Showing papers in "Shock Waves in 2005"

DDT in a smooth tube filled with a hydrogen–oxygen mixture

Abstract: Results of experimental study on DDT in a smooth tube filled with sensitive mixtures having detonation cell size from 1 to 3 orders of magnitude smaller than the tube diameter are presented. Stoichiometric hydrogen–oxygen mixtures were used in the tests with initial pressure ranging from 0.2 to 8 bar. A dependence of the run-up distance to DDT on the initial pressure is studied. This dependence is found to be close to the inverse proportionality. It is suggested that the flow ahead of the flame results in formation of the turbulent boundary layer. This boundary layer controls the scale of turbulent motions in the flow. A simple model to estimate the maximum scale of the turbulent pulsations (boundary layer thickness) at flame positions along the tube is presented. The largest scale of the turbulent motions at the location of the onset of detonation is shown to be 1 order of magnitude greater than the detonation cell widths, λ, in all the tests. It is suggested that the onset of detonation is triggered during flame acceleration as soon as the maximum scale of the turbulent pulsations increases up to about 10 λ. The model to estimate the maximum size of turbulent motions, δ, and the correlation δ≈ 10λ, give a basis for estimations of the run-up distances to DDT in tubes with internal diameter D > 20λ.

Unsteady drag on a sphere by shock wave loading

Abstract: The dynamic drag coefficient of a sphere by shock wave loading is investigated numerically and experimentally. The diameter of the sphere is varied from 8 \({\mathrm{\mu}}\)m to 80 mm in numerical simulation. The axisymmetric Navier-Stokes equations are solved on a fine grid, and the grid convergence of the drag coefficient is achieved. The numerical result is validated by comparing the experimental data of a 80 mm sphere, measured by the accelerometer in a vertical shock tube. It is found that the sphere experiences in the early interaction one order higher drag than in the steady state. A transient negative drag, mainly resulting from the focusing of shock wave on the rear side of the sphere, is observed only for high Reynolds number flows, and the drag becomes positive because of increased skin friction for low Reynolds number flows.

Effect of obstacle size and spacing on the initial stage of flame acceleration in a rough tube

Abstract: Experiments were conducted to study flame acceleration in an orifice plate laden detonation tube. Orifice plate area blockage and spacing were varied to determine their affect on flame acceleration. The tube used in the study was 3.05 m long with an inner diameter of 14.0 cm. Experiments were primarily carried out with stoichiometric propane-air, however the affect of mixture reactivity was also investigated by varying the mixture equivalence ratio. The distance required for the flame to achieve a velocity equal to the speed of sound in the unburned gas mixture was measured. This run-up distance is used to characterize the early stage of the flame acceleration process. It was found that in all cases, the flame run-up distance decreased with increased blockage ratio and with increased mixture reactivity. It was found that for higher blockage ratios plates flame acceleration was greatest for a plate spacing of one tube diameter, but for lower blockage ratio plates the results obtained for one-half, one, and one and one-half tube diameter plate spacing were very similar. The most rapid flame acceleration was observed when the ratio of the orifice plate spacing and the orifice plate height (half of the difference between the tube and orifice plate diameter) is on the order of 5. It is proposed that this optimum acceleration corresponds to the condition where the plate spacing is roughly equal to the length of the unburned gas re-circulation zone downstream from the orifice plate.

Shock wave impacts on deforming panel, an application of fluid-structure interaction

Abstract: Because of the lack of fluid-structure interaction FSI test cases, particularly for transient compressible flows, the present paper deals with a numerical and experimental study of the behaviour of a cantilever panel submitted to a shock tube flow. Our purpose is to confront our numerical model to an unsteady FSI problem. This study hopes to initiate a reference data bank in this domain. After a description of FSI numerical model, experimental device and diagnostic, a comparison is presented through the fluid flow structure and the panel deformations. A good agreement between numerical results and the experiments is obtained.

