Oncoming Shock Research Articles

Shock wave interaction with a polygonal bubble containing two different gases, a numerical investigation

The interaction between a planar shock wave propagating in air and a polygonal bubble (composed of two triangles) containing two different gases is studied numerically. Studying the interaction between an oncoming shock wave with front- and rear-facing triangles containing light and heavy gases is of great importance in understanding the complex shock wave propagation, interaction and hydrodynamic instabilities as well as their effect on mitigating or enhancing the colliding shock/blast wave. Two different cases were studied: in the first case, the front triangle contained sulfur hexafluoride (SF6) and the rear one contained helium (He); while in the second case, He is in the front and SF6 in the rear triangle. As the speed of sound in He is significantly higher than that in SF6 and in air, different flow fields were evolved. When SF6 is placed in the front triangle, the shock wave transmitted through the SF6 is reflected back from the interface separating the two gases and starts propagating downstream; over the He segment of the bubble, the incident shock wave (in the open air) is already seen over the He section and it submits compression waves into the He gas. These compression waves travel upstream and downstream; in their upstream movement they generate compression waves into the ambient air ahead of the incident shock wave. The part moving downstream will hit the interface separating SF6 and He, resulting in a complex wave pattern. A completely different wave pattern is visible when He is placed in the front triangle. Now the fastest shock is the transmitted shock wave in the He section; it reaches the membrane separating the two gases well before the incident shock wave reaches this location. Unlike the previous case, now the resulting flow in the rear triangle of the bubble is affected not only by the incident shock wave but also by the transmitted compression waves from the helium section. Furthermore, when helium is placed in the front section of the bubble, the compression waves in the He impacts the rear triangle of the bubble (containing SF6) almost like a planar shock wave. This is different from the previous case where SF6 was in the front section; then the shock wave impacting on the rear bubble containing He had a completely different shape due to its propagation into the SF6 bubble. This resulted in completely different peak pressures.

Experimental research into the influence of two­spark ignition on the deflagration to detonation transition process in a detonation tube

The paper reports a study into the initiation of detonation in pulse detonation engines. The chosen direction to resolve this issue is the use of detonation tubes with multifocal ignition. We applied two spark discharges as a source of ignition, which ignited synchronously. The spark discharges were ignited at a distance, which provided for the intensive gas-dynamic interaction between the discharges. The interaction implied a collision between shock waves generated by the spark discharges. As a result, the growth in temperature of gas was ensured in the region between spark intervals, due to a collision between the oncoming shock waves.The influence of dual spark ignition on the time and length of the section where deflagration transfers into detonation along a detonation tube was studied by comparing the transition parameters for cases of single-spark and dual spark ignition all other conditions for research being equal. The study involved a detonation tube with a length of 2.3 m and an inner diameter of 22 mm. Spark plugs were located at the closed end of the tube. We applied a stoichiometric mixture of propane with oxygen, diluted with nitrogen by 50 % at the initial pressure in the mixture equal to 50 kPa. To register the time of propagation of the flame front and to measure the process rate, the tube was equipped with 22 ionization sensors. Distance between the sources of ignition was 6 mm. Length of the discharge interval at each source of ignition was 2.5 mm. Sources of ignition in the form of spark plugs were connected to high-voltage units with a total discharge energy of 3.3 J.The results of our study helped establish a reduction in the distance of deflagration to detonation transition by 1.6…2 times, and in the time of transition ‒ from 3.9 ms to 1.2 ms for the case of the transition from single-spark to dual spark ignition.The results obtained could be used when designing ignition systems for pulse detonation engines.

Experimental and Numerical Investigation of Shock Wave Attenuation by Dynamic Barriers

An explosion at the entrance of an underground bunker and a suicide bomber inside an airplane are examples of scenarios in which blast waves propagate in tunnels and corridor-type structures. The need to attenuate the shock/blast wave propagating downstream a corridor and mitigate the developed loads inside the structure is essential. The interaction of a shock/blast wave with an obstacle inside a tunnel can dramatically reduce its strength. Earlier researches revealed that the dominant parameter in attenuating a shock wave by rigid barriers is the barrier opening ratio (i.e., the cross section that is open to the flow divided by the total cross section of the tunnel). Decreasing the opening ratio from 0.6 to 0.2 increased the attenuation by about 40%. Based on strong dependence of the attenuation on the opening ratio, a barrier designed to adjust its opening ratio to the loads exerted upon it is essential. In our previous study, we found that the effect of the rigid barrier geometry becomes more significant when the barrier inclination angle is larger, i.e., the barriers inclined toward the oncoming shock wave were found to be more effective in reducing the transmitted shock wave intensity than those inclined in the opposite direction. The pressure difference between both sides of the barrier exerts massive loads on the barrier. In the present ongoing research, based on a numerical approach using a commercial solver (msc.dytran), we focus on the geometry of a dynamic barrier, which changes its orientation as a response to the loads exerted on it. As a result, the barrier opening ratio, which as mentioned earlier strongly affects the shock wave attenuation, changes too. In this study, the feasibility of a dynamic barrier and the complex flow regime around it are investigated. The rapid pressure drop downstream of the barrier depends both on the shock wave strength and the barrier material and geometrical properties. Barriers with various geometries and properties are used to investigate the concept of a deflecting/rotating barrier as a response to the shock wave loads exerted upon it. For the first time, a new and exciting proven concept of a dynamic barrier, which reacts to the loads exerted upon it from a passing shock wave, and dramatically reduces the shock-induced pressure jump downstream of the barrier, is demonstrated.

