIP)

A shock tube study of the reaction of the hydroxyl radical with propane

Abstract: The rate coefficient for the reaction of the hydroxyl radical, OH, with propane has been measured at 1220 K in shock tube experiments, and a value of (1.58 ± 0.24) × 1013 cm3/mol s was obtained. This measured value is compared with previous experimental results and a transition-state theory calculation.

Shock tube and modeling study of acetylene oxidation

Abstract: The oxidation of C/sub 2/H/sub 2/ was studied behind incident shock waves in the temperature range 1300-2200 K with laser schlieren techniques. A computer simulation study was performed to determine the important elementary reactions. Data analysis using a 23-reaction mechanism showed that relatively few steps play significant roles in determining ignition behavior. The mechanism was tested against a variety of experimental data in the literature.

Schlieren photography of second‐sound shock waves in superfluid helium

Abstract: A conventional schlieren system has been used to study second‐sound shock waves produced in a shock tube with optical windows. Initially planar shocks are observed to remain so, even after several reflections from the end walls. No interaction of the shocks with the side wall boundary layers is seen, attesting to the thinness of these layers. Second‐sound shocks are reflected from the liquid–vapor interface, producing transmitted gasdynamic shocks and reflected second‐sound shocks. Several strong shocks are fired with short separation times, generating observable fluctuations in the fluid.

A diaphragmless shock tube

Abstract: A pneumatic valve has been designed to replace diaphragms in shock tubes and a diaphragmless shock tube has been constructed for use with the pneumatic valve. Performance tests have been carried out for the shock tube. The present apparatus is shown to be capable of producing shock waves up to 3.5 in Mach number and be convenient to use. It is also shown that the flow in the driven section of the shock tube is equivalent to that in a conventional shock tube.

Oscillation modes in single‐step Hartmann–Sprenger tubes

Abstract: When a high velocity jet is directed toward the mouth of a tube closed at the downstream end, large, nonlinear flow oscillations may occur within the tube. Shock waves propagate up and down the tube and generate strong heating of the tube walls. The device is called a Hartmann–Sprenger tube (in short, H–S tube). It has been proposed by several investigators to strengthen the shock waves and consequently the heating effects by tapering the tube. Conical and multistepped configurations have been investigated, but also tubes having a sudden area contraction (single step). This latter geometry produces remarkable pressure and thermal heating amplification compared to a constant area tube. The paper presents theoretical and experimental results obtained for the single‐step H–S tube. It is shown that several oscillation modes can exist. These are separated by intervals of instability. The influence of geometrical parameters (upstream to downstream cavity diameter and length ratios) and of jet Mach number on osc...

Turbulent boundary layer behind constant velocity shock including wall blowing effects

Abstract: A theory, previously presented by the author, concerning turbulent boundary-layer development behind a shock moving with uniform speed is used to obtain numerical results for turbulent boundary-layer properties in air. Numerical results are tabulated for shock propagation at Mach numbers in the range 1.01

The gas-phase decomposition of methylsilane. Part I. Mechanism of decomposition under shock-tube conditions

Abstract: The gas-phase decompositions of methylsilane and methylsilane-d3 have been investigated in a single-pulse shock tube at 4700 torr total pressure in the temperature range of 1125–1250 K. For CH3SiD3 at 1200 K three primary steps occur in the homogeneous decomposition with efficiencies in parentheses: , , and . For CH3SiH3 at 1200 K the primary CH4 elimination efficiency is 0.09 while the total primary H2 elimination efficiency is 0.91. Minor product formations of C2H4, acetylene, dimethylsilane, and SiH4 are discussed.

Shock tube study of the thermal decomposition of cyanogen

Abstract: The thermal decomposition of cyanogen behind incident shock waves has been studied in the temperature range 2500–3450 K, at pressures between 0.23 and 0.58 bar. The course of the reaction was followed by monitoring the CN (B 2Σ+, v=0←X 2Σ+, v=0) absorption at 388 nm. The low‐pressure rate coefficient for the reaction C2N2+Ar→2CN+Ar was fitted by the Arrhenius expression k1=1016.8±0.17 exp[−(50 040±1070)/T]cm3/mol s. Comparisons of the present work with previously published results are discussed.