Detonation diffraction from circular tubes to cones

Abstract: An experimental study of the detonation diffraction from 26- and 52-mm inner diameter tubes to cones of various angles α in stoichiometric acetylene/oxygen mixture allowed us to determine critical conditions for diffraction and to detail the mechanisms involved. All soot-foil records show that critical transmission is due to super-detonation propagating transversally in shocked gas before the decoupled flame front. However, at large cone angles (α > 40∘), super-detonation originates at the axis of the flow and propagates tangentially to the cone wall (this situation is close to detonation transmission to a space and a half-space). At smaller angles (i.e. α < 40∘), on the opposite, super-detonation originates at the cone wall and propagates toward the axis. In addition the soot plates often give some evidence that, during escape of detonation products from the tube, a Mach disk is formed at a distance of about one tube diameter from the tube exit. Numerical two-dimensional simulations of detonation diffraction favorably agree with the observations.

Numerical study of water mitigation effects on blast wave

Abstract: The mitigating effect of a water wall on the generation and propagation of blast waves of a nearby explosive has been investigated using a numerical approach. A multimaterial Eulerian finite element technique is used to study the influence of the design parameters, such as the water-to-explosive weight ratio, the water wall thickness, the air-gap and the cover area ratio of water on the effectiveness of the water mitigation concept. In the computational model, the detonation gases are modelled with the standard Jones–Wilkins–Lee (JWL) equation of state. Water, on the other hand, is treated as a compressible fluid with the Mie–Gruneisen equation of state model. The validity of the computational model is checked against a limited amount of available experimental data, and the influence of mesh sizes on the convergence of results is also discussed. From the results of the extensive numerical experiments, it is deduced that firstly, the presence of an air-gap reduces the effectiveness of the water mitigator. Secondly, the higher the water-to-explosive weight ratio, the more significant is the reduction in peak pressure of the explosion. Typically, water-to-explosive weight ratios in the range of 1–3 are found to be most practical.

The Structure and Evolution of a Two-Dimensional H2/O2/Ar Cellular Detonation

Abstract: This paper reports on two-dimensional numerical simulation of cellular detonation wave in a \(\rm {H_2}\)/\(\rm {O_2}\)/\(\rm {Ar}\) mixture with low initial pressure using a detailed chemical reaction model and high order WENO scheme. Before the final equilibrium structure is produced, a fairly regular but still non-equilibrium mode is observed during the early stage of structure formation process. The numerically tracked detonation cells show that the cell size always adapts to the channel height such that the cell ratio is fairly independent of the grid sizes and initial and boundary conditions. During the structural evolution in a detonation cell, even as the simulated detonation wave characteristics suggest the presence of an ordinary detonation, the evolving instantaneous detonation state indicates a mainly underdriven state. As a considerable region of the gas mixture in a cell is observed to be ignited by the incident wave and transverse wave, it is further suggested that these two said waves play an essential role in the detonation propagation.

Experimental analysis of unsteady separated flows in a supersonic planar nozzle

Abstract: The unsteady aspects of shock-induced-separation patterns have been investigated inside a Mach 2 planar nozzle. The mean location of the shock can vary by changing, relatively to the nozzle throat, the height of the second throat which is positioned downstream of the square test section. This study focuses on the wall pressure fluctuations spectra and the unsteady behaviour of the shock. Symmetric shock configurations appear both for the largest openings of the second throat, and for the smallest openings. For an intermediate opening the shock system exhibits asymmetrical configurations. A coating with roughnesses sticked on the throat part of the nozzle in order to modify the state of the incoming boundary layers (from smooth to rought turbulent statement) is a driver for the asymmetry. The fluctuating displacements of the shock patterns were analysed by using an ultra fast shadowgraph visualization technique. A spectral analysis of the unsteady wall pressure measurements has revealed low frequency phenomena governed by large structure dynamics in the separated flows.

On the origin of the double cellular structure of the detonation in gaseous nitromethane and its mixtures with oxygen

Abstract: An experimental study of the detonation in gaseous nitromethane (NM) and nitromethane-oxygen mixtures has exhibited unambiguously the existence of a double cellular structure in the range of equivalence ratio \(\phi\) from 1.3 to 1.75 (NM). Calculations of the reaction zone of the detonation in the same range of equivalence ratio, using a detailed chemical scheme in the ZND model, demonstrate that the chemical energy is released in two main successive distinct exothermic reaction steps characterized by their own induction length which justifies the existence of a two levels detonation cellular structure. This result strengthens the idea that the cellular detonation structure finds its origin in instabilities amplified by delayed local high energy release rate inside the reaction zone.