Interaction between shocks and clumps with self-contained magnetic fields

Problems involving magnetized clouds and clumps, especially their interaction with shocks are common in astrophysical environments and have been a topic of research in the past decade. Many previous numerical studies have focused on the problem of clumps immersed in a globally uniform magnetic field subject to an oncoming shock. However, realistic clumps may have tangled magnetic field self-contained within them. This magnetic field will be compressed by the shock and its energy spectrum and spatial structure may affect the evolution of the clump during the shock encounter. Using our parallel MHD code AstroBEAR, we set up an initial state with magnetized clumps of different contained magnetic field configurations. We then drive strong shocks through these clumps (including the effects of radiative cooling) and compare our results to previous studies of global uniform field scenarios.

Polarization and Fragmentation of Solar Type II Radio Bursts

Solar type II radio bursts are the electromagnetic signatures of collisionless shocks propagating radially outward in the solar atmosphere. The Zurich radio spectrograph has detected a fragmented type II burst at high frequencies. The Trieste polarimeter found this event to contain a high degree of left-hand polarization. These observations indicate that (1) the fragmented type II event is probably emitted at the fundamental of the electron plasma frequency fpe as one of the magnetoionic modes, namely, the ordinary (O) mode, (2) the fragmentation is due to fluctuations in the efficiency of linear coupling of Langmuir waves with escaping radiation at sharp density gradients associated with the oncoming shock, and (3) the key role of backscattering Langmuir waves in the direction of the oncoming shock is probably played by one of the saturation mechanisms, such as induced scattering of Langmuir waves on thermal ions or the electrostatic decay instability.

Shock-induced mixing of nonhomogeneous density turbulent jets

An experimental study of the mixing enhancement and changes in flow structure arising from the interaction of weak normal shock waves with turbulent jets was conducted. The experimental configuration was an axisymmetric jet processed by weak normal shock waves propagating in a shock tube along the jet axis. Experiments involved three different jet gases: helium, air, and carbon dioxide, each in a coflowing air stream, with nominal jet fluid to ambient density ratios of 0.14, 1.00, and 1.52, respectively. The jet local Reynolds number was Reδ≈25 000 and the nominal oncoming shock Mach numbers were 1.23 and 1.45. Planar laser Mie light scattering from mineral oil smoke was utilized for flow visualization and for obtaining jet fluid concentration distributions across diametric planes of jets. Analysis of the spatial probability density function (pdf) of jet fluid concentration indicates that the average helium jet fluid concentration levels decrease and become more uniform in the regions processed by the shock waves. The degree of mixing enhancement increases with increasing shock strength, and amounts to nearly 30% for the stronger shock (M=1.45). The passage of a shock through low-density (helium) jets induces the formation of a flow structure that resembles a large-scale, toroidal vortex. The air and carbon dioxide jets exhibit neither a vortex-like structure or a significant change in mixing upon shock passage, unlike the helium jets. A comparison of the results for the helium and carbon dioxide jets indicates that the reversal of the density ratio between the jet and the surroundings, and the consequent change in the sign of baroclinic vorticity does not yield similar effects in terms of flow structure or mixing enhancement. The average concentration behind the shock wave decreases for both air and helium jets with increasing distance behind the shock. These features are explained qualitatively in terms of a simple characteristic time scale argument. The spatially averaged scalar dissipation calculated from the concentration data decreases for both the air and helium jets due to shock passage. The change is less marked for the air jet than for the helium jet for a given shock strength.

The interaction of quasiperpendicular shock waves in a collisionless plasma

Hybrid simulations with kinetic ions and massless fluid electrons are used to investigate the interaction of collisionless shocks. The interaction between pairs of sub- and supercritical shocks are studied for a wide range of parameters relevant to both the solar and interplanetary plasma. These parameters include the shock Mach number, the electron and ion plasma betas, and the angle between the shock normal and the magnetic field. In the collision of two subcritical quasiperpendicular shocks, the shock electromagnetic field structures pass through each other, but do not carry the plasma associated with them. When unequal shocks collide, there is no evidence of the contact discontinuity predicted by fluid theory. In the collision of supercritical quasiperpendicular shocks, significant acceleration of ions occurs. This acceleration arises when ions reflected at one shock encounter the electric fields of the other shock and takes place in a number of distinct phases. For a wide range of parameters, the maximum ion energy is roughly an order of magnitude larger than the ram kinetic energy of the colliding shocks. This ion energization will subsequently manifest itself as a high ion to electron temperature ratio in the doubly shocked plasma. Finally, it was found that the values of the Mach number and angle between the shock normal and magnetic field play a significant role in the collisions. For shock geometries significantly different from perpendicular, ions from each oncoming shock can escape upstream and undergo considerable acceleration.

