Showing papers in "Shock Waves in 2002"

DDT in methane-air mixtures

Abstract: Experimental results from a study on the critical condition for deflagration-to-detonation transition (DDT) in methane-air mixtures are presented Experiments were carried out at 293 K and 1 atm using methane-air mixtures with methane concentrations ranging from 55 to 17% vol The tests were performed in detonation tubes with inner-diameters of 174 mm and 520 mm Detonation cell widths \(\lambda\) were determined in the tests for a range of methane concentrations The results of DDT tests indicate that for a tube cross-sectional area blockage ratio (BR) of 03 the critical condition for DDT can be characterized by the \(d/\lambda = 1\) criterion However, for a BR = 06 the critical value \(d/\lambda\) was significantly higher The data also show that the critical condition for DDT can be described by \(L/\lambda = 7\), where L is the characteristic length-scale of the channel volume between orifice plates This length-scale is defined by a grouping of the orifice plate dimensions (inner and outer-diameter) and plate spacing

On the collapse of cavities

Abstract: The collapse of a single cavity, or a cloud of bubbles has several physical consequences when in proximity to a structure or resident within a material during deformation. The earliest recognized of these was cavitation erosion of the propellers of steam ships. However, other processes include the rapid collapse of cavities leading to hot spots in explosives from which reaction ensues, or the more recent phenomenon of light generation by oscillating single bubbles or clouds. In the collapse of a cavity, the least considered but the most important mechanism is asymmetric closure. One of the consequences of this is the formation of jets leading to local high pressures and shears that result in the damage or reaction mechanisms observed. The challenge for the future remains in understanding the effects of cloud cavitation since it is likely that only one bubble in perhaps millions in a cloud catalyses an event. The review follows the author's work in the understanding of shock-induced cavity collapse and highlights several results which indicate the importance of this problem in a variety of fields.

Size of craters produced by explosive charges on or above the ground surface

Abstract: The results of a series of tests performed with different amounts of explosive at short distances above and below ground level, as well as on the soil surface are briefly described. After an introductory description of both the main features of the blast wave and the mechanics of crater formation, a brief review of empirical methods for crater size prediction is presented. Next, the experimental design and the results obtained are described. The crater dimensions for underground explosions coincide with those found in the literature. For explosions at ground level the results are qualitatively described by empirical equations. For explosive charges situated above ground level, the dimensions of the craters are smaller than those observed in underground and near the surface explosions. Two new single prediction equations for this case are presented.

Investigations into water mitigation using a meshless particle method

Abstract: It is very difficult for traditional numerical methods to simulate the problems of water mitigation which has been increasingly used to reduce blast effects. This paper studies water mitigation problems by using smoothed particle hydrodynamics (SPH), which is a meshless, Lagrangian method appealing in treating large deformation explosion events with significant inhomogeneities. Numerical verifications considering high explosive detonation and underwater explosion shock waves have demonstrated the effectiveness of the SPH method, the solution procedure and the code. Contact and non-contact water mitigation simulations have been carried out and are compared with the case without mitigation. For either contact or non-contact water shield, the peak shock pressure and the equilibrium gas pressure are reduced to different levels according to the relevant geometry of the system setup. An optimum contact water shield thickness is found to produce the best mitigation effect for a given high explosive charge, while the non-contact water shield, if properly designed, can result in further reduction of the peak shock pressure and the equilibrium gas pressure.

Detonation interaction with wedges and bends

Abstract: The paper reports the results of a series of studies on the interaction of gaseous detonations with obliquely inclined surfaces. Interactions of increasing complexity are described in turn: at a planar inclined wall, two-dimensional propagation in a curved channel and finally three-dimensional interaction with a bend in a cylindrical pipe. The role of detonation structure is discussed as well as the magnitude and duration of potentially damaging overpressures.

The delivery of particulate vaccines and drugs to human skin with a practical, hand-held shock tube-based system

Abstract: A unique form of powdered vaccine and drug delivery has been developed. The principle behind the concept is to accelerate vaccine and drug particles, using a gas flow, so that they attain sufficient velocities to enter the skin and achieve a pharmaceutical effect. This paper presents the Contoured Shock Tube (CST), configured to deliver particles to the skin with a narrow and controlled velocity distribution and uniform spatial distribution. The gas and particle flows of a prototype CST are explored experimentally and compared with Computational Fluid Dynamics (CFD) calculations. Some key steps in converting the prototype into a practical hand-held vaccine and drug delivery system are discussed. The ability of this system to deliver particles to the skin is illustrated by sample penetration data into excised human tissue.

