Showing papers on "Shock tube published in 1998"

Shock oscillation in underexpanded screeching jets

Abstract: The periodic oscillation of the shock waves in screeching, underexpanded, supersonic jets, issuing from a choked, axisymmetric, nozzle at fully expanded Mach numbers (Mj) of 1.19 and 1.42, is studied experimentally and analytically. The experimental part uses schlieren photography and a new shock detection technique which depends on a recently observed phenomenon of laser light scattering by shock waves. A narrow laser beam is traversed from point to point in the flow field and the appearance of the scattered light is sensed by a photomultiplier tube (PMT). The time-averaged and phase-averaged statistics of the PMT data provide significant insight into the shock motion. It is found that the shocks move the most in the jet core and the least in the shear layer. This is opposite to the intuitive expectation of a larger-amplitude shock motion in the shear layer where organized vortices interact with the shock. The mode of shock motion is the same as that of the emitted screech tone. The instantaneous profiles of the first four shocks over an oscillation cycle were constructed through a detailed phase averaged measurement. Such data show a splitting of each shock (except for the first one) into two weaker ones through a ‘moving staircase-like’ motion. During a cycle of motion the downstream shock progressively fades away while a new shock appears upstream. Spark schlieren photographs demonstrate that a periodic convection of large organized vortices over the shock train results in the above described behaviour. An analytical formulation is constructed to determine the self-excitation of the jet column by the screech sound. The screech waves, while propagating over the jet column, add a periodic pressure fluctuation to the ambient level, which in turn perturbs the pressure distribution inside the jet. The oscillation amplitude of the first shock predicted from this linear analysis shows reasonable agreement with the measured data. Additional reasons for shock oscillation, such as a periodic perturbation of the shock formation mechanism owing to the passage of the organized structures, are also discussed.

Thermal Decomposition of 2,5-Dimethylfuran. Experimental Results and Computer Modeling

Abstract: The thermal reactions of 2,5-dimethylfuran were studied behind reflected shock waves in a pressurized driver single pulse shock tube over the temperature range 1070−1370 K and overall densities of ∼3 × 10-5 mol/cm3. A large number of products resulting from unimolecular cleavage of the ring and consecutive free radical reactions were obtained under shock heating. A methyl group migration from C(2) to C(3) in the ring with the elimination of CO produces four isomers of C5H8 in unimolecular processes. An additional unimolecular process is the decomposition of 2,5-dimethylfuran to CH3CO and C4H5 which is an important initiator of free radical reactions. Ejection of a hydrogen atom from the methyl group in the molecule is another channel for initiation of free radical reactions in the system. The 2,5-dimethylfuryl radical, which is obtained in the process of H-atom ejection, decomposes in channels similar to those of 2,5-dimethylfuran to produce, among other products, C5H7, which is the precursor of cyclopent...

A Finite - Volume Particle Method for CompressibleFlows

Abstract: We derive a new class of particle methods for conservation laws, which are based on numerical flux functions to model the interactions between moving particles. The derivation is similar to that of classical Finite-Volume methods; except that the fixed grid structure in the Finite-Volume method is substituted by so-called mass packets of particles. We give some numerical results on a shock wave solution for Burgers equation as well as the well-known one-dimensional shock tube problem.

Experimental and numerical investigation of the shock-induced fluidization of a particles bed

Abstract: An experimental study on unsteady two phase flow is conducted in a vertical shock tube. Shock Mach numbers range from 1.3 to 1.5 in 1 bar. The particles are initially positioned in horizontal beds of various thicknesses. Our research covers a large domain of void fraction from 1 (single particles) to 0.35 (compact beds). The experiments provide shadowgraph images for the recording of particle trajectories (effect of the gas on the particles) and side-wall pressures (action of the particles on the gas). A dense two phase flow model has been elaborated and numerically solved using a finite difference scheme with pseudoviscosity. The simulated shock-induced fluidization of a 2 cm thick bed of 1.5 mm diameter glass particles is compared to the experiment.

Velocity measurements in turbulent gaseous mixtures induced by Richtmyer–Meshkov instability

Abstract: Instantaneous velocity measurements in a gaseous mixture arising from the shock wave-induced Richtmyer–Meshkov instability are conducted for the first time in a shock tube. Laser Doppler anemometry gives us the evolution of the axial velocity fluctuations of the mixing zone, before and after its interactions with reflected shock waves from the shock tube end wall. Experimental results demonstrate that a turbulent mixing zone is generated by the incident shock wave. Afterwards, the axial variance decreases before being amplified by the first reshock interaction through a baroclinic effect. Before the second reshock arrival, we measure once again a decrease of the turbulence level which is explained by both diffusion and dissipation.

