ALBERT

Description: 〈h3〉Abstract〈/h3〉 〈p〉In this study, a less-dissipative hybrid AUSMD scheme considering the linearized approximated solution around the material interfaces of compressible multi-component flows is proposed. A high-resolution reconstruction scheme, so-called MUSCL + THINC, has been devised by combining the MUSCL method with the Tangent of Hyperbola for Interface Capturing technique (THINC) under the boundary variation diminishing concept, which is used to determine the cell-interface values to evaluate the AUSMD flux. Several perfect gas and multi-component flow problems are selected as the benchmark test cases. The flow models we use here are the perfect gas Euler equations and the multi-phase five-equation flow model. We compared the proposed MUSCL + THINC-type AUSMD scheme with the original MUSCL-type AUSMD scheme to verify its capability of capturing shock waves, expansion fans, and material interfaces, which are identified as a well-defined sharp jump in volume fraction. Numerical results of all benchmark tests show that the MUSCL + THINC-type AUSMD solver is superior to the original MUSCL-type AUSMD in resolving shock waves, expansion fans, and interfaces. In particular, the solution quality for expansion fans and interfaces on coarse grids is greatly improved by the MUSCL + THINC-type AUSMD scheme. 〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉SLAU2 and AUSMPW〈span〉 〈span〉\(+\)〈/span〉 〈/span〉, both categorized as AUSM-type Riemann solvers, have been extensively developed in gasdynamics. They are based on a splitting of the numerical flux into advected and pressure parts. In this paper, these two Riemann solvers have been extended to magnetohydrodynamics (MHD). The SLAU2 Riemann solver has the favorable attribute that its dissipation for low-speed flows scales as 〈span〉 〈span〉\(O(M^{2})\)〈/span〉 〈/span〉, where 〈em〉M〈/em〉 is the Mach number. This is the physical scaling required for low-speed flows, and the dissipation in SLAU2 for MHD is engineered to have this low Mach number scaling. The AUSMPW〈span〉 〈span〉\(+\)〈/span〉 〈/span〉, when its pressure flux is replaced with that of SLAU2, has the same low Mach number scaling. At higher Mach numbers, however, the pressure-split Riemann solvers were found not to function well for some MHD Riemann problems, despite the fact that they were engineered to have a dissipation that scales as 〈span〉 〈span〉\(O(\vert M\vert )\)〈/span〉 〈/span〉 for high Mach number flows. The HLLI Riemann solver (Dumbser and Balsara in J Comput Phys 304:275–319, 〈span〉2016〈/span〉) has a dissipation that scales as 〈span〉 〈span〉\(O(\vert M\vert )\)〈/span〉 〈/span〉, which makes it unsuitable for low Mach number flows. However, it has very favorable performance for higher Mach number MHD flows. Since the two families of Riemann solvers perform very well over a range of intermediate Mach numbers, the best way to benefit from the mutually complementary strengths of both these Riemann solvers is to hybridize between them. The result is an all-speed Riemann solver for MHD. We, therefore, document hybridized SLAU2–HLLI and AUSMPW〈span〉 〈span〉\(+\)〈/span〉 〈/span〉–HLLI Riemann solvers. The hybrid Riemann solvers suppress the oscillations that appeared in single-solver solutions, and they also preserve contact discontinuities, as well as Alfvén waves, very well. Furthermore, their better resolution at low speeds has been demonstrated. We also present several stringent one-dimensional test problems.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉The HLLC (Harten–Lax–van Leer contact) approximate Riemann solver for computing solutions to hyperbolic systems by means of finite volume and discontinuous Galerkin methods is reviewed. HLLC was designed, as early as 1992, as an improvement to the classical HLL (Harten–Lax–van Leer) Riemann solver of Harten, Lax, and van Leer to solve systems with three or more characteristic fields, in order to avoid the excessive numerical dissipation of HLL for intermediate characteristic fields. Such numerical dissipation is particularly evident for slowly moving intermediate linear waves and for long evolution times. High-order accurate numerical methods can, to some extent, compensate for this shortcoming of HLL, but it is a costly remedy and for stationary or nearly stationary intermediate waves such compensation is very difficult to achieve in practice. It is therefore best to resolve the problem radically, at the first-order level, by choosing an appropriate numerical flux. The present paper is a review of the HLLC scheme, starting from some historical notes, going on to a description of the algorithm as applied to some typical hyperbolic systems, and ending with an overview of some of the most significant developments and applications in the last 25 years.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Presented are results from a parametric investigation of wind-tunnel test-section configurations with a goal of stabilization of normal-shock-wave structure. The test section includes a two-flow-passage arrangement, where each passage is separated by a shock-wave-holding plate. The top wall for the top passage is contoured relative to the streamwise-flow direction, and a choking flap is located at the downstream portion of the bottom flow passage. Altered are the streamwise and spanwise positions of the shock-wave-holding plate, angle of the choking flap, and amount of venting. Of interest are shadowgraph flow visualization images, grayscale spectral energy variations, and integrated grayscale spectral energy levels. Higher static-pressure ratio downstream of the shock wave (caused by higher choking-flap angle, lower shock-wave-holding-plate position, and less venting) is associated with greater shock-wave-standoff distance (relative to the shock-wave-holding plate) and decreased flow unsteadiness. The most optimal arrangement includes a stabilized normal shock wave and lambda foot, which are largely two dimensional over the shadowgraph visualization volume.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Experiments have been conducted in a supersonic rectangular duct (Mach 1.4), with a normal shock wave/boundary layer interaction. The duct is designed in a modular fashion so that its aspect ratio can be varied without a change in the flow geometry. The shock location, duct height, and Mach number are kept constant, while varying the aspect ratio. Conventional and inclined schlieren techniques have been used to visualize the normal shock. The height and width of the “normal part” of the normal shock have been measured. A parameter termed 〈em〉area fraction of the normal shock〈/em〉 is used to quantify the extent of shock bifurcation, and the effect of the duct aspect ratio on this parameter is studied. It has been found that a nearly square duct is the preferred geometry for maximizing the 〈em〉area fraction of the normal shock.〈/em〉〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉It is well known that hypersonic boundary-layer transition is sensitive to a surface roughness since the roughness may either trigger an early transition or delay the transition. Hypersonic transition is still poorly understood as there are a very limited number of studies in the literature. In the present work, we conduct a computational study on the transition process of a hypersonic Mach 6 flow over a flat plate with a gap. An implicit large eddy simulation approach based on the flux reconstruction/correction procedure via a reconstruction method is used to investigate the interaction between the hypersonic boundary layer and a gap. Flow structures with and without the gap are compared to analyze the local skin friction coefficient overshoots before and after the gap. Two inlet angles of attack are investigated. The evolution of the skin friction coefficient shows that the gap has a very limited influence on the oblique transition at a zero angle of attack. In contrast, the gap can trigger an early transition at a negative angle of attack. In this case, the transverse feedback mechanism is believed to be the main cause, which amplifies the broadband instability waves.