Single pulse shock tube study on the thermal stability of ketones

Abstract: 5-Methyl-hexanone-2, 3-methyl-pentanone-2, and hexanone-2 have been decomposed in comparative rate single pulse shock tube experiments The mechanism of decomposition involves the breaking of carbon-carbon bonds as well as molecular processes involving 6-center complexes The following rate expressions at 1100 K have been obtained: These results lead to ΔHf(CH3ĊO) = − 138 kJ and ΔHf(CH3COCH2·) = − 126 kJ at 300 K They are compared with existing literature values and some generalizations are made with regard to the stability of carbonyl compounds

Temperatures in Imploding Detonation Waves

Abstract: The spectroscopic and electron temperatures in spherically-imploding detonations in a stoichiometric propane-oxygen mixture were measured and the gas temperature behind the shock waves at cylindrical and spherical implosion fronts was estimated from the probability of triple shock formation. Both the spectroscopic and electron temperatures increase with the propagation of detonation as rapidly as the pressure. The temperature behind the shock waves is in agreement with the theoretical value. The results suggest that the high-pressure peak appears not behind the shock waves but in the combustion zone of the detonation. The reason for this is attributed to reactions caused by shock waves reflected at the vessel wall releasing more heat than in normal combustion, by other exothermic processes such as recombination.

High Temperature Combustion of Selected Chlorinated Hydrocarbons.

Abstract: A study combining ignition delay measurements and quenched product distributions from shock-tube experiments with chlorinated hydrocarbons has been conducted to develop a better understanding of the combustion characte ristics of these compounds which are candidates for incineration. The ignition de lay times of selected C lr C 2 and C 6 chlorinated hydrocarbons toichiometric oxygen mixtures have been measured behind reflected shock waves at a pressure of 1.8 atm over the temperature range 12001700 K. Studies were also conducted with mixtures of chlorinated and n o n ­ chlorinated hydrocarbons to examine the effect of chlorine atom/ hydrogen atom ratio on ignition delay behavior. The results indicate that contrary to conventional wisdom the chlorinated hydrocarbons are not more difficult to ignite than the analogous hydrocarbon. Quenched product distri butions in shocktube studies of the pyrolysis and oxidation of methane, methyl chloride and dichloromethane were determined at a total density of 7.5i 0.5x 10'7 mol/cc over the temperature range 12002700 K. The product distrib utions indicate that there is a much larger propensity to pro duce soot and prior ity organic pollutants as the chlorine atom/ hydrogen atom ratio of the reactants increased. The first chemical kinetic m e c hanism including detailed chem istry for the C] and C 2 chlorinated hydrocarbons has been developed. The m e c hanism contains 432 reactions and 59 chemical species. This model was used to identify the importance of C 2 chlorinated hydrocarbons during the pre-ignition oxidation of methyl chloride.

A shock tube study of vibrational relaxation of O2 in argon by small amounts of H2 (860–1290 K), D2 (890–1070 K), and He (1000–1500 K)

Abstract: Vibrational relaxation times of O2 in a mixture of 2% O2 in Ar were measured behind reflected shock waves at temperatures of 1000–1600 K and total pressures of 2–3 atm. The effects of small amounts (0.05%–0.3%) of H2 (860–1290 K), D2 (890–1070 K), and He (1000–1500 K) on the relaxation times were measured, the effects being consistent with the linear mixture rule. At the D Ly‐α wavelength (121.6 nm), new values of the absorption cross sections for vibrationally excited O2 are σ1 (for v=1)=(3.4±1)×10−19 cm2, and σ2 (for v=2)=(5±2)×10−18 cm2.

The dependence of thermal shock wave velocity on heat flux in helium II

Abstract: When thermal shock waves propagate in helium II their velocity is related to the heat flux behind the wave. The authors derive this relation theoretically and compare it with the theoretical approach of Lutset and co-workers (1981) and the experimental results of Cummings and co-workers (1978). They also comment on the remarks made by Turner (1981) concerning the observation of a critical velocity in shock wave experiments.

