Shock Waves最新文献

Large yield airbursts generate powerful outdoor blast waves. Over long propagation distances, the blast is significantly altered by the topographical relief. Usually, the terrain effects are quantified by running accurate but expensive hydrodynamics or CFD codes. We present an alternative approach based on artificial neural networks, which is applicable wherever the blast–relief interaction can be approximated by an axisymmetric configuration. A database of overpressures associated with a very large sample of the French topography is constructed by running a high-fidelity hydrodynamics code. The proposed neural networks then learn the relationship between the relief geometry and the ground overpressures. The predictive ability of the networks is assessed extensively over a test database for several error metrics. ({97}{%}) of the peak overpressure predictions can be considered accurate for most practical purposes, and the pressure impulse predictions are even more accurate. Finally, specific artificial neural networks able to estimate the model uncertainties are presented and their performances are discussed.

This study presents an experimental and numerical investigation to characterize the plume pattern of a high-aspect-ratio rectangular convergent/divergent nozzle with an aft deck in under-expanded conditions. The function of an aft deck is to shield the infrared signal of an exhaust plume at its strongest intensity located at the immediate downstream region of the nozzle exit. However, this practice may cause undesirable plume deflection, which needs to be reduced as much as possible. The nozzle pressure ratios ranged from 2 to 4, and the effect of the nozzle exit aspect ratio was examined using wall static pressure measurements and schlieren visualization for cold flows. The experimental setup involved a 3D-printed aft deck nozzle made of acrylonitrile butadiene styrene material, which underwent surface smoothing using acetone vapor. Numerical simulations were conducted using the commercial STARCCM(^{mathrm {+}}) software to analyze static pressure ratio variations at the aft deck. The investigation revealed that a nozzle pressure ratio of 3 induced a downward plume deflection at aspect ratio values of 6.77 and 7.54, while an increased aspect ratio of 8.35 resulted in the horizontal ejection of the plume. Moreover, at an aspect ratio of 8.35, the plume was ejected horizontally for nozzle pressure ratios ranging from 2 to 4. At a nozzle pressure ratio of 4, the flow separated from the deck without reattaching, and the plume moved horizontally with minimal deflection. The findings suggest that a combination of a high aspect ratio and sufficiently high nozzle pressure ratio can effectively reduce plume deflection.

Considered are interactive relationships between a normal shock wave and the downstream shock wave leg of the associated lambda foot, as well as between a normal shock wave and time-varying static pressure as measured along the bottom surface of the test section. Such relationships are investigated as they vary with two different magnitudes of inlet unsteady Mach wave intensity and are characterized using shadowgraph flow visualization data, as well as power spectral density, magnitude-squared coherence, and time lag data. Employed for the investigation is a specialty test section with an inlet Mach number of 1.54, as utilized within a transonic/supersonic wind tunnel. The resulting data provide evidence of distinct interactions over a wide range of frequencies between the normal shock wave and the downstream shock wave leg of the lambda foot for low inlet unsteady Mach wave intensity. Note that these are not present in the same form and over the same ranges of frequency with high inlet unsteady Mach wave intensity. These differences are partially due to the location where flow events originate. The most significant sources of flow unsteadiness within the present investigation are mostly associated with the normal and oblique shock waves (with low inlet unsteady Mach wave intensity), and mostly with inlet flow disturbances from unsteady Mach waves (with high inlet unsteady Mach wave intensity). The present experimental results additionally evidence important connections between the normal shock wave and unsteady flow events within lower portions of the lambda foot, especially near the adjacent boundary layer separation region.

The blast wave interactions with the helmet–head assembly can result in localized pressure amplification at certain locations around the head. The underwash effect is a typical example of such interaction in the gap between the suspension-type combat helmet and the head. There are hypotheses in the literature that suspect an increase in the severity of blast-induced traumatic brain injury due to combat helmet usage under blast loading. But the literature lacks concrete experimental visual evidence for the underwash effect and the cause–effect relationship between the underwash effect and brain injury. Firstly, in this study, shock wave interactions causing the underwash effect are visualized using the schlieren imaging technique. Secondly, a reasonable correlation between a significantly large, localized pressure amplification due to the underwash effect and the brain’s mechanical stress response was observed with an idealized helmet–head model in a coupled Eulerian–Lagrangian framework. But further studies are needed with more realistic models to prove their significance in the design of blast-resistant combat helmets.

It is extremely important to predict the growth, aggregation, and coalescence failure of voids during the dynamic tensile fracture of ductile metals. In the present work, we used the finite element—smoothed particle hydrodynamics (FE-SPH) adaptive method to simulate the plate impact of tantalum simultaneously from macro- and meso-scales. For macro simulation results, the spallation phenomena and free-surface velocity were in good agreement with the experimental results, verifying the correctness of the simulation method and material model. Moreover, the free surface velocity profiles simulated by the FE-SPH adaptive method is closer to the experiment than those by the finite element method. According to the magnified details of the damage, we envisaged that the deleted elements are converted to SPH particles to represent the formation of voids. By comparing the porosity, we demonstrated the rationality of this envisagement and determined the fine mesh size to simulate growth, aggregation, and coalescence of actual meso-voids. On this basis, we proposed a void-position tracking method to accurately track the temporal and spatial information of voids. Such information would provide a detailed range of damage and describe the features and macro factors affecting void evolution. In general, the fine mesh FE-SPH method can well predict the damage distribution of spallation simultaneously in macro- and meso-scales, and this simple method has important applications.

