Showing papers on "Shock tube published in 2012"

Investigation of Cavitation as a Possible Damage Mechanism in Blast-Induced Traumatic Brain Injury

Abstract: Cavitation was investigated as a possible damage mechanism for war-related traumatic brain injury (TBI) due to an improvised explosive device (IED) blast. When a frontal blast wave encounters the head, a shock wave is transmitted through the skull, cerebrospinal fluid (CSF), and tissue, causing negative pressure at the contrecoup that may result in cavitation. Numerical simulations and shock tube experiments were conducted to determine the possibility of cranial cavitation from realistic IED non-impact blast loading. Simplified surrogate models of the head consisted of a transparent polycarbonate ellipsoid. The first series of tests in the 18-inch-diameter shock tube were conducted on an ellipsoid filled with degassed water to simulate CSF and tissue. In the second series, Sylgard gel, surrounded by a layer of degassed water, was used to represent the tissue and CSF, respectively. Simulated blast overpressure in the shock tube tests ranged from a nominal 10–25 pounds per square inch gauge (psig; ...

On the Chemical Kinetics of Ethanol Oxidation: Shock Tube, Rapid Compression Machine and Detailed Modeling Study

Abstract: Abstract Auto-ignition characteristics of ethanol were experimentally investigated using two Shock Tube (ST) facilities and a Rapid Compression Machine (RCM). Ignition delay times for stoichiometric ethanol-air mixtures were measured for temperatures between 775–1300 K in a High Pressure Shock Tube (HPST) at pressures close to 80 bar by probing pressure time histories and CH* emission. In some experiments the HPST was additionally employed for schlieren imaging to visualize ignition behavior by probing density gradients during ignition for ethanol-air mixtures. The ignition delay experiments in HPST were complemented by RCM measurements for extending the temperature regime to the Low Temperature Combustion (LTC) regime, down to 705 K, providing kinetic model validation data over a very wide temperature and pressure range. The current results also extend the earlier shock tube measurements performed in the same laboratory for pressures around 40 bar for temperatures down to 800 K [Heufer et al., Shock Waves 20 (2010) 307]. Furthermore, a Rectangular Shock Tube (RST) was solely used for additional schlieren imaging experiments to acquire information on ignition modes in stoichiometric ethanol-air mixtures around 10 bar. An improved chemical kinetic model was developed based on the Li et al. mechanism [Li et al., “Ethanol Model v1.0”, Princeton University, 2009] which was updated with evaluated rate parameters from the literature and validated through results obtained from the aforementioned facilities. The model predictions were compared to previously published low-pressure, premixed flat flame molecular beam mass spectrometry speciation data [Kasper et al., Combust. Flame 150 (2007) 220; Wang et al., J. Phys. Chem. A 112 (2008) 9255] where reasonable agreement is obtained considering the uncertainties in experiments and model. However, the model provides excellent agreement for the auto-ignition results obtained in the RCM and the high temperature shock induced ignition delays. Significant disparities with the model predictions are obtained for the shock tube results at temperatures below 1000 K as it transitions from the intermediate to the low temperature regime. The reasons for these deviations are assigned to strong fuel specific “pre-ignition” effects observed in ethanol auto-ignition, in contrast to other investigated fuels, which was satisfactorily explained through schlieren experimental results. To our knowledge this work is first of its kind that combines results from complementary experimental methods from three different facilities providing a holistic description on the auto-ignition behavior of ethanol. Furthermore, this paper reports ignition delay measurements for ethanol in air, at the highest pressures applicable to practical combustors.

Behavior of shock trains in a hypersonic inlet/isolator model with complex background waves

Abstract: The background flow field of a scramjet isolator that accommodates a shock train contains complex compression and expansion waves, referred as “background waves,” causing large streamwise and transverse parameter gradients upstream of the shock train. Therefore, the available results of shock train research obtained by direct-connect methods might be not applicable for real scramjet isolators. Special tests are therefore performed for an inlet/isolator model. Close coupling is found between the shock train and the background shocks. The pointing direction of the leading shock switches upwards and downwards repeatedly during the upstream propagation of the shock train. Three unstable stages with substantial oscillations are also observed, interlaced with four stable stages. In addition, the interference of the background shock waves increases the sustainable back-pressure ratio and decreases the length of the shock train. However, this does not mean that the background waves in the isolator should be intensified intentionally.

