Shock Waves最新文献

Variations in the experimental constraints applied within blast simulations can result in dramatically different measured biomechanical responses. Ultimately, this limits the comparison of data between research groups and leads to further inquisitions about the “correct” biomechanics experienced in blast environments. A novel bilayer surrogate brain was exposed to blast waves generated from advanced blast simulators (ABSs) where detonation source, boundary conditions, and ABS geometry were varied. The surrogate was comprised of Sylgard 527 (1:1) as a gray matter simulant and Sylgard 527 (1:1.2) as a white matter simulant. The intracranial pressure response of this surrogate brain was measured in the frontal region under primary blast loading while suspended in a polyurethane spherical shell with 5 mm thickness and filled with water to represent the cerebrospinal fluid. Outcomes of this work discuss considerations for future experimental designs and aim to address sources of variability confounding interpretation of biomechanical responses.

Experimental data obtained in shock tubes, including ignition delay-time and species concentration profiles, are among the most significant parameters in combustion studies. Although shock tubes are widely considered as a quasi-ideal reactor for high-temperature studies, it involves a number of non-ideal effects such as a time-dependent pressure increase within the test section. This non-ideal pressure rise induces inaccuracy in the shock tube measurements. To overcome this issue, the driver insert strategy has proven to be successful. Nevertheless, the approaches presented in the literature to design such a driver insert either are not self-sufficient, i.e., they rely on external software, or lack flexibility. In this study, a simple, self-sufficient, fully analytical approach implemented in a MATLAB code has been developed to design a driver insert for the control of the rate of pressure rise in the test volume. The tip and end positions of the insert, as well as the effect of area change ratio on pressure behind reflected shock are obtained by the code. Extensive validation is performed against previous results from the literature and new data generated with several numerical codes.

Ammonium dinitramide (ADN, ([textrm{NH}_{4}]^{+}[textrm{N}(textrm{NO}_{2})_{2}]^{-})) and its related propellants are promising high energy density materials for new-generation space propulsion. In order to ensure their safe utilization, it is of primary importance to assess the risk of accidental combustion events such as detonation. Thus, focusing on ADN and its related propellant composed of ADN, monomethylamine nitrate, and urea with weight percentages of 40:40:20 (AMU442), we have studied the properties and steady structure of detonation propagating in gaseous mixtures formed by their thermal decomposition. The AMU442-based mixture exhibits higher von Neumann and Chapman–Jouguet temperatures and pressures than the ADN-based mixture. The study of their steady detonation structure reveals that the gaseous species resulting from the decomposition of AMU442 have higher detonability than the ones resulting from the decomposition of ADN alone. This is in contrast to a previous study about the safety of these propellants in their original (solid or liquid) phase, i.e., AMU442 has lower sensitivity/reactivity to incident impact than ADN. Thermochemical analyses performed for both mixtures show that the decomposition of (textrm{HNO}_{3}) plays a dominant role for the energy consumption and initiation of the reaction by releasing both OH and (textrm{NO}_{2}). For the ADN-based mixture, the reactions involving (textrm{HN}(textrm{NO}_{2})_{2}) and (textrm{HNO}_{3}) are the most sensitive, whereas for the AMU442-based mixture, the most sensitive reactions involve (textrm{CH}_{3}textrm{NH}_{2}), (textrm{CH}_{2}textrm{NH}_{2}), and (textrm{HNO}_{3}). Reaction pathway diagrams emphasize the higher complexity of the chemical pathways for the AMU442-based mixture because of the presence of ([textrm{CH}_{3}textrm{NH}_{3}]^{+}[textrm{NO}_{3}]^{-}) and (textrm{CH}_{4}textrm{N}_{2}textrm{O}) in the initial mixture. An uncertainty quantification study demonstrated that the calculated induction lengths exhibit an uncertainty on the order of 50%.