Experimental Investigation of Air Radiation from Behind a Strong Shock Wave

Abstract: Radiation phenomena behind a strong shock wave have been experimentally investigated using a freepiston double-diaphragm shock tube. The shock front is detected by pressure sensors, whose rise time is calibrated by a laser schlieren technique. Spatial distribution of emission spectra correlated with the shock front is obtained for a 270-520 nm wavelength range by means of one-dimensional imaging spectroscopy, in which a wavelength vs. position image is taken by an ICCD camera with the gate time of 100 ns. Molecular spectra of N2 second positive and N^~ first negative band are observed immediately after the shock front, while atomic line spectra from N become dominant shortly after it. The rotational and vibrational temperatures of molecules, and the electronic excitation temperature of atoms are evaluated, and their spatial profiles in the streamwise direction are obtained for a shock velocity of 11.9 km/s and an ambient pressure of 0.3 torr, using N2 as the test gas. The measured temperatures seem to be in significant nonequilibrium with the translational temperature. Also, the relaxation rates for Njj" seem to be much higher than those for Ng.

Shock waves in a uniform bubbly flow

Abstract: An experimental and numerical study of transient shock wave phenomena in a liquid containing noncondensable gas bubbles is presented. Experiments are done in a shock tube with an upwardly directed bubbly flow to obtain a uniform spatial distribution of bubbles. The bubbly flow has an initial gas volume fraction of 0.2%. The bubbles have a radius of 0.6 mm. The liquid used is a silicone oil whose kinematic viscosity is 50×10−6 m2/s. Nitrogen and SF6 gas bubbles are tested to bring out the thermal effects of the bubble interior. The numerical calculation is performed using a modified mathematical model based on Kameda and Matsumoto [Phys. Fluids 8, 322 (1996)]. The transient pressure profiles determined in the experiment for the upwardly bubbly flow agree well quantitatively with those obtained by the numerical calculation using a uniform spatial distribution of bubbles. The SF6 experiment shows that the radial motion of the bubbles should be estimated by solving an equation in which the liquid compressibil...

Experimental and Numerical Study of Nonequilibrium Ultraviolet NO and N Emission in Shock Layer

Abstract: The results on an investigation of nonequilibrium uv radiation in the systems of molecular bands NO and N 2 + (1-) in a shock layer in air are presented. The studies included the experiments on a measurement of the nonequilibrium radiation behind a strong shock wave in an electric arc-driven shock tube within the range of shock-wave velocity change V s =5 to 10 km/s at the initial air pressure P 1 = 0.1 torr, and numerical simulation of radiation processes behind the shock wave and in a hypersonic viscous shock layer, where the flow was simulated on the basis of Navier-Stokes equations. A numerical model of the radiation behind the strong shock wave has been used to verify a kinetic scheme of radiation processes by comparison with experimental results. Examples of emission calculations in the NO and N 2 + (1-) systems for the conditions of a flight test are presented

Numerical study on blast flowfields induced by supersonic projectiles discharged from shock tubes

Abstract: In this paper we report on a numerical study of the blast flowfield generated by a supersonic projectile released from the open-end of a shock tube into ambient air. The Euler equations, assuming axisymmetric flows, were solved using a dispersion-controlled scheme implemented with moving boundary conditions. Two initial test cases were calculated. One of them is for validation of the numerical method and the other for verification of the moving boundary conditions. After good agreement was achieved, four further cases were calculated for examining effects of various projectile speeds and different release times of the projectile after the precursor shock wave was discharged. The present numerical study confirms that complicated transient phenomena exist in the initial stages shortly after projectile release, and that the blast flowfield is much more complex than that which can be inferred from muzzle blast studies where combustion products obscure the flow.

Attenuation of weak shock waves along pseudo-perforated walls

Abstract: In order to attenuate weak shock waves in ducts, effects of pseudo-perforated walls were investigated. Pseudo-perforated walls are defined as wall perforations having a closed cavity behind it. Shock wave diffraction and reflection created by these perforations were visualized in a shock tube by using holographic interferometer, and also by numerical simulation. Along the pseudo-perforated wall, an incident shock wave attenuates and eventually turns into a sound wave. Due to complex interactions of the incident shock wave with the perforations, the overpressure behind it becomes non-uniform and its peak value can locally exceed that behind the undisturbed incident shock wave. However, its pressure gradient monotonically decreases with the shock wave propagation. Effects of these pseudo-perforated walls on the attenuation of weak shock waves generated in high speed train tunnels were studied in a 1/250-scaled train tunnel simulator. It is concluded that in order to achieve a practically effective suppression of the tunnel sonic boom the length of the pseudo-perforation section should be sufficiently long.

