Showing papers in "Shock Waves in 1992"

Gaseous detonation driver for a shock tunnel

Abstract: The concept of a shock tunnel with gaseous detonation driver is discussed. A detonation driver presents an alternative to a free-piston driver because comparable values of high enthalpy can be attained, however, without the fast movement of a heavy piston. Wave diagrams, pressure and temperature distributions are presented. Finally, first experimental results are given.

Nonideal thermoequilibrium calculations using a large product species data base

Abstract: Thermochemical data fits for approximately 900 gaseous and 600 condensed species found in the JANAF tables (Chase et al. 1985) have been completed for use with the TIGER non-ideal thermoequilibrium code (Cowperthwaite and Zwisler 1973). The TIGER code has been modified to allow systems containing up to 400 gaseous and 100 condensed constituents composed of up to 50 elements. Gaseous covolumes have been estimated following the procedure outlined by Mader (1979) using estimates of van der Waals radii for 48 elements and three-dimensional molecular mechanics. Molecular structures for all gaseous components were explicitly defined in terms of atomic coordinates in A (Hobbs and Baer 1992a). The Becker-Kistiakowsky-Wilson equation of state (BKW-EOS) has been calibrated near C-J states using detonation temperatures measured in liquid and solid explosives and a large product species data base. Detonation temperatures for liquid and solid explosives were predicted adequately with a single set of BKW parameters. Values for the empirical BKW constants α,β, κ, and θ were 0.5, 0.174, 11.85, and 5160, respectively. Values for the covolume factors, κi, were assumed to be invariant. The liquid explosives included mixtures of hydrazine nitrate with hydrazine, hydrazine hydrate, and water; mixtures of tetranitromethane with nitromethane; liquid isomers ethylnitrate and 2-nitroethanol; and nitroglycerine. The solid explosives included HMX, RDX, PETN, Tetryl, and TNT. Color contour plots of HMX equilibrium products as well as thermodynamic variables are shown in pressure and temperature space. Similar plots for a pyrotechnic reaction composed of TiH2 and KClO4 are also reported. Calculations for a typical HMX-based propellant are also discussed.

A numerical study of shock wave reflections on low density foam

Abstract: A continuum mixture theory is used to describe shock wave reflections on low density open-cell polyurethane foam. Numerical simulations are compared to the shock tube experiments of Skews (1991) and detailed wave fields are shown of a shock wave interacting with a layer of foam adjacent to a rigid wall boundary. These comparisons demonstrate that a continuum mixture theory describes well the shock interactions with low density foam.

The phenomena of shock wave reflection — a review of unsolved problems and future research needs

Abstract: Although the phenomenon of shock wave reflection was discovered more than a hundred years ago, active research related to this phenomenon still goes on in many countries in the world (e.g., Australia, Canada, China, Germany, Israel, Japan, Poland, Russia and United States of America). As a matter of fact the research activity increased so drastically in the past decade and a half that a special scientific meeting dedicated to better understanding the reflection phenomena of shock waves, namely “The International Mach Reflection Symposium” was initiated in 1981 and was held since then in the major research centers actively involved in the research of shock wave reflections. In the present paper the status of the research of the phenomenon of shock wave reflection will be discussed in general, and unresolved problems and future research needs will be pointed out.

Vorticity produced by shock wave diffraction

Abstract: Numerical simulations with a monotonicity preserving flow solver have been performed to study shock diffraction phenomena and shock wave generated vorticity. The computations were performed using the conservative Finite Element Method-Flux Corrected Transport (FEM-FCT) scheme, which has been shown to have an excellent predictive capability for various compressible flows with both strong and weak shocks. Adaptive unstructured methodology based on adapting to high density and entropy gradients was used in conjunction with a conservative shock-capturing scheme to adequately resolve storng and weak flowfield gradients. The chief interest was therefore, a topic of vortices (original paper:the formation of vorticity) arising from shock wave propagation over a sharp corner and the high accuracy and resolution of the interacting compressible wave features. Numerical simulations were compared with previous experimental results and exhibited remarkably good agreement in terms of compressible wave propagation, as well as vorticity development and transport. The computations also allowed insight to the fundamental fluid dynamics, spencifically shock diffraction, vortex convection and shock-vortex interactions.

