Shock Waves最新文献

We identify six scenarios of detonation re-initiation downstream of an obstacle with a single-centered hole in square and round tubes. The tubes have the same cross-sectional area of (16,hbox {cm}^2), and the hole has the same shape as the tube cross section, but different open area ratios. The test mixtures investigated were 2 H₂ + O₂ + 2 Ar and 2 H₂ + O₂. We used soot foil recordings, high-speed schlieren, and chemiluminescence imaging to obtain longitudinal and frontal views of the diffraction phenomena. Depending on the initial pressure, one supercritical, four critical, and one subcritical scenarios were observed. The supercritical and critical scenarios were more likely to occur in the square tube than in the round tube, all other parameters being equal. Most of these transient, three-dimensional effects of the cross-sectional shape can be observed even at initial pressures for which there is no effect on steady propagation, e.g., without an obstacle or far from it. This raises the question of what dimensionality refers to in experiments in terms of global and local detonation dynamics.

In the field of blast protection engineering, it remains challenging to validate large, complex numerical models and the implications of modelling assumptions relating to how structures are represented (e.g., geometric fidelity) are not well understood. This paper presents experimental work addressing these two issues, in the context of the 2020 Beirut explosion, which remains an important case study for understanding urban blast effects. A series of reduced-scale (1:250) blast tests examined shielding effects caused by the Beirut grain siloes and investigated the influence of the siloes’ geometric fidelity on blast loading. Rigid obstacles were constructed at two geometric fidelities: “rectangular” (i.e., cuboid) and “accurate”, with closer resemblance to the siloes. Pressure gauges were mounted at multiple locations but at fixed blast scaled distances to examine blast–obstacle interaction behaviour. Additionally, Viper::Blast was used to perform computational fluid dynamics analyses of the tests. Experimental findings confirmed significant shielding (reduced pressure and specific impulse) locally behind the siloes ((Z) (<) 3 m/kg^1/3), although models indicated that these effects ceased further afield ((Z) (>) 5 m/kg^1/3). Overall, blast wave parameters did not exhibit significant differences between the rectangular and accurate representation of the siloes geometry, except for minor differences (10%) in peak overpressures in localised zones. Numerical models confirmed that these discrepancies were caused by differing blast wave scattering, diffraction, and superposition behaviour attributed to the siloes outer geometry. The results suggest that city-scale blast loading analyses can yield reliable results through idealising structures as simplified cuboidal obstacles. These findings will be of direct relevance to blast protection practitioners and researchers concerned with modelling urban blast scenarios.

To develop a detonation combustion chamber intake channel with a short axial length, minimal resistance to the incoming flow, and effective suppression of back-propagating pressure waves, this study investigated a novel intake structure consisting of a U-shaped channel. The study investigated the effects of obstacles with different geometric parameters installed at different positions within the U-channel on the attenuation of pressure waves. To standardize the channel structure, the concept of a gas retention volume ratio is introduced and systematically studied as a key parameter. The findings reveal that the flow area ratio of the channel is the most significant factor influencing the attenuation of pressure waves, while variations in the gas retention volume ratio also affect the wave propagation process. Furthermore, the study reveals that installing obstacle structures in the downstream leg of the U-channel results in bidirectional anisotropy, characterized by different total pressure recovery coefficients for forward and reverse flows.

It is important to understand the effects of explosions to ensure the safety of civilians and military personnel. Rapid City Planner (RCP) is a comprehensive software tool for predicting the effects of conventional and improvised explosive devices with geographic information system-based outcomes for munition safety, building/structure damage, protection of assets, and human vulnerability in real cities. Modelling cased explosives requires the consideration of casing fragmentation, which is modelled in RCP using three different methods having a different level of detail. The present study focuses on introducing and validating one of the methods, namely the fast primary fragmentation method. The fast fragmentation solver is used to simulate casing breakup of steel-cased cylindrical charges with various TNT- and RDX-based explosives. The predicted velocities were within less than one percent of theoretical Gurney velocities, and the fragment size distributions compared well with experimental data for each explosive. A TNT-filled artillery shell trial was used to validate the fast fragmentation solver in terms of polar distributions of initial fragment speed and number, as well as the spatial spread of fragment throw in a free-field environment. The RCP hydrocode solver is used to assess the equivalent bare charge model used by the fast fragmentation approach. The detailed and fast-modelling approaches produced similar peak overpressures, but underpredicted impulses, at standoff distances between 5 and 10 m (mid-to-far field range). The failure strain and fragment size distribution are shown to have little effect on the blast wave, whereas both would have a significant impact on subsequent fragment effects. Finally, a full-scale scenario was modelled in RCP to show blast and primary fragmentation effects in a coastal urban environment, including a demonstration of urban blast effects from blast pressure and primary fragment trajectories outcomes.

An experimental and modeling study of the autoignition of (hbox {CH}_{4})–(hbox {O}_{2})–(hbox {Ar}) mixtures with 0.25–4.0% (hbox {CH}_{textrm{4}}) and 2.0% (hbox {O}_{textrm{2}}) was performed at reflected shock wave conditions of (sim )2 atm and (sim )1600–2300 K. The process was monitored by recording the absorption time profiles of (hbox {CH}_{textrm{3}}) and OH radicals at 216.6 nm and 306.772 nm, respectively. The ignition delay time was determined in two ways: as the times it takes to reach the peak (hbox {CH}_{textrm{3}}) concentration or one-half of the maximum OH concentration. Kinetic simulations were carried out using a number of reaction mechanisms, and the predictions were compared to the measurements. An analysis of the sensitivity of the ignition delay time to the rate constants of various elementary stages was conducted, and the main reactions controlling the ignition process were identified. It was demonstrated that uncertainties in experimental conditions, such as the initial temperature and pressure rise rate, produce an effect comparable with that stemming from uncertainties in the rate constants of the key reactions.

The effects of temperature on cavitation behavior have been examined in non-biological engineering applications, e.g., water purification, propeller degradation. However, there is a lack of detailed results on how temperature may affect shock-induced cavitation modeling for biological systems. In particular, it is essential to establish if experiments of biological systems (and surrogate biological systems) conducted at room temperature are accurate representations of cavitation at body temperatures. Additionally, many existing works on biological cavitation utilize distilled water. While distilled water is purified, it is not guaranteed to be free from all ions and is still slightly conductive. Since the impact of ion concentration on cavitation behavior has also yet to be quantified, it is preferable to utilize deionized (DI) water for such experiments. As a result, the present study examines the effect of temperature on shock-induced cavitation using a novel shock tube model to visually record the cavitation events in deionized water. Results show a statistically significant relationship between temperature and cavitation bubble number. Although deionized water was used in this study, the results highlight the need to incorporate temperature into future simulations and experiments involving biological fluids in shock wave environments.

To prevent flow separation under overexpanded conditions in traditional large-area-ratio nozzles of rocket engines at sea level, the method of characteristics for wall pressure control is adopted. This method, which is based on thrust-optimized contours, can be implemented to redesign the latter half of a divergent contour to ensure that the wall pressure of the new contour is not less than the critical separation pressure of 0.03 MPa. The newly generated nozzle is named the full-flow nozzle. Then, the design method is verified by simulations, and the performance of full-flow nozzles is evaluated. The results show that the method of wall pressure control can achieve the intended purpose, and the newly generated contour ensures that the nozzle is not only in the full-flow state at sea level but also able to withstand combustion chamber or ambient pressure fluctuations. The combustion chamber pressure is 8.5 MPa, and the specific heat ratio of hot gas is 1.144. Compared with the thrust-optimized contour with an area ratio of 40, in which the flow tends to separate at sea level, the full-flow nozzle can increase the area ratio to 60. Thus, the vacuum specific impulse can be increased by approximately 5.24 s. Compared with the thrust-optimized contour nozzle with an area ratio of 60, the vacuum specific impulse of the full-flow nozzle with an equal area ratio is decreased by 1.57 s.

Detonation diffraction leads to either successful transmission of the detonation or quenching wherein the propagation mechanism is attenuated. The transmission behavior is governed by competing effects of energy release, curvature, and unsteadiness. There is a potentially unique critical diameter that will determine the diffraction outcome for every combustible mixture composition at each set of initial conditions. The critical diffraction diameter has been correlated to several detonation parameters to date; however, these correlations all have limitations. Analytical or quasi-analytical solutions to the diffraction problem, specifically those able to predict the critical diameter, are scarce. The present work develops several critical diameter models by uniting previous work on diffraction phenomena and the critical initiation energy problem. Curvature, decay rate, and energy-based models are established, and their critical diameter predictions are compared against a wide range of experimental critical diameter data. While detonation diffraction is a complex multifaceted phenomenon, a curvature-based one-dimensional model in this work is shown to accurately reproduce empirical critical diameter behavior at relatively low computational cost.

In recent years, terrorist attacks and accidental explosions in urban environments have occurred frequently, causing severe damage, even collapse, of building structures, and have become a major concern of modern society. The need to design and evaluate the blast resistance of building structures is rising markedly. The utmost requirement is the determination of blast loads acting on building structures, i.e., the reflected overpressure of blast waves. To better keep the balance between computational efficiency and prediction accuracy of complex blast wave propagation and its interactions with buildings, a practical numerical simulation approach integrating multiple existing techniques including the multi-stage method, graded mesh, mapping, and un-refinement technique is proposed based on ANSYS/AUTODYN. Firstly, the propagation of blast waves is simplified into three stages, i.e., propagation in the free air from the explosion center to ground zero, propagation after the ground reflection, and interaction with building structures. These three stages are modeled by 1D uniform meshes and 2D/3D graded meshes with increasing mesh sizes. Then, the mapping technique, including mesh un-refinement, is adopted to transfer the predicted results at the previous stage into the next stage. The corresponding meshing strategy against the scaled distances Z ((Z = R / root 3 of {W}), where R is the distance between the detonation point and the target surface, W is the equivalent charge weight of TNT) for each stage is recommended through mesh sensitivity analyses. Finally, the proposed approach and mesh sizes are validated against four series of explosive tests for a single house, an intersection, and two city blocks by comparing with both the overpressures and impulses of blast waves. Additionally, two solvers, i.e., Euler FCT and Euler multi-material, are compared. The former solver is recommended due to its greater efficiency and accuracy. The present work could provide a helpful reference for the blast-resistant design and evaluation of urban building structures.