Showing papers on "Shock tube published in 2014"

Polar-coordinate lattice Boltzmann modeling of compressible flows.

Abstract: We present a polar coordinate lattice Boltzmann kinetic model for compressible flows. A method to recover the continuum distribution function from the discrete distribution function is indicated. Within the model, a hybrid scheme being similar to, but different from, the operator splitting is proposed. The temporal evolution is calculated analytically, and the convection term is solved via a modified Warming-Beam (MWB) scheme. Within the MWB scheme a suitable switch function is introduced. The current model works not only for subsonic flows but also for supersonic flows. It is validated and verified via the following well-known benchmark tests: (i) the rotational flow, (ii) the stable shock tube problem, (iii) the Richtmyer-Meshkov (RM) instability, and (iv) the Kelvin-Helmholtz instability. As an original application, we studied the nonequilibrium characteristics of the system around three kinds of interfaces, the shock wave, the rarefaction wave, and the material interface, for two specific cases. In one of the two cases, the material interface is initially perturbed, and consequently the RM instability occurs. It is found that the macroscopic effects due to deviating from thermodynamic equilibrium around the material interface differ significantly from those around the mechanical interfaces. The initial perturbation at the material interface enhances the coupling of molecular motions in different degrees of freedom. The amplitude of deviation from thermodynamic equilibrium around the shock wave is much higher than those around the rarefaction wave and material interface. By comparing each component of the high-order moments and its value in equilibrium, we can draw qualitatively the main behavior of the actual distribution function. These results deepen our understanding of the mechanical and material interfaces from a more fundamental level, which is indicative for constructing macroscopic models and other kinds of kinetic models.

Towards a shock tube method for the dynamic calibration of pressure sensors

Abstract: In theory, shock tubes provide a pressure change with a very fast rise time and calculable amplitude. This pressure step could provide the basis for the calibration of pressure transducers used in highly dynamic applications. However, conventional metal shock tubes can be expensive, unwieldy and difficult to modify. We describe the development of a 1.4 MPa (maximum pressure) shock tube made from unplasticized polyvinyl chloride pressure tubing which provides a low-cost, light and easily modifiable basis for establishing a method for determining the dynamic characteristics of pressure sensors.

An experimental investigation of the turbulent mixing transition in the Richtmyer–Meshkov instability

Abstract: The Richtmyer–Meshkov instability (RMI) is experimentally investigated in a vertical shock tube using a broadband initial condition imposed on an interface between a helium–acetone mixture and argon ( ). The interface is created without the use of a membrane by first setting up a flat, gravitationally stable stagnation plane, where the gases are injected from the ends of the shock tube and exit through horizontal slots at the interface location. Following this, the interface is perturbed by injecting gas within the plane of the interface. Perturbations form in the lower portion of this layer due to the shear between this injected stream and the surrounding gas. This shear layer serves as a statistically repeatable broadband initial condition to the RMI. The interface is accelerated by either a or planar shock wave, and the development of the ensuing mixing layer is investigated using planar laser-induced fluorescence (PLIF). The PLIF images are processed to reveal the light-gas mole fraction by accounting for laser absorption and laser-steering effects. The images suggest a transition to turbulent mixing occurring during the experiment. An analysis of the mole-fraction distribution confirms this transition, showing the gases begin to homogenize at later times. The scalar variance energy spectra exhibits a near inertial range, providing further evidence for turbulent mixing. Measurements of the Batchelor and Taylor microscales are made from the mole-fraction images, giving and 4 mm, respectively, by the latest times. The ratio of these scales implies an outer-scale Reynolds number of .

An experimental and numerical investigation of the dependency on the initial conditions of the Richtmyer-Meshkov instability

Abstract: The nonlinear evolution of 2D single-mode Richtmyer-Meshkov instabilities is investigated through experiments in shock tube and numerical simulations. In our shock tube, the interface is materialized by a thin membrane attached to a stereo-lithographed grid. The purpose of this study is to compare experimental and numerical results, verify that using a higher Mach number for the incident shock wave (Misw) than in a previous study [C. Mariani, M. Vandenboomgaerde, G. Jourdan, D. Souffland, and L. Houas, “Investigation of the Richtmyer-Meshkov instability with stereolithographed interfaces,” Phys. Rev. Lett. 100, 254503 (2008)] drastically reduces the deleterious effects of the membrane remnants, explore the effect of a high initial amplitude at the interface on the growth of the perturbation, and understand the lack of roll-up structures in the nonlinear phase of the instability. Using grayscale gradient rather than gray level, a new processing of the raw pictures is developed. Numerical simulations run wi...

