Shock Waves最新文献

The current study numerically evaluates the detonation inhibition effects of a range of halogenated compounds on hydrogen-air gaseous detonations. The halogenated compounds investigated in this research encompass halogen acids (HI, HBr, HCl, HF), halomethanes ((hbox {CH}_{{3}}hbox {I}), (hbox {CH}_{{3}}hbox {Br}), (hbox {CH}_{{3}}hbox {Cl}), (hbox {CH}_{{3}}hbox {F})), haloethenes ((hbox {C}_{{2}}hbox {H}_{{3}}hbox {I}), (hbox {C}_{{2}}hbox {H}_{{3}}hbox {Br}), (hbox {C}_{{2}}hbox {H}_{{3}}hbox {Cl}), (hbox {C}_{{2}}hbox {H}_{{3}}hbox {F})), haloethanes ((hbox {C}_{{2}}hbox {H}_{{5}}hbox {I}), (hbox {C}_{{2}}hbox {H}_{{5}}hbox {Br}), (hbox {C}_{{2}}hbox {H}_{{5}}hbox {Cl}), (hbox {C}_{{2}}hbox {H}_{{5}}hbox {F})), and complex halogenated compounds ((hbox {CF}_{{3}}hbox {I}), (hbox {CF}_{{3}}hbox {Br}), (hbox {CF}_{{3}}hbox {Cl}), (hbox {CF}_{4})). The study employs a one-dimensional ZND model with detailed chemical kinetics to examine the impact on detonation propagation by adding these halogenated compounds to hydrogen-air mixtures. The effectiveness of these inhibitors is evaluated based on their capacity to increase the induction length, the amount of inhibitor needed to attenuate a detonation wave, and their influence on the detonability of the gaseous mixture under both lean and rich conditions. The results indicate that several halogenated compounds exhibit superior inhibition properties compared to Halon 1301 ((hbox {CF}_{{3}}hbox {Br})). Specifically, (hbox {C}_{{2}}hbox {H}_{{5}}hbox {Br}) leads to the most significant increase in the induction length, with HBr and (hbox {C}_{{2}}hbox {H}_{{5}}hbox {I}) following closely, particularly at 20,000 ppmv concentration levels. However, it is worth noting that the inhibition efficiency also varies depending on the concentration of the inhibitor added to the gaseous (hbox {H}_{{2}})-air mixture. Moreover, based on retardant weight analysis, fluorinated compounds were found to be the most effective inhibitors, followed by chlorinated, brominated, and iodinated compounds across all categories of halogenated inhibitors.

Characteristic timescales for rotating detonation rocket engines (RDREs) are described in this study. Traveling detonations within RDREs create a complex reacting flow field involving processes spanning a range of timescales. Specifically, characteristic times associated with combustion kinetics (detonation and deflagration), injection (e.g., flow recovery), flow (e.g., mixture residence time), and acoustic modes are quantified using first-principle analyses to characterize the RDRE-relevant physics. Three fuels are investigated including methane, hydrogen, and rocket-grade kerosene RP-2 for equivalence ratios from 0.25 to 3 and chamber pressures from 0.51 to 10.13 MPa, as well as for a case study with a standard RDRE geometry. Detonation chemical timescales range from 0.05 to 1000 ns for the induction and reaction times; detonation-based chemical equilibrium, however, spans a larger range from approximately 0.5 to (200~upmu )s for the flow condition and fuel. This timescale sensitivity has implications regarding maximizing detonative heat release, especially with pre-detonation deflagration in real systems. Representative synthetic detonation wave profiles are input into a simplified injector model that describes the periodic choking/unchoking process and shows that injection timescales typically range from 5 to (50~upmu )s depending on injector stiffness; for detonations and low-stiffness injectors, target reactant flow rates may not recover prior to the next wave arrival, preventing uniform mixing. This partially explains the detonation velocity deficit observed in RDREs, as with the standard RDRE analyzed in this study. Finally, timescales tied to chamber geometry including residence time are on the order of 100–10,000 (upmu )s and acoustic resonance times are 10–(1000~upmu )s. Overall, this work establishes characteristic time and length scales for the relevant physics, a valuable step in developing tools to optimize future RDRE designs.