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Regimes of continuous spin detonation and continuous multifront detonation in a hydrogen–oxygen mixture are obtained in a plane-radial combustor with an inner diameter of 100 mm and exhaustion toward the periphery. The fuel-lean limits of detonation in terms of the specific flow rate of the mixture are determined. For continuous spin detonation, transverse detonation waves and the flow in their vicinity in the combustor plane are reconstructed. The detonation wave is found to be significantly curved because of the increase in the tangential component of the velocity along the combustor radius. It is demonstrated that the scale effect is manifested only in the number of rotating waves. However, their velocity increases with increasing the combustor size. The velocity deficit of continuous detonation is 20–40% as compared to the velocity of the ideal Chapman–Jouguet detonation. (The smaller value corresponds to the fuel-lean mixture.) 〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉The Hydrogen Unconfined Combustion Test Apparatus (HUCTA) was designed and built to study the blast waves produced from unconfined hydrogen/oxygen deflagrations. The HUCTA uses evacuated balloons of up to 2 m in diameter which are filled with a combustible combination of gaseous hydrogen–oxygen mixtures. The well-mixed gases are ignited with an electric spark at the center of the sphere, resulting in a gaseous deflagration propagating through the mixture and a shock wave produced in the air. The combinations of balloon size and fuel/oxidizer ratios allow for a wide range of blast waves to be produced. Overpressures are measured with standard blast gauges at a variety of locations, demonstrating a high degree of radial symmetry and repeatability in the shock wave pressures, as well as the ability to produce non-ideal shock wave pressure profiles under some conditions. The range of peak pressures and explosive impulses obtainable is described as a function of mixture ratio. High-speed retroreflective shadowgraphy is used to visualize shock wave propagation and coalescence in individual frames and digital streak images. Since HUCTA is elevated approximately 2 m off the ground, there is a significant area around the apparatus where non-noisy, un-reflected, symmetric blast waves propagate; this area is ideal for testing items whose response to blast waves is desired for safety considerations.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Projectile accelerations above 〈span〉 〈span〉\(500~\hbox {ms}^{-2}\)〈/span〉 〈/span〉 are commonly encountered in aerodynamic applications, but suitable validation data are rare in this regime. Experimental transonic velocity range data for a sphere decelerating under its own drag have been used to validate a numerical model for decelerating objects. The validated model is then used to explore the flow field ahead of objects decelerating from supersonic to subsonic Mach numbers. To model the non-inertial frame of the projectile, source terms were included in the momentum and energy equations in a computational fluid dynamics model. In decelerating cases, the bow shock formed in supersonic flight persists in the subsonic regime. The differences in the flow field between the steady and unsteady cases are explained using the concept of flow history. In the experiment, a tubular insert was present near the observation window in the ballistic range. The insert was numerically modelled, and it is shown that the resulting bow shock behaviour can be explained in terms of the Kantrowitz criterion, in conjunction with flow history. The RAE2822 aerofoil was used to explore the effects of shock overtaking and propagation during deceleration from supersonic to low subsonic Mach numbers. In this case, the bow shock wave persists from the initial supersonic speed to projectile Mach numbers lower than 0.4. The expansion wave and tail shock are shown to overtake the decelerating projectile and propagate forward, behind the bow shock.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Although several mechanisms have been suggested as explanations for the low-frequency unsteadiness in shock wave/turbulent boundary layer interactions, questions remain on causes and effects. In this effort, we examine the observed asymmetry in large-scale shock motions to highlight which of the suggested mechanisms is most consistent with shock-speed observations and accompanying separation dynamics. The analysis is based on a flowfield obtained from a validated large eddy simulation of a fully separated interaction. A statistical analysis is used to determine the speed of bubble collapse relative to dilation. The low-pass filtering required to separate upstream from downstream motions in the presence of higher-frequency jitter is accomplished with a relatively new technique, empirical mode decomposition, that is very appropriate for this purpose. The dynamics of bubble dilation versus collapse are then elaborated with conditional dynamic mode decomposition (DMD) analyses on the respective pressure fields. Bubble breathing is shown to have a different structure during dilation than during collapse—larger structures are observed during collapse when fluid is expelled from the bubble. The nature of the DMD mode associated with Kelvin–Helmholtz (K–H) shedding in the mixing layer also differs between dilation and collapse: When the bubble is dilating, the structures at the dominant K–H frequency are larger than when the bubble is collapsing. Additionally, a link is established between the convecting K–H structures and corrugation observed along the reflected shock. Some aspects of the nature of the asymmetry are linked to the ease of eddy formation (K–H structures), which plays an important role in the collapse of the bubble.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Detonation velocity as a function of charge diameter is reported for Alliant Bullseye powder. Results are compared to those of mixtures of ammonium nitrate mixed with aluminum and ammonium nitrate mixed with fuel oil. Additionally, measurements of free surface velocity of flyers in contact with detonating Bullseye are presented and results are compared to those of hydrocode calculations using a Jones–Wilkins–Lee equation of state generated in a thermochemical code. Comparison to the experimental results shows that both the free surface and terminal velocities were under-predicted. 〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉Experimental investigations and numerical simulations of normal shock waves of different strengths propagating inside ducts with roughness are presented. The roughness is added in the form of grooves. Straight and branching ducts are considered in order to better explore the mechanisms causing attenuation of the shock and the physics behind the evolution of the complex wave patterns resulting from diffraction and reflection of the primary moving shock. A well-established finite-volume numerical method is used and further validated for several test cases relevant to this study. The computed results are compared with experimental measurements in ducts with grooves. Good agreement between high-resolution simulations and the experiment is obtained for the shock speeds and complex wave patterns created by the grooves. High-frequency response time histories of pressure at various locations were recorded in the experiments. The recorded pressure histories and shock strengths were found in fair agreement with the two-dimensional simulation results as long as the shock stays in the duct. Overall, the physics of the interactions of the moving shock and the diffracted and reflected waves with the grooves are adequately captured in the high-resolution simulations. Therefore, shocks propagating in ducts with different groove geometries have been simulated in order to identify the groove shape that diminishes shock strength.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉The constrained reaction volume (CRV) method for shock-tube experiments makes it possible to conduct chemical kinetics studies at nearly constant pressure while inhibiting remote ignition. The application of end-wall imaging revealed, however, that CRV experiments are susceptible to vertical stratification at the interface of the test and buffer gases. This work identifies gravity currents as the mechanism leading to the test gas stratification, providing both a theoretical development and experimental investigation of their behavior in a shock tube. Parametric studies are conducted with both the gate valve and stage-filled CRV methods using a novel beam-split laser absorption diagnostic. The speed of gravity-current-induced mixing in both gate valve and stage-filled CRV experiments is shown to depend on the molecular weight matching of the gases and the fill pressure. Mixing velocity in gate valve experiments is shown not to depend on the gate valve speed. In stage-filled experiments, mixing time is seen to be a strong function of the test gas length. Recommended practices for avoiding gravity-current-induced mixing and stratification are described, including gas density matching, utilizing double diaphragms, increasing test gas length in stage-filled experiments, and implementing a gas interface diagnostic.