Shock Waves最新文献

Time-resolved schlieren photography was used to visualise mixing zones induced by the Richtmyer–Meshkov instability. These were initiated with four different initial conditions: three of them with monotonic, single-mode shapes and one with a non-monotonic, multi-mode shape. These initial conditions were generated by an innovative experimental concept, the Micro Rotating Shutter System. The results of this experimental campaign reveal that the shape of the initial air–helium interface influences the subsequent development of the resulting mixing zone. Over the measurement time range, the width of the mixing zone induced by this instability is correctly fitted by a power law. Its growth exponent depends on the monotonicity of the initial air–helium interface: while mixing widths originating from single-mode initial conditions are almost superimposed, a lesser growth exponent is found for the multi-mode initial condition. The Reynolds number based on the width of the mixing zone suggests that both flows initiated with single- and multi-mode initial conditions reach a fully turbulent state after the interaction with the reflected shock wave (reshock). The schlieren photography visualisations presented here also allow to illustrate the structure of the induced mixing and highlight the effect of the initial conditions on the large-scale structures of the Richtmyer–Meshkov instability-induced mixing.

An ongoing rotating detonation rocket engine program is investigating the influence of combustor annulus radii on RDRE operating characteristics with flat-faced impinging injectors. To facilitate the isolation of all but the radius of curvature effects in the experiments, the annular gap was kept constant at 5 mm in combustors having either 25-mm or 51-mm outer diameter. The mixing processes were kept similar by utilizing injectors with the same net injector-to-annular gap area ratio (AR = 0.11), same radial separation distance of the orifices, and same center-of-gap impingement distance from the front-end wall. The wave dynamics, plenum pressure, and axial pressure profiles in these RDREs were compared over the mass flux and equivalence ratio ranges of (80{-}400,text {kg/s/m}^{2}) and 0.26(-)2.6, respectively, with gaseous methane–oxygen propellant. Experiments showed that stable one-wave operation would occur in the 25-mm RDRE at most mass fluxes where stable two-wave operation was established in the 51-mm RDRE. Stable one-wave operation with a single counter-rotating wave was maintained in the 51-mm RDRE at mass fluxes of (240,text {kg/s/m}^{2}) and below. Under these fueling conditions in the 25-mm RDRE, a counter-rotating wave also appeared while it operated with a single dominant wave. The wave spin speeds were typically 20–40% less than the Chapman–Jouguet detonation speed of the propellant and depended only on mass flux and wave number rather than the annulus diameter.

We studied the real gas effect on the ignition characteristics in chemical reactors with one-step irreversible reaction. The real gas effects were characterized by the inter-molecular attraction term ((alpha )) and the finite molecular volume term ((beta )). The Noble-Abel and van der Waals equations of state were employed to derive non-dimensional reactor models. In addition to ideal reactors, i.e., constant volume and constant pressure, non-ideal reactors that account for the non-ideal pressure variation in shock tube and rapid compression machine were also considered. For all reactors, low value of (alpha /beta ) and high value of (beta ) (approximately (alpha /beta <{{1.0}}) and (beta >{{0.1}})) induce a decrease of the ignition delay-time, while high value of both (alpha /beta ) and (beta ) (approximately (alpha /beta >{{2.0}}) and (beta >{{0.1}})) induces an increase of the ignition delay-time. The variations of the ignition delay-time induced by real gas effects are mainly related to the change of the fugacity coefficient with (alpha ) and (beta ). Additional contributions are due to the real gas heat capacity at constant pressure when considering a constant pressure reactor and to non-ideal volume variation when considering non-ideal reactors. The impact of various parameters was also investigated, including the heat capacity ratio of perfect gas, the reduced activation energy of the one-step reaction, and the heat content of the mixtures. Comparison with simulation performed with detailed reaction mechanisms and considering real gas models demonstrates that the present approach constitutes a rapid and simple, yet qualitatively or even quantitatively accurate method to assess the need of accounting for real gas effects to model chemical kinetics under high-pressure conditions.

To study the shock wave initiation characteristics of 2,4,6-trinitrotoluene (TNT) under different charging types, the shock wave pressure and shock wave attenuation of standard Pentolite explosives under different diaphragm thicknesses were quantitatively studied using the ion probe method. The gap tests of three explosives were carried out, including pressed TNT without restraint, pressed TNT with steel pipe restraint, and cast TNT with steel pipe restraint. The shock wave initiation pressures of TNT under the three different conditions were compared. Moreover, combined with the numerical simulation technology, the critical initiation pressure and the pressure cloud diagram of the gap test of TNT were obtained, and the dynamic change process of the shock wave in the diaphragm was acquired, which was difficult to measure in the experiments. The results showed that the critical initiation pressure of pressed TNT was significantly lower than that of cast TNT and that restraint can reduce the measured critical initiation pressure of TNT under certain conditions. Therefore, the research results may provide a basis for the damage range of TNTs with different charging types and the determination of the safety protection distance of shock wave initiation.