Evidence suggests that low-level blast (LLB) overpressure exposure from military heavy weapons training is associated with subclinical adverse brain health and performance (H &P) outcomes. Existing DOD safety policies related to blast overpressure exposure are not specific to LLB-related brain health effects. This study sought to synthesize the available literature and analyze the relevancy of a specific blast metric to LLB exposures and the manifestation of adverse brain H &P outcomes. A literature search yielded 311 unique articles, from which 220 were identified as human studies on LLB published from 2010 to 2021. After more exhaustive exclusion criteria were applied, 14 articles met the criteria for inclusion. Findings on brain H &P changes were examined in relation to quantified LLB measurements (e.g., peak overpressure) to identify trends. Overall, the included studies suggested that alterations of reaction time, a metric for neurocognitive performance, as well as symptom reporting can occur following cumulative LLB exposures above 4 psi (27.6 kPa). Biomarkers and neurosensory changes have not demonstrated consistent associations with LLB exposures. These findings suggest that cumulative blast overpressure exposures above 4 psi (27.6 kPa) based on current measurement methodologies for body-worn sensors may be associated with adverse brain H &P outcomes. Current research efforts seek to better quantify LLB exposure, the relationships between LLB (e.g., intensity, duration, dose) and brain health, as well as to assess brain H &P domains more comprehensively. These efforts will serve to promote a better understanding of the interaction between LLB exposures and adverse brain H &P outcomes.

This note presents a vapor-based seeding apparatus, named the external alkali seeding instrument (EASI), which is designed to introduce alkali metal vapors into experimental facilities without using precursors or large auxiliary equipment. The device vaporizes small amounts of alkali metals, potassium in this work, which are then carried away by an inert gas. In a benchtop flow cell, carrier gas flow rate (6–(200~hbox {cm}^3/hbox {s})) and device temperature (150–(250,^{circ }hbox {C})) most strongly affected potassium-vapor concentrations. Higher values of either quantity lead to increased potassium-vapor concentrations. When using the EASI to seed a shock tube experiment, vapor-phase potassium was detected immediately after the incident and reflected shockwaves using a laser absorption diagnostic. Mole fraction time histories stay within a factor of 2 over the test time as compared with those from a precursor-based seeding approach, which may span multiple orders of magnitude. This suggests potassium is nearly homogeneously distributed throughout the test gas. This design can be extended to other low-vapor-pressure elements, such as other alkalis or sulfur, with minimal modifications. The EASI simplifies seeding for laboratory experiments targeting potassium and other alkali metals—enabling advances in fundamental spectroscopy, diagnostic development, and chemical kinetics.

An experimental investigation was carried out to study the fluid–structure interactions on a compliant panel subjected to an impinging shock wave and an incoming turbulent boundary layer. These experiments were aimed at understanding the time-averaged and unsteady characteristics of fluid–structure interaction at Mach 2. Two shock impingement locations on the panel (aspect ratio of 2.82), namely the central and three-fourths of the panel length, were tested. The shock boundary layer interactions on a rigid flat plate served as a baseline case. Measurements include shadowgraph and surface oil flow visualizations, panel deflections using a capacitance probe, cavity acoustics using a pressure sensor, surface pressures using discrete pressure sensors, and pressure-sensitive paints. Results show that the interaction on the compliant panel is relatively three-dimensional as compared to a rigid plate with a nominally two-dimensional interaction. Pressure fluctuations on the compliant panel are significantly higher than on the rigid plate, and the fluctuation spectra are multi-modal. Strong coupling at some frequencies was observed between the shock and the panel for both shock impingement locations. The present study suggests that for a compliant panel, the shape of pressure spectra is sensitive to the measurement location on the panel, the panel modifies the pressure distribution around the interaction, and the energy in dominant modes depends on the shock impingement location.

Far-field blast loading has been studied extensively for decades. Close-in, confined, and semi-confined detonations less so, partly because it is difficult to obtain good experimental data. The increase in computational power in recent years has made it possible to conduct studies of this kind numerically, but the results of such simulations ultimately depend on experimental validation and verification. This work thus aims at using reliable experiments to validate and verify numerical models developed to represent blast loading in general. Test rigs consisting of massive steel cylinders with pressure sensors were used to measure the pressure profiles of semi-confined detonations with different charge sizes. The experimental data set was then used to assess numerical models appropriate for simulating blast loading. In general, the numerical results were in excellent agreement with the experimental data, in both qualitative and quantitative terms. These results may in turn be used to analyse structures exposed to internal blast loads, which constitutes the next phase of this research project.

Numerical simulation results of a convecting shielded vortex interacting with a normal shock using a compact scheme in the convecting upwind and split pressure framework are presented. We explore the parameter space spanned by vortex Mach number and incident Mach number to look for combinations of the parameters which lead to vortex breakdown. The incident and vortex Mach numbers covered are on the higher side, where relatively less information is available. It is well known that for a weak shock, the vortex retains its original shape and for stronger shocks it breaks down. In-between these two extremes, there is a region where the vortex neither retains its original shape nor does it break into small pieces. We determine the vortex breakdown and transition regions that have not so far been reported in shock–vortex interaction studies. A number of cases have been studied, and a vortex breakdown criterion for the cases considered is proposed.