The ongoing study of blast waves and blast wave mitigation continues to play an essential role in protecting structures and personnel. The methodology, however, for capturing far-field blast waves in large-scale tests has remained largely unchanged for three decades, relying on large arrays of pressure transducers connected by hundreds of meters of cabling and requiring a considerable amount of time to set up. This paper evaluates the use of a modern low-cost microprocessor with high computational power to capture blast waves with sufficient fidelity to provide scientists and engineers with credible data. The system utilizes an ARM Cortex M7 processor as an experimental data acquisition (DAQ) system for measuring far-field blast waves in an open-air blast arena at sampling speeds of up to 1.8 Msps (megasamples per second). The experimental system’s performance was evaluated by comparing it to a traditional commercial system used for measuring blast waves. The comparison showed an average Spearman correlation coefficient r of 0.928 between the two systems, suggesting a low variance between the commercial and experimental DAQ systems. This suggests that, despite its simplicity, the experimental system is an effective and low-cost alternative for accurately measuring blast waves.

Shock wave reforming, or the use of shock waves to achieve the necessary high-temperature conditions for thermal cracking, has recently gained commercial interest as a new approach to clean hydrogen (H(_2)) generation. Presented here is an analysis of the chemical kinetic and gasdynamic processes driving the shock wave reforming process, as applied to methane (CH(_4)) reforming. Reflected shock experiments were conducted for high-fuel-loading conditions of 11.5–35.5% CH(_4) in Ar for 1790–2410 K and 1.6–4 atm. These experiments were used to assess the performance of five chemical kinetic models. Chemical kinetic simulations were then carried out to investigate the thermal pyrolysis of 100% CH(_4) across a wide range of temperature and pressure conditions (1400–2600 K, 1–30 atm). The impact of temperature, pressure, and reactor assumptions on H(_2) conversion yields was explored, and conditions yielding optimal H(_2) production were identified. Next, the gasdynamic processes needed to achieve the target temperature and pressure conditions for optimal H(_2) production were investigated, including analysis of requisite shock strengths and potential driver gases. The chemical kinetic and gasdynamic analyses presented here reveal a number of challenges associated with the shock wave reforming approach, but simultaneously reveal opportunities for further research and innovation.

Abstract

The dynamics of shock-induced unsteady separated flow past a three-dimensional square-faced protuberance are investigated through wind tunnel experiments. Time-resolved schlieren imaging and unsteady surface pressure measurements are the diagnostics employed. Dynamic mode decomposition (DMD) of schlieren snapshots and analysis of spectrum and correlations in pressure data are used to characterize and resolve the flow physics. The mean shock foot in the centreline is found to exhibit a Strouhal number of around 0.01, which is also the order of magnitude of the Strouhal numbers reported in the literature for two-dimensional shock–boundary layer interactions. The wall pressure spectra, in general, shift towards lower frequencies as one moves away (spanwise) from the centreline with some variation in the nature of peaks. The cross-correlation analysis depicts the strong dependence of the mean shock oscillations and the plateau pressure region, and disturbances are found to travel upstream from inside the separation bubble. Good coherence is observed between the spanwise mean shock foot locations till a Strouhal number of about 0.015 indicating that the three-dimensional shock foot largely moves to-and-fro in a coherent fashion.

Abstract

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.

Passive flow control devices, such as vortex generators (VGs), have shown to be successful in controlling flows associated with shock-wave/boundary-layer interactions. In the present work, we investigate the effectiveness of micro-VGs in controlling the interactions of the boundary layer with a swept shock wave generated by a semi-infinite fin placed in a Mach 2 freestream generated in a wind tunnel with a rectangular cross section. The strength of the interaction is varied by changing the angle of attack of the fin in the range (alpha = 3^circ )–(15^circ ). Arrays of micro-VGs are placed upstream of the interaction zone in two different configurations: (I) along a line perpendicular to the freestream and (II) along a line inclined to the freestream following the conical topology of the interaction zone. A parametric analysis is done for the rectangular, ramp, and Anderson-type micro-VGs for three different heights. Unsteady and time-averaged pressure measurements are done using arrays of ports spanned radially across the interaction zone. Surface flow patterns are obtained using the oil-flow visualisation technique. It is observed that VGs offer significantly better control effectiveness when placed inclined to the freestream along the interaction region. The rectangular VGs demonstrate a maximum shift (as much as (8^circ )) in the upstream influence line azimuthally towards the fin resulting in a decrease in the size of the separation region. Footprints obtained from the oil-flow experiments give important signatures of the vortices that are shed from the VGs and are responsible for the flowfield distortion in the interaction zone.

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.