DDT in fuel–air mixtures at elevated temperatures and pressures

Abstract: An experimental study was carried out to investigate flame acceleration and deflagration-to-detonation transition (DDT) in fuel–air mixtures at initial temperatures up to 573 K and pressures up to 2 atm. The fuels investigated include hydrogen, ethylene, acetylene and JP-10 aviation fuel. The experiments were performed in a 3.1-m long, 10-cm inner-diameter heated detonation tube equipped with equally spaced orifice plates. Ionization probes were used to measure the flame time-of-arrival from which the average flame velocity versus propagation distance could be obtained. The DDT composition limits and the distance required for the flame to transition to detonation were obtained from this flame velocity data. The correlation developed by Veser et al. (run-up distance to supersonic flames in obstacle-laden tubes. In the proceedings of the 4th International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions, France (2002)) for the flame choking distance proved to work very well for correlating the detonation run-up distance measured in the present study. The only exception was for the hydrogen–air data at elevated initial temperatures which tended to fall outside the scatter of the hydrocarbon mixture data. The DDT limits obtained at room temperature were found to follow the classical d/λ = 1 correlation, where d is the orifice plate diameter and λ is the detonation cell size. Deviations found for the high-temperature data could be attributed to the one-dimensional ZND detonation structure model used to predict the detonation cell size for the DDT limit mixtures. This simple model was used in place of actual experimental data not currently available.

Mass spectrometric measurements of driver gas arrival in the T4 free-piston shock-tunnel

Abstract: Available test time is an important issue for ground-based flow research, particularly for impulse facilities such as shock tunnels, where test times of the order of several ms are typical The early contamination of the test flow by the driver gas in such tunnels restricts the test time This paper reports measurements of the driver gas arrival time in the test section of the T4 free-piston shock-tunnel over the total enthalpy range 3–17 MJ/kg, using a time-of-flight mass spectrometer The results confirm measurements made by previous investigators using a choked duct driver gas detector at these conditions, and extend the range of previous mass spectrometer measurements to that of 3–20 MJ/kg Comparisons of the contamination behaviour of various piston-driven reflected shock tunnels are also made

Hydrogen detonation and fast deflagration triggered by a turbulent jet of combustion products

Abstract: Initiation of detonation by a turbulent jet of combustion products has been studied in a detonation tube of 141 mm inner diameter. Jet formation techniques based on either a perforated plate or bursting membrane subjected to the impact of a stable detonation wave were utilized. Critical conditions of detonation initiation in hydrogen–air and hydrogen–oxygen–nitrogen mixtures have been found to depend on both the mixture sensitivity and the geometrical parameters of the arrangement.

Laboratory-scale blast wave phenomena – optical diagnostics and applications

Abstract: Laboratory-scale experiments with explosive charges in the milligram range are a useful tool to investigate basic blast wave phenomena and to replicate, to some extent, large-scale explosions. This paper reviews and discusses the optical diagnostics that can be applied in these experiments and outlines how these techniques can be used to obtain new information about the propagation and interaction of blast waves. Performance criteria for the required instrumentation are established. Several examples illustrate the potential and the limitations of this approach to blast wave research.

High-speed time-resolved color schlieren visualization of shock wave phenomena

Abstract: A newly developed high-speed color video camera, which can record 103 frames at rates of up to 1 million frames per second, has been used to obtain time-resolved color schlieren visualizations of shock wave phenomena. These trials constitute the first successful time-resolved application of the direction-indicating color schlieren method with frame rates up to 125 kHz. The instabilities of a supersonic flow over a double cone were made visible in unprecedented clarity, and the potential of the camera for schlieren visualizations was further demonstrated in experiments showing the explosion of a small firecracker and the bursting of a helium-filled toy balloon.

Attenuation of shock waves propagating over arrayed baffle plates

Abstract: The paper reports results of shock tube experiments of the attenuation of shock waves propagating over arrayed baffle plates, which is motivated to simulate shock wave attenuation created accidentally at the acoustic delay line in synchrotron radiation factory upon the rupture of a metal membrane separating the acceleration ring at high vacuum and atmospheric test chambers. Experiments were carried out, by using double exposure holographic interferometry with double path arrangement, in a 100 mm×180 mm shock tube equipped with a test section of 180 mm×1100 mm view field. Two baffle plate arrangements were tested: Oblique and staggered baffle plates; and vertical symmetric ones. Pressures were measured along the shock tube sidewall at individual compartments for shock Mach numbers ranging from 1.2 to 3.0 in air. The results were compared with a numerical simulation. The rate of shock attenuation over these baffle plates was compared for vertical and oblique baffle plates. Shock wave attenuation is more pronounced in the oblique baffle plate arrangements than in the vertical ones.