Hypersonic aerothermodynamic and scramjet research using high enthalpy shock tunnel

Abstract: A high enthalpy shock tunnel is a potential facility for gaining knowledge to develop modern aerothermodynamic and propulsion technologies. The largest high enthalpy shock tunnel HIEST was built at NAL Kakuda in 1997, aiming for aerothermodynamic tests of Japan's space vehicle HOPE and scramjet propulsion systems. Selected topics from the experimental studies carried out using HIEST so far, such as the nonequilibrium aerodynamics of HOPE, the surface catalytic effect on aerodynamic heating and scramjet performance are described.

Low hydrocarbon mixtures ignition delay times investigation behind reflected shock waves

Abstract: Ignition delays for low alkanes/oxygen mixtures highly diluted with argon were measured behind a reflected shock wave using ultraviolet emission spectrometry in wide ranges of temperature (1200–2700 K), pressure (0.1–1.8 MPa), equivalence ratio (0.5–2) and dilution (89–99%). For each alkane (methane, ethane and propane), a correlation between ignition delay time, temperature, pressure and concentration is proposed and compared with those obtained in previous studies. This correlation enables the estimation of the delay time with an accuracy better than 20% for all measurement ranges. Results are compared with those from a recent study of the detailed kinetic modelling of the alkane oxidation.

Towards the miniaturization of explosive technology

Abstract: Condensed phase explosives used in conventional explosive systems have a charge size on the order of a meter or a sizable fraction of a meter. This paper addresses a range of issues required to scale downwards the size of explosive systems by a factor of one hundred to one thousand.

Upstream porthole injection in a 2-D scramjet model

Abstract: Injection from portholes upstream of the combustion chamber was investigated as a method of delivering fuel into a scramjet. This method reduces the viscous drag on a model by allowing a reduction in the length of the combustion chamber. At experimental enthalpies of 3.0 MJ/kg in the T4 shock tunnel, there was no evidence of combustion in the intake, either by shadowgraph or pressure measurements. Combustion was observed in the combustion chamber. A theoretical extension of these results is made to a hot wall scenario.

Diffraction and re-initiation of detonations behind a backward-facing step

Abstract: Diffraction phenomena of gaseous detonation waves behind a backward-facing step in a tube are observed by using high-speed schlieren photography and soot-track records as well as by pressure measurements on the sidewall. Mixtures are stoichiometric oxyhydrogen and those diluted by argon at sub-atmospheric pressures. Three types of phenomena are observed, that is, continuous propagation of detonation, re-initiation after a temporal extinction of detonation and complete extinction of detonation. The continuous propagation means that the diffracted wave does not affect the main propagation although reflected shock waves from the bottom surface of the tube may affect it. The re-initiation occurs at a wall surface of the tube behind a reflected shock wave after the main detonation wave has been extinguished. Positions and conditions of the re-initiation are discussed. The complete extinction is defined as disappearance of detonation cells behind the step within a certain length of the tube. Cases exist where an ignition occurs after several reflections off the bottom and top surface of the tube.

Shock tunnel study of spiked aerodynamic bodies flying at hypersonic Mach numbers

Abstract: A three-component accelerometer balance system is used to study the drag reduction effect of an aerodisc on large angle blunt cones flying at hypersonic Mach numbers. Measurements in a hypersonic shock tunnel at a freestream Mach number of 5.75 indicate more than 50% reduction in the drag coefficient for a 120degrees apex angle blunt cone with a forward facing aerospike having a flat faced aerodisc at moderate angles of attack. Enhancement of drag has been observed for higher angles of attack due to the impingement of the flow separation shock on the windward side of the cone. The flowfields around the large angle blunt cone with aerospike assembly flying at hypersonic Mach numbers are also simulated numerically using a commercial CFD code. The pressure and density levels on the model surface, which is under the aerodynamic shadow of the flat disc tipped spike, are found very low and a drag reduction of 64.34% has been deduced numerically.

