Showing papers on "Shock tube published in 2008"

Formulation of Kinetic Energy Preserving Conservative Schemes for Gas Dynamics and Direct Numerical Simulation of One-Dimensional Viscous Compressible Flow in a Shock Tube Using Entropy and Kinetic Energy Preserving Schemes

Abstract: This paper follows up on the author's recent paper "The Construction of Discretely Conservative Finite Volume Schemes that also Globally Conserve Energy or Enthalpy". In the case of the gas dynamics equations the previous formulation leads to an entropy preserving (EP) scheme. It is shown in the present paper that it is also possible to construct the flux of a conservative finite volume scheme to produce a kinetic energy preserving (KEP) scheme which exactly satisfies the global conservation law for kinetic energy. A proof is presented for three dimensional discretization on arbitrary grids. Both the EP and KEP schemes have been applied to the direct numerical simulation of one-dimensional viscous flow in a shock tube. The computations verify that both schemes can be used to simulate flows with shock waves and contact discontinuities without the introduction of any artificial diffusion. The KEP scheme performed better in the tests.

A shock tube study of cyclopentane and cyclohexane ignition at elevated pressures

Abstract: Ignition delay times for cyclopentane/air and cyclohexane/air mixtures were measured in a shock tube at temperatures of 847–1379 K, pressures of 11–61 atm, and equivalence ratios of ϕ = 1.0, 0.5, and 0.25. Ignition times were determined using electronically excited OH emission monitored through the shock tube endwall and piezoelectric pressure measurements made in the shock tube sidewall. The dependence of ignition time on pressure, temperature, and equivalence ratio is quantified and correlations for ignition time formulated. Measured ignition times are compared to kinetic modeling predictions from four recently published mechanisms. The data presented provide a database for the validation of cycloalkane kinetic mechanisms at the elevated pressures found in practical combustion engines. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 624–634, 2008

Shock-bubble interactions: Features of divergent shock-refraction geometry observed in experiments and simulations

Abstract: The interaction of a planar shock wave with a spherical bubble in divergent shock-refraction geometry is studied here using shock tube experiments and numerical simulations The particular case of a helium bubble in ambient air or nitrogen (A≈−08) is considered, for 14

Analysis and Model Validation of Shock Layer Radiation in Air

Abstract: This paper analyzes the shock layer radiative heating environment for a large entry vehicle on a lunar return trajectory. Modeling results show that much of the shock layer plasma is in local thermodynamic equilibrium (LTE) and is not optically thin. The ionization level is generally high (15%) and the air is almost fully dissociated. A significant amount of vacuum ultraviolet (VUV) radiation is produced due to bound-bound and bound-free transitions of N and O atoms. The sensitivity of total radiation to Stark broadening, which dominates over other line broadening mechanisms, is quantified. The latter part of this paper reports the status of ongoing validation of the current radiation models with measurements in the Electric-Arc Shock Tube (EAST) facility at NASA Ames Research Center. Model predictions are compared with the calibrated radiation spectra measured in the equilibrium portion of the shock layer at 0.3 Torr. The reasons for discrepancy between model and measurements are also discussed with possible hypotheses presented for further investigation.

A Comparison of EAST Shock-Tube Radiation Measurements with a New Air Radiation Model

Abstract: This paper presents a comparison between the recent EAST shock tube radiation measurements (Grinstead et al., AIAA 2008-1244) and the HARA radiation model. The equilibrium and nonequilibrium radiation measurements are studied for conditions relevant to lunar-return shock-layers; specifically shock velocities ranging from 9 to 11 kilometers per second at initial pressures of 0.1 and 0.3 Torr. The simulated shock-tube flow is assumed one-dimensional and is calculated using the LAURA code, while a detailed nonequilibrium radiation prediction is obtained in an uncoupled manner from the HARA code. The measured and predicted intensities are separated into several spectral ranges to isolate significant spectral features, mainly strong atomic line multiplets. The equations and physical data required for the prediction of these strong atomic lines are reviewed and their uncertainties identified. The 700-1020 nm wavelength range, which accounts for roughly 30% of the radiative flux to a peak-heating lunar return shock-layer, is studied in detail and the measurements and predictions are shown to agree within 15% in equilibrium. The plus or minus 1.5% uncertainty on the measured shock velocity is shown to cause up to a plus or minus 30% difference in the predicted radiation. This band of predictions contains the measured values in almost all cases. For the highly nonequilibrium 0.1 Torr cases, the nonequilibrium radiation peaks are under-predicted by about half. This under-prediction is considered acceptable when compared to the order-of-magnitude over-prediction obtained using a Boltzmann population of electronic states. The reasonable comparison in the nonequilibrium regions provides validation for both the non-Boltzmann modeling in HARA and the thermochemical nonequilibrium modeling in LAURA. The N2 (+)(1-) and N2(2+) molecular band systems are studied in the 290 480 nm wavelength range for both equilibrium and nonequilibrium regimes. The non-Boltzmann rate models for these systems, which have significant uncertainties, are tuned to improve the comparison with measurements.