This technical note reports the expression of selected higher-order moments associated with the Mott-Smith solution of the shock-wave profile. The considered moments are the pressure tensor, the heat-flux vector and tensor, the fourth-order double-tensor, its full contraction, and the fifth-order moment vector. The resulting shock profiles are shown for Mach 2 and Mach 10 conditions.

The influence of real gas effects and a turbulent boundary layer on shock wave attenuation in the expansion tube is studied by numerically solving the axisymmetric compressible Navier–Stokes equations with an adaptive mesh refinement technique. Numerical simulation results reveal that the ideal gas assumption is not applicable to the expansion tube, and the turbulent boundary layer plays a major role in decreasing the shock wave speed in the acceleration tube of the expansion tube. Shock wave attenuation is attributed to the turbulent boundary layer decreasing the pressure behind the shock wave. The numerical simulations that include the real gas effects and the development of turbulent boundary layers qualitatively agree with analytical solutions in the shock tube, and they show good agreement with the experimental results, especially for the shock speed in the acceleration tube of the expansion tube. Both effects should be considered in the numerical simulation model aimed to support experiments in expansion tubes.

Accurate prediction of the shock–boundary layer interactions (SBLIs) region, encompassing boundary layer separation, reattachment, and transition, is crucial for high-speed flows due to its impact on the aerothermodynamics and performance, particularly at hypersonic speed. Among various types of compression ramp SBLI (laminar, turbulent, or transitional), several experimental and numerical investigations on turbulent SBLI are available in the literature. However, very few RANS-based numerical studies exist on the high-speed laminar/transitional SBLI due to the complexity of modeling the boundary layer transition in hypersonic flows. This study numerically analyzes boundary layer transition and the SBLI interaction region of a double-wedge configuration at hypersonic speeds using a modified (gamma )-transition model. An in-house solver developed with a transition model and SST k–(omega ) turbulence model is utilized for this study. A parametric analysis is also carried out to study the effect of wall temperature, wedge length, and wedge angle on the interaction region and transition for various types of compression ramp SBLI. The separation region of the boundary layer and the transition location were estimated using numerical schlieren and Stanton numbers for different parameters. The results show that the modified (gamma )-model predicts the boundary layer separation, reattachment, and transition of laminar/transitional SBLI appropriately compared to a fully turbulent model for all considered parameters.

Experiments were conducted to investigate detonation propagation in a curved tube filled with stoichiometric 2H(_{2}+)O(_{2}+)7Ar and CH(_{4}+)2O(_{2}). The test section of the experimental setup was a semicircular channel with an internal radius of 500 mm. Detonation velocities were calculated based on the arrival time of the wave front, monitored by pressure transducers. The detonation cellular evolution was recorded using smoked foils. The results revealed that after crossing the obstacle, the detonation wave failed and promptly re-initiated. It then decayed from an overdriven detonation to a steady-state detonation. The detonation development processes were divided into five regimes. The formation of the boundary behind the obstacle and the generation mechanism of the overdriven detonation were thoroughly analyzed. The formation of the boundary behind the obstacle is associated with the curved shock front and the non-uniform cellular structure. The re-initiation distance for an unstable mixture in a curved tube was significantly shorter than that in a straight channel. In the absence of the obstacle, the cell width decreased radially outward, a linear relationship was determined. The speed of the detonation wave initially decreased and then gradually increased.

This paper focuses on the blast resistance performance and protection mechanism of a polyurea-sprayed vehicle armor composite structure under blast impact. We study the blast resistance performance of a steel plate with composite structures in four different spray configurations, which depend on whether spraying occurs and the spray position on the steel plate. First, near-field airburst tests are conducted for four different sprayed types of composite structures for a 2-kg TNT equivalent, and then, the test conditions are simulated using LS-DYNA software. Based on the verification of the accuracy of the calculation model, the dynamic response of the back-sprayed structure at different standoff distances is compared and analyzed. The test and simulation results reveal that compared with other spraying configurations, the back-sprayed structure has better blast resistance, and the low impedance ratio of the front-sprayed structure is the major cause of the aggravation of the structure damage. With the decrease in the standoff distance, the deformation range and flatness factor of the structure are constantly reduced, and the damage mode and protection mechanism of the composite structure keep changing. The blast resistance performance of polyurea is mainly based on the energy absorption and storage during the tensile phase and the energy release and dissipation during the rebound phase. For the back-sprayed structure, the steel plate is always the main energy-absorbing structure. In a certain load range, the energy absorption ratio of polyurea is proportional to the strength of the blast load. Additionally, when the load strength exceeds the tolerance limit of the surface steel plate, the blast resistance of polyurea cannot be effectively exerted. In such a case, the damage modes of steel plate and polyurea tend to be similar.