Experiments on the Richtmyer-Meshkov instability with an imposed, random initial perturbation

Abstract: A vertical shock tube is used to perform experiments on the Richtmyer–Meshkov instability with a three-dimensional random initial perturbation. A membraneless flat interface is formed by opposed gas flows in which the light and heavy gases enter the shock tube from the top and from the bottom of the shock tube driven section. An air/SF $$_{6}$$ gas combination is used and a Mach number $$ M = 1.2$$ incident shock wave impulsively accelerates the interface. Initial perturbations on the interface are created by vertically oscillating the gas column within the shock tube to produce Faraday waves on the interface resulting in a short wavelength, three-dimensional perturbation. Planar Mie scattering is used to visualize the flow in which light from a laser sheet is scattered by smoke seeded in the air, and image sequences are captured using three high-speed video cameras. Measurements of the integral penetration depth prior to reshock show two growth behaviors, both having power law growth with growth exponents in the range found in previous experiments and simulations. Following reshock, all experiments show very consistent linear growth with a growth rate in good agreement with those found in previous studies.

Measurement and Characterization of Mid-wave Infrared Radiation in CO2 Shocks

Abstract: We present the characterization of infrared radiation obtained in the NASA Ames Electric Arc Shock Tube at velocities from 3-7.5 km/s and freestream densities from 4.724 g/m (corresponding to ground test pressures of 0.2 to 1.0 Torr), which are relevant to Mars entry conditions. The IR radiation is shown to decrease with increasing velocity over this range, and is expected to be largest at 3 km/s. Based on this data, an estimated relationship for (near) peak radiative heating is given as 20 W/cm for Mars Science Laboratory, which compares well to the 18 W/cm discrepancy (out of 35 W/cm) observed on its stagnation line sensor. Analysis of the experimental data shows that spectral profiles are predicted well by NEQAIR for most conditions. The only exception is the 2.7 μm band at high temperature, which is underpredicted. Spatially resolved data are used for comparison against proposed kinetic models for CO2 shock environments. No one model matches the data at all conditions, but each may agree over some velocity range. A simplified three reaction model is shown to predict the post-shock decay well, while matching the radiative magnitude to within 30-70% in the velocity range 3.0-5.7 km/s. Attempts to extract reaction rates directly from the data shows the dissociation rate to be controlled by O atom exchange, and show good consistency with published kinetic rates for a velocity of 3 km/s. At higher velocity, the initial kinetics occur in thermal non-equilibrium, suggesting further study of these mechanisms are warranted.

Measurement of the Rate of Hydrogen Peroxide Thermal Decomposition in a Shock Tube Using Quantum Cascade Laser Absorption Near 7.7 μm

Abstract: Hydrogen peroxide (H2O2) is formed during hydrocarbon combustion and controls the system reactivity under intermediate temperature conditions. Here, we measured the rate of hydrogen peroxide decomposition behind reflected shock waves using midinfrared absorption of H2O2 near 7.7 µm. We performed the experiments in diluted H2O2/Ar mixtures between 930 and 1235 K and at three different pressures (1, 2, and 10 atm). Under these conditions, the decay of hydrogen peroxide is sensitive only to the decomposition reaction rate, H2O2 + M 2OH + M (k1). The second-order rate coefficient at low pressures (1 and 2 atm) did not exhibit any pressure dependence, suggesting that the reaction was in the low-pressure limit. The rate data measured at 10 atm exhibited falloff behavior. The measured decomposition rates can be expressed in Arrhenius forms as follows:

Sensitive and rapid laser diagnostic for shock tube kinetics studies using cavity-enhanced absorption spectroscopy.

Abstract: We report the first application of cavity-enhanced absorption spectroscopy (CEAS) using a coherent light source for sensitive and rapid gaseous species time-history measurements in a shock tube. Off-axis alignment and fast scanning of the laser wavelength were used to minimize coupling noise in a low-finesse cavity. An absorption gain factor of 83 with a measurement time resolution of 20 µs was demonstrated for C2H2 detection using a near-infrared transition near 1537 nm, corresponding to a noise-equivalent detection limit of 20 ppm at 296 K and 76 ppm at 906 K at 50 kHz. This substantial gain in signal, relative to conventional single-pass absorption, will enable ultra-sensitive species detection in shock tube kinetics studies, particularly useful for measurements of minor species and for studies of dilute reactive systems.

Modeling shock waves generated by explosive volcanic eruptions

Abstract: Atmospheric shock waves induced by explosive volcanic eruptions can provide valuable information about eruption characteristics. Shock waves are manifested as pressure-density gradients that can be remotely observed with relatively little noise. Field measurements of expanding shock waves can be directly recorded by pressure transducers or imaged under the proper illumination and atmospheric conditions. In this paper, an open-ended shock tube was used to generate weak shock waves in the laboratory that are representative of explosive volcanic eruptions. They indicate that strong shock wave theory can be used for modeling moderate volcanic eruptions. Based on that finding, we use strong shock theory to estimate the sudden explosive energy released from several explosive eruptions. Our energy calculations are well correlated with total energy estimates derived from plume height or erupted mass.