Application of background oriented schlieren for quantitative measurements of shock waves from explosions

Abstract: This paper describes application of a background oriented schlieren technique in order to obtain quantitative measurements of shock waves from explosions by processing high speed digital video recordings. The technique is illustrated by an analysis of two explosions, a high explosive test and a hydrogen gas explosion test. The visualization of the shock front is utilized to calculate the shock Mach number, leading to a predicted shock front pressure. For high explosives the method agreed quite well with a standard curve for side-on shock pressures. In the case of the gas explosion test we can also show that the shock front is non-spherical. It should be possible to develop this technique to investigate external blast waves and external explosions from vented gas explosions in more details.

Two-Dimensional Simulation of Stripping Breakup of a Water Droplet

Abstract: The breakup of a liquid droplet induced by a high-speed gas stream is a known multiphase flow problem. Among various breakup modes of liquid droplets impinged by shock waves, the stripping breakup takes place over a wide range of Weber numbers, from 100 to approximately 20,000. In this study, the evolution of the stripping breakup of a water droplet is simulated by using a multiphase flow solver with a five-equation model. Several test cases such as gas-gas shock tube, water-air shock tube, and underwater explosion problems are performed to validate the present numerical methods. To compare with the experimental results, the water droplets with diameters of 6.4 and 4.8 mm and Mach numbers of 1.3 and 1.47 are chosen in this study. The stripping breakup of a water droplet, including the shape deformation, vortex shedding, unsteady drag force, and flow instability, is investigated. The computed displacement, acceleration, and volume change of the water droplet are in agreement with the experimental data in dimensionless form. The evolution of a water droplet during the stripping breakup for an inviscid flow is presented by flow visualization.

A diaphragmless shock tube for high temperature kinetic studies.

Abstract: A novel, diaphragmless shock tube (DFST) has been developed for use in high temperature chemical kinetic studies. The design of the apparatus is presented along with performance data that demonstrate the range and reproducibility of reaction conditions that can be generated. The ability to obtain data in the fall off region, confined to much narrower pressure ranges than can be obtained with a conventional shock tube is shown, and results from laser schlieren densitometry experiments on the unimolecular dissociation of phenyl iodide (P2=57±9 and 122±7 torr, T2=1250–1804 K) are presented. These are compared with results similar to those that would be obtained from a classical shock tube and the implications for extrapolation by theoretical methods are discussed. Finally, the use of the DFST with an online mass spectrometer to create reproducible experiments that can be signal averaged to improve signal/noise and the quality of mass peaks is demonstrated; something that is not possible with a conventional s...

Design of the Dense Gas Flexible Asymmetric Shock Tube

Abstract: This paper presents the conceptual design of the flexible asymmetric shock tube (FAST) setup for the experimental verification of the existence of nonclassical rarefaction shock waves in molecularly complex dense vapors. The FAST setup is a Ludwieg tube facility composed of a charge tube that is separated from the discharge vessel by a fast-opening valve. A nozzle is interposed between the valve and the charge tube to prevent disturbances from the discharge vessel to propagate into the tube. The speed of the rarefaction wave generated in the tube as the valve opens is measured by means of high-resolution pressure transducers. The provisional working fluid is siloxane D6 (dodecamethylcyclohexasiloxane, C12H36O6Si6). Numerical simulations of the FAST experiment are presented using nonideal thermodynamic models to support the preliminary design. The uncertainties related to the thermodynamic model of the fluid are assessed using a state-of-the-art thermodynamic model of fluid D6. The preliminary design is confirmed to be feasible and construction requirements are found to be well within technological limits.

