Shock Waves最新文献

An extension to the normal shock relations for a thermally perfect, calorically imperfect gas, modelling the vibrational excitation with an anharmonic oscillator model and including the influence of electronic modes, is derived and studied. Such additional considerations constitute an extension to the work achieved in the past, which modelled the caloric imperfections with a harmonic oscillator for vibrational energy and did not consider the effect of electronic energy. Additionally, the newly derived expressions provide physical insights into the limitations of experimentation for replicating flight conditions, which is demonstrated through providing solutions at different upstream temperatures. The results are compared with direct simulation Monte Carlo simulations for nitrogen and air, with the extent of the caloric imperfection of the gas showing excellent agreement. For low upstream temperatures, the extended relations are found to be in good agreement with the original normal shock wave expressions, but the results diverge for higher upstream temperatures that would be more representative of real flows. The results show that the new expressions depart from ideal gas theory for Mach numbers in excess of 4.9 at wind-tunnel conditions and for any Mach number above 3.0 at flight conditions. It is also shown that the traditional harmonic oscillator model and the anharmonic oscillator model begin to diverge at Mach number 3.0 for molecular oxygen gas and at Mach number 5.0 for an air mixture at flight conditions.

YAG transparent ceramic has great potential in the applications to transparent armour protection modules. To study the dynamic behaviour and obtain the parameters for the equation of state of YAG under the load of longitudinal stress ranging from 0 to 20 GPa, ramp wave and shock compression experiments were conducted based on the electromagnetic loading test platform. The Hugoniot data, isentropic data, dynamic strength, and elastic limit of YAG were obtained. The results showed that the relationship between the longitudinal wave speed and the particle velocity of YAG was linear when the longitudinal stress was lower than the elastic limit. The quasi-isentropic compression and shock Hugoniot compression curves were coincident when the stress in YAG was below 10 GPa; however, a separation of the two curves occurred when the stress in YAG ranged from 10 GPa to the elastic limit. Moreover, the effect of strain rate on the fracture stress of YAG under a moderate strain rate of 10(^{textrm{5}})–10(^{textrm{6}}) (hbox {s}^{mathrm {-1}}) was more evident than in other strain rate ranges. The amplitude of the precursor wave decayed with increasing sample thickness.

As a method of initiating detonation in a short distance with a small amount of energy, the combination of laser ignition and shock focusing in an elliptical cavity was proposed and experimentally demonstrated with a (hbox {C}_{2}hbox {H}_{4}{-}hbox {O}_{2}) mixture at 100 kPa and 297 K. In the experiment, an elliptical cavity and single rectangular cavities of different heights were used, and their flow-field patterns were visualized using high-speed schlieren imaging. Detonation initiation was achieved in the case of the elliptical cavity, and based on the Mach number change of the leading shock wave, two propagation phases were verified: the deceleration and acceleration phases. The deceleration phase was driven merely by the gasdynamic effect, wherein the initial shock wave (ISW) expanded spherically, and the acceleration phase began when the ISW shifted to planar propagation. In the acceleration phase, although gradual acceleration was observed in rectangular cavities, rapid acceleration occurred in the elliptical cavity. From the schlieren images, the second acceleration was caused not only by the concave reflected shock wave’s catching up with the ISW, but also by the fast-flames that were generated along the cavity corners and engulfed the ISW in the converging section of the elliptical cavity.

Contrary to the conventional chemical propulsion systems based on the controlled relatively slow (subsonic) combustion of fuel in a combustor, the operation process in pulsed detonation engines (PDEs) and rotating detonation engines (RDEs) is based on the controlled fast (supersonic) combustion of fuel in pulsed and continuous detonation waves, respectively. One of the most important issues for such propulsion systems is the choice of fuel with proper reactivity and exothermicity required for a sustained and energy-efficient operation process. Presented in the paper are the results of thermodynamic calculations of the detonation parameters of boron- and aluminum-containing compounds (B, B(_{{2}})H(_{{6}}), B(_{{5}})H(_{{9}}), B(_{{10}})H(_{{14}}), Al, AlH(_{{3}}), Al(C(_{{2}})H(_{{5}})_{{3}}), and Al(CH(_{{3}})_{{3}})) in air and water. The results demonstrate the potential feasibility of using the considered compounds as fuels for both air- and water-breathing transportation vehicles powered with PDEs and RDEs. As a verification of the reliability of the calculated results, the detonation parameters of diborane, aluminum, and isopropyl nitrate in air were compared with experimental data available in the literature.

