Shock Waves最新文献

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.

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.

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.

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.