Shock Tube Modelling With L1d

Abstract: L1d is a computer program for the simulation of transient-flow facilities such as light-gas launchers and free-piston driven shock tunnels. The numerical modelling embodied within L1d is based on a quasi-one-dimensional Lagrangian description of the gas dynamics coupled with engineering correlations for viscous effects and point-mass dynamics for poston motion. This report describes the governing equations and a set of four example simulations: * Sod's classic shock tube problem; * a ideal gas gun; * a fixed-driver shock tunnel; and * a free-piston driven shock tunnel.

Ignition dynamics of boron particles in a shock tube

Abstract: Small crystalline boron particles (1-15 (J,m) in a pure oxygen atmosphere are ignited at the endwall of a high pressure (8.5 atm) shock tube to study the effect of particle size on ignition delay time. An infrared detector is used to observe 6263 oxide layer removal from the boron particles, while a visible wavelength photodiode is used simultaneously to observe BO2 emission. Both infrared and visible data are presented showing particle size and temperature effects. Results compare favorably to theoretical predictions from a recent model.

Studies of the Transient Flows in High Enthalpy Shock Tunnels

Abstract: The nature of high enthalpy shock tunnels dictates that only very limited run time can be achieved during experiments. Due to the extreme flow conditions and the severe time constraint involved, it is very difficult to conduct diagnostic measurements to determine the flow condition produced in the facilities. A series of numerical studies has therefore been carried out to provide a quantitative description of the transient flow in the shock tunnel. The flows in the HEG shock tunnel are analyzed using time-dependent viscous computations. The studies focused on the events subsequent to shock reflection at the downstream end of the shock tunnel, namely, the reservoir and the nozzle regions. For the shock tube, the interactions of the reflected shock with the wall boundary layer and the contact surface are examined for their contribution to driver gas contamination and the generation of flow disturbances. This includes interaction with the reflecting wall geometry, that is, in the presence of the nozzle entrance and “particle stopper”. For the starting flow in the nozzle, the propagation of the initial shock system and the development of the boundary layers during the starting transient are examined and the time required to established the steady flow is analyzed to help determine the proper “test window” in the experiments.

A computational study of shock speeds in high-performance shock tubes

Abstract: This paper describes U2DE, a finite-volume code that numerically solves the Euler equations. The code was used to perform multi-dimensional simulations of the gradual opening of a primary diaphragm in a shock tube. From the simulations, the speed of the developing shock wave was recorded and compared with other estimates. The ability of U2DE to compute shock speed was confirmed by comparing numerical results with the analytic solution for an ideal shock tube. For high initial pressure ratios across the diaphragm, previous experiments have shown that the measured shock speed can exceed the shock speed predicted by one-dimensional models. The shock speeds computed with the present multi-dimensional simulation were higher than those estimated by previous one-dimensional models and, thus, were closer to the experimental measurements. This indicates that multi-dimensional flow effects were partly responsible for the relatively high shock speeds measured in the experiments.

Development of serial bio-shock tubes and their application.

Abstract: OBJECTIVE To design and produce serial shock tubes and further examine their application to experimental studies on blast injury. METHODS Bio-medical engineering technique was used for the design and development of the serial shock tubes. One thousand four hundred and fifty nine animals (757 rats, 105 guinea pigs, 335 rabbits, 240 dogs and 22 sheep) were then used to test the wounding effects of the shock tubes. RESULTS Three types of bio-shock tubes, that is, large-, medium- and small-scale shock tubes were made in our laboratory. The large-scale shock tube is 39 meters long; the inner diameter of the test section is 1 meter; and the maximum overpressure in the driving section is 10.3 MPa. A negative pressure could be formed by means of the reflected rarefactive wave produced by the end plate. The medium-scale shock tube is 34.5 meters long; the maximum overpressure in the driving section is 22 MPa; the test section is designed to be a knockdown, showing 5 basic types with inner diameter of 77 to 600 millimeters, which could be used for researches on overpressure, explosive decompression, underwater explosion, and so on. The small-scale shock tube is 0.5 meter long with the maximum endured overpressure of 68.6 MPa. Results from animal experiments showed that this set of shock tubes could induce various degrees of systemic or local blast injury in large or small animals. CONCLUSIONS This set of bio-shock tubes can approximately simulate typical explosive wave produced by nuclear or charge explosion, and inflict various degrees of blast injury characterized by stability and reproducibility. Therefore, they can meet the needs of blast research on large and small animals.