The reflection of weak shock waves from absorbent surfaces

Abstract: Previous models of the transient behaviour of air within the cavities of a layer of porous material on a solid backplate dealt only with the influence of viscous resistance to motion within the porosities. The short timescales implicit in the reflection of a weak blast wave from such a layer require the inclusion of inertia effects together with those of viscous resistance. The first part of the present study is devoted to a calculation of the velocity of inflow to the porous layer, from which all other quantities, such as reflected shock strength, can be calculated. The model assumes that the porous material consists of an aggregate of long thin tubes, of different and irregular lengths; the porous layer is attached to a solid backplate. The results of this analysis are compared with experimental observations that were made specifically to test the theory, by using a small shock tube that had its low-pressure end closed by a solid-backed porous plug of polyurethane foam.

Interaction of Weak Shock Waves and Discrete Gas Inhomogeneities

Abstract: An experimental investigation of the interaction of shock waves with discrete gas inhomogeneities is conducted in the GALCIT 15 cm diameter shock tube. The gas volumes are cylindrical refraction cells of 5 cm diameter with a 0.5 µm thick membrane separating the test gas (helium or Freon 22) from the ambient air and large spherical soap bubbles containing the same gases. The incident wave Mach numbers are nominally 1.09 and 1.22. The wave pattern and the deformation of the gas volumes are documented by shadowgraphs. The transmitted and diffracted wave pressure profiles are recorded by pressure transducers at various distances behind the cylinders. The basic phenomena of acoustic wave refraction, reflection and diffraction by cylindrical acoustic lenses, with indices of refraction appropriate to the gases used in the experiments, are illustrated with computer-generated ray and wave-front diagrams. In the case of a Freon 22-filled cylinder, the wave diffracted externally around the body precedes the wave transmitted from the interior which goes through a focus just behind the cylinder, while in the case of the helium-filled cylinder the expanding transmitted wave runs ahead of the diffracted wave. Both sets of waves merge a few cylinder diameters downstream. The wave patterns inside the cylinder, showing initially the refracted waves and later the same waves reflected internally, present some interesting phenomena. The mechanisms by which the gas volumes are transformed into vertical structures by the shock motion are observed. The unique effect of shock acceleration and Rayleigh-Taylor instability on the spherical volume of helium leads to the formation of a strong vortex ring which rapidly separates from the main volume of helium. Measurements of the wave and gas-interface velocities are compared to values calculated for one-dimensional interactions and for a simple model of shock-induced Taylor instability. The behavior of thin liquid membranes accelerated by shocks under varying conditions is documented by high speed photography. In a related experiment, shock waves of Mach number between 1.005 and 1.36 interact with a dense random array of 2 mm diameter helium filled soap bubbles. Experimental results (based on shadowgraphs and pressure measurements) show that very weak shock waves (Ms ≤ 1.01) are strongly scattered by the array, which is left undisturbed by the shock, and that stronger shock waves, only locally disturbed by each bubble, maintain undisturbed pressure profiles because of nonlinear effects, while the array undergoes shock-induced mixing. A simple criterion for multiple scattering shows that the combined effect of many bubbles is necessary in order to produce important modifications on the shock wave pressure profile.

Propagation Velocity and Mechanism of Bubble Detonation

Abstract: In a previous paper (Hasegawa and Fujiwara 1982), observations of shock wave propagation in glycerin with 70%Ar+30%(2H2+02) bubbles and associated chain explosion of the bubbles in a vertical shock tube were reported. This phenomenon was called "bubble detonation," because the blast wave generated by a bubble explosion compressed the nearest bubble to explode and this sequence sustained the propagation of explosion. In this work, the bubble detonation was further investigated from the viewpoint of steady propagation by measurements of the decay of the pressure amplitudes at three points, and by observations of the interaction between the two adjacent bubbles using a high-speed framing camera. The results obtained were as follows: 1) The propagation of bubble detonations was observed to be nonsteady, and the velocity and pressure amplitudes decreased conceivably due to the insufficient length of the shock tube. However, observations revealed that the pressure amplitude approached a constant value. 2) The pressure wave accompanying bubble detonations had two typical features, a weak precursor wave followed by a strong shock wave. 3) The reciprocal compression-explosion process of bubbles provided the propagation mechanism of bubble detonation. 4) A model of the propagation velocity, which was reduced from the observed behavior of the bubbles, explained the measured detonation velocity and suggested a relation on the velocity dependency upon the pressure amplitude.