Experimental investigations of compressible vortex loops

Abstract: The present study involves the shock wave and consequent vortex loop generated when a shock tube with various nozzle geometries is employed. It aims to provide quantitative and qualitative insight into the physics of these compressible phenomena. The geometries included two elliptic nozzles with minor to major axis ratios of 0.4 and 0.6, a 15 mm circular nozzle and a 30×30 mm2 square nozzle. The experiments were performed for driver gas (air) pressures of 4, 8 and 12 bars. Schlieren, shadowgraphy, and particle image velocimetry techniques were employed to visualize and quantify the induced flow field.

Application of an Aerosol Shock Tube for the Kinetic Studies of n-Dodecane/Nano-Aluminum Slurries

Abstract: Aerosol shock tube methods were used to measure the ignition delay times of slurries of n-dodecane and nano-aluminum particles. The aerosol shock tube has the potential to allow the study of gel propellant chemical kinetics without interference from other physicalchemical processes. A Sono-Tek ultrasonic Spray nozzle was used to produce slurry aerosol droplets which were uniformly dispersed in the driven section of the shock tube. An array of laser extinction and absorption diagnostics was used to track species time histories, droplet breakup and evaporation, and measure ignition delay times. These experiments were conducted behind reflected shock waves for temperatures ranging from 1127 to 1249 K and pressures from 4.7 to 9.5 atm.

Industrial applications of shock waves

Abstract: Shock waves have been traditionally considered to be an integral part of flow field features in the area of high-speed aerodynamics. Physically the propagation of shock waves in any media is invariably associated with instantaneous increase in pressure and temperature behind the shockwave. The capability of shock waves to generate non-linear pressure and temperature spikes in the medium of propagation finds very interesting applications in variety of areas such as medicine, biological sciences, material processing, manufacturing, and microelectronic industries. This paper reviews four new industrial applications of shock waves that have been developed in the Shock Waves Laboratory (SWL), Department of Aerospace Engineering, IISc, Bangalore. They are shock wave assisted (a) cell transformation, (b) preservative injection into Bamboos, (c) sandal oil extraction, and (d) removal of micron size dust from silicon wafer surfaces. The shock waves generated in an underwater shock wave generator are exploited in successfully injecting the desired deoxyribonucleic acid into Escherichia coli and Agrobacterium cells. The vertical shock wave reactor is applicable in successfully injecting the water-soluble chemical preservative (copper–chrome–arsenic) to samples of bamboo. The exposure of sandalwood samples to shock wave loading in the horizontal diaphragmless shock tube resulted in the drastic reduction (40 per cent) of time required for oil extraction. Further, a new shock wave assisted technique for micron-size dust removal from silicon wafer surfaces has been developed in collaboration with the Interdisciplinary ShockWave Research Center, Tohoku University, Sendai, Japan. The strong vortex field generated behind theMach reflection of a shock wave has been used to remove micron size dust particles from the surface of silicon wafers. The salient features of these new industrial applications of shock waves are described in this paper along with some important results.

Analysis of the FIRE II Flight Experiment by Means of a Collisional Radiative Model

Abstract: An accurate investigation of the behavior of electronically excited states of atoms and molecules in the post shock relaxation area is carried out by means of the Collisionalradiative model. The model is applied to a 1D shock tube code and the operating conditions are taken from three points in the trajectory of the FIRE II flight experiment. We account for thermal nonequilibrium between the translational and vibrational energy modes of individual molecular species and treat the electronic states of atoms and molecules as separate species. Relaxation of free-electrons is also accounted for by making use of a separate conservation equation for their energy. Non-Boltzmann distributions of the electronic state populations of atoms and molecules are allowed. Deviations from Boltzmann distributions are expected to occur in a rapidly ionizing regime behind a strong shock wave, due to the depletion of the high lying bound electronic states. In order to quantify the extent of departure from equilibrium of the electronic state populations, results are compared with those obtained assuming a Boltzmann distribution.