Shock wave structure in a mixture of gas, vapour and droplets

Abstract: The structure of the relaxation zone behind a shock wave of moderate strength in a mixture of gas, vapour and droplets is analysed. A model is presented for shock induced evaporation, which is based on wet-bulb equilibrium and on the absence of relative motion between droplets and gas. Experimental and numerical data on heterogeneous condensation induced by an unsteady rarefaction wave and on re-evaporation due to shock wave passage are reported for a mixture of water vapour, nitrogen gas and condensation nuclei. Pressure, temperature, saturation ratio and droplet size are experimentally obtained and are very well predicted by a numerical simulation based on the non-linear quasisteady wet-bulb model for phase transition, as well for the expansion wave as for the shock wave. During expansion, droplet number density decays much faster than predicted, which is not yet satisfactorily explained. Shock induced droplet evaporation is studied for post-shock saturation ratios ranging from 5×10−3 to 0.2, corresponding to shock Mach numbers of 1.2 to 1.9. The evaporation times are well predicted by the theoretical model. No evidence is found for droplet break-up for Weber numbers up to 13, and droplet radii of the order of 1μm.

A weak shock wave reflection over wedges

Abstract: When a weak shock wave reflects from wedges its reflection pattern does not appear to be a simple Mach reflection. This reflection pattern is known to be von Neumann Mach reflection in which a Mach stem can not necessarily be straight. In this paper the local change of the Mach stem curvature was experimentally and numerically investigated. A distinct triple point, at which the curvature becomes infinite as appears in a simple Mach reflection, was not observed but the Mach stem curvature became a maximum between foot of the Mach stem and a point, P1, at which an incident shock met with a reflected shock. Maximum curvature point P2 and P1 do not coincide for small wedge angles and tend to merge over a certain wedge angle. Experimental results agreed with numerical results. The trajectory angle of P2 was found to be expressed well by Whitham's shock-shock angle.

A revised three-shock solution for the Mach reflection of weak shocks (1.1<Mi<1.5)

Abstract: It is well known that the classical three-shock theory of von Neumann (1943) does not adequately describe the configuration of the shocks close to the triple-point of a Mach reflection of an incident shock with a Mach number less than about 1.5. The assumptions on which the three-shock theory is based have been examined and several of them are shown to be invalid. The assumption that may be of most significance is that the normal components of the flows behind the reflected and the Mach stem shocks are parallel. Dropping this assumption removes an essential equation in the three-shock solution. An alternative assumption, based on experimental observation, is that there is an approximate linear relationship between the pressure behind the reflected shock and the triple-point trajectory angle. This assumption permits a revised three-shock solution which gives results that are in agreement with experimental observations of reflections of incident shocks with Mach numbers between 1.1 and 1.5.

Seismic excitation by space shuttles

Abstract: Shock waves generated by the space shuttles Columbia (August 13, 1989), Atlantis (April 11, 1991) and Discovery (September 18, 1991) on their return to Edwards Air Force Base, California, were recorded by TERRAscope (Caltech's broadband seismic network), the Caltech-U.S.G.S Southern California Seismic Network (SCSN), and the University of Southern California (USC) Los Angeles Basin Seismic Network. The spatial pattern of the arrival limes exhibits hyperbolic shock fronts from which the path, velocity and altitude of the space shuttle could be determined. The shock wave was acoustically coupled to the ground, converted to a seismic wave, and recorded clearly at the broadband TERRAscope stations. The acoustic coupling occurred very differently depending on the conditions of the Earth's surface surrounding the station. For a seismic station located on hard bedrock, the shock wave (N wave) was clearly recorded with little distortion. Aside from the N wave, very little acoustic coupling of the shock wave energy to the ground occurred at these sites. The observed N wave record was used to estimate the overpressure of the shock wave accurately; a pressure change of 0.5 to 2.2 mbars was obtained. For a seismic station located close to the ocean or soft sedimentary basins, a significant amount of shock wave energy was transferred to the ground through acoustic coupling of the shock wave and the oceanic Rayleigh wave. A distinct topography such as a mountain range was found effective to couple the shock wave energy to the ground. Shock wave energy was also coupled to the ground very effectively through large man made structures such as high rise buildings and offshore oil drilling platforms. For the space shuttle Columbia, in particular, a distinct pulse having a period of about 2 to 3 seconds was observed, 12.5 s before the shock wave, with a broadband seismograph in Pasadena. This pulse was probably excited by the high rise buildings in downtown Los Angeles which were simultaneously hit by the space shuttle shock waves. The proximity of the natural periods of the high rise buildings and the modal periods of the Los Angeles basin enabled efficient energy transfer from shock wave to seismic wave.