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉To further refine the existing high-temperature combustion chemistry mechanisms of Sarin surrogates or to develop new ones, the ignition delay times of mixtures containing various Sarin surrogates have been studied in the authors’ laboratory, namely dimethyl-methylphosphonate, diethyl-methylphosphonate (DEMP), and triethylphosphate, and the results are compared for the first time herein. They were each measured in a heated shock tube, with the DEMP-related ignition delay times being the new data reported in this paper. The Sarin surrogates were studied in neat mixtures with oxygen or seeded to baseline mixtures of hydrogen or methane at around 1.5 atm. Noticeable differences were observed between the ignition delay times of the three simulants, whether sole or mixed with a fuel. Comparisons of OH* time histories obtained from each surrogate highlight the similarities and differences in chemical structure among the different compounds. In mixtures with oxygen and Ar, the three surrogates present similar ignition delay times below 1380 K, whereas the ignition delay time results rapidly diverge above this temperature. When the surrogates were added into 〈span〉 〈span〉\(\hbox {H}_{2}/\hbox {O}_{2}\)〈/span〉 〈/span〉 or 〈span〉 〈span〉\(\hbox {CH}_{4}/\hbox {O}_{2}\)〈/span〉 〈/span〉 mixtures, large changes in the reactivity of the mixtures were observed. These changes in reactivity are however dependent on the surrogate, for each fuel investigated.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉This paper investigates the structure of a normal shock wave using the continuum model for steady one-dimensional flow of a viscous non-ideal gas under heat conduction. The coefficients of viscosity and heat conductivity are assumed to be directly proportional to a power of the temperature. The simplified van der Waals equation of state for the non-ideal gas has been assumed in this work. The smooth and rough sphere models of the gas molecules in the kinetic theory of gases are used for the viscosity of a non-ideal gas. Assuming the Prandtl number to be 3 / 4, the complete integral of the energy equation, exact velocity, density, pressure, Mach number, change in entropy, viscous stress, and heat flux across the shock transition zone have been obtained in a perfect and a non-ideal gas under both constant and variable properties of the medium. The validity of the continuum hypothesis with respect to Mach number is examined for the study of shock wave structure in both the smooth and rough sphere models of ideal and non-ideal gas molecules. It has been shown that the continuum theory gives reasonably valid results for flows with higher Mach numbers in the case of a non-ideal gas in comparison with an ideal gas. The inverse thickness of the shock wave is calculated and compared for constant and variable properties of the gases. The shock wave thickness is also discussed as a function of mean free path of the gas molecules computed at different points between the boundary states. It is found that the inverse shock thickness decreases with the increase in non-idealness of the gas. In the rough sphere model of gas molecules, the increase in the non-idealness of the gas and the temperature exponent in the coefficients of viscosity and heat conductivity significantly increases the validity limit of the continuum model.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉The use of conjugate circular arcs in rocket nozzle contour design has been investigated by numerically comparing three existing sub-scale nozzles to a range of equivalent arc-based contour designs. Three performance measures were considered when comparing nozzle designs: thrust coefficient, nozzle exit wall pressure, and a transition between flow separation regimes during the engine start-up phase. In each case, an equivalent arc-based contour produced an increase in the thrust coefficient and exit wall pressure of up to 0.4 and 40% respectively, in addition to suppressing the transition between a free and restricted shock separation regime. A general approach to arc-based nozzle contour design has also been presented to outline a rapid and repeatable process for generating sub-scale arc-based contours with an exit Mach number of 3.8–5.4 and a length between 60 and 100% of a 15〈span〉 〈span〉\(^{\circ }\)〈/span〉 〈/span〉 conical nozzle. The findings suggest that conjugate circular arcs may represent a viable approach for producing sub-scale rocket nozzle contours, and that a further investigation is warranted between arc-based and existing full-scale rocket nozzles.〈/p〉

Description: 〈h3〉Abstract〈/h3〉 〈p〉A time delay is created between elastic and plastic wave fronts because of the difference between the elastic longitudinal sound speed and the plastic shock wave velocity. Over a short propagation distance, the time delay between the elastic and plastic wave fronts at the Hugoniot elastic limit (HEL) is nonlinear, while at larger distances, the time delay is linear. In this work, a new time delay model is introduced that is based on the distance traveled by the waves and using the Rayleigh–Hugoniot jump relations for elastic–perfectly plastic materials. The results of the model have shown in FCC metals the subsonic shock velocity is due to the reduction of shear stress in an unsteady wave being greater than the one in the steady wave. The reduction of the plastic shock wave speed and formation of the elastic shock at the moment of impact are found to result in the nonlinear relationship of the lag between elastic and plastic wave fronts. For calculating the nonlinear time delay in a relaxing material, the lower HEL must be used; the elastic shock is important when the difference between the longitudinal elastic sound speed and the plastic shock wave speed is very small or when the ratio of the HEL to the applied stress is high. In BCC metals, V, Cr, and W, a different behavior has been observed which is in contrast to FCC metals, Ag, Al, and Cu. Therefore, the different behavior is due to a different mechanism that occurs in BCC metals.〈/p〉

Description:    This paper investigates the flow field near three intersecting shock waves appearing in steady Mach reflection. Results of numerical computations reveal a “von Neumann Paradox”—like feature for weak shock waves, in which the flow field between the reflected and the Mach stem is smooth with no distinct slip flow region and changes rather smoothly. An analytical solution of the Navier–Stokes equations constructed using a polar–coordinate system gives a flow field with the same properties as the numerical simulation. Content Type Journal Article Pages 1-6 DOI 10.1007/s00193-011-0329-8 Authors A. Sakurai, Tokyo Denki University, Kanda, Tokyo, Japan M. Tsukamoto, Tokyo Denki University, Kanda, Tokyo, Japan D. Khotyanovsky, Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia M. Ivanov, Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    The propagation and attenuation of an initial shock wave through a mm-scale channel of circular cross-section over lengths up to 2,000 diameters is examined as a model problem for the scaling of viscous effects in compressible flows. Experimental wave velocity measurements and pressure profiles are compared with existing data and theoretical predictions for shock attenuation at large scales and low pressures. Significantly more attenuation is observed than predicted based on streamtube divergence. Simulations of the experiment show that viscous effects need to be included, and the boundary layer behavior is important. A numerical model including boundary layer and channel entrance effects reproduces the wave front velocity measurements, provided a boundary layer transition model is included. A significant late-time pressure rise is observed in experiments and in the simulations. Content Type Journal Article Pages 1-11 DOI 10.1007/s00193-011-0330-2 Authors J. M. Austin, Department of Aerospace Engineering, University of Illinois, Urbana, IL 