Impulse Generation by an Open Shock Tube

Abstract: We perform experimental and numerical studies of a shock tube with an open end. The purpose is to investigate the impulse due to the exhaust of gases through the open end of the tube as a model for a partially filled detonation tube as used in pulse detonation engine testing. We study the effects of the pressure ratio (varied from 3 to 9.2) and the volume ratio (expressed as fill fractions) between the driver and driven section. Two different driver gases, helium and nitrogen, and fill fractions between 5 and 100% are studied; the driven section is filled with air. For both driver gases, increasing the pressure ratio leads to larger specific impulses. The specific impulse increases for a decreasing fill fraction for the helium driver, but the impulse is almost independent of the fill fraction for the nitrogen driver. Two-dimensional (axisymmetric) numerical simulations are carried out for both driver gases. The simulation results show reasonable agreement with experimental measurements at high pressure ratios or small fill fractions, but there are substantial discrepancies for the smallest pressure ratios studied. Empirical models for the impulse in the limits of large and small fill fractions are also compared with the data. Reasonable agreement is found for the trends with fill fractions using the Gurney or Sato model at large fill fractions, but only Cooper’s bubble model is able to predict the small fill fraction limit. Computations of acoustic impedance and numerical simulations of unsteady gas dynamics indicate that the interaction of waves with the driver-driven gas interface and the propagation of waves in the driven gas play an essential role in the partial-fill effect.

Kinetics of 1,2-Dimethylbenzene Oxidation and Ignition: Experimental and Detailed Chemical Kinetic Modeling

Abstract: New experimental results were obtained for the oxidation of 1,2-dimethylbenzene in a jet-stirred reactor (JSR) at atmospheric pressure in dilute conditions over the temperature range 900–1400 K, and variable equivalence ratio (0.5 ≤ ϕ ≤ 1.5). The data consisted of concentration profiles vs. temperature for the reactants, stable intermediates and final products, measured by sonic probe sampling followed by on-line GC-MS analyses and off-line GC-TCD-FID and GC-MS analyses. The ignition of 1,2-dimethylbenzene-oxygen-argon mixtures was measured behind reflected shock waves over the temperature range 1400–1830 K, at 1 atm, and variable equivalence ratio (0.5 ≤ ϕ ≤ 2.0), using a shock tube (ST). The oxidation and ignition of 1,2-dimethylbenzene under respectively JSR and ST conditions were modeled using a detailed chemical kinetic reaction mechanism (219 species and 1545 reactions, most of them reversible) deriving from a previous scheme proposed for the ignition, oxidation, and combustion of simple aromatics (...

Shock Tube Flows Past Partially Opened Diaphragms

Abstract: Unsteady compressible flows resulting from the incomplete burst of the shock tube diaphragm are investigated both experimentally and numerically for different initial pressure ratios and opening diameters. The intensity of the shock wave is found to be lower than that corresponding to a complete opening. A heuristic relation is proposed to compute the shock strength as a function of the relative area of the open portion of the diaphragm. Strong pressure oscillations past the shock front are also observed. These multi-dimensional disturbances are generated when the initially normal shock wave diffracts from the diaphragm edges and reflects on the shock tube walls, resulting in a complex unsteady flow field behind the leading shock wave. The limiting local frequency of the pressure oscillations is found to be very close to the ratio of acoustic wave speed in the perturbed region to the shock tube diameter. The power associated with these pressure oscillations decreases with increasing distance from the diaphragm since the diffracted and reflected shocks partially coalesce into a single normal shock front. A simple analytical model is devised to explain the reduction of the local frequency of the disturbances as the distance from the leading shock increases.

Flow visualization of shock/water column interactions

Abstract: The breakup of a liquid droplet induced by a high speed gas stream is a typical multiphase flow problem. The shock/droplet interaction is the beginning stage of the droplet breakup. Therefore, investigation of the shock/droplet interactions would be a milestone for interpreting the mechanism of the droplet breakup. In this study, a compressible multiphase solver with a five-equation model is successfully developed to study shock/water column interactions. For code validation, interface-only, gas–gas shock tube, and gas–liquid shock tube problems are first computed. Subsequently, a planar shock wave interacting with a water column is simulated. The transmitted wave and the alternative appearances of local high- and low-pressure regions inside the water column are observed clearly. Finally, a planar shock wave interacting with two water columns is investigated. In this work, both horizontal and vertical arrangements of two water columns are studied. It is found that different arrangements can result in the diversity of the interacting process. The complex flow structures generated by shock/water column interactions are presented by flow-visualization techniques.

Unsteady Wave Phenomena on a Supercritical Airfoil

Abstract: Transonic flow investigations are performed in a modified shock tube with a rectangular test section. The investigated model is a BAC3-11 airfoil with a constant cord length and a sharp trailing edge. Time-resolved shadowgraphs and schlieren pictures show pressure waves initiated near the trailing edge and propagating upstream, where they become apparently weaker near the leading edge. These wave processes are accompanied by wake fluctuations and vortex generation in the boundary layer. The observed waves are also captured by pressure transducers mounted in the airfoil model. The dominant frequencies range between approximately 0.7 and 1.5 kHz. Using statistical analysis of the pressure histories, wave propagation direction and wave speed are determined. For higher flow Mach numbers, a strong wave/shock interaction is also observed in which the shock, depending on the shock strength, is attenuated and degenerated into compression waves.