Numerical study of spherical blast-wave propagation and reflection

Abstract: The objective of this study is to understand the flow structures of weak and strong spherical blast waves either propagating in a free field or interacting with a flat plate. A 5th-order weighted essentially non-oscillatory scheme with a 4th-order Runge-Kutta method is employed to solve the compressible Euler/Navier-Stokes equations in a finite volume approach. The real-gas effects are taken into account when high temperature occurs. A shock-tube problem with the real-gas effect is first tested in order to verify the solver accuracy. Moreover, unsteady shock waves moving over a stationary wedge with various wedge angles, resulting in different types of shock wave reflections, are also tested. It is found that the computed results agreed well with the existing data. Second, the propagation of a weak spherical blast wave, created by rupture of a high-pressure isothermal sphere, in a free field is studied. It is found that there are three minor shock waves moving behind the main shock. Third, the problem of a strong blast wave interacting with a flat plate is investigated. The flow structures associated with single and double Mach reflections are reported in detail. It is found that there are at least three local high-pressure regions near the flat plate.

Numerical investigations of transition between regular and Mach reflections caused by free-stream disturbances

Abstract: Numerical simulations have been performed to study the influence of the free-stream disturbances on the alternation of the steady shock wave reflection configurations in the dual solution domain. Different types of disturbances have been considered. The analysis of interaction between disturbances and the incident shock wave can be substantially simplified for the localized density disturbances. It is shown that such disturbances can indeed cause the transition from regular reflection to Mach reflection and back, so that within a certain range of angles of incidence the shock wave reflection configuration can be considered as a bi-stable system. The threshold amplitude of the localized density disturbance, able to induce the transition, has been estimated theoretically. The results of numerical computations convince of higher stability of the Mach reflection in the dual solution domain compared to the regular reflection, which is in accordance with available experimental data.

An experimental and theoretical study of converging polygonal shock waves

Abstract: An experimental investigation was carried out to explore the possibility of producing converging polygonal shocks in an essentially two-dimensional cavity. Previous calculations by Apazidis and Lesser (1996) suggested that such configurations could be produced by reflecting a cylindrical outgoing shock from a smoothly altered circular boundary, the alteration having n-gonal symmetry. In the experiments the outgoing shock was produced by a spark discharge which yielded shocks in the Mach number range from 1.1 to 1.7 at a radius just prior to the reflection. Polygonal shocks were observed as predicted by using a modified form of geometrical shock dynamics, derived in the above paper. In addition, the modified theory was used to calculate the results of an experiment carried out by Sturtevant and Kulkarny (1976). The results of the numerical calculations were found to be in substantial agreement with both experiments, suggesting that the modifications in geometrical shock dynamics for non-uniform flow ahead of an advancing shock are useful in the case of shock focusing. The experiment also showed that the polygonal shapes were stable in the examined range of shock Mach numbers, a result that may be of importance for a number of practical situations in which shock focusing is present.

An investigation of shock induced temperature rise and melting of bismuth using high-speed optical pyrometry

Abstract: Temperature is a basic parameter in physics, but in the case of shock-compressed metals its measurement remains difficult and controversial Improvements in accuracy of a high-speed optical pyrometer enabled us to study interesting features of bismuth: temperatures of shock-loaded samples measured through a lithium-fluoride anvil and (indirectly) part of the fusion curve An improved method of analyzing optical pyrometry data is also discussed

Self-ignition and ignition of aluminum powders in shock waves

Abstract: Ignition of fine aluminum powders in reflected shock waves has been studied. Two ignition regimes are found: self-ignition observed at temperatures higher than 1800 K and “low-temperature” ignition at temperatures of 1000–1800 K. The possibility of initiating the ignition of aluminum powders in air using combustible liquids has been studied too.

Experimental study of gaseous detonation propagation over acoustically absorbing walls

Abstract: The paper presents results front an experimental investigation of the propagation of gaseous detonation waves over tube sections lined with acoustically absorbent materials. The measurements were compared with results from control tests in a smooth wall section. The results show the increasing effectiveness of a perforated steel plate, wire mesh and steel wool in attenuating detonation.