61801, USA D. J. Bodony, Department of Aerospace Engineering, University of Illinois, Urbana, IL 61801, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    An investigation into a three-dimensional, curved shock wave interacting with a three-dimensional, curved boundary layer on a slender body is presented. Three different nose profiles mounted on a cylindrical body were tested in a supersonic wind tunnel and numerically simulated by solving the Navier–Stokes equations. The conical and hemispherical nose profiles tested were found to generate shock waves of sufficient strength to separate the boundary layer on the cylinder, while the shock wave generated by the ogival profile did not separate the boundary layer. For the separated flow, separation was found to occur predominantly on the windward side of the cylinder with the lee-side remaining shielded from the direct impact of the incident shock wave. A thickening of the boundary layer on the lee-side of all the profiles was observed, and in the conical and hemispherical cases this leads to the re-formation of the incident shock wave some distance away from the surface of the cylinder. A complex reflection pattern off the shock wave/boundary layer interaction (SWBLI) was also identified for the separated flow cases. For comparative purposes, an inviscid simulation was performed using the hemispherical profile. Significant differences between the viscous and inviscid results were noted including the absence of a boundary layer leading to a simplified shock wave reflection pattern forming. The behaviour of the incident shock wave on the lee-side of the cylinder was also affected with the shock wave amalgamating on the surface of the cylinder instead of away from the surface as per the viscous case. Test data from the wind tunnel identified two separation lines present on the cylindrical surface of the hemispherical SWBLI generator. The pair of lines were not explicitly evident in the original CFD simulations run, but were later identified in a high-resolution simulation. Content Type Journal Article Pages 1-16 DOI 10.1007/s00193-011-0322-2 Authors S. Mowatt, Flow Research Unit, School of Mechanical, Industrial and Aeronautical Engineering, University of the Witwatersrand, Private Bag 3, Johannesburg, Wits 2050, South Africa B. Skews, Flow Research Unit, School of Mechanical, Industrial and Aeronautical Engineering, University of the Witwatersrand, Private Bag 3, Johannesburg, Wits 2050, South Africa Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Most gas dynamic computations in industrial ducts are done in one dimension with cross-section-averaged Euler equations. This poses a fundamental difficulty as soon as geometrical discontinuities are present. The momentum equation contains a non-conservative term involving a surface pressure integral, responsible for momentum loss. Definition of this integral is very difficult from a mathematical standpoint as the flow may contain other discontinuities (shocks, contact discontinuities). From a physical standpoint, geometrical discontinuities induce multidimensional vortices that modify the surface pressure integral. In the present paper, an improved 1D flow model is proposed. An extra energy (or entropy) equation is added to the Euler equations expressing the energy and turbulent pressure stored in the vortices generated by the abrupt area variation. The turbulent energy created by the flow–area change interaction is determined by a specific estimate of the surface pressure integral. Model’s predictions are compared with 2D-averaged results from numerical solution of the Euler equations. Comparison with shock tube experiments is also presented. The new 1D-averaged model improves the conventional cross-section-averaged Euler equations and is able to reproduce the main flow features. Content Type Journal Article Pages 1-16 DOI 10.1007/s00193-011-0321-3 Authors R. Menina, LESEI, Department of Mechanical Engineering, Faculty of Technology, Batna University (UHLB), 1 Avenue Chahid Mohamed El Hadi, 05000 Batna, Algeria R. Saurel, IUSTI, Polytech Marseille, Aix-Marseille University, 5 rue E. Fermi, 13453 Marseille Cedex 13, France M. Zereg, LPEA, Department of Physics, Faculty of Sciences, Batna University (UHLB), 1 Avenue Chahid Mohamed El Hadi, 05000 Batna, Algeria L. Houas, IUSTI, Polytech Marseille, Aix-Marseille University, 5 rue E. Fermi, 13453 Marseille Cedex 13, France Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    An experimental investigation has been carried out to study dual-bell transition behavior in different set ups inside a high-altitude test facility. Cold gas tests were carried out under two different operating conditions namely (i) test nozzle operating in self-evacuation mode and, (ii) test nozzle operating with an additional ejector nozzle (pre-evacuated condition). Although forward transition nozzle pressure ratio does not show any change in its value irrespective of the type of test facility and test set up, the re-transition nozzle pressure ratio shows a significant increase (7–8%) in its value when tested in the high-altitude facility. The latter is caused due to plume blowback effect which dominates during shut down transients in such facilities. Driven by the high atmospheric pressure, the jet exhaust is pushed backwards into the altitude chamber causing the re-transition to occur earlier than that observed in sea-level tests. Further the reduced mass flow rates for nozzle operation in different test set ups in a high-altitude test facility also reduces the magnitude of side-load peaks during the dual-bell transitions. Content Type Journal Article Pages 1-10 DOI 10.1007/s00193-011-0302-6 Authors S. B. Verma, Experimental Aerodynamics Division, CSIR-National Aerospace Laboratories, Bangalore, 560017 India R. Stark, Nozzle Flow Group, DLR Lampoldshausen, Lampoldshausen, Germany C. Génin, Nozzle Flow Group, DLR Lampoldshausen, Lampoldshausen, Germany O. Haidn, Nozzle Flow Group, DLR Lampoldshausen, Lampoldshausen, Germany Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Results of one-dimensional numerical simulations of the parameters of the converging strong shock wave generated by electrical underwater explosions of a cylindrical wire array with different array radii and different deposited energies are presented. It was shown that for each wire array radius there exists an optimal duration of the energy deposition into the exploding array, which allows one to maximize the shock wave pressure and temperature in the vicinity of the implosion axis. The simulation results agree well with the 130-GPa pressure in the vicinity of the implosion axis that was recently obtained, which strongly indicates the azimuthal symmetry of the converging shock wave at these extreme conditions. Also, simulations showed that using a pulsed power generator with a stored energy of ~200 kJ, the pressure and temperature at the shock wave front reaches ~220 GPa and 1.7 eV at 0.1 mm from the axis of implosion in the case of a 2.5 mm radius wire array explosion. It was found that, in spite of the complicated equation of state of water, the maximum pressure at the shock wave front at radius r can be estimated as P ≈ ( P *( r */ r ) α , where P * is the known value of pressure at the shock wave front at radius r * ≥ r and α is a parameter that equals 0.62±0.02. A rough estimate of the implosion parameters of the hydrogen target after the interaction with the converging strong shock wave is presented as well. Content Type Journal Article Pages 1-9 DOI 10.1007/s00193-011-0320-4 Authors G. Bazalitski, Physics Department, Technion, Haifa, 32000 Israel V. Ts. Gurovich, Physics Department, Technion, Haifa, 32000 Israel A. Fedotov-Gefen, Physics Department, Technion, Haifa, 32000 Israel S. Efimov, Physics Department, Technion, Haifa, 32000 Israel Ya. E. Krasik, Physics Department, Technion, Haifa, 32000 Israel Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description: Report on the 28th International Symposium on Shock Waves Content Type Journal Article Category Report Pages 1-2 DOI 10.1007/s00193-011-0337-8 Authors K. Kontis, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester, M13 9PL UK Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    A quantitative thermometry technique, based on planar laser-induced fluorescence (PLIF), was applied to image temperature fields immediately next to walls in shock tube flows. Two types of near-wall flows were considered: the