Comparison of pressure-based velocity and momentum procedures for shock tube problem

Abstract: In this work, a one-dimensional investigation is performed to compare the performance of velocity-based and momentum-based procedures. Both procedures are formulated based on a control-volume approach with pressure as a dependent variable. The related integration point operators are derived based on incorporating the correct physical influence of the flow and other relevant couplings. The formulations are free from employing any explicit artificial viscosity and damping mechanism. They are applied to highly compressible flow by testing the shock tube problem in order to investigate two aspects of linearization and constant mass flux advantage within the developed procedures. The results show that the proposed momentum-based procedure (MBP) is more stable and accurate than the velocity-based procedure (VBP) without damping.

Gas-Phase Pyrolysis of 1,3,3-Trinitroazetidine: Shock Tube Kinetics

Abstract: Vapors of 1,3,3-trinitroazetidine (TNAZ) were pyrolyzed in a single-pulse shock tube, under high dilution in Ar, over the temperature range 750−1100 K (reflected shocks). The decay of TNAZ and the appearance of the reactive intermediate, NO2, were followed spectrophotometrically at 271 and 405 nm, respectively, in real time via a multiple-pass quartz extension of the shock tube terminus. Samples of the major products that were generated during 1.5 ms residence time and wave quenched were identified and quantitated by GC and FTIR. The unimolecular rate constant (high-pressure limit) for dissociation of TNAZ under our experimental conditions is kuni = 1013.96±0.63 exp[(−19900 ± 1190)/T], s-1. Successive fissions of NO2 groups were indicated by the time-dependent absorption levels at 405 nm. A gas-phase FTIR spectrum of TNAZ recorded at ∼110 °C provided the missing data for computing the thermochemical parameters for this compound. Then the partition of its decomposition products (minimal free energy) could ...

Quantitative Visualization of the Change of Turbulence Structure Caused by a Normal Shock Wave

Abstract: A speckle photographic technique is used for visualizing the planar distribution of the refractive deflection angles of light transmitted through the compressible turbulent flow in a shock tube. Illumination by a short laser pulse allows to freeze the instantaneous pattern of the deflection angles as caused by the turbulent fluctuations of the gas density. Turbulent structures are visible in the patterns of the deflection angles' isolines. A normal shock wave propagating through the turbulent density field causes structural changes of the observed patterns, which is quantified by determining the spatial correlation functions of the deflection angles.

Shock Tube Measurements of the Equation of State of Argon

Abstract: We have investigated the equation of state of argon at elevated temperatures and pressures using a new shock tube method. Temperatures in the range of 1280 to 1830 K, and pressures from 6 to 50 MPa were generated behind reflected shock waves in test gas mixtures of argon with trace amounts of CO and H2 added. Density was determined from reflected shock pressure and incident shock speed measurements using the shock-jump relations. Temperature was determined from the modeling of the 4.7 sum infrared emission of the fundamental vibrational band of thermally-equilibrated CO. The experimentally determined argon P-p-T data points are in good agreement with the static-cell data of LeCocq and an extrapolation of the equation of state of Stewart and Jacobsen.

Density reconstruction from laser schlieren signal in shock tube experiments

Abstract: Using a diffraction approach the convolution-type integral equation for the laser schlieren signal created by an arbitrary disturbance at low pressure, where refractive index of disturbance is close to unity, in a shock tube (thin optical layer) has been deduced. In the equation electric circuit relaxation processes were taken into account by a response function. The equation was solved with the aid of the regularization method worked out for ill-posed problems. The density structures of the strong shock waves in air have numerically been reconstructed from experimental data ranging shock wave Mach number of $M_s=20$ –30, and $P_0=10$ –30 Pa.

Holographic interferometric observation of shock waves discharged from an open-end of a square cross-sectional shock tube

Abstract: This paper describes results of holographic interferometric observation of the evolution of shock waves and the primary vortex loop discharged from the open-end of a 40 mm × 40 mm square tube. Experiments were conducted by using the square tube connected to a 60 mm × 1500 mm diaphragmless shock tube. Holographic interferometric flow visualization viewed from the axial direction was carried out using a diffusively-reflected beam as the object beam. In order to interpret the observed three-dimensional shock-wave motion, numerical simulations were also carried out by solving the Euler equations with the dispersion-controlled scheme. The experimental interferogram was compared with numerical visualization results. The three-dimensional transition of the shock wave configuration from a planar to a spherical shape and the development of the primary vortex loop from a square shaped to the three-dimensional structure were clearly observed.