A multiphase shock tube for shock wave interactions with dense particle fields

Abstract: Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes. To close this gap, a novel multiphase shock tube has been constructed to drive a planar shock wave into a dense gas–solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 20%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 are reported. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30–40% by the presence of the particle field.

Turbulent mixing in a Richtmyer–Meshkov fluid layer after reshock: velocity and density statistics

Abstract: The properties of turbulent mixing in a Richtmyer‐Meshkov (RM) unstable fluid layer are studied under the impact of a single shock followed by a reshock wave using simultaneous velocity‐density measurements to provide new insights into the physics of RM mixing. The experiments were conducted on a varicose SF6 fluid layer (heavy fluid) interposed in air (light fluid) inside a horizontal shock tube at an incident Mach number of 1.21 and a reflected reshock Mach number of 1.14. The light‐heavy‐light fluid layer is observed to develop a nonlinear growth pattern, with no transition to turbulence upon impact by a single shock (up to tU= D 23:4). However, upon reshock, enhanced mixing between the heavy and light fluids along with a transition to a turbulent state characterized by the generation of significant turbulent velocity fluctuations ( u=U 0:3) is observed. The streamwise and spanwise root-mean-squared velocity fluctuation statistics show similar trends across the fluid layer after reshock, with no observable preference for the direction of the shock wave motion. The measured streamwise mass flux ( 0 u 0 ) shows opposing signs on either side of the density peak within the fluid layer, consistent with the turbulent material transport being driven along the direction of the density gradient. Measurements of three of the six independent components of the general Reynolds stress tensor (RijD u 00 u 00 ) show that the self-correlation terms R11 and R22 are similar in magnitude across much of the fluid layer, and much larger than the cross-correlation term R12. Most importantly, the Reynolds stresses (Rij) are dominated by the mean density, cross-velocity product term ( u 0u 0), with the mass flux product and triple correlation terms being negligibly smaller in comparison. A lack of homogeneous mixing (and, possibly, a long-term imprint of the initial conditions) is observed in the spanwise turbulent mass flux measurements, with important implications for the simulation and modelling of RM mixing flows.

Evolution of blast wave profiles in simulated air blasts: experiment and computational modeling

Abstract: Shock tubes have been extensively used in the study of blast traumatic brain injury due to increased incidence of blast-induced neurotrauma in Iraq and Afghanistan conflicts. One of the important aspects in these studies is how to best replicate the field conditions in the laboratory which relies on reproducing blast wave profiles. Evolution of the blast wave profiles along the length of the compression-driven air shock tube is studied using experiments and numerical simulations with emphasis on the shape and magnitude of pressure time profiles. In order to measure dynamic pressures of the blast, a series of sensors are mounted on a cylindrical specimen normal to the flow direction. Our results indicate that the blast wave loading is significantly different for locations inside and outside of the shock tube. Pressure profiles inside the shock tube follow the Friedlander waveform fairly well. Upon approaching exit of the shock tube, an expansion wave released from the shock tube edges significantly degrades the pressure profiles. For tests outside the shock tube, peak pressure and total impulse reduce drastically as we move away from the exit and majority of loading is in the form of subsonic jet wind. In addition, the planarity of the blast wave degrades as blast wave evolves three dimensionally. Numerical results visually and quantitatively confirm the presence of vortices, jet wind and three-dimensional expansion of the planar blast wave near the exit. Pressure profiles at 90° orientation show flow separation. When cylinder is placed inside, this flow separation is not sustained, but when placed outside the shock tube this flow separation is sustained which causes tensile loading on the sides of the cylinder. Friedlander waves formed due to field explosives in the intermediate-to far-field ranges are replicated in a narrow test region located deep inside the shock tube.