A study of shock and detonation waves propagating in gaseous two-fuel (hbox {CH}_{{4}}/hbox {H}_{{2}})/air mixtures and heterogeneous three-fuel (hbox {CH}_{{4}}/hbox {H}_{{2}})/air/coal mixtures with the mean bulk density of the coal dust suspension equal to (95{-}560,hbox {g/m}^{{3}}) and with a particle size of (0{-}200,upmu hbox {m}) is performed. The experiments are conducted in a vertical shock tube with a length of 6.75 m and a diameter of 70 mm. The detonation parameters measured in the experiments are compared with the calculated equilibrium thermodynamic values. It is found that the detonation wave parameters are mainly affected by methane and hydrogen rather than by the coal dust suspension. Decaying shock waves are as dangerous as detonation waves because blast wave reflections can initiate detonation. An increase in the hydrogen fraction in the mixture decreases the energy of initiation of (hbox {CH}_{{4}}/hbox {H}_{{2}})/air and (hbox {CH}_{{4}}/hbox {H}_{{2}})/air/coal mixtures, resulting in a greater hazard for the generation of shock and detonation waves in these mixtures.

Explosively driven shock wave radius versus time profiles are frequently used to document energy release and relative explosive performance. Recently, two universal shock wave radius versus time profiles have been presented in the literature, which demonstrate the ability to represent explosively driven shock wave profiles for all explosive sources in any fluid environment. These two universal shock wave profiles are examined here relative to each other and relative to a commonly used nonlinear shock wave profile, which is fit to experimental data for individual explosive materials. The nonlinear profile, originally developed by Dewey, is examined here, and a universal non-dimensional form of the equation is proposed. The universal shock wave profiles are all found to be relatively similar, but with slight variations in a transition region of non-dimensional radii (0.15lesssim R^*lesssim 2). The variations in this region result in different estimations of energy release or blast strength between the curve fits.

The present work is a numerical follow-up on our published experimental paper on shock ignition of aluminium particle clouds in the low-temperature regime. The in-house multi-phase regularized smoothed particle hydrodynamics (MP-RSPH) code is used to perform numerical simulations with an increasing degree of complexity, looking at single-phase, inert, and reactive particles in separate simulations. The first part of the paper gives a short description of the additional physics added to the code. Based on the experimental results, the numerical code is then used to estimate the particle temperature at the time of ignition. Results from simulations with three different numerical descriptions, the diffusive, kinetic, and total burn rates, are then compared to the experimental results. The two diffusive burn rate simulations (K &H and O &H) show the best fit to the experimental results. The burn rate formula based on our experimental data (O &H) is preferred, since it has the gas temperature dependency included and does not require additional parameter adjustments. The results from the numerical simulations support the theory that the observed aluminium particle cloud burning process is diffusive, as indicated in the experimental paper.

The inerting of a detonable mixture through thermal pretreatment or parasitic combustion is critical to understand for advanced detonation-based combustor design and safety. This work addresses the inerting effects of low temperature chemistry (LTC) on detonations. LTC was induced in both ozoneless DME/O(_{textrm{2}}) and 1.0 mol% O(_{{3}})-enhanced DME/O(_{2}) mixtures over a range of detonation tube temperatures ((T_{textrm{o}})) from 423 to 648 K for reactant mixture equivalence ratios ((phi )) of 0.6–1.8. Upon filling the detonation tube, reactant gas temperatures increased by over 100 K in some cases but never exceeded a maximum gas temperature of 700 K, suggesting a limiting behavior such as the RO(_{2}) ceiling temperature. Zero-dimensional constant-volume simulations were conducted to identify chemical composition changes and heat releasing reactions with LTC pretreatment, and ZND simulations were conducted to show the evolution of thermicity with LTC pretreatment. Prolonged pretreatment at (T_{textrm{o}}) greater than 573 K prior to spark ignition of detonation was observed to inert DME/O(_{2}) mixtures and inhibit detonation transition for all tested (phi ). Additionally, detonation cell sizes were measured, and increased DDT distances and detonation cellular instability at near-limit conditions due to LTC pretreatments were observed using soot foils. Numerical cell sizes were estimated using a correlation model based on center-of-exothermic-length from ZND thermicity simulations, and results showed good agreement with experimental cell sizes. Stability parameter and DDT distance analyses based on correlation models supported the observed reduction in mixture detonability and increase in DDT distances with LTC pretreatment progression.

