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The use of micro shock tubes has become common in many instruments requiring a high velocity and temperature flow field, for example in micro-propulsion systems and drug delivery devices for medical systems. A shock tube has closed ends, and the flow is generated by the rupture of a diaphragm separating a driver gas at high pressure from a driven gas at relatively low pressure. The rupture results in the movement of a shock wave and contact discontinuity into the low-pressure gas, and an expansion wave into the high pressure gas. The characteristics of the resulting unsteady flow for micro shock tubes are not well known as the physics of such tubes includes additional phenomena such as rarefaction and complex viscous effects at low Reynolds numbers. In the present study, computational fluid dynamics (CFD) calculations are made for unsteady compressible flow within a micro shock tube using the van-Leer MUSCL scheme and the two-layer k-ε turbulence model. Novel results have been obtained and discussed of the effects of using different diaphragm pressure ratios, shock tube diameters and wall boundary conditions, namely no slip and slip walls.

The present study aims to investigate the reflection and refraction of a curved shock front as it slides along an air–water interface, using the time-resolved shadowgraph technique. The curved shock front is generated from a free-piston shock tube. The study successfully captured the propagation of a refracted shock wave in water along with that of the reflected shock wave in the air. The refracted shock moves much faster than the incident shock due to a higher acoustic speed in the water. It is seen that the reflected shock initially exhibits a regular reflection (RR), which then transitions to a Mach reflection (MR) as it propagates along the interface. As the shock wave propagates along the air–water interface, the incident shock wave angle with the interface keeps on increasing, leading to RR–MR transition. Shock polar analysis shows that as the Mach reflection structure propagates further along the interface, it transitions from a standard Mach reflection to a non-standard Mach reflection. It is seen that the distance the shock wave propagates along the interface before it transitions from RR to MR increases with the increase in the interface distance (distance between the water surface and the shock tube axis). It is also found that the reflection surface (water or solid) does not seem to have a significant effect on the shock transition criterion, especially the distance at which the shock wave transitions from RR to MR.

Studying the nature of transient reflections of shock waves from surfaces is important in many engineering fields, e.g. blast protection, supersonic flights, shock focusing, medical and industrial applications and more. The recent advancements in this field reveal that the major obstacle in better understanding this phenomenon by means of experimental investigations is the limited temporal and spatial resolution. An alternative approach to commonly used high-speed photography is based on the use of a single-lens reflex (SLR) camera that captures only one image per experiment. Using this method to study a transient reflection process necessitates repeating each experiment many times while retaining extremely high repeatability. In the present study, we present a solution to this obstacle by means of a fully automated shock tube facility, which has been developed in the course of this study. A typical experiment can be executed a few hundred times with a repeatability of less than 0.01 in the incident-shock-wave Mach number at moderate shock strengths ( $M=1.2{-}1.4$ ). The system offers a very high spatial and temporal resolution description of the transient reflection process of a shock wave over a coupled convex–concave surface. The study of this complex configuration using a fully automated shock tube enables one to observe, in greater detail than ever before, both the transient transition from regular reflection, RR, to Mach reflection, MR, and the reverse transient transition from MR to RR. The geometry studied can also be found in blunt leading-edge reflectors in which higher pressures were recorded, and the results presented also describe in detail the shock reflection process inside such a reflector. The results highlight and strengthen the recent understanding of the importance of high spatial and temporal resolution in determining the transition process from RR to MR over a coupled concave–convex surface. However, despite achieving very high statistical certainty in the experimental measurements, the question of the difference between the pseudo-steady transition criterion and the experimental results remains unresolved.

We previously developed a novel needle-free reagent injection method based on electrically induced microbubbles. The system generates microbubbles and applies repetitive mechanical oscillation associated with microbubble dynamics to perforate tissue and introduce a reagent. In this paper, we propose improving the reagent injection depth by reflecting the shock wave through microbubble dynamics. Our results show that the developed shock wave reflection method improves the ability of the electrically induced microbubble injection system to introduce a reagent. The method extends the application potential of electrically induced microbubble needle-free injection.

The paper presents an experimental study on spatiotemporal dynamics of conical shock waves focusing in water. A multichannel pulsed electrohydraulic discharge source with a cylindrical ceramic-coated electrode was used. Time-resolved visualizations revealed that cylindrical pressure waves were focused to produce conical shock wave reflection over the axis of symmetry in water. Positive and negative pressures of 372 MPa and -17 MPa at the focus with 0.48 mm lateral and 22 mm axial extension ( -6 dB) were measured by a fiber-optic probe hydrophone. The results clearly show the propagation process leading to the high-intensity underwater shock wave. Such strong and sharp shock wave focusing offers better localization for extracorporeal lithotripsy or other non-invasive medical shock wave procedures.

In any supersonic intake, the flow decelerates from supersonic to subsonic speed through a constant or divergent channel \"isolator\" by a series of bifurcated compression shock waves referred to as a shock train. It is important to understand the characteristics of the shock train which occur inside the isolator to improve the performance of scramjet engines. In the present work, numerical simulations were carried out to investigate the characteristics of the shock train occurring in the divergent channels using coupled implicit Reynolds Averaged Navier-Stokes (RANS) equations along with the two-equation k-w SST turbulence model. Results show that the downstream pressure variation causes the shock train length to decrease and the shock structure phenomenon varies from Mach reflection to Regular reflection. The variation of the inlet Mach number has less influence on the shock train length and the location of the shock train is determined by the area ratio. In comparison with the constant area duct, the shock train structure phenomena varies from Mach reflection to regular reflection in the divergent channel. Also, the increase in divergent angle raises the total pressure loss.