Propagation mechanism of dust detonations

Abstract: This paper summarizes the studies on dust detonations at the Stosswellenlabor of RWTH Aachen since 1987. The onset and propagation mechanism of heterogeneous dust detonations are similar to those of marginal gas phase detonations. A self-sustained dust detonation has transverse wave structures that provide the coupling between shock and reaction. Large transition distances and transverse wave spacings require large sized tubes for the propagation of self-sustained dust detonations. The Hugoniot analysis of the Chapman-Jouguet detonation predicts equilibrium detonation states being in reasonable agreement with the self-sustained dust detonations observed. Shock matching calculations at the triple point adequately determine the wave structures of those stable dust detonations.

Stagnation point heat flux in hypersonic high enthalpy flow

Abstract: The paper describes stagnation point heat flux measurements at a range of enthalpies relevant to re-entry speeds of aero-assisted space transfer vehicles (ASTVs) and proposed space planes, using the Australian National University Free Piston Shock Tunnel T3. The unique feature of these experiments is that they were conducted in the ‘straight through’ (reflectionless) mode which enabled higher enthalpies and densities hitherto unattained.

Blast wave measurements close to explosive charges

Abstract: The paper reports the results of experimental measurements of the reflection pressures close to spherical charges of TNT. These measurements were made using a pressure bar technique. Charge weights of up to 3.6 kg were used, with the reflecting plane in the range 25 to 300 mm from the charge surface.

Ignition and growth of a detonation by a high energy plasma

Abstract: Many experimental and numerical studies have been achieved to describe the transition process of deflagration to detonation when a projectile impacts an explosive. Also a large work has been done for the determination of various parameters — such as the impact pressure, the efficiency factors, etc. of the laser — material interaction. When a laser beam impacts an explosive, the P2 τ criterion, characteristic of shock initiated detonations, is no longer valid due to the generated hot plasma whose effect is to decrease the DDT (Deflagration to Detonation Transition) duration. The present paper deals with a modelling of the plasma-explosive medium allowing the determination of distances and times of the DDT process. The two phase modelling of the granular explosive takes into account the creation of hot spots. The pressure of the plasma is computed using a semi empirical model, while the temperature is obtained from Maxwell Boltzmann statistics. The authors focused their attention on the equation of state for the detonation products and the numerical process.

In the footsteps of Ernst Mach — A historical review of shock wave research at the Ernst-Mach-Institut

Abstract: The aim of this paper is to recall some of the historical work on shock waves and to give a brief survey of research activities at the Ernst-Mach-Institut (EMI). Some fundamental results of Ernst Mach (1838 – 1916) are demonstrated and historical remarks are given to the shock tube as an important tool in shock wave research. The activity at EMI in this field was initiated by Prof. H. Schardin (1902 – 1965) in 1955 and has since been continued. Propagation processes of shock and blast waves, blast loading phenomena, shock attenuation, shock reflection at various surfaces, development of new types of blast simulators, electromagnetically driven T-tubes, precursor and decursor phenomena are only a few examples of research topics at EMI that will be discussed.

The Life and Work of Pierre Henri Hugoniot

Abstract: None of the scientific shock wave community will deny that the name of Hugoniot has become, as it were, a password But who was this man, whose life was as brief as W A Mozart’s or V Van Gogh’s, and whose most famous work appeared posthumously one century ago, two years after being written? Which point did he make, that B Riemann and W J M Rankine failed to? Through which later contributions - J Crussard, E Jouguet, H A Bethe, H Weyl - did we get to our present understanding of the jump relations? This historical perspective is a modest attempt to better the knowledge of a field which has so deeply influenced theoretical physics, and has also allowed tremendous applications which still control the history of the world

The Mach reflection triple-point locus for internal and external conical diffraction of a moving shock wave

Abstract: This paper examines the different behavior that occurs for the Mach reflection triple-point loci between the two fundamental axisymmetric cases, these being the external diffraction by a cone and the internal diffraction within a conically contracting channel. From equations derived in this paper using a shock dynamics approach, it has been shown that, for external diffraction over a cone, a possible solution is that the triple-point locus is a straight line which corresponds to the experimental results available, while for internal diffraction along a conically converging channel, it cannot be straight and is, in fact, a convex curve. In the latter case, a transition point is noted on the triple-point locus before which the locus is nearly straight but after which the curvature becomes marked. The second region diminishes as a proportion of the total locus with decreasing half cone angle.