Shock Layer Radiation Modeling and Uncertainty for Mars Entry

Abstract: A model for simulating nonequilibrium radiation from Mars entry shock layers is presented. A new chemical kinetic rate model is developed that provides good agreement with recent EAST and X2 shock tube radiation measurements. This model includes a CO dissociation rate that is a factor of 13 larger than the rate used widely in previous models. Uncertainties in the proposed rates are assessed along with uncertainties in translational-vibrational relaxation modeling parameters. The stagnation point radiative flux uncertainty due to these flowfield modeling parameter uncertainties is computed to vary from 50 to 200% for a range of free-stream conditions, with densities ranging from 5e-5 to 5e-4 kg/m3 and velocities ranging from of 6.3 to 7.7 km/s. These conditions cover the range of anticipated peak radiative heating conditions for proposed hypersonic inflatable aerodynamic decelerators (HIADs). Modeling parameters for the radiative spectrum are compiled along with a non-Boltzmann rate model for the dominant radiating molecules, CO, CN, and C2. A method for treating non-local absorption in the non-Boltzmann model is developed, which is shown to result in up to a 50% increase in the radiative flux through absorption by the CO 4th Positive band. The sensitivity of the radiative flux to the radiation modeling parameters is presented and the uncertainty for each parameter is assessed. The stagnation point radiative flux uncertainty due to these radiation modeling parameter uncertainties is computed to vary from 18 to 167% for the considered range of free-stream conditions. The total radiative flux uncertainty is computed as the root sum square of the flowfield and radiation parametric uncertainties, which results in total uncertainties ranging from 50 to 260%. The main contributors to these significant uncertainties are the CO dissociation rate and the CO heavy-particle excitation rates. Applying the baseline flowfield and radiation models developed in this work, the radiative heating for the Mars Pathfinder probe is predicted to be nearly 20 W/cm2. In contrast to previous studies, this value is shown to be significant relative to the convective heating.

IR laser absorption diagnostic for C2H4 in shock tube kinetics studies

Abstract: An IR laser absorption diagnostic has been further developed for accurate and sensitive time-resolved measurements of ethylene in shock tube kinetic experiments. The diagnostic utilizes the P14 line of a tunable CO2 gas laser at 10.532 μm (the (0 0 1) (1 0 0) vibrational band) and achieves improved signal-to-noise ratio by using IR photovoltaic detectors and accurate identification of the P14 line via an MIR wavemeter. Ethylene absorption cross sections were measured over 643–1959 K and 0.3–18.6 atm behind both incident and reflected shock waves, showing evident exponential decay with temperature. Very weak pressure dependence was observed over the pressure range of 1.2–18.6 atm. By measuring ethylene decomposition time histories at high-temperature conditions (1519–1895 K, 2.0–2.8 atm) behind reflected shocks, the rate coefficient of the dominant elementary reaction C2H4 + M C2H2 + H2 + M was determined to be k1 = (2.6 ± 0.5) × 1016exp(−34,130/T, K) cm3 mol−1 s−1 with low data scatter. Ethylene concentration time histories were also measured during the oxidation of 0.5% C2H4/O2/Ar mixtures varying in equivalence ratio from 0.25 to 2. Initial reflected shock conditions ranged from 1267 to 1440 K and 2.95 to 3.45 atm. The measured time histories were compared to the modeled predictions of four ethylene oxidation mechanisms, showing excellent agreement with the Ranzi et al. mechanism (updated in 2011). This diagnostic scheme provides a promising tool for the study and validation of detailed hydrocarbon pyrolysis and oxidation mechanisms of fuel surrogates and realistic fuels. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 423–432, 2012

A Multiscale Approach to Blast Neurotrauma Modeling: Part I – Development of Novel Test Devices for in vivo and in vitro Blast Injury Models

Abstract: The loading conditions used in some current in vivo and in vitro blast-induced neurotrauma models may not be representative of real-world blast conditions. To address these limitations, we developed a compressed-gas driven shock tube with different driven lengths that can generate Friedlander-type blasts. The shock tube can generate overpressures up to 650 kPa with durations between 0.3 and 1.1 ms using compressed helium driver gas, and peak overpressures up to 450 kPa with durations between 0.6 and 3 ms using compressed nitrogen. This device is used for short duration blast overpressure loading for small animal in vivo injury models, and contrasts the more frequently used long duration/high impulse blast overpressures in the literature. We also developed a new apparatus that is used with the shock tube to recreate the in vivo intracranial overpressure response for loading in vitro culture preparations. The receiver device surrounds the culture with materials of similar impedance to facilitate the propagation of a single overpressure pulse through the tissue. This method prevents pressure waves reflecting off the tissue that can cause unrealistic deformation and injury. The receiver performance was characterized using the longest helium-driven shock tube, and produced in-fluid overpressures up to 1500 kPa at the location where a culture would be placed. This response was well correlated with the overpressure conditions from the shock tube (R2 = 0.97). Finite element models of the shock tube and receiver were developed and validated to better elucidate the mechanics of this methodology. A demonstration exposing a culture to the loading conditions created by this system suggest tissue strains less than 5% for all pressure levels simulated, which was well below functional deficit thresholds for strain rates less than 50 s-1. This novel system is not limited to a specific type of culture model and can be modified to reproduce more complex pressure pulses.