Numerical analysis of pressure oscillations over axisymmetric spiked blunt bodies at Mach 6.80

Abstract: The pressure oscillations over a forward facing spike attached to an axisymmetric blunt body are simulated by solving time-dependent compressible Navier–Stokes equations. The governing fluid flow equations are discretized in spatial coordinates employing a finite volume approach which reduces the equations to semidiscretized ordinary differential equations. Temporal integration is performed using the two-stage Runge–Kutta time stepping scheme. A global time step is used to obtain a time-accurate numerical solution. The numerical computation is carried out for a freestream Mach number of 6.80 and for spike length to hemispherical diameter ratios of 0.5, 1.0 and 2.0. The flow features around the spiked blunt body are characterized by a conical shock wave emanating from the spike tip, a region of separated flow in front of the hemispherical cap, and the resulting reattachment shock wave. Comparisons of the numerical results are made with the available experimental results, such as schlieren pictures and the surface pressure distribution along the spiked blunt body. They are found to be in good agreement. Spectral analysis of the computed pressure oscillations are performed employing fast Fourier transforms. The surface pressure oscillations over the spike and phase plots exhibit a behaviour analogous to that of the Van der Pol equation for a self-sustained oscillatory flow.

An approximate solution of the Riemann problem for a realisable second-moment turbulent closure

Abstract: An approximate solution of the Riemann problem associated with a realisable and objective turbulent second-moment closure, which is valid for compressible flows, is examined. The main features of the continuous model are first recalled. An entropy inequality is exhibited, and the structure of waves asso- ciated with the non-conservative hyperbolic convective system is briefly described. Using a linear path to connect states through shocks, approximate jump conditions are derived, and the existence and uniqueness of the one-dimensional Riemann problem solution is then proven. This result enables to construct exact or approximate Riemann-type solvers. An approximate Riemann solver, which is based on Gallouet's recent proposal is eventually presented. Some computations of shock tube problems are then discussed.

Radiative Interaction Between Driver and Driven Gases in an Arc-Driven Shock Tube

Abstract: An electric-arc driven shock tube was operated with hydrogen as the driven gas and either hydrogen or helium as the driver gas. Electron density was measured behind the primary shock wave spectroscopically from the width of the Beta line of hydrogen. The measured electron density values were many times greater than the values calculated by the Rankine - Hugoniot relations. By accounting for the radiative transfer from the driver gas to the driven gas, the measured electron density values were numerically recreated.

Recent work on gaseous detonations

Abstract: The paper reviews recent progress in the field of gaseous detonations, with sections on shock diffraction and reflection, the transition to detonation, hybrid, spherically-imploding, and galloping and stuttering fronts, their structure, their transmission and quenching by additives, the critical energy for initiation and detonation of more unusual fuels. The final section points out areas where our understanding is still far from being complete and contains some suggestions of ways in which progress might be made.

On critical conditions for detonation initiation by shock reflection from obstacles

Abstract: A series of experiments supported by numerical simulations are reported on the interaction between a planar incident shock and a single obstacle The test mixtures used were stoichiometric hydrogen and oxygen diluted with either argon or nitrogen at sub-atmospheric pressures The main aim of the study was to determine the conditions under which a reflected detonation was generated Observed critical conditions are compared with a simple predictive criterion based on the ratio of auto-ignition delay time behind an ideal reflected shock to the acoustic transit time across the face of the obstacle

Simulation of detonation wave interaction using an ignition and growth model

Abstract: A kinetics based detonation model has been integrated with an existing object oriented hydrocode. The model has been demonstrated to correctly predict the shock initiation of explosives and captures key features such as the von Neumann pressure spike and reaction zone. Comparisons to experimental flyer plate data for both primary and secondary class explosives have been performed and key features such as detonation wave form and resulting target disk velocities are reproduced. Simulations have also been performed representing the interaction of various mechanical waves. First, the interaction of detonation waves, a key process in energetic systems, is simulated. Next, the response of an established detonation wave to an explosive that has been preconditioned by a weak compression wave is modeled. These have been carried out to determine the utility of the existing kinetic schemes for such problems.

Supersonic liquid jets: Their generation and shock wave characteristics

Abstract: The generation of high-speed liquid (water and diesel fuel) jets in the supersonic range using a vertical single-stage powder gun is described. The effect of projectile velocity and mass on the jet velocity is investigated experimentally. Jet exit velocities for a set of nozzle inner profiles (e.g. straight cone with different cone angles, exponential, hyperbolic etc.) are compared. The optimum condition to achieve the maximum jet velocity and hence better atomization and mixing is then determined. The visual images of supersonic diesel fuel jets (velocity about 2000 m/s) were obtained by the shadowgraph method. This provides better understanding of each stage of the generation of the jets and makes the study of their characteristics and the potential for auto-ignition possible. In the experiments, a pressure relief section has been used to minimize the compressed air wave ahead of the projectile. To clarify the processes inside the section, additional experiments have been performed with the use of the shadowgraph method, showing the projectile travelling inside and leaving the pressure relief section at a velocity of about 1100 m/s.

Dynamics study of detonation-wave cellular structure 1. Statistical properties of detonation wave front

Abstract: We have investigated the evolution of cellular detonation-wave structure as a gaseous detonation travels along a round tube and measured cell lengths as a function of the initial pressure of the gas. We have tested acetylene-containing combustible gas mixtures with different degrees of regularity. Along with the smoked-foil technique, an emission method has been used to the measure current and average values of the detonation cell length. The method is based on the detection of an emission spectrum behind the detonation front in the spectral range corresponding to local gas temperatures that are much higher than those for the Chapman-Jouguet equilibrium condition. This technique provides quasi-continuous cell-length measurements along the normal to the detonation front over the length of several factors of ten times the tube.

Numerical study of the lift force influence on two-phase shock tube boundary layer characteristics

Abstract: The importance of the lift force acting on the dispersed phase in the boundary layer of a laminar gas-particle dilute mixture flow generated by a shock wave is investigated numerically. The particle phase is supposed to form a continuum and is described by an Eulerian approach. The ability of the Eulerian model to simulate particle flows and the importance of the two-way coupling are proven by comparison with experimental data as well as with the numerical results from schemes based on a Lagrangian approach. The models used for the lift force are discussed through comparisons between numerical and experimental results found in the literature. Some results about the formation of a dust cloud are numerically reproduced and show the major role of the lift force. Simulations of two-dimensional two-phase shock tube flows are also performed including the lift force effects. Although the wave propagation is weakly influenced by the lift force, the force modifies substantially the dynamics of the flow near the wall.

Aerodynamic calibration of TCM2 facility and study of a bow shock layer by emission and laser spectroscopy

Abstract: Flow properties in the TCM2 free piston shock tube/tunnel are determined by time-resolved pressure and heat flux measurements in numerous points of the shock tube and the nozzle, and in the free flow for two stagnation enthalpy conditions (35 and 11 MJ/kg) These measurements demonstrate the homogeneity of the flow during more than 1 ms The cleanness of the useful test time is shown with time-resolved emission measurements at critical wavelengths NO fluorescence profiles are established with local and planar laser-induced fluorescence in the shock layer around a cylindrical model It allows to determine the shock stand-off distance for both enthalpy conditions The problems of quenching and amplified spontaneous emission are considered The importance of atomic oxygen and atomic nitrogen densities as well as temperature effects is also shown Evaluation of the temperatures behind the shock front through spectroscopic data agrees with calculations The proof of the presence of vibrationally excited NO ahead of the shock layer is given

Fluorescence imaging of mixing flowfields and comparisons with computational fluid dynamic simulations

Abstract: This paper compares computational and experimental fluorescence images of the mixing flows associated with a number of fuel injectors, injecting hydrogen fuel into a supersonic coflow. The favourable comparison allows us to establish the reliability of the computational fluid dynamic modelling on which the theoretical images are based. Theoretical calculations of mixing performance parameters are then used to assess the mixing characteristics and performance of the injectors.

Investigations involving shock waves generation and shock pressure measurement in direct ablation regime and confined ablation regime

Abstract: Recent investigations involving shock waves generation and shock pressure measurement in direct ablation regime and confined ablation regime for aluminium, copper, titanium and steel (40C130) materials are reported. Experimental measurements demonstrated that in direct ablation regime the peak pressures typically are less than 10\(^{9}\) Pa when the incident laser intensity is about 10\(^{9}\) W/cm\(^{2}\) and the time duration of the applied pressure is roughly equal to the laser pulse duration. It is shown that confinement of the surface with a transparent overlayer provided an effective method of enhancing laser-induced shock waves pressure in the target material with an order of magnitude for same laser intensity. Also, in this second regime, the pressure is applied over a period much longer than the laser-pulse duration. As an application measurements of the hardness of target surface before and after laser irradiation in direct ablation regime and confined ablation regime are given, and it is shown that the maximum value of surface hardness is obtained in confined ablation regime.