The present work is focused on the numerical analysis of the flow structures of high-speed underwater air jets. In an earlier work (Jana et al., J. Fluids Eng. 144(11):111208, 2022), the authors presented an analysis of the unsteady behavior of different flow variables of the jets. The present work is further extended to analyze the temporal evolution of the flow structures of different jet regions. The numerical simulations are conducted with the unsteady Reynolds-averaged Navier–Stokes equations with a homogeneous mixture model. The previous work rendered the effect of pressure ratio (the ratio of nozzle exit pressure to back pressure) on the behavior of the jet flow. In the present analysis, jet exit Mach number is also considered as another operating parameter. The results for three pressure ratios, 0.8, 1, and 1.2, and two exit Mach numbers, 2 and 3, are presented. Temporal behavior of the three major regions, namely, the core, shear layer, and mixing layer of the jet due to its interaction with surrounding water, is discussed. The flow physics of shock and expansion waves in the core region is analyzed, and the effects of the underwater ambience on the structures of shock waves are also explained. Various phenomena, such as necking, back-attack, and expansion, are also visualized and explained from the simulated flow variables. Given the limitations of experimental flow visualizations, these analyses aid to understand the major flow behavior of supersonic underwater jets.

Aerobreakup of fluid droplets under the influence of impulsively generated high-speed gas flow using an open-ended shock tube is studied using experiments and numerical simulations. Breakup of millimeter-sized droplets at high Weber numbers was analyzed for water and two-phase nanofluids consisting of dispersions of ({{hbox {Al}}_{2}{hbox {O}}_{3}}) and ({{hbox {TiO}}_{2}}) nanoparticles in water with high loading of 20 and 40 wt%, respectively. Droplet breakup is visualized using high-speed imaging in the experimental setup, where an open-ended shock tube generates impulsive high-speed flow impinging on a droplet held stationary using an acoustic levitator. Axisymmetric simulations using the volume-of-fluid technique are conducted to capture the gas dynamics of the flowfield and droplet deformation at the initial stages. Fluid droplets are subject to a transient flowfield generated by the open-ended shock tube, characterized by a propagating incident shock wave, a recirculating vortex ring, and standing shock cells. Droplet breakup for all fluids proceeds through an initial flattening of the droplet followed by generation of a liquid sheet at the periphery in the presence of a curved detached shock front at the leading edge. The breakup appears to follow a sheet stripping process whereby stretched ligaments undergo secondary atomization through viscous shear. Mist generated in the wake of the droplet appears to expand laterally due to the unconstrained expansion of the high-speed gas jet. The breakup morphology of droplets for all fluids appears consistent with previous observations using conventional shock tubes. Lateral deformation of the coherent droplet mass is observed to be higher for nanofluids as compared to water. This is attributed to higher viscosity and Ohnesorge number of nanofluid droplets, which results in delayed breakup and increased lateral stretching. When plotted as a function of non-dimensionalized time, the same effects are also attributed to generate the highest non-dimensional velocities for the ({{hbox {TiO}}_{2}}) nanofluid, followed by ({{hbox {Al}}_{2}{hbox {O}}_{3}}) nanofluid, and water, which mirrors the order of viscosity and Ohnesorge number for the three fluids. An area of spread, which can be interpreted as a measure of dispersion, plotted as a function of non-dimensionalized time also shows the highest value for the ({{hbox {TiO}}_{2}}) nanofluid, followed by ({{hbox {Al}}_{2}{hbox {O}}_{3}}) nanofluid, and water. Overall, current results indicate that droplet breakup for two-phase fluids appears to be similar to those for single-phase fluids with effectively higher viscosity. Furthermore, an open-ended shock tube proves to be an effective tool to study droplet aerobreakup, with some differences observed in the droplet wake due to the unconfined expansion of gas flow.