The blast loading from a detonation of a high explosive charge at high altitude is quite different from that at sea level. Due to diminished ambient pressure, the damage caused by the blast load may be more minor at high altitude. However, the shock wave parameters at diminished ambient pressure have not yet been thoroughly studied. In this research, experiments were carried out to study the relation between ambient air pressure and shock wave parameters. The explosion experiments were carried out in a sealed explosion chamber with an initial pressure of 95 kPa, 74 kPa, and 57 kPa. For these three atmospheric conditions, the history profiles of incident shock wave pressure generated by TNT charges of 106 g and 292 g were recorded. The influence of ambient pressure and temperature on the shock wave parameters was analyzed through numerical simulations. By analyzing the experimental and numerical data, it was found that ambient pressure is the main factor affecting the shock wave parameters, while the effect of temperature is not so obvious. Furthermore, based on the analysis of experimental data, formulas for evaluating shock wave overpressure, specific impulse, and arrival time using the Sachs variables are given, and the shock wave parameters at an altitude of 5000 m are calculated using these formulas. The observed maximum reduction in the shock wave overpressure was 23%, in specific impulse 27%, and in arrival time 12%, compared to the results calculated at sea level. The results can be applied to blast-resistant analyses of buildings in low-pressure environment.

Rotating detonation propulsion technologies have the potential to create highly efficient engines in a small form factor. However, the detonation dynamics and complex flowfields inside the combustion chamber are greatly dependent on geometry; in particular, the downstream nozzle design affects dynamics inside the combustion chamber. In this work, three-dimensional large eddy simulations of a gaseous methane–oxygen rotating detonation rocket engine are presented for two geometries. The geometries match experimental tests previously conducted at the Air Force Research Laboratory and are chosen to compare engine operation with and without a converging–diverging nozzle. It is shown that flow in the unconstricted chamber exceeds Mach 1 behind the generated oblique shock structure, but that the addition of a 4.4(^circ ) converging section results in supersonic flow existing only in the diverging section of the nozzle. The formation enthalpy of the flow is calculated inside the chamber and demonstrates that the difference in pressures and detonation structures associated with the chamber area constriction do not result in a significant change in energy released through combustion.

The effects of ozone addition and low-temperature chemistry (LTC) progression on DME/O(_{2}) detonations are evaluated with experimental detonation velocity and cell size measurements and one-dimensional ZND simulations. For ( phi = 1.2) and (P_{textrm{o}}= 22.7) kPa, detonations are experimentally investigated over a range of ozone enhancement levels (0.0–1.6-mol%), initial reactant temperatures (293 K and 468 K), and LTC progression times (250–6000 ms). A 33-K gas temperature rise from LTC heat release is observed for mixtures with 1.0-mol% ozone enhancement and initial temperature of 468 K, suggesting a limited extent of LTC progression in this study. Experiments showed minimal detonation velocity dependence on ozone enhancement level or LTC progression despite the increased radical pool. Average cell size is found to decrease 15–30% with 1.6-mol% ozone addition, indicating a greater reactant mixture sensitivity to detonation. To estimate the cell size, a center-of-exothermic-length induction length is defined and used with an empirical correlation to calculate a singular cell size when multiple thermicity peaks are present in ZND modeling. This approach shows good agreement with experimental findings. Cell size dependence on LTC progression is found to have a statistically insignificant variance for LTC progression times at the temperatures used in this study.

The interaction and coalescence of shock waves originating from multiple explosion sources were studied using numerical simulations and theoretical analysis. The effects of the mass distribution, layout, and quantity of explosion sources were considered, and an engineering calculation model for shock wave parameters at the focus center was established. The results show that the peak overpressure at the focus center is significantly changed only when the mass ratio of the two explosion sources increases beyond two. Overall, the peak overpressure at the focus center decreases with the increase in mass ratio. The focus effect of multiple explosion sources is the greatest when the sources are uniformly distributed on a circle. When the number of explosion sources is less than four, the peak overpressure and specific impulse at the focus center increase with the increase of the number of explosion sources. Increasing the number of explosion sources from one to four results in an increase in the peak overpressure by a factor of 0.73–5.23 and an increase in the specific impulse gain by a factor of 1.59–4.71. The results from simulations and experiments verify the validity of the model used to characterize multiple explosion sources.

This paper examines the effect of an asymmetrical energy source impact on the flow around supersonic aerodynamic bodies in a viscous heat-conducting gas (air) at Mach 2.5. The simulations are based on the Navier–Stokes equations with temperature-dependent viscosity and thermal conductivity. The dynamics of density, pressure, temperature, and heat fluxes were analyzed. Specific emphasis is placed on the effects of viscosity and thermal conductivity. Self-sustained oscillations of the flow parameters, lift and drag forces, and heat fluxes were obtained and studied. The mechanism of these oscillations was established, and the conditions of their presence in a flow in relation to the energy source characteristics and location were researched. Possible approaches for elimination of these oscillations were discussed.