Reaction Rate Constant of CH2O + H = HCO + H2 Revisited: A Combined Study of Direct Shock Tube Measurement and Transition State Theory Calculation

Abstract: The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, has been studied behind reflected shock waves with use of a sensitive mid-IR laser absorption diagnostic for CO, over temperatures of 1304–2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxane as the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintain relatively constant H atom concentrations, the current study significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304–2006 K, 1 atm) = 1.97 × 1011(T/K)1.06 exp(−3818 K/T) cm3 mol–1s–1, with uncertainty limits estimated to be +18%/–26%. A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12 level of theory, is in good agreement with the shock tube measurement and ext...

A parametric approach to shape field-relevant blast wave profiles in compressed-gas-driven shock tube.

Abstract: Detonation of a high explosive produces shock-blast wave, shrapnel, and gaseous products. While direct exposure to blast is a concern near the epicenter, shock-blast can affect subjects even at farther distances, which is termed as primary blast injury, which is the theme of this work. The shock-blast profile is characterized with blast overpressure, positive time duration, and impulse as shock-blast wave parameters (SWPs). These parameters in turn are a function of field factors, such as the strength of high explosive and the distance of the human subjects from the epicenter. The shape and magnitude of the profile determine the severity of injury to the subjects. As shown in some of our recent works (Chandra et al., 2011;Sundaramurthy et al., 2012;Skotak et al., 2013), the profile not only determines the survival of the animal but also the acute and chronic biomechanical injuries along with the following bio-chemical sequelae. It is extremely important to carefully design and operate the shock tube to produce field relevant SWPs. Furthermore, it is vital to identify and eliminate the artifacts that are inadvertently introduced in the shock-blast profile that may affect the results. In this work, we examine the relationship between shock tube adjustable parameters (SAPs) and SWPs that can be used to control the blast profile; the results can be easily applied to many of the laboratory shock tubes. Further, exact replication of shock profile (magnitude and shape) can be related to field explosions and can be a standard in comparing results across different laboratories. 40 experiments are carried out by judiciously varying SAPs such as membrane thickness, breech length (66.68 to 1209.68 mm), measurement location, and type of driver gas (nitrogen, helium). The relationships between SAPs and the resulting shock-blast profiles are characterized. Finally, shock-blast profiles of a TNT explosion from ConWep software is compared with the profiles obtained from the tube.

Shock Waves in Dispersive Eulerian Fluids

Abstract: The long-time behavior of an initial step resulting in a dispersive shock wave (DSW) for the one-dimensional isentropic Euler equations regularized by generic, third-order dispersion is considered by use of Whitham averaging. Under modest assumptions, the jump conditions (DSW locus and speeds) for admissible, weak DSWs are characterized and found to depend only upon the sign of dispersion (convexity or concavity) and a general pressure law. Two mechanisms leading to the breakdown of this simple wave DSW theory for sufficiently large jumps are identified: a change in the sign of dispersion, leading to gradient catastrophe in the modulation equations, and the loss of genuine nonlinearity in the modulation equations. Large amplitude DSWs are constructed for several particular dispersive fluids with differing pressure laws modeled by the generalized nonlinear Schrodinger equation. These include superfluids (Bose–Einstein condensates and ultracold fermions) and “optical fluids.” Estimates of breaking times for smooth initial data and the long-time behavior of the shock tube problem are presented. Detailed numerical simulations compare favorably with the asymptotic results in the weak to moderate amplitude regimes. Deviations in the large amplitude regime are identified with breakdown of the simple wave DSW theory.

Shock boundary layer interaction under the influence of a normal suction slot

Abstract: The phenomenon of shock boundary layer interaction of a shock train under the influence of a normal suction slot is studied In previous work, it was found that a normal, circumferential suction slot is sufficient to stabilize the primary shock of a shock train in as much as that the back pressure of the shock train can be increased until the shock train gradually changes into a single normal shock Based on the experimental and numerical results, a flow model was derived which explains the transition of a shock train into a single shock under the influence of boundary layer suction In this work, the normal shock boundary layer interaction model is validated against flow cases with different upstream Mach and Reynolds numbers For that purpose three different nozzle flows are investigated at various total pressure levels In a second step, the flow model is extended to the oblique shock case, correlating the suction mass flow with the total pressure distribution of the incoming boundary layer and the static pressure downstream of the oblique shock Finally, the influence of the suction cavity pressure onto the shock boundary layer interaction is considered