Shock Wave Transition Research Articles

Experimental study on the physical mechanism of pulse noise in the tail of a recoilless gun

During the firing process of a recoilless gun, there is a high-intensity pulse noise phenomenon in its rear breech area. In order to obtain the physical mechanism of the pulse noise formation, this article designed experiments and used high-speed cameras and overpressure sensors to discover a “three peaks” phenomenon in the pulse noise at the shooter’s position. Using resistance strain sensors, the changes of strain with time in the chamber and nozzle expansion section of the recoilless gun during the firing process were recorded. By comparing the micro-strain data and overpressure data, the shock waves at the shooter’s position can be categorized into two types: internal shock waves and external shock waves. Internally, the opening of the blocking sheet initiates the formation of the initial shock wave, followed by the formation of a second pressure wave within the combustion chamber. The peak-to-peak interval between these two shock waves is nearly fixed, and the ratio of the dual peaks increases with the distance of the measurement position. The third shock wave forms externally to the nozzle, induced by the explosion of unburned propellant. As the weapon’s distance from the ground increases, the peak values of the overpressure data decrease, and the waveform of this explosion-induced shock wave transitions from a single-peak phenomenon to a double-peak phenomenon. This study can provide a reference for the study of the potential risk of injury to operators using single soldier recoilless guns and similar weapons.

Experimental study on the heat flux of transitional shock wave–boundary layer interaction at Mach 6

The shock wave–boundary layer interaction (SWBLI) phenomenon was investigated experimentally to explore the heat flux distribution characteristics of SWBLIs under different boundary layer flow regimes and the influence/mechanism of different transition positions on peak heat flux of SWBLI. Experiments were conducted in a flow of Mach number 6. The shock generator deflected the flow by 20 °, 22 °, 25 ° and 30 ° resulting in an oblique shock impinging on a flat plate. The rough elements with different heights were arranged to achieve different transition positions. The influence of the relative position of transition and interference zone on the peak heat flux and its physical mechanism are revealed. The results demonstrated that the transitional SWBLI has a higher peak heat flux than the turbulent SWBLI, however the overshoot phenomenon is not reflected in the peak pressure. The increment of heat flux peak in transitional SWBLI is related to the relative positions of transition and interference zones. The increment of heat flux peak of transitional SWBLI compared to turbulent SWBLI can reach 25.5–38.9% and 51.9–65.1% when transition approaches the separation zone and reattachment zone, respectively. This overshoot phenomenon is caused by the streamwise vortex in transitional and so-called laminar SWBLIs, which enhances the energy exchange in the boundary layer and leads to an increase in heat flux. However, in the turbulent SWBLI, the energy mixing in the boundary layer is not significantly enhanced due to the breakup of the large scale streamwise vortex into turbulent small-scale structure.

The role of tangential velocity in explosive initiation by fragment impact

An initiation study is presented in which we examine the effect of the normal and the tangential velocity components of a fragment that impacts a covered and a bare Comp-B explosive. The study is based on calibrated numerical Lagrangian simulations using the LS-Dyna hydrocode. A special procedure was developed to present the estimated initiation point and the shock wave transition into detonation for various impact angles. The influence of the fragment strength and the steel cover thickness on the detonation threshold were also examined. The velocity threshold results are presented and used to modify the Jacobs-Roslund initiation model.

TNT equivalent of the shock wave energy generated during the supersonic flight of an aircraft

To assess the impact on human health of the sonic boom that occurs when an aircraft is flying at supersonic speed, and, accordingly, to solve the problem of noise reduction by optimizing the aircraft design, it is proposed to evaluate the shock wave energy using the TNT equivalent of a cylindrical explosion. An example of calculating the shock wave energy during flights of F4 and F18 aircraft at different altitudes is considered. To calculate the evolution of an acoustic pulse during its propagation from the boundary of the shock wave transition to the acoustic one, the wave equation and its solution are used, taking into account the inhomogenei-ty of the atmosphere, nonlinear effects, absorption and expansion of the wave front, as well as the results of ground-based measurements of acoustic pulses. The results of calculations of the dependence of the explosion energy on the flight altitude, as well as on the type of aircraft are explained on the basis of the formula for the atmospheric resistance force.