Turbulent mixing measurements in the Richtmyer-Meshkov instability

Abstract: The Richtmyer-Meshkov instability is experimentally investigated in a vertical shock tube using a new type of broadband initial condition imposed on an interface between a helium-acetone mixture and argon (A = 0.7). The initial condition is created by first setting up a gravitationally stable stagnation plane between the gases and then injecting the same two gases horizontally at the interface to create a shear layer. The perturbations along the shear layer create a statistically repeatable broadband initial condition. The interface is accelerated by a M = 1.6 planar shock wave, and the development of the ensuing turbulent mixing layer is investigated using planar laser induced fluorescence. By the latest experimental time, 2.1 ms after shock acceleration, the layer is shown to be fully turbulent, surpassing both turbulent transition criteria based on the Reynolds number and shear layer scale. Mixing structures are nearly isotropic by the latest time, as seen by the probability density function of gradient angles within the mixing layer. The scalar variance energy spectrum suggests a k−5/3 inertial range by the latest time and an exponential region at higher wavenumbers.

Using a gel/plastic surrogate to study the biomechanical response of the head under air shock loading: a combined experimental and numerical investigation

Abstract: A combined experimental and numerical study was conducted to determine a method to elucidate the biomechanical response of a head surrogate physical model under air shock loading. In the physical experiments, a gel-filled egg-shaped skull/brain surrogate was exposed to blast overpressure in a shock tube environment, and static pressures within the shock tube and the surrogate were recorded throughout the event. A numerical model of the shock tube was developed using the Eulerian approach and validated against experimental data. An arbitrary Lagrangian-Eulerian (ALE) fluid–structure coupling algorithm was then utilized to simulate the interaction of the shock wave and the head surrogate. After model validation, a comprehensive series of parametric studies was carried out on the egg-shaped surrogate FE model to assess the effect of several key factors, such as the elastic modulus of the shell, bulk modulus of the core, head orientation, and internal sensor location, on pressure and strain responses. Results indicate that increasing the elastic modulus of the shell within the range simulated in this study led to considerable rise of the overpressures. Varying the bulk modulus of the core from 0.5 to 2.0 GPa, the overpressure had an increase of 7.2%. The curvature of the surface facing the shock wave significantly affected both the peak positive and negative pressures. Simulations of the head surrogate with the blunt end facing the advancing shock front had a higher pressure compared to the simulations with the pointed end facing the shock front. The influence of an opening (possibly mimicking anatomical apertures) on the peak pressures was evaluated using a surrogate head with a hole on the shell of the blunt end. It was revealed that the presence of the opening had little influence on the positive pressures but could affect the negative pressure evidently.

Experimental investigation of reshocked spherical gas interfaces

Abstract: The evolution of a spherical gas interface under reshock conditions is experimentally studied using the high-speed schlieren photography with high time resolutions. A number of experimental sets of helium or SF6 bubble surrounded by air for seven different end wall distances have been performed. Distinct flow structures are observed due to the additional vorticity and wave configuration caused by the reshock. In the air/helium case, the deformation of the reshocked bubble is dependent on the development of the penetrating air jet along the symmetry axis of the bubble. In general, two separate vortex rings can be observed, i.e., one develops slowly, and the other approaches and eventually impinges on the shock tube end wall. In the air/SF6 case, two SF6 jets moving in opposite directions are generated and the oscillation of the interface is observed for small end wall distances, while small scale vortex morphologies on the gas interface are found for large end wall distances. The physical mechanisms of the...

Numerical simulation and PIV study of compressible vortex ring evolution

Abstract: Formation and evolution of a compressible vortex ring generated at the open end of a short driver section shock tube has been simulated numerically for pressure ratios (PR) of 3 and 7 in the present study. Numerical study of compressible vortex rings is essential to understand the complicated flow structure and acoustic characteristics of many high Mach number impulsive jets where simultaneously velocity, density and pressure fields are needed. The flow development, incident shock formation, shock diffraction, vortex ring formation and its evolution are simulated using the AUSM+ scheme. The main focus of the present study is to evaluate the time resolved vorticity field of the vortex ring and the shock/expansion waves in the starting jet for short driver section shock tubes—a scenario where little data are available in existing literature. An embedded shock and a vortex induced shock are observed for PR = 7. However the vortex ring remains shock free, compact and unaffected by the trailing jet for PR = 3. Numerical shadowgraph shows the evolution of embedded shock and shock/expansion waves along with their interactions. The velocity and vorticity fields obtained from simulation are validated with the particle image velocimetry results and these data match closely. The translational velocity of the vortex ring, velocity across the vortex and the centre line velocity of the jet obtained from simulation also agree well with the experimental results.

Electron Density Measurement in Reentry Shocks for Lunar Return

Abstract: Results of electron density measurements in the Electric Arc Shock Tube at NASA Ames Research Center are reported here. Measurements are made at conditions relevant for lunar return of the Orion Command Module. Normal shocks are produced in the Electric Arc Shock Tubewith freestreampressures of 0.1–1.0 Torr and velocities from 8–12 km=s. Nonintrusive electron densitymeasurements aremade by observing optical emission from various lines at high resolution and employing analysis of Stark broadening effects. Themeasurements show electron density in the shock to be up to several times larger than equilibrium density up to a few centimeters behind the shock front. The disagreement with equilibrium improves at higher velocities.

Time-resolved stereo PIV measurements of shock–boundary layer interaction on a supercritical airfoil

Abstract: Time-resolved stereo particle-image velocimetry (TR-SPIV) and unsteady pressure measurements are used to analyze the unsteady flow over a supercritical DRA-2303 airfoil in transonic flow. The dynamic shock wave–boundary layer interaction is one of the most essential features of this unsteady flow causing a distinct oscillation of the flow field. Results from wind-tunnel experiments with a variation of the freestream Mach number at Reynolds numbers ranging from 2.55 to 2.79 × 106 are analyzed regarding the origin and nature of the unsteady shock–boundary layer interaction. Therefore, the TR-SPIV results are analyzed for three buffet flows. One flow exhibits a sinusoidal streamwise oscillation of the shock wave only due to an acoustic feedback loop formed by the shock wave and the trailing-edge noise. The other two buffet flows have been intentionally influenced by an artificial acoustic source installed downstream of the test section to investigate the behavior of the interaction to upstream-propagating disturbances generated by a defined source of noise. The results show that such upstream-propagating disturbances could be identified to be responsible for the upstream displacement of the shock wave and that the feedback loop is formed by a pulsating separation of the boundary layer dependent on the shock position and the sound pressure level at the shock position. Thereby, the pulsation of the separation could be determined to be a reaction to the shock motion and not vice versa.

Mechanism of the Shock Wave Generation and Energy Efficiency by Underwater Discharge

Abstract: We are developing the rice powder manufacturing system using an underwater shock wave. The purpose of this study is to research a mechanism of the shock wave generation and energy efficiency by underwater discharge in order to increase energy of the underwater shock wave. We observed the shock wave generation using the visualization device with a high speed camera, and measured voltage current characteristics at the same time. As a result, it was clarified that countless underwater shock waves were generated at the time of water plasma expansion by discharge. But, the shock wave was not confirmed at the time of after a second peak of the damping oscillation. It was clarified that one part of charging energy was used to generation of the shock wave. Therefore, it was clarified that to release energy by the critical oscillation is desirable for efficient generation of the shock wave.

Behaviour of a shock train under the influence of boundary-layer suction by a normal slot

Abstract: The interaction of a shock train with a normal suction slot is presented. It was found that when the pressure in the suction slot is smaller or equal to the static pressure of the incoming supersonic flow, the pressure gradient across the primary shock is sufficient to push some part of the near wall boundary layer through the suction slot. Due to the suction stabilized primary shock foot, the back pressure of the shock train can be increased until the shock train gradually changes into a single normal shock. During the experiments, the total pressure and therewith the Reynolds number of the flow were varied. The structure and pressure recovery within the shock train is analysed by means of Schlieren images and wall pressure measurements. Because the boundary layer is most important for the formation of a shock train, it has been measured by a Pitot probe. Additionally, computational fluid dynamics is used to investigate the shock boundary-layer interaction. Based on the experimental and numerical results, a simplified flow model is derived which explains the phenomenology of the transition of a shock train into a single shock and derives distinct criteria to maintain a suction enhanced normal shock. This flow model also yields the required suction mass flow in order to obtain a single normal shock in a viscous nozzle flow. Furthermore, it allows computation of the total pressure losses across a normal shock under the influence of boundary-layer suction.