side wall thermal boundary layer behind an incident shock wave, and the end wall thermal layer behind a reflected shock wave. These thin layers are imaged with high spatial resolution (15μm/pixel) in conjunction with fused silica walls and near-UV bandpass filters to accurately measure fluorescence signal levels with minimal interferences from scatter and reflection at the wall surface. Nitrogen, hydrogen or argon gas were premixed with 1–12% toluene, the LIF tracer, and tested under various shock flow conditions. The measured pressures and temperatures ranged between 0.01 and 0.8 bar and 293 and 600 K, respectively. Temperature field measurements were found to be in good agreement with theoretical values calculated using 2-D laminar boundary layer and 1-D heat diffusion equations, respectively. In addition, PLIF images were taken at various time delays behind incident and reflected shock waves to observe the development of the side wall and end wall layers, respectively. The demonstrated diagnostic strategy can be used to accurately measure temperature to about 60 μm from the wall. Content Type Journal Article Category Original Article Pages 1-10 DOI 10.1007/s00193-011-0338-7 Authors J. Yoo, High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA D. Mitchell, High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA D. F. Davidson, High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA R. K. Hanson, High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    The existence and characteristics of shock wave triple points are examined. The analysis utilizes a single flow plane for the three shocks and is local to the triple point. It applies when the flow is unsteady, three-dimensional, and the upstream flow is nonuniform. Under more restrictive conditions, a relation is also derived for the ratio of the curvature of the Mach stem to that of the reflected shock. For given values of the ratio of specific heats, γ , and the upstream Mach number, M 1 , a solution window is established. A parametric set of solutions is generated within the window for γ  = 1, 1.4, and 5/3 and for 16 values of M 1 ranging from solution onset to M 1 = 6.A solution can be one of three types, these stem from the velocity tangency condition along the slip stream. Topics are addressed such as solution multiplicity, shock wave and slip stream orientation, the nature of the reflected wave (weak, strong, inverted, normal), the nature of the Mach stem (weak, strong, normal), and differences due to changes in γ and M 1 . Content Type Journal Article Category Original Article Pages 1-11 DOI 10.1007/s00193-011-0339-6 Authors H. Hekiri, Department of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019-0390, USA G. Emanuel, Department of Mechanical and Aerospace Engineering, University of Texas at Arlington, Arlington, TX 76019-0018, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    It is well accepted that the persistence of regular reflection of a shock wave off a wedge beyond the ideal theoretical prediction is due to viscous and thermal boundary layers induced behind the reflection point. Experiments have been done by reflecting two shock waves of equal strength off each other so that the plane of symmetry between them becomes an ideal inviscid and adiabatic reflection plane thereby experimentally mimicking the assumptions of the theory. There is one definitive experiment done at a wall angle of 40° using a bifurcated shock tube that indicates that the actual transition angle is the theoretical detachment condition. This paper extends these results to two cases near limiting conditions; one at a very low incidence shock Mach number and one at a wall angle very close to the theoretical transition limit. The first confirms the reasons for the von Neumann Paradox but cannot discriminate between sonic and detachment conditions, but is within about 0.5% of them, and the second shows transition much closer to the sonic than the detachment condition but with both within the experimental error bounds. In both cases, the results are notably different from transition conditions off a wedge and confirm the effects of transport properties being the cause of persistence of regular reflection. Content Type Journal Article Category Technical Note Pages 1-6 DOI 10.1007/s00193-011-0341-z Authors T. Herron, Flow Research Unit, School of Mechanical, Industrial and Aeronautical Engineering, University of the Witwatersrand, PO WITS, 2050 Johannesburg, South Africa B. Skews, Flow Research Unit, School of Mechanical, Industrial and Aeronautical Engineering, University of the Witwatersrand, PO WITS, 2050 Johannesburg, South Africa Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    The use of a non-steady ejector based on wave rotor technology is modeled for pulse detonation engine performance improvement and for compatibility with turbomachinery components in hybrid propulsion systems. The rotary wave ejector device integrates a pulse detonation process with an efficient momentum transfer process in specially shaped channels of a single wave-rotor component. In this paper, a quasi-one-dimensional numerical model is developed to help design the basic geometry and operating parameters of the device. The unsteady combustion and flow processes are simulated and compared with a baseline PDE without ejector enhancement. A preliminary performance assessment is presented for the wave ejector configuration, considering the effect of key geometric parameters, which are selected for high specific impulse. It is shown that the rotary wave ejector concept has significant potential for thrust augmentation relative to a basic pulse detonation engine. Content Type Journal Article Category Original Article Pages 1-16 DOI 10.1007/s00193-011-0348-5 Authors M. R. Nalim, Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202-5132, USA Z. A. Izzy, Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202-5132, USA P. Akbari, Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    By the discrete element method (DEM), we perform numerical simulations of shock-induced load transfer processes in granular layers composed of spherical particles packed in vertical channels. In order to isolate the load transfer through the grains’ contact points from the complicated load transfer processes, we simulate the shock wave interactions with granular layers having no permeability for gas. The shock loading is achieved by applying a downward step force on the top of the granular layers. Complex, three-dimensional load transfer processes in the granular media, which are extremely difficult to understand from experiments, are visualized based on the results from the present DEM simulation. The numerical results show that highly concentrated load transfer paths, through which shock loads are transferred mainly, exist in the granular media, and that the dimensions of the container of the granular media considerably affect the shock-induced load transfer processes. From a coarse-grained representation of intergranular stress, wave-like load transfer processes are clearly observed. For relatively deep granular layers, however, the wave fronts became unclear as they propagated. Content Type Journal Article Category Original Article Pages 1-12 DOI 10.1007/s00193-011-0347-6 Authors Y. Sakamura, Department of Mechanical Systems Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama, 939-0398 Japan H. Komaki, Department of Mechanical Systems Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama, 939-0398 Japan Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Micro-blast waves emerging from the open end of a detonation transmission tube were experimentally visualized in this study. A commercially available detonation transmission tube was used (Nonel tube, M/s Dyno Nobel, Sweden), which is a small diameter tube coated with a thin layer of explosive mixture (HMX + traces of Al) on its inner side. The typical explosive loading for this tube is of the order of 18 mg/m of tube length. The blast wave was visualized using a high speed digital camera (frame rate 1 MHz) to acquire time-resolved schlieren images of the resulting flow field. The visualization studies were complemented by computational fluid dynamic simulations. An analysis of the schlieren images showed that although the blast wave appears to be spherical, it propagates faster along the tube axis than along a direction perpendicular to the tube axis. Additionally, CFD analysis revealed the presence of