Propagation of a planar shock wave along a convex–concave ramp

The propagation of a normal shock wave along a coupled convex–concave surface of equal radii has been analysed experimentally and numerically in this study. The experimental and numerical studies were conducted using a similar geometry as of that used by Ram et al. (J. Fluid Mech., vol. 768, 2015, pp. 219–239) for studying the shock wave transition from regular reflection to Mach reflection. Many interesting flow features such as shock wave transitions over the ramp, characteristics of the induced flow behind the shock wave and the development of a stationary separation shock wave have been observed in the study. The numerical results are validated with experimental data. While the shock wave transitions over the ramp are found to depend mainly on the ramp geometry, the characteristics of the stationary shock wave and the flow separation in the concave region of the ramp surface have been found to vary with the shock wave Mach numbers.

Interaction of metastable shock waves with perturbations

The shock wave metastable behavior in equilibrium media is detected in numerical experiments. This phenomenon is directly related to the ambiguous representation of a shock wave discontinuity when the shock belongs to the S-shaped fragment of the pressure-velocity Hugoniot. After interaction with weak perturbations the metastable shock wave recovers its original form and parameters (like stable shock). If the amplitude of the perturbations exceeds some critical value, the shock wave instability is developed. The instability is accompanied by formation of the transverse secondary waves switching local post-shock parameters between two Hugoniot segments separated from each other by the absolute instability region. The problem of the metastable shock wave interaction with local entropy perturbation passing through the shock front is studied in the framework of three-dimensional problem formulation. The shock wave transition to the limiting self-oscillating mode is described. Some features of this mode are completely determined by the shape of the Hugoniot and isentropes and does not depend on the shock wave structure. Namely, the shape of the shock front corrugations and flow structure evolution are analyzed. These features may be served as marker of the corresponding thermodynamic anomalies.

Fundamental limits to the transfer of energy harvested from ferroelectric materials under shock loading

Ferroelectrics are capable of producing megawatt power levels under shock loading due to stress-induced phase transformations, resulting in depolarization of the ferroelectric materials. This power can be used for generation of high voltages, high currents, or ultrahigh-power electromagnetic radiation. The results are reported herein on an experimental study of limitations on energy harvested from shocked Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 and PbZr0.52Ti0.48O3 ferroelectrics and transferred to external electrical systems. The experimental results indicate that one of the limits to the energy transfer is electric breakdown that occurs within ferroelectric specimens during shock wave transit and depolarization, interrupting the energy transfer process and resulting in energy losses. It was revealed that the mechanism for breakdown in shocked ferroelectrics differs depending on the energy transfer time range, making a significant impact on the energy transfer process. High-speed photography and analysis of outputs for the two ferroelectrics indicate that for energy transfer times exceeding eight microseconds, the mechanical fragmentation of the ferroelectric material caused by the shock and resulting release waves following the shock wave front plays an important part in the breakdown process, while a thermal runaway dominates the breakdown at shorter energy transfer times. The heretofore disregarded mechanism of electric breakdown of the mechanically fragmented dielectric media is an unavoidable time-limiting factor for energy transfer from ferroelectrics under shock loading. The results obtained in this study are important for understanding the behavior of ferroelectrics during shock wave transit under high electric fields and for ultrahigh-power applications of ferroelectric materials.

Control of transitional shock wave boundary layer interaction using structurally constrained surface morphing

The potential of surface morphing techniques, including passive shock control bumps (SCB) and active surface morphing, is explored to control transitional shock wave boundary layer interactions (SWBLI). In addition to reducing the size of the separation bubble, a key objective is to mitigate the low-frequency unsteadiness that can cause detrimental structural response. To this end, three-dimensional flow simulations are performed using direct numerical simulations (DNS) at Mach 2 and Reynolds number based on inflow boundary layer thickness Reδin=996. An incident oblique shock of angle σ=35deg and strength p2/p1=1.4 impinges on a laminar boundary layer that evolves from a Blasius profile. The resulting boundary layer separation leads to transition and a Görtler-like instability is observed; the nominally two-dimensional and steady flow becomes three-dimensional and unsteady. The goal of this work is to develop a surface modification method to mitigate separation and low-frequency unsteadiness, which can trigger structural response and flow distortion. An aