Shock Waves In Air Research Articles

Contact and non-contact ultrasonic diagnostics of shock waves driven by single-shot femtosecond laser ablation of titanium

Initial pressures and expansion speeds of air shock waves, driven by single-shot femtosecond laser ablation of titanium surface at fluences up to 6 J/cm2, were acquired by means of front-side non-contact time-of-flight ultrasonic measurements. Similar contact ultrasonic measurements on the rear side of the titanium target demonstrate general correlation of ultrasonic amplitudes the measured in the contact and non-contact modes at high ablative pressures (above 100 GPa, superelastic propagation regime), and strong deviation of these dependences at lower ablative pressures (strong dissipation regime) without any indications of new titanium phases, besides the initial α-Ti phase.

Characterization of spark-generated N-waves in air using an optical schlieren method.

Accurate measurement of high-amplitude, broadband shock pulses in air is an important part of laboratory-scale experiments in atmospheric acoustics. Although various methods have been developed, specific drawbacks still exist and need to be addressed. Here, a schlieren optical method was used to reconstruct the pressure signatures of nonlinear spherically diverging short acoustic pulses generated using an electric spark source (2.5 kPa, 33 μs at 10 cm from the source) in homogeneous air. A high-speed camera was used to capture light rays deflected by refractive index inhomogeneities, caused by the acoustic wave. Pressure waveforms were reconstructed from the light intensity patterns in the recorded images using an Abel-type inversion method. Absolute pressure levels were determined by analyzing at different propagation distances the duration of the compression phase of pulses, which changed due to nonlinear propagation effects. Numerical modeling base on the generalized Burgers equation was used to evaluate the smearing of the waveform caused by finite exposure time of the high-speed camera and corresponding limitations in resolution of the schlieren technique. The proposed method allows the study of the evolution of spark-generated shock waves in air starting from the very short distances from the spark, 30 mm, up to 600 mm.

Mach-Zehnder interferometry method for acoustic shock wave measurements in air and broadband calibration of microphones.

A Mach-Zehnder interferometer is used to measure spherically diverging N-waves in homogeneous air. An electrical spark source is used to generate high-amplitude (1800 Pa at 15 cm from the source) and short duration (50 μs) N-waves. Pressure waveforms are reconstructed from optical phase signals using an Abel-type inversion. It is shown that the interferometric method allows one to reach 0.4 μs of time resolution, which is 6 times better than the time resolution of a 1/8-in. condenser microphone (2.5 μs). Numerical modeling is used to validate the waveform reconstruction method. The waveform reconstruction method provides an error of less than 2% with respect to amplitude in the given experimental conditions. Optical measurement is used as a reference to calibrate a 1/8-in. condenser microphone. The frequency response function of the microphone is obtained by comparing the spectra of the waveforms resulting from optical and acoustical measurements. The optically measured pressure waveforms filtered with the microphone frequency response are in good agreement with the microphone output voltage. Therefore, an optical measurement method based on the Mach-Zehnder interferometer is a reliable tool to accurately characterize evolution of weak shock waves in air and to calibrate broadband acoustical microphones.

Time-resolved imaging of filamentary damage on the exit surface of fused silica induced by 1064 nm nanosecond laser pulse

Laser-induced damage on the exit surface of fused silica with a filament was observed. The filament has a central hollow core surrounded by molten materials and no obvious cracks could be observed. The critical intensity for the transition from pure surface damage (SD) to filamentary damage (FD) was measured. Time-resolved shadowgraphic microscopy with nanosecond time resolution was employed to compare the propagation of shock wave and material response in the SD and FD process. The main different features during the material response process include: (i) thermoelastic shock waves launched in FD were multiple and a column envelope was observed in the lateral direction; (ii) more energy is deposited in the bulk for FD resulting to a lower speed of shock wave in air; (iii) the overall time for establishing the main character of the damage site for FD was shorter because of the absence of crack expansion. Self-focusing and temperature-activated optical absorption enhancement of the bulk material are discussed to explain the morphology difference between SD and FD and the evolution of filament length under different incident intensities.

Self-ignition and explosion of a 13-MPa pressurized unsteady hydrogen jet under atmospheric conditions

The process of unsteady high-pressure hydrogen release observed on local vessel rupture is investigated experimentally. The flow structure during the formation of a supersonic free hydrogen jet is studied. The obtained data correlates well with the numerical simulation of sonic underexpanded hydrogen jet released into the atmosphere. In particular, it has been confirmed that the Mach disk fluctuates in the case of unsteady outflow of underexpanded hydrogen jet. It has been shown that in order to provide diffusion self-ignition, it is necessary to focus an air shock wave, formed after the vessel rupture, within the studied initial hydrogen tank pressure range. Lab and ground tests on self-ignition and explosion of hydrogen that outflows through a 15-mm opening when the flow interacts with a 37-mm-radius hemispherical cavity are described. A comparison of the diffusion combustion dynamics of releasing hydrogen in self-ignition and induced ignition scenarios is made. The observed visible flame front velocities of 150–220 m/s can result in dangerous blast waves in the case of large-scale hydrogen release. Measures to reduce the risk of explosion of unsteady high-pressure hydrogen jets releasing into an encumbered space are proposed.

