Shock Waves最新文献

In shock tubes, some non-ideal effects can result in a gradual rise in pressure behind the reflected shock wave. This variation in the reflected shock pressure (P(_text {5})) can significantly affect the accurate determination of kinetic parameters. Among the various techniques available to address the non-ideal pressure rise in the reflected shock wave region, the use of driver inserts has emerged as the most effective and reliable method for compensating the non-ideal increase in P(_text {5}). In this study, a step-size driver insert was implemented in a shock tube, and the resulting pressure measurements were analysed for two different initial temperatures T(_text {1}). The results reveal a notable difference in the geometry of the driver inserts required to achieve a uniform pressure profile behind the reflected shock wave for initial temperatures of 300 K and 353 K. While the direct design strategy of the driver insert did not yield optimal performance in one case, the final design of the driver insert was refined empirically based on the experimental pressure trace, guided by the principle of expansion fan reflection. Distinct insert shapes were ultimately developed for each temperature condition to maintain a stable pressure profile behind the reflected shock wave. Accurate chemical kinetic measurements require well-defined temperature and pressure conditions, and appropriately designed driver inserts play a crucial role in achieving these conditions. This manuscript presents the design methodology and experimental validation of driver inserts for improved performance under varying thermal conditions.

This study is motivated by concerns for the safety of goods and people, as well as the need to provide individuals with the means to prevent and protect against accidental risks and terrorist threats. The research is one of the tasks in the ANR research project (mathrm{URB(EX)}^{{3}}), which aims at developing a fast-running, breakthrough model for blast consequences in urban configurations. The objective is to characterize the propagation of a shock wave along a straight street using experimental and numerical approaches. The shock wave results from the detonation of a gaseous explosive charge. The experiments are carried out at laboratory scale by applying the laws of similarity. Shock waves are studied using pressure profiles recorded by regularly distributed pressure sensors. Visualization is also used to illustrate various shock wave interactions between the two walls. The explosive charge is placed on the central axis of a street between two parallel walls. The shock wave propagation is analysed in terms of street width and height. It is demonstrated that the shock wave changes its propagation mode from 3D to 2D. It is also shown that several planar shock waves are correlated with the junction of two Mach stems. This study reveals secondary shock waves, as well as multiple shock waves, which can lead to a certain complexity in interpreting the measured pressure signals. The 3D to 2D mode transition zone is determined for each configuration, and an empirical law is established based on the different experimental results obtained. The law considers three parameters, namely the diameter of the explosive charge and the dimensions of the street (height and width).

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.