The stabilization of oblique detonation waves (ODWs) in an engine combustor is important for the successful applications of oblique detonation engines, and comprehensively understanding the effects of the inviscid reflection of ODWs on their stabilization and the relevant mechanisms is imperative to overall combustor design. In this study, the flow fields of ODW reflections in a space-confined combustor are numerically studied by solving the two-dimensional time-dependent multispecies Euler equations in combination with a detailed hydrogen combustion mechanism. The inviscid Mach reflections of ODWs before an expansion corner are emphasized with different flight Mach numbers, Ma, and different dimensionless reflection locations, ζ ≥ 0 (ζ = 0: the ODW reflects precisely at the expansion corner; ζ > 0: the ODW reflects off the wall before the expansion corner). Two kinds of destabilization phenomena of the inviscid Mach reflection of an ODW induced by different mechanisms are found, namely wave-induced destabilization at large ζ > 0 for moderate (not very low) Ma and inherent destabilization at any ζ > 0 for low Ma. Wave-induced destabilization is attributed to the incompatibility between the pressure ratio across the Mach stem and its relative propagation speed, which is triggered by the action of the secondary reflected shock wave or the transmitted Mach stem on the subsonic zone behind the Mach stem. Inherent destabilization is demonstrated through an in-depth theoretical analysis and is attributed to geometric choking of the flow behind the Mach stem.

Shock refraction in a gas–liquid interface is ubiquitous in nature and engineering. This study investigates the shock refraction phenomena in air–water interfaces for various inclination angles. The interface inclination angles are achieved using a tiltable vertical shock tube. The time-resolved schlieren images are compared with numerical simulations performed using the BlastFoam solver in the OpenFOAM software. The stiffened gas equation of state is used to model water in the simulations. The shock polar analysis using modified shock relations for a stiffened gas is used to elucidate the refraction patterns. A regular refraction pattern with a reflected shock wave and a bound precursor refraction with a regular reflection are observed experimentally for the first time in an air–water system. Further, a new free precursor refraction pattern with a Mach reflection is observed. The transition criteria and the corresponding boundaries for each refraction pattern are demarcated in the ($M_S, \\theta _w^c$)-plane. The refraction sequence and the range for various incident shock strength regimes are also identified.

The reflection of rightward moving shocks (RMSs) belonging to the first and second families, over an initially steady oblique shock wave (SOSW) produced by a wedge, is studied in this paper. Various possible combinations of primary reflection (reflection at the intersection point of the RMS and the SOSW) and secondary reflection (reflection, on the wedge, of reflected shock waves of the primary reflection) are identified and the transition conditions are studied. For an RMS of the first family, the shock reflection problem can be shown to be equivalent to a shock interference problem. If the wedge angle is large, then the problem is equivalent to a shock interaction problem with two incident shock waves of the same family so that we have type VI, type V and type IV shock interferences. Interestingly, when the wedge angle is small enough, deflection angle reversal is observed for the SOSW so that the right part of the SOSW can no longer be regarded as one incident shock wave. It is now the left part of the SOSW that becomes one incident shock wave. As a result, for a small wedge angle, type I or type II shock interference is observed. If the RMS belongs to the second family, then the primary reflection may have regular and Mach reflections, and one reflected shock of this primary reflection reflects over the wall as another pseudo-steady shock reflection, while the other reflected shock wave may be smooth or have a kink or a triple point, as in single, transitional and double Mach reflection of pseudo-steady shock reflection.

Experiments are presented on the Richtmyer–Meshkov instability (RMI) with a three-dimensional, multi-mode initial perturbation. The experiments use a vertical shock tube, where a stably stratified interface is formed between air and sulphur hexafluoride (SF$_6$) via counterflow. A perturbation is imposed at the interface by vertical oscillation of the gas column, forming Faraday waves. The interface is accelerated by a Mach 1.17 (in air) shock wave, and the development of the mixing region between the gases is investigated using particle image velocimetry. Following shock acceleration, a reflected shock wave from the bottom of the shock tube interacts with the mixing layer a second time (reshock). The experiment is initialized with both high and low amplitude perturbations to examine the effect of the perturbation amplitude on measured quantities. The instability growth exponent ($\\theta$) is determined from the kinetic energy field using the width of the mixing layer and the decay of kinetic energy, which are found to be in agreement when the flow is most strongly excited. A growth exponent of $\\theta \\approx 0.5$ is found for all cases except the high-amplitude reshocked regime (where $\\theta \\approx 0.33$). High-amplitude experiments exhibit the transitional outer Reynolds number $(Re \\equiv {h \\dot {h}}/{\\nu } > 10^4)$ required for mixing transition following the incident shock, and both experiments are elevated well above this threshold following reshock. However, neither set of experiments meet the more stringent requirements proposed by Zhou et al. (Phys. Rev. E, vol. 67, issue 5, 2003) which include the time dependent aspect of the RMI, an observation which is also made when examining the spectra.