Numerical simulation for propagation characteristics of shock wave and gas flow induced by outburst intensity

In order to analyze the propagation characteristics of shock wave and gas flow induced by outburst intensity, the governing equations of shock wave and gas flow propagation were put forward, and the numerical simulation boundary condition was obtained based on outburst characteristics. The propagation characteristics of shock wave and gas flow were simulated by Fluent software, and the simulation results were verified by experiments. The results show that air shock wave is formed due to air medium compressed by the transient high pressure gas which rapidly expands in the roadway; the shock wave and gas flow with high velocity are formed behind the shock wave front, which significantly decays due to limiting effect of the roadway wall. The attenuation degree is greater in the early stage than that in the late stage, and the velocity of gas convection transport is lower than the speed of the shock wave. The greater the outburst intensity is, the greater the pressure of the shock wave front is, and the higher the speed of the shock wave and gas flow is.

Numerical Simulations of Droplet Aerobreakup

The work presented in this thesis aims to bridge an existing gap in the state of droplet aerobreakup knowledge associated with the fundamental flow physics that govern the experimentally observable droplet morphologies. Using direct numerical simulations of the aerobreakup of water cylinders and droplets in the flow behind shock waves in air, we investigate the behavior of the surrounding gas flow to gain insight into the droplet’s deformation and evolution in the stripping breakup regime. The compressible multicomponent Navier-Stokes equations are solved using the Multicomponent Flow Code — a high-order accurate structured finite-volume flow solver with shock- and interface-capturing. Following qualitative descriptions of the aerobreakup process, comparisons are made with available experimental data. In 2D, accurate measurements of the cylinder’s center-of-mass acceleration across a range of incident shock Mach numbers allow characterization of the unsteady drag coefficient. Additionally, mass loss measurements from viscous simulations refute a well-known boundary layer stripping theory. The results of a 3D nonaxisymmetric aerobreakup simulation are presented with an emphasis on describing the intricate flow phenomena observable in the wake region. Subsequent analyses of the surface instabilities and a Fourier decomposition of the flow field reveal asymmetrical azimuthal modulations and broadband instability growth that result in the devolution of the wake region into chaotic flow.

Numerical Analysis of Rigid and Flexible Reflection of Air Shock Wave

To study the difference between the reflected pressures of air shock wave on the rigid and flexible walls, the FE models of RC anti-blast wall and high-strength polyethylene fiber textile anti-blast wall were set up and the FSI solver of LS-DYNA was used to perform the numerical simulation. The TNT charge weight used in simulation is 5kg, 10kg, 15kg and 20kg, respectively. The peak reflected overpressures of air shock wave on the two kinds of walls were obtained. The variation of reflection coefficient of air shock wave on the anti-blast wall with scaled explosion distance and the wall stiffness was analyzed.

On the influence of low initial pressure and detonation stochastic nature on Mach reflection of gaseous detonation waves

The two-dimensional, time-dependent and reactive Navier–Stokes equations were solved to obtain an insight into Mach reflection of gaseous detonation in a stoichiometric hydrogen-oxygen mixture diluted by 25 % argon. This mixture generates a mode-7 detonation wave under an initial pressure of 8.00 kPa. Chemical kinetics was simulated by an eight-species, forty-eight-reaction mechanism. It was found that a Mach reflection mode always occurs for a planar detonation wave or planar air shock wave sweeping over wedges with apex angles ranging from \(5^\circ \) to \(50^\circ \). However, for cellular detonation waves, regular reflection always occurs first, which then transforms into Mach reflection. This phenomenon is more evident for detonations ignited under low initial pressure. Low initial pressure may lead to a curved wave front, that determines the reflection mode. The stochastic nature of boundary shape and transition distance, during deflagration-to-detonation transition, leads to relative disorder of detonation cell location and cell shape. Consequently, when a detonation wave hits the wedge apex, there appears a stochastic variation of triple point origin and variation of the angle between the triple point trajectory and the wedge surface. As the wedge apex angle increases, the distance between the triple point trajectory origin and the wedge apex increases, and the angle between the triple point trajectory and the wedge surface decreases exponentially.