Shock tunnel studies on cowl/ramp shock interactions in a generic scramjet inlet

Abstract: An experimental study is presented to show the effect of contraction ratio (CR) on shock interaction phenomena for a two-dimensional, planar scramjet inlet model. Experiments are conducted in a hypersonic shock tunnel, at Mach 8, at three CRs: 8.4, 5.0, and 4.3. CR here is defined as the ratio of the projected inlet area to the throat area. Investigations include Schlieren flow visualization around the cowl region and heat transfer rate measurement inside the inlet chamber. Various ramp/cowl shock interaction processes ahead of the inlet have been visualized using a high-speed camera. Edney type II interference pattern is observed for a CR of 4.3 with all its typical features resulting because of the forebody shock/cowl shock interaction. Peaks in the heat transfer rate measured inside the chamber show possible locations of shock impingement because of the shock interaction inside the inlet.

Two-wavelength mid-IR diagnostic for temperature and n-dodecane concentration in an aerosol shock tube

Abstract: A two-wavelength, mid-IR optical absorption diagnostic is developed for simultaneous temperature and n-dodecane vapor concentration measurements in an aerosol-laden shock tube. FTIR absorption spectra for the temperature range 323 to 773 K are used to select the two wavelengths (3409.0 and 3432.4 nm). Shock-heated mixtures of n-dodecane vapor in argon are then used to extend absorption cross section data at these wavelengths to 1322 K. The sensor is used to validate a model of the post-evaporation temperature and pressure of shock-heated fuel aerosol, which can ultimately be used for the study of the chemistry of low-vapor-pressure compounds and fuel blends. The signal-to-noise ratio of the temperature and concentration are ∼20 and ∼30, respectively, illustrating the sensitivity of this diagnostic. The good agreement between model and measurement provide confidence in the use of this aerosol shock tube to provide well-known thermodynamic conditions. At high temperatures, pseudo-first-order decomposition rates are extracted from time-resolved concentration measurements, and data from vapor and aerosol shocks are found to be in good agreement. Notably, the n-dodecane concentration measurements exhibit slower decomposition than predicted by models using two published reaction mechanisms, illustrating the need for further kinetic studies of this hydrocarbon. These results demonstrate the potential of multi-wavelength mid-IR laser sensors for hydrocarbon measurements in environments with time-varying temperature and concentration.

Design of a double diaphragm shock tube for fluid disintegration studies

Abstract: A double diaphragm shock tube facility for studying liquid-spray atomization and combustion-related phenomena at elevated pressures and temperatures is described. The present shock tube is specifically intended for the investigation of fundamental processes related to fluid disintegration and mixing under realistic engine conditions. Special features of the facility include a variable-area driver section to compensate for shock attenuation, a square test section to allow flow visualization in the postshock region, a skimmer to dispose part of the boundary layer, a heated, fast-response injector, a fully automated gas-filling system, and a new control system and electronics. Test times of the order of 2–5 ms are possible with reflected shock pressures up to 50 bar and temperatures of 2000 K. Details on the setup design, construction and operation are given. Particular emphasis is placed on the accuracy and reproducibility of the test conditions. To that aim, qualification tests have been performed to assess the shock tube performance in terms of effectiveness of the skimmer concept, the capability to compensate for boundary layer effects and the generation of uniform and reproducible test and injection conditions.

High-temperature shock tube study of the reactions CH3 + OH products and CH3OH + Ar products

Abstract: The reaction between methyl and hydroxyl radicals has been studied in reflected shock wave experiments using narrow-linewidth OH laser absorption. OH radicals were generated by the rapid thermal decomposition of tert-butyl hydroperoxide. Two different species were used as CH3 radical precursors, azomethane and methyl iodide. The overall rate coefficient of the CH3 + OH reaction was determined in the temperature range 1081–1426 K under conditions of chemical isolation. The experimental data are in good agreement with a recent theoretical study of the reaction. The decomposition of methanol to methyl and OH radicals was also investigated behind reflected shock waves. The current measurements are in good agreement with a recent experimental study and a master equation simulation. © 2008 Wiley Periodicals, Inc. 40: 488–495, 2008