An exact Riemann solver for detonation products

Abstract: Numerical methods based upon the Riemann Problem are considered for solving the general initial-value problem for the Euler equations applied to real gases. Most of such methods use an approximate solution of the Riemann problem when real gases are involved. These approximate Riemann solvers do not yield always a good resolution of the flow field, especially for contact surfaces and expansion waves. Moreover, approximate Riemann solvers cannot produce exact solutions for the boundary points. In order to overcome these shortcomings, an exact solution of the Riemann problem is developed, valid for real gases. The method is applied to detonation products obeying a 5th order virial equation of state, in the shock-tube test case. Comparisons between our solver, as implemented in Random Choice Method, and finite difference methods, which do not employ a Riemann solver, are given.

Control of pressure distribution in shock generators with elliptic cross-sections

Abstract: Theoretical investigation of a liquid shock generating device with an elliptically shaped chamber is presented. A device of such kind has been previously fabricated and tested experimentally. Experimental observations confirmed the results of earlier theoretical analysis of the problem, showing that shock waves produced by an electric discharge at one of the foci of the elliptic chamber will converge at the second focus after the reflection off the cavity wall. In the present paper a previous two-dimensional model is extended to account for the height variation in the chamber. Expression for the pressure distribution behind the converging wave front is obtained and a relation between the shape of the upper bounding surface of the chamber and pressure distribution behind the converging wave front is investigated. It is shown that a desired pressure distribution may be obtained by an appropriate choice of the upper surface of the chamber.

The vector refraction law for shock waves

Abstract: The law is formulated in vector form and is shown to be a powerful principle for studying the refraction of shock waves. A variety of criteria for the onset of irregular refraction are discussed. The refraction index matrix is defined and it is shown that it arises naturally from the law. A projection matrix is also defined and it is found to be useful for operating on the vector wave impedance. It is expected that the methods described here will be useful for the numerical solution of problems in the refraction of shocks by materials with continuous changes in properties. The refraction law is violated in fast-slow refraction by the reflected wave over-running the incident shock to produce an irregular refraction which is either the anomalous type or the Mach-reflection-refraction type. For slow-fast refraction the law is violated by the transmitted wave becoming a precursor and also over-running the incident shock. The precursor may either be a shock or an evanescent compression wave band.

X-Ray diffraction evidence for the role of stacking faults in plastic deformation of solids under shock loading

Abstract: A mechanism responsible for the high speed shear relaxation immediately behind shock fronts is suggested The shear stress generated by the shock front causes the growth of two-dimensional defects in the crystal lattice, known as stacking faults (SF) Increasing the SF concentration and area leads to the absorption of impact energy A breach of the lattice symmetry due to the SF presence causes an additional shift in peaks of the x-ray diffraction pattern obtained from the shock compressed material Thus pulse x-ray diffraction is the only method that experimentally measures both the dilatational and deviatoric components of the deformation, which takes place during shock wave passage

Oblique shock wave with sweep

Abstract: An attached planar, oblique shock with sweep is investigated for the inviscid flow of a perfect gas. The ratio of specific heats, freestream Mach number, and wedge angle in the plane of the freestream velocity are prescribed, with the sweep angle as a free parameter. Explicit relations are provided for jump and detachment conditions. A number of trends, some non-intuitive, are discussed, e.g., the downstream Mach number may increase with sweep.

On the performance of adaptive mesh refinement computation

Abstract: A finite difference scheme for the unsteady Euler equations using an adaptive mesh refinement (AMR) algorithm was applied to the time-dependent flowfield of shock diffraction problems. The effectiveness of the AMR algorithm was evaluated against a uniform mesh algorithm. Computational results showed that to obtain solutions with equivalent resolution, the AMR algorithm requires much less processing time, when compared with a uniform mesh algorithm.