a barrel shock and Mach disc, showing structures that are typical of an underexpanded jet. A theory in use for centered large–scale explosions of intermediate strength (10  〈 D p / p 0 〈~ 0.02) gave good agreement with the blast trajectory along the tube axis. The energy of these micro-blast waves was found to be 1.25 ± 0.94 J and the average TNT equivalent was found to be 0.3 . The repeatability in generating these micro-blast waves using the Nonel tube was very good ( ± 2 %) and this opens up the possibility of using this device for studying some of the phenomena associated with muzzle blasts in the near future. Content Type Journal Article Category Original Article Pages 1-10 DOI 10.1007/s00193-012-0416-5 Authors I. Obed Samuelraj, Department of Aerospace Engineering, Indian Institute of Science, Bangalore, 560 012 India G. Jagadeesh, Department of Aerospace Engineering, Indian Institute of Science, Bangalore, 560 012 India K. Kontis, School of MACE, University of Manchester, Manchester, UK Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    A phenomenological study of the process occurring when a plane shock wave reflected off an aqueous foam column filling the test section of a vertical shock tube has been undertaken. The experiments were conducted with initial shock wave Mach numbers in the range 1.25 £ M s £ 1.7 and foam column heights in the range 100–450 mm. Miniature piezotrone circuit electronic pressure transducers were used to record the pressure histories upstream and alongside the foam column. The aim of these experiments was to find a simple way to eliminate a spatial averaging as an artifact of the pressure history recorded by the side-on transducer. For this purpose, we discuss first the common behaviors of the pressure traces in extended time scales. These observations evidently quantify the low frequency variations of the pressure field within the different flow domains of the shock tube. Thereafter, we focus on the fronts of the pressure signals, which, in turn, characterize the high-frequency response of the foam column to the shock wave impact. Since the front shape and the amplitude of the pressure signal most likely play a significant role in the foam destruction, phase changes and/or other physical factors, such as high capacity, viscosity, etc., the common practice of the data processing is revised and discussed in detail. Generally, side-on pressure measurements must be used with great caution when performed in wet aqueous foams, because the low sound speed is especially prone to this effect. Since the spatial averaged recorded pressure signals do not reproduce well the real behaviors of the pressure rise, the recorded shape of the shock wave front in the foam appears much thicker. It is also found that when a thin liquid film wet the sensing membrane, the transducer sensitivity was changed. As a result, the pressure recorded in the foam could exceed the real amplitude of the post-shock wave flow. A simple procedure, which allows correcting this imperfection, is discussed in detail. Content Type Journal Article Category Original Article Pages 1-14 DOI 10.1007/s00193-012-0402-y Authors A. Britan, Protective Technologies R&D Center, Faculty of Engineering Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel M. Liverts, Protective Technologies R&D Center, Faculty of Engineering Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel H. Shapiro, Protective Technologies R&D Center, Faculty of Engineering Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel G. Ben-Dor, Protective Technologies R&D Center, Faculty of Engineering Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    An experimental investigation of the elastic–plastic nature of shock wave propagation in foams was undertaken. The study involved experimental blast wave and shock tube loading of three foams, two polyurethane open-cell foams and a low-density polyethylene closed-cell foam. Evidence of precursor waves was observed in all three foam samples under various compressive wave loadings. Experiments with an impermeable membrane are used to determine if the precursor wave in an open-cell foam is a result of gas filtration or an elastic response of the foam. The differences between quasi-static and shock compression of foams is discussed in terms of their compressive strain histories and the implications for the energy absorption capacity of foam in both loading scenarios. Through a comparison of shock tube and blast wave loading techniques, suggestions are made concerning the accurate measurements of the principal shock Hugoniot in foams. Content Type Journal Article Category Original Article Pages 1-13 DOI 10.1007/s00193-012-0414-7 Authors O. E. Petel, Department of Mechanical Engineering, McGill University, 817 Sherbrooke W., Montréal, QC H3A 2K6, Canada S. Ouellet, Defence Research and Development Canada Valcartier, 2459 Pie XI Nord, Val-Bélair, QC G3J 1X5, Canada A. J. Higgins, Department of Mechanical Engineering, McGill University, 817 Sherbrooke W., Montréal, QC H3A 2K6, Canada D. L. Frost, Department of Mechanical Engineering, McGill University, 817 Sherbrooke W., Montréal, QC H3A 2K6, Canada Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description: Report on the 20th international shock interaction symposium Content Type Journal Article Category Report Pages 1-1 DOI 10.1007/s00193-012-0415-6 Authors N. Apazidis, Department of Mechanics, KTH, 100 44 Stockholm, Sweden Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Characteristics of the unsteady type IV shock/shock interaction of hypersonic blunt body flows are investigated by solving the Navier–Stokes equations with high-order numerical methods. The intrinsic relations of flow structures to shear, compression, and heating processes are studied and the physical mechanisms of the unsteady flow evolution are revealed. It is found that the instantaneous surface-heating peak is caused by the fluid in the “hot spot” generated by an oscillating and deforming jet bow shock (JBS) just ahead of the body surface. The features of local shock/boundary layer interaction and vortex/boundary layer interaction are clarified. Based on the analysis of flow evolution, it is identified that the upstream-propagating compression waves are associated with the interaction of the JBS and the shear layers formed by a supersonic impinging jet, and then the interaction of the freestream bow shocks and the compression waves results in entropy and vortical waves propagating to the body surface. Further, the feedback mechanism of the inherent unsteadiness of the flow field is revealed to be related to the impinging jet. A feedback model is proposed to reliably predict the dominant frequency of flow evolution. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to this complex flow. Content Type Journal Article Category Original Article Pages 1-11 DOI 10.1007/s00193-012-0366-y Authors Y.-B. Chu, Department of Modern Mechanics, University of Science and Technology of China, Hefei, 230026 Anhui, China X.-Y. Lu, Department of Modern Mechanics, University of Science and Technology of China, Hefei, 230026 Anhui, China Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    This study is an investigation into the effects of temperature and pressure within a test chamber on the dynamic characteristics of injected supersonic diesel fuel jets. These jets were generated by the impact of a projectile driven by a horizontal single stage powder gun. A high speed video camera and a shadowgraph optical system were used to capture their dynamic characteristics. The test chamber had controlled air conditions of temperature and pressure up to 150 °C and 8.2 bar, respectively. It was found experimentally that, at the highest temperature, a maximum jet velocity of around 1,500 m/s was obtained. At this temperature, a narrow pointed jet appeared while at the highest pressure, a thick, blunt headed jet was obtained. Strong shock waves were generated in both cases at the jet head. For analytical prediction, equations of jet tip velocity and penetration from the work of Dent and of Hiroyasu were employed to describe the dynamic characteristics of the experiments at a standard condition of 1 bar, 30 °C. These analytical predictions show reasonable agreement to the experimental results, the experimental trend differing in slope because of the effect of the pressure, density fluctuation of the injection and the shock wave phenomena occurring during the jet generation