Influence of a Patch on the Deformation of an Elastic Plate with an Edge Notch Under the Action of a Shockwave

The behavior of a thin elastic plate with an edge notch under the action of a weak shock wave in air is studied experimentally. The effect of a patch placed either at the bottom or at the top of the notch on the deformation process is analyzed

Experiments and numerical simulations of plate gap model for high energetic materials

Experimental and numerical study for detonation propagation was conducted using the system in which the high energy materials and air gaps alternately stacked. The aim of this simulation is an extraction of information of the EOS for detonation product at arbitrary initial density from the numerical simulation with EOS at TMD (theoretical maximum density). In this report, we described the numerical procedure and an example of the calculation result. On the other hand, the experiment was designed for confirmation of the validity of our reactive flow simulation. The experimental system for this study consists of the pellet explosives and PMMA rings, PMMA pipe and booster explosive part. The pellets and the rings were alternately stacked in the PMMA pipe to make the system. The diameter of the pellet was 20 mm and the thicknesses were 10 or 5 mm. The thickness of the ring was varied to adjust the size of the air gaps between the pellets. The sample explosive was a composition A5 (RDX 98.8 wt%). The relationship between the bulk density which was estimated by the thicknesses of the pellets and the air gaps and the average detonation velocity was compared with the data for RDX. The slopes of those relationships differed mutually. Although the experimental results can be used for confirmation of the validity of the numerical procedure, it does not simulate the detonation wave in the powdered explosive. It may show the interesting process that consists of the shock wave in air, shock to detonation transition and steady detonation.

WAMR: An adaptive wavelet method for the simulation of compressible reacting flow. Part II. The parallel algorithm

The Wavelet Adaptive Multiresolution Representation (WAMR) algorithm is parallelized using a domain decomposition approach suitable to a wide range of distributed-memory parallel architectures. The method is applied to the solution of two unsteady, compressible, reactive flow problems and includes detailed diffusive transport and chemical kinetics models. The first problem is a cellular detonation in a hydrogen–oxygen–argon mixture. The second problem corresponds to the ignition and combustion of a hydrogen bubble by a shock wave in air. In both cases, results agree favorably with previous computational results.

Investigation of the shock wave propagation characteristics and cavitation effects of underwater explosion near boundaries

The shock wave propagation characteristics of underwater explosion have been of great interest to researchers. While the physical processes during an underwater explosion near boundaries are extremely complex, which involve lots of complex issues such as the explosion, shock wave propagation, water–air or water–structure interaction, etc. After the underwater detonation of an explosive, the shock wave may approach two main types of boundaries: the free surface of the water and the fluid–structure interface. The presence of these boundaries will significantly affect the wave propagation phenomena, and lead to bulk cavitation near the free surface or the structure surface. This paper deals with the behavior of shock wave propagation and cavitation from underwater explosion near different boundaries. A coupled numerical approach with combined Lagrangian and Eulerian methods is used to simulate the water–air interface and shock wave–structure interaction. A numerical model of free-field explosion in water is established, and the results have been compared with the published empirical formulas to verify the results of numerical solutions. The shock wave propagation characteristics from explosions in water near different boundaries are simulated and compared. In addition, the unsteady cavitations just near the free surface and the structure surface are described and captured. The water–air and water–structure interaction effects are examined. The results show that the free surface and structure surface boundaries have significant influence on the shock wave propagation characteristics.

Influence of the tunnel wall surface condition on the methane-air explosion

Based on numerical methods and theoretical analysis, the influence of the tunnel wall surface conditions on the methane-air explosion is evaluated. A rough tunnel wall causes stronger turbulence in the methane-air explosion. In a straight tunnel where some part of the space is filled with the methane-air mixture, the turbulence intensity varies with distance along the tunnel axis: it is higher in the methane-air premixing region and also in the far region of air shock wave propagation; between these regions, the turbulence intensity is lower. In the methane-air premixing region, the effect of turbulence is manifested as a significant increase in the explosion pressure. In the far region of air shock wave propagation, turbulence makes the shock wave strength decrease, but its effect is indistinctive among others. In the original methane-air premixing region, the explosion pressure of the methane-air mixture in a tunnel with rough walls is higher than that in a tunnel with smooth walls. However, the air shock wave beyond the premixing region in a tunnel with rough walls is weaker than that in a tunnel with smooth walls.