Cardiac Responses in Aversive Situation With and Without Avoidance Possibility

ABSTRACTThis experiment is concerned with evaluating the cardiac responses of human Ss in an aversive situation. Two main conditions were investigated: Control of shock occurrence (Avoidance) and Passivity, that is, Ss were instructed to passively receive the shocks. Sixty Ss were used. The Avoidance condition was further divided into 3 specific groups. One group of Ss had to avoid the aversive stimulus by a proper task (depressing keys) without any certainty about the result of their action until the habitual moment of shock occurrence. Ss in a second group were provided immediate feedback from their action via a light bulb. A third group of Ss kept receiving shocks whatever they tried. After completion of 30 trials the conditions for each Ss were reversed by appropriate instructions and run for another 20 trials. A decelerative cardiac response was found in anticipation of shock for Ss in the Passivity condition. In the Avoidance condition, such a deceleration was found for trials during which Ss had had a feedback of their failure to avoid (second group) and were expecting an oncoming shock. Furthermore, only the Ss in Avoidance condition displayed HR acceleration after the signal onset of the ISI. Specific variations were found in the second phase of the experiment for the third group of Avoidance condition.

Sound-Shadow Effect in Impulse Noise

The sound-shadow effect of the human head in an impulse-noise field was studied by exposing 27 subjects to gunfire noise so their left ears were normal to the oncoming shock wave (near ear) and their right ears were protected by the shadow of the head (far ear). Noise exposure was continued until the subject's near ear demonstrated 15 dB TTS and the postexposure TTS in near and far ears was compared. Peak pressure level at the entrance of the far-ear canal was less than one-half that found at the near ear (153 vs 161 dB re 0.0002 μbar). Mean TTS was significantly smaller in the far ears than in the near ears. The mean “protection” afforded the far ear by the head's shadow ranged from 3 dB at 1 kHz to 12 dB at 6 kHz. The implications of the findings for the protection of weapon crewmen and others is discussed.

Measurements of the free-bound and free-free continua of nitrogen, oxygen and air

Measurements have been made of the equilibrium radiation behind incident and reflected shocks produced in an electric arc-driven shock tube over a temperature range of 8000–14000°K. Experiments are described which used a tungsten photoelectric gauge to make photometric measurements in the vacuum ultraviolet. This gauge was positioned at the axis of the shock tube and cored the oncoming shock wave. By the use of optical stops, this gauge was capable of resolving the shock front. A knowledge of the photoelectric yield of the tungsten provided a photometric measurement. The photoionization cross-section for the ground-state nitrogen atom was inferred from these measurements to be of the order of 10 -17 cm 2. Data were also obtained of the continua at 0·51 μ in nitrogen, oxygen and air which gave information on the component of the radiation produced by electrons being captured by ions and neutral atoms. The measurements agree well with free-bound calculations assuming a hydrogen-like model for N and O. Measurements were also obtained in the infrared at 6·1 μ in air over a range of shock conditions which infer an effective Z 2 g ( Z 2 times Gaunt factor) of 1·5 for the air-constituent free-free transitions.

An Improved Blast Pressure Gauge for Shock Measurements

The measurement of transient pressures associated with blast or shock waves propagated in gaseous media imposes stringent requirements on transducer designs that are not usually encountered in most electroacoustic devices. Reasonably accurate reproduction of the shock pressure transient requires a flat response from a fraction of a cps to 100 kcps or better. Pressure peaks encountered may be several hundreds or even thousands of psi. Field testing conditions are rugged and pyroelectric effects due to compressive heating of the shock front must be avoided. Superimposed on these difficulties are perturbations of the local pressure field due to interaction of the gage and its housing with the oncoming shock wave. Most of these difficulties can be overcome through the use of a “streamlined” housing, and through measuring the transverse or “side-on” pressure rather than “face-on” The electroacoustic design and aerodynamic shape of several gages have been studied, both in a shock tube, and in the field, using spherical pentolite charges. A gauge design based on the use of a barium titanate cylinder mounted in, but electrically and mechanically isolated from, a pencil-shaped, streamlined housing can reproduce a theoretical shock within about 2 percent. The sensitive element is sealed and pyroelectric response avoided by means of a bonded Neoprene sheath. Shadow photographs showing the interaction of a shock with several different gauge housings, as well as calibration data, are to be presented.