Ground testing of the HyShot supersonic combustion flight experiment in HEG

Abstract: The first phase of the HyShot supersonic combustion ramjet (scramjet) flight exper- iment program of The University of Queensland in Australia was designed to provide benchmark data on supersonic combustion for a flight Mach number of approximately M=8. The second flight of the HyShot program, performed on July 30th 2002, was successful and supersonic com- bustion was observed along the specified trajectory range. The operating range of the High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Centre (DLR) was recently extended. The facility has now the capability of testing a complete scramjet engine with internal combustion and external aerodynamics at M=7.8 flight conditions in altitudes of about 30 km. A post flight analysis of the HyShot flight experiment was performed using an operational scramjet wind tunnel model with a geometry which is identical to that of the flight configuration.

Performance of a detonation driven shock tunnel

Abstract: A detonation driven shock tunnel is useful as a ground test facility for hypersonic flow research. By attaching a convergent section ahead of the primary diaphragm in the driver section, the downstream operation mode became available to generate a high-enthalpy test flow. A 100 mm diameter shock tunnel was for the first time installed in the Laboratory of High-Temperature-Gas Dynamics (LHD), Institute of Mechanics, Chinese Academy of Sciences, and after its continuous refitments, a high performance detonation driven shock tunnel was achieved to generate high-enthalpy and high-Reynolds number test flows. A new method to burst a metal diaphragm with the downstream operation mode is discussed.

Oscillatory Behavior of Supersonic Impinging Jet Flows

Abstract: Oscillatory flows of a choked underexpanded supersonic impinging jet issuing from a convergent nozzle have been computed using the axisymmetric unsteady Navier--Stokes system. This paper focuses on the oscillatory flow features associated with the variation of the nozzle-to-plate distance and nozzle pressure ratio. Frequencies of the surface pressure oscillation and flow structural changes from computational results have been analyzed. Staging behavior of the oscillation frequency has been observed for both cases of nozzle-to-plate distance variation and pressure ratio variation. However, the staging behavior for each case exhibits different features. These two distinct staging behaviors of the oscillation frequency are found to correlate well if the frequency and the distance are normalized by the length of the shock cell. It is further found that the staging behavior is strongly correlated with the change of the pressure wave pattern in the jet shear layer, but not with the shock cell structure.

Optimization study of spray detonation initiation by electric discharges

Abstract: Development of air-breathing pulse detonation engines is faced with a challenging problem of detonation initiation in fuel sprays at distances feasible for propulsion applications. Extensive experimental study on initiation of a confined n-hexane spray detonation in air by electric dis- charges is reported. It is found that for direct initiation of spray detonation with minimal energy requirements (1) it is worth to use one discharger located near the closed end of a detonation tube and at least one additional discharger down- stream from it to be triggered in-phase with primary shock wave arrival; (2) the discharge area should be properly insu- lated to avoid electric loss to metal tube walls; (3) discharge duration should be minimized to at least 50 µs; (4) discharge channel should preferably occupy a large portion of a tube cross-section; (5) test tube should be preferably of a diame- ter close to the limiting tube diameter; (6) gradual transition between the volume with electric discharger and the tube should be used; and (7) a powerful electric discharger uti- lized for generating a primary shock wave can be replaced by a primary shock wave generator comprising a relatively low-energy electric discharger, Shchelkin spiral, and tube coil. With all these principles implemented, the rated elec- tric energy of about 100 J was required to initiate n-hexane spray-air detonation in a 28-mm tube at a distance of about 1 m from the atomizer.

Pseudo-schlieren CT measurement of three-dimensional flow phenomena on shock waves and vortices discharged from open ends

Abstract: 1A pseudo-schlieren technique is applied to the interferometric computed tomography (CT) measurement of three-dimensional (3-D) shock waves discharged from a square open end and a pair of circular open ends in a shock tube experiment. The experiment is performed for incident shock Mach numbers of 2.0 and 2.2 in nitrogen gas under supersonic post shock flow conditions at the open end. The 3-D density-gradient distributions are evaluated from the CT data of the 3-D density distributions, and are depicted in gray-scale CT images of the gradient magnitude and in pseudo-color CT images of the gradient component. The resultant pseudo-schlieren CT images clearly illustrate the 3-D flow features of shock waves, contact surfaces, and the other sharp density fronts. Their image characteristics and meaning in gas dynamics are discussed in comparison with the pseudo-color images of the density. We demonstrate that the pseudo-schlieren CT technique is a useful tool for studying 3-D problems in shock dynamics.

Uncertainty analysis of deflagration-to-detonation run-up distance

Abstract: Three methods were adopted to estimate the deflagration-to-detonation run-up distance in a smooth tube, which comprise (1) the measurement of the propagation speed of pressure or combustion waves compared with the C–J detonation speed, (2) the time of the onset of detonation by the emission of visible light and the trajectory of pressure wave or combustion waves, and (3) the trajectory intersection with the presence of retonation wave. A nonstationary cross-correlation technique was applied to evaluate the uncertainty in estimating the run-up distance. Evaluation of the pressure wave (pressure wave speed or the pressure wave trajectory) appears to be more suitable to determine the deflagration-to-detonation run-up distance.

Real-time determination and controlof the laser-drilled holes depth

Abstract: We present an optodynamical method for the real-time determination of the depth of laser-drilled holes. The method consists of the detection of shock waves generated during the interaction of the laser beam with the irradiated material using a piezoelectric transducer (PZT), and measurement of the shock waves propagation time through the sample using the PZT signal. The experimental observations reveal that the propagation time has an almost exponential decay with the number of pulses and is strongly dependent on the laser radiation wavelength. The quantity of ablated substrate material per laser pulse is a nonlinear function of the number of consecutive laser pulses incident on the same spot at the irradiated sample surface.

Hypersonic flow over a multi-step afterbody

Abstract: Effect of a multi-step base on the total drag of a missile shaped body was studied in a shock tunnel at a hypersonic Mach number of 5.75. Total drag over the body was measured using a single component accelerometer force balance. Experimental results indicated a reduction of 8% in total drag over the body with a multi-step base in comparison with the base-line (model with a flat base) configuration.The flow fields around the above bodies were simulated using a 2-D axisymmetric Navier-Stokes solver and the simulated results on total drag were compared with the measured results. The simulated flow field pictures give an insight into the involved flow physics.

Multiple pulsed hypersonic liquid diesel fuel jetsdriven by projectile impact

Abstract: Further studies on high-speed liquid diesel fuel jets injected into ambient air conditions have been carried out. Projectile impact has been used as the driving mechanism. A vertical two-stage light gas gun was used as a launcher to provide the high-speed impact. This paper describes the experimental technique and visualization methods that provided a rapid series of jet images in the one shot. A high-speed video camera (106 fps) and shadowgraph optical system were used to obtain visualization. Very interesting and unique phenomena have been discovered and confirmed in this study. These are that multiple high frequency jet pulses are generated within the duration of a single shot impact. The associated multiple jet shock waves have been clearly captured. This characteristic consistently occurs with the smaller conical angle, straight cone nozzles but not with those with a very wide cone angle or curved nozzle profile. An instantaneous jet tip velocity of 2680 m/s (Mach number of 7.86) was the maximum obtained with the 40\(^\circ\) nozzle. However, this jet tip velocity can only be sustained for a few microseconds as attenuation is very rapid.

The propagation mechanism of cellular detonation

Abstract: The present lecture examined the available experimental and numerical results on cellular detonations. It is concluded that apart from special mixtures where the detonation is only “weakly unstable”, the classical shock ignition and thermal explosion mechanism cannot describe the physical and chemical processes in the highly complex reaction zone of unstable detonations in general. Turbulence will play an important role in both the ignition and the combustion mechanism in the reaction zone of highly unstable detonations. Vorticity and turbulence generation from shock-shock, shock-vortex, shock-density interactions and the baroclinic torque mechanism are considered important in contrast to the velocity gradient shear flow mechanism of turbulence production in incompressible flows. It is recommended that the experimental determination of the hydrodynamic thickness, its correlation with chemical, thermodynamic and transport properties of the mixture and the formulation of a turbulence model to describe the steady mean flow properties of cellular detonation structure are important problems in detonation research for the immediate future.

A study on jet initiation of detonation using multiple tubes

Abstract: A detonator consisting of a dense bundle of small-diameter tubes (4.4–19 mm) is tested experimentally using stoichiometric mixtures of hydrogen–oxygen and hydrogen–air. Tests are conducted in a 5,200-mm long detonation tube fitted with a schlieren photograph section and smoked foil to record the deflagration to detonation (DDT) transition. It is confirmed that the flame jet emanating from the tube assembly causes detonation initiation immediately downstream of the detonator, with little dependence on the size of the detonation tube. For the fuel–air mixture, the insertion of Shchelkin spirals into each of the smaller tubes enhances the development of the turbulent flame jet, leading to a shorter DDT distance. Multi-point spark ignition is also shown to provide a further reduction in the DDT distance compared to single-point ignition.

Alternative kernel functions for Smoothed Particle Hydrodynamics in cylindrical symmetry

Abstract: In this paper we present an alternative generic kernel function for use in Smoothed Particle Hydrodynamics (SPH) based methods. The alternative kernel has been implemented in a standard Cartesian description, and the results for the Sod shock tube problem is presented both with the alternative kernel and the traditional B-spline kernel. Kernels for cylindrically symmetric systems are developed based on fundamental interpolation theory for the cylindrical symmetry. A set of modified equations of motions are further derived through the use of Lagrangian formalism. Previous results have shown that the use of the traditional B-spline kernel function as a generic kernel implies that the cylindrically symmetric solution must be solved by table interpolation. By introducing an alternative generic kernel function, we derive simple analytical solution also for cylindrically symmetric systems. When compared to the B-spline function, the computational efficiency has been increased and the results improved. The results show good agreement with the analytical solution both for the Noh infinite shock problem and the Sedov point source explosion.

Modeling and simulation of cavitation in hydraulic pipelines based on the thermodynamic and caloric properties of liquid and steam

Abstract: The present paper focuses on a homogeneous cavitation model based on the thermodynamic equilibrium model of liquid and steam, which has significant importance for the development of modern hydraulic tools and injection systems. Subsequent to the derivation of the mathematical model a numerical method is described. Computations are carried out for a variety of test cases. The results are compared with analytic solutions, experimental data, and simulations obtained with a different numerical scheme. The investigation proves the ability of the method to predict cavitation in hydraulic pipelines in a reliable and successful manner. The lucidity of the procedure enables a simple extension of the model and the numerical method to higher space dimensions as well as applications in the context of complex hydraulic systems.

Simulation of the Mach reflection in supersonic flows by the CE/SE method

Abstract: This study employs the Space-Time Conservation Element and Solution Element (CE/SE) method to determine the influence of downstream flow conditions on Mach stem height. The results indicate that the Mach stem height depends on the incident shock wave angle and the distance between the trailing edge and the symmetry plane. Furthermore, it is shown that the downstream length ratio and the trailing edge angle do not affect the Mach stem height nor the Mach reflection (MR) configuration, and the Space-Time Conservation Element and Solution Element method is able to simulate the MR as well as many other numerical schemes.

Shock wave diffraction on multi-facetted and curved walls

Abstract: The two-dimensional diffraction of a shock wave over a wall made up of a series of plane and/or curved sections is considered. The analysis is based on the theory presented by, for the interaction of an originally plane shock wave with a corner. A method is presented by which the shock profile may be determined for a wall of any shape and for any incident Mach number, in regions where the characteristics form a simple wave. Comparisons are made between experimental measurements and theoretical predictions for convex walls consisting of a number of facets, and for circular arcs, for a range of incident shock wave Mach numbers. It is shown that the theory gives a satisfactory prediction of the wave shape, which improves as the Mach number increases. Modifications in the flow field behind the shock, compared to that for a simple corner made up of two plane walls is discussed, particularly relating to flow separation. For circular arc concave walls a inverse Mach reflection results experimentally, leading to regular reflection, for which the theory is of no use.