process. Content Type Journal Article Category Original Article Pages 1-9 DOI 10.1007/s00193-012-0364-0 Authors W. Sittiwong, Department of Mechanical Engineering, Faculty of Engineering, Ubon Ratchathani University, Ubonratchathani, Thailand K. Pianthong, Department of Mechanical Engineering, Faculty of Engineering, Ubon Ratchathani University, Ubonratchathani, Thailand W. Seehanam, Department of Mechanical Engineering, Faculty of Engineering, Ubon Ratchathani University, Ubonratchathani, Thailand B. E. Milton, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, Australia K. Takayama, Interdisciplinary Shock Wave Research Center, Institute of Fluid Science, Tohoku University, Sendai, Japan Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    This paper outlines our research on a multimode warhead in which we adopted center point and annular initiation modes to form multimode penetrators. Using LS-DYNA software, we studied the effect of the configuration parameters, namely the length/diameter ratio of the shaped charge, on the formation parameters, such as the velocity and length/diameter ratio, of multimode penetrators. We found that when the charge length was in the range of 0.9–1.2 times the charge diameter, the same structure of shaped charge can form suitable multimode penetrators. Either an explosively formed penetrator (EFP) or a long stretchy rod-shaped EFP penetrator can be formed. We establish an optimum charge length for penetrator formation of 1.4 times the charge diameter. Simulation results were validated using X-ray imaging experiments and they were in good agreement. The results found that by increasing the charge length from 0.9 to 1.4 times the charge diameter, the penetration depth of the EFP increased by 74.5%, while increasing the charge length from 1.4 to 1.6 times the charge diameter only increased the penetration depth by 1.9%. Content Type Journal Article Category Original Article Pages 1-10 DOI 10.1007/s00193-012-0365-z Authors Weibing Li, ZNDY of Ministerial Key Laboratory, Nanjing University of Science and Technology, Nanjing, 210094 People’s Republic of China X. Wang, ZNDY of Ministerial Key Laboratory, Nanjing University of Science and Technology, Nanjing, 210094 People’s Republic of China Wenbin Li, ZNDY of Ministerial Key Laboratory, Nanjing University of Science and Technology, Nanjing, 210094 People’s Republic of China Y. Zheng, ZNDY of Ministerial Key Laboratory, Nanjing University of Science and Technology, Nanjing, 210094 People’s Republic of China Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    The Richtmyer–Meshkov instability after reshock is investigated in shock tube experiments at the Wisconsin Shock Tube Laboratory using planar laser imaging and a new high-speed interface-tracking technique. The interface is a mixture of helium and argon (50% each by volume) stratified over pure argon. This interface has an Atwood number of 0.29 and a near single-mode, two-dimensional, standing wave perturbation with an average amplitude of 0.35 cm and a wavelength of 19.4 cm. The incident shock wave of Mach number 1.92 accelerates the interface before reflecting from the shock tube end wall with M = 1.70 and accelerating the interface in the opposite direction. The amplitude growth after reshock is reported for variations in this initial amplitude, and several amplitude growth rate models are compared to the experimental growth rate after reshock. A new growth model is introduced, based on a model of circulation deposition calculated from one-dimensional gas dynamics parameters. This model compares well with the amplitude growth rate after reshock and the circulation over one-half wavelength of the interface after the first shock wave and after reshock. Content Type Journal Article Category Original Article Pages 1-9 DOI 10.1007/s00193-012-0367-x Authors C. Weber, Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Dr., Madison, WI 53706, USA N. Haehn, Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Dr., Madison, WI 53706, USA J. Oakley, Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Dr., Madison, WI 53706, USA M. Anderson, Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Dr., Madison, WI 53706, USA R. Bonazza, Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Dr., Madison, WI 53706, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    We consider a model for the interaction of a gas with photons. In the article (Lin et al. Phys D 218:83–94, 2006 ), smooth traveling wave solutions called shock profiles have been constructed under a suitable smallness assumption between the asymptotic states. In this work, we construct piecewise smooth traveling wave solutions that connect two asymptotic states with a large jump. In particular, we give a rigorous mathematical justification to the formation of the so-called Zeldovich spike. Content Type Journal Article Category Original Article Pages 1-17 DOI 10.1007/s00193-012-0368-9 Authors J.-F. Coulombel, Laboratoire de Mathématiques Jean Leray UMR 6629 CNRS-Université de Nantes, 2, rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France T. Goudon, Team COFFEE Inria Sophia Antipolis Méditerranée, Sophia Antipolis Cedex, France P. Lafitte, Laboratoire MAS-Ecole Centrale Paris, Grande Voie des Vignes, 92290 Chatenay-Malabry, France C. Lin, Department of Mathematics, Shanghai Jiaotong University, Shanghai, 200240 China Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Shock-focusing concave reflector is a very simple and effective tool to obtain a high-pressure pulse wave near the physical focal point. In the past, many optical images were obtained through experimental studies. However, measurement of field variables is not easy because the phenomenon is of short duration and the magnitude of shock waves is varied from pulse to pulse due to poor reproducibility. Using a wave propagation algorithm and the Cartesian embedded boundary method, we have successfully obtained numerical schlieren images that resemble the experimental results. By the numerical results, various field variables, such as pressure, density and vorticity, become available for the better understanding and design of shock focusing devices. Content Type Journal Article Category Technical Note Pages 1-5 DOI 10.1007/s00193-012-0404-9 Authors Y. G. Jung, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejeon, 305-701 Republic of Korea K. S. Chang, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejeon, 305-701 Republic of Korea Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Experiments addressing the effect of energy deposition via arc discharge on 15 ° half angle-truncated cone-cylinder configurations at Mach 5 flow were carried out. The arc discharge was accomplished using a setup that consisted of a power supply, a high voltage unit and tungsten electrodes. Discharge-on tests were compared to discharge-off tests to evaluate the net effect of the energy deposition. Flow visualisation revealed the presence of a heated wake downstream of the energy spot. Compression waves were observed on top of the wake of the heated channel, which were oscillatory in nature. The deposited energy of 7 W shows a repeatable influence on the measured drag force for all the models at close arc-to-nose distances. Content Type Journal Article Category Original Article Pages 1-14 DOI 10.1007/s00193-012-0405-8 Authors E. Erdem, School of MACE, The University of Manchester, Manchester, UK K. Kontis, School of MACE, The University of Manchester, Manchester, UK L. Yang, School of MACE, The University of Manchester, Manchester, UK Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    A numerical study of propagation and interaction of cylindrical blast waves in an enclosure at different blast intensities is presented. The interest to study such flows stems from the need to bring in an updated description of the flow field and to predict the pressure loads on the structure. An implicit-unfactored high-resolution hybrid Riemann solver for the two-dimensional Euler equations is used. The characteristic values at the cell faces are evaluated by a modified MUSCL scheme. Numerical schlieren-type images are used for understanding the flows qualitatively. The investigation indicated that the resulting flow field is dominated by complex interacting shock systems due to the complex series of shock focusing events, shock-structure and shock-shock interactions. The pressure-load distribution and maximum overpressure are estimated. Content Type Journal