Effect of Explosion and Fire on Underground Structures

It appears that explosion in a closed space of underground works together with extreme increase of temperature influences the bearing capacity of underground structures, particularly road and rail tunnels, underground power stations, underground parkings, etc. The explosion is caused by various resources (terrorist attack, explosion of inflammable material positioned on freight cars, and such). This paper is focused on the effect of a fast change of load in tunnels and especially on the tunnel linings from fiber reinforced concrete. Shock wave caused by both the explosion and increased temperature leads to a spalling of concrete, i.e. to a reduction of thickness of the bearing structure and decrease of feasibility of the support. Relatively complicated system, describing an interaction between the air (shock wave described by gas dynamics), and the solid part (tunnel lining and the rock surrounding the tunnel obeying the equations of elasto-plasticity) is to be simplified in this paper. First, laminated cylinder describing the solid structure of the tunnel and the non-linear Winkler subsoil simulates the rock. The cylinder is loaded by the influence of explosion and change of temperature. It is to be shown that the superheated vapor together with interacting waves of the open charge inside of the underground space may cause an extensive occurrence of fissures on the surface exposed to the external loading envisaged. On the other hand the rock surrounding the underground structure interacts with the underground structure as a dumper. The simplified model starts with axisymmetric geometry and axisymmetric load by the shock wave and change of temperature, and moreover, generalized plain strain assumption is considered with a constant change of pressure in the axial direction of the tunnel. The governing equations adopt this assumption and the solution is then easier. The generalization to more complicated non-symmetric problems is straightforward. Couple of examples is to verify the theoretical considerations.

Interaction of the air shock waves with ground objects

This note considers the interaction and behavior of the shock wave on an element surface object (building, construction, equipment, apparatus and other items), and a cell reaction to this force. Since explosive impact in the general case - variable in time and space. Their feature is the complex nature of the interactions with the construction and development of stress - causing oscillating structures, so the work in assessing the impact of the shock wave on an element of the object (building, structure, equipment, apparatus and other items) to take into account the force resulting from the action shock, and cell reaction expressed as a deformation of its structure. Analyzes the dynamic load of the shock wave and the law of its change over time, depending on the location of the considered building, structure or a single subject design features of the item, its shape, size, strength characteristics, internal structure, as well as the parameters of the incident shock wave, and action loads flow, defined mainly by the maximum excess pressure in the shock wave, and braking loads generated by the action of the ram. There submitted formulas pressure reflection and propagation velocity of the shock wave changes graphic illustration of the shock wave pressure and reaction time airblast the object.

Modeling of Explosion Gas Dynamics with Account of Detonation

The physical and hydrodynamic processes in the initial phase of explosion of condensed explosives in the air have been considered. The role of the processes of energy release connected with the explosive detonation has been analyzed. The equations of formal kinetics for modeling the processes of transformation of the original substance into detonation products have been described. The results obtained with the use of the equation of state of an ideal gas with a constant adiabatic index have been compared with calculations, where for the equation of state wide-range tables of properties of the air and explosion products were used. The stage of detonation of an explosive is included in the self-consistent hydrodynamic model used for describing the explosion processes from the moment of initiation of the detonation wave to the moment the air shock wave is formed, as well as in describing its propagation and attenuation.

Destruction Analysis of Broadside Multi-Cabin Protective Structure of Warship under Explosion Loading

Shock wave parameters of cabins for shipboard defensive structure are studied based on shock wave theory. The destroy of defensive structure can be estimated by impulse of shock wave. In the process of air shock wave propagating, isentropic suction wave is reflected from void cabin into defense structure. The solution of shock wave attenuation of void cabin can be reached by using isentropic line to replace the shock adiabatic of the reflected shock. It can be seen from the example that the multi-layers defense structure system of warship is very important to decrease the damage from explosive shock wave. The method can be used to predict the extent of damage of naval vessel.

Numerical Simulation of Shock Wave in Air Based on FCT Method

Blast wave is numerical simulated based on FCT method. According to the comparative analysis, taking Henrych empirical formula as a standard, FCT method is more accuracy than Godunov method. Moreover, it has been found that the numerical accuracy is insufficient when the distance is small, it is necessary to develop and modify the numerical method continuously.

Analysis of the Movement Trajectory of Blasting Flyrock in the Bottom of Open Pits

The main hazards caused by blasting include vibration, flyrock, air shock wave, noise and dust. And the blasting flyrock is one of the dangerous factors to surrounding environment. Small and medium open pits are usually surrounded with residential districts, so controlling the casting distance of flyrock is very important. It is difficult to predict the casting direction and flying distance of blasting flyrock because of its more influence factors. Based on Sihe stope of Fangmayu Iron Mine, the paper analyses the causes, movement rule and casting direction of the bottom flyrock of open pit caused by bench blasting, calculates the casting distance of bottom flyrock, obtains the actual casting initial velocity and the biggest safe distance of blasting flyrock in Sihe stope. The study results can provide references for reducing the flyrock harzards of open pit caused by bench blasting.