Protecting pressure gauges with grease in high temperature facilities

Abstract: The thermal protection of pressure gauges used in shock tubes has to avoid inducing losses in response time and accuracy. A study was undertaken to this purpose, and a procedure for mounting and covering the gauges with a protection layer of grease has been developed. Results up to date show satisfactory behaviour.

Shock wave attenuation in partially confined channels

Abstract: An approximate analytical solution is presented for the attenuation of planar shock waves in channels with perforated walls. The problem is considered as quasi-one-dimensional. Good agreement is found between the theoretical results and available experimental data regarding the rate of shock wave attenuation within the range of initial shock Mach numbers between 1.1 and 4 and perforation ratios between 4.5 × 10−3 and 0.53. A correlation for the discharge coefficient of a single hole perforation is presented which gives quantitatively good agreement with particular experimental observations.

Nonequilibrium excitation of internal molecular degrees of freedom in the shock layer during hypersonic flight

Abstract: A kinetic scheme of processes including the formation and quenching of electronically and vibrationally excited particles is proposed for the shock layer adjacent to the surface of a body flying at hypersonic speed. We present results of a numerical calculations for the stagnation point obtained under the thin viscous shock layer approximation for space shuttle flight conditions.

Shock induced temperature in MgO, Ag, and Ta

Abstract: From a system-independient formulation of the thermodynamics of high pressures, it is posible to predict the shock induced temperatures at a given shock compressed state. The formalism is applied, and results are presented in detailed form for magnesium oxide, silver and tantalum. Calculated values for pressure and temperature compare well with values estimated from radiation intensities recently reported by Svendsen and Ahrens for Ag and Ta. For MgO, however, strong discrepancies are found between values reported by these authors, and values calculated using our formalism. There is not apparent reason for this inconsistency.

Head-on collision between normal shock waves and a rubber supported plate , a parametric study

Abstract: The equations governing the flow resulting from a head-on collision between a normal shock wave and a rubber-supported plate are listed. The non-dimensional parameters that may affect the resulting flow are specified and their influence on the post-collision flow and waves is studied numerically. It is shown that changes in: the area-ratio between the gas and the rubber cross-sections, the incident shock wave Mach number and the mass ratio between the rubber and the plate it supports, all have significant effects on the post-collision gas and rubber responses. Changes in the rubber elasticity constant also affect the post-collision flow. The extent of the effect that changes in the above mentioned parameters have on the post-collision flow responses depends upon the loading mode used. Three different modes were studied; uni-axial stress loading, bi-axial stress loading and uni-axial strain loading.

Holographic interferometric investigation of shock wave interaction with a ramp

Abstract: The internal flowfield including shock reflections structure inside a 2-D half inlet model was examined by a double exposure holographic interferometry using a Q-switched pulse laser. The interferograms give detailed flow visualization pictures. The structures of regular reflection, lambda reflection and Mach reflection in the inlet channel were observed. From the analysis of fringe patterns, the quantitative distributions of flow density, pressure and Mach number of the internal flowfield have been reconstructed. The boundary layer thickening and separation in the case of lambda reflection and the Mach stem in the case of Mach reflection were clearly demonstrated in the holographic fringe patterns. The results show that the shock interacitons with the ramp and the wall introduce a complicated flow structure which could strongly affect the efficiency of the inlet.

Laminar and turbulent burning velocities of propane in spherical vessels

Abstract: Laminar and turbulent burning velocities of C3H8/air mixtures under moderately elevated temperature and pressure conditions have been determined experimentally using spherical combustion bombs. The results are reported here and the effects of temperature, pressure, turbulence intensity and stoichiometric ratio are included. The results obtained are compared with those of other researchers. The laminar burning velocity obtained from the pressure-time curves without compensation for the reaction zone thickness is lower than those determined using burners. A reaction zone thickness correction which reduces the difference is suggested and this relies on an assessment based on the different histories found experimentally when different initial temperatures were used for the isentrope through the standard state. For mixtures with initially low turbulence levels, both the turbulent burning velocity and the burning velocity ratio, turbulent to laminar, increase as the combustion progresses. The rate of increase becomes larger with increase in the initial and instantaneous turbulence intensity, temperature and stoichiometric ratio. Deviation from the Damkohler relationship is noted.