Article Category Original Article Pages 1-10 DOI 10.1007/s00193-012-0406-7 Authors A. M. Bagabir, Faculty of Engineering, Jazan University, P. O. Box 706, Jazan, Saudi Arabia Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    The induction time measured in shock tube experiments is typically converted into kinetic data assuming that the reaction takes place in a constant volume process, thus neglecting spatial gradients. The actual process of shock ignition is, however, both time- and space-dependent; ignition takes place at a well-defined location, and subsequently a front travels, which may couple with the pressure wave that it created and forms a detonation wave behind the shock that reflects off the wall. To assess how different the actual processes are compared with the constant volume assumption, a numerical study was performed using a simplified three step chain-branching kinetic scheme. To overcome the difficulties that arise when simulating shock-induced ignition due to the initial absence of a domain filled with shocked reactive mixture, the problem is solved in a transformed frame of reference. Furthermore, initial conditions are derived from short-time asymptotics, which resolves the initial singularity. The induction times obtained using the full unsteady formulation with those of the homogeneous explosion are compared for various values of the heat release. Results for the spatially dependent formulation show that the evolution of the post-shock flow is complex, and that it leads to a gradient in induction times, after the passage of the reflected shock. For all cases simulated, thermal explosion initially occurs very close to the wall, and the corresponding induction time is found to be larger than that predicted under the constant volume assumption. As the measurement is made further away however, the actual time interval between passage of the reflected shock, and the specified pressure increase denoting ignition, decreases to a value close to zero, corresponding to that obtained along a Rayleigh line matching that of a steady ZND process (assuming a long enough tube). In situations where the constant volume assumption is expected to be weak, more accurate kinetic data will be obtained using a spatially resolved computation such as the one used in the current comparison. Content Type Journal Article Category Original Article Pages 1-11 DOI 10.1007/s00193-012-0403-x Authors J. Melguizo-Gavilanes, Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Dr. NW, Calgary, T2N 1N4 Canada L. Bauwens, Department of Mechanical and Manufacturing Engineering, University of Calgary, 2500 University Dr. NW, Calgary, T2N 1N4 Canada Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Shock-induced oxidation of two higher-order linear alkanes was measured using a heated shock tube facility. Experimental overlap in stoichiometric ignition delay times obtained under dilute (99 % Ar) conditions near atmospheric pressure was observed in the temperature-dependent ignition trends of n -nonane ( n -C 9 H 20 ) and n -undecane ( n -C 11 H 24 ). Despite the overlap, model predictions of ignition using two different detailed chemical kinetics mechanisms show discrepancies relative to both the measured data as well as to one another. The present study therefore focuses on the differences observed in the modeled, high-temperature ignition delay times of higher-order n -alkanes, which are generally regarded to have identical ignition behavior for carbon numbers above C 7 . Comparisons are drawn using experimental data from the present study and from recent work by the authors relative to two existing chemical kinetics mechanisms. Time histories from the shock-tube OH* measurements are also compared to the model predictions; a double-peaked structure observed in the data shows that the time response of the detector electronics is crucial for properly capturing the first, incipient peak near time zero. Calculations using the two mechanisms were carried out at the dilution level employed in the shock-tube experiments for lean ( f = 0.5) , stoichiometric, and rich ( f = 2.0) equivalence ratios, 1230–1620 K, and for both 1.5 and 10 atm. In general, the models show differing trends relative to both measured data and to one another, indicating that agreement among chemical kinetics models for higher-order n -alkanes is not consistent. For example, under certain conditions, one mechanism predicts the ignition delay times to be virtually identical between the n -nonane and n -undecane fuels (in fact, also for all alkanes between at least C 8 and C 12 ), which is in agreement with the experiment, while the other mechanism predicts the larger fuels to ignite progressively more slowly. Content Type Journal Article Category Original Article Pages 1-15 DOI 10.1007/s00193-012-0387-6 Authors B. Rotavera, Texas A&M University, College Station, TX, USA E. L. Petersen, Texas A&M University, College Station, TX, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    Porous materials have long been known to be effective in blast mitigation strategies. Nano-structured materials appear to have an even greater potential for blast mitigation because of their high surface-to-volume ratio, a geometric factor which substantially attenuates shock wave propagation. A molecular dynamics approach was used to explore the effects of this remarkable property on the behavior of traveling shocks impacting on solid materials. The computational setup included a moving piston, a gas region, and a target solid wall with and without a porous structure. The materials involved were represented by realistic interaction potentials. The results indicate that the presence of a nano-porous material layer in front of the target wall reduced the stress magnitude and the energy deposited inside the solid by about 30 %, while at the same time substantially decreasing the loading rate. Content Type Journal Article Category Original Article Pages 1-12 DOI 10.1007/s00193-012-0397-4 Authors A. K. Al-Qananwah, Experimental Fluid Mechanics and Aerodynamics Laboratory, Department of Mechanical Engineering, The City College of New York/CUNY, New York, NY 10031, USA J. Koplik, Levich Institute and Department of Physics, The City College of New York/CUNY, New York, NY 10031, USA Y. Andreopoulos, Experimental Fluid Mechanics and Aerodynamics Laboratory, Department of Mechanical Engineering, The City College of New York/CUNY, New York, NY 10031, USA Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287

Description:    The deformation and instability of a low-density spherical bubble induced by an incident and its reflected shock waves are studied by using the large eddy simulation method. The computational model is firstly validated by experimental results from the literature and is further used to examine the effect of incident shock wave strength on the formations and three-dimensional evolutions of the vortex rings. For the weak shock wave case ( Ma  = 1.24), the baroclinic effect induced by the reflected shock wave is the key mechanism for the formation of new vortex rings. The vortex rings not only move due to the self-induced effect and the flow field velocity, but also generate azimuthal instability due to the pressure disturbance. For the strong shock wave case ( Ma  = 2.2), a boundary layer is formed adjacent to the end wall owing to the approach of vortex ring, and unsteady separation of the boundary layer near the wall results in the ejection and formation of new vortex rings. These vortex rings interact in the vicinity of the end wall and finally collapse to a complicated vortex structure via azimuthal instability. For both shock wave strength cases, the evolutions of vortex rings due to the instability lead to the formation of the complicated structure dominated by the small-scale streamwise vortices. Content Type Journal Article Category Original Article Pages 1-15 DOI 10.1007/s00193-012-0393-8 Authors Y. J. Zhu, State Key Laboratory of Transient Physics, Nanjing University of Science and Technology, Nanjing, 210094 China G. Dong, State Key Laboratory of Transient Physics, Nanjing University of Science and Technology, Nanjing, 210094 China B. C. Fan, State Key Laboratory of Transient Physics, Nanjing University of Science and Technology, Nanjing, 210094 China Y. X. Liu, State Key Laboratory of Transient Physics, Nanjing University of Science and Technology, Nanjing, 210094 China Journal Shock Waves Online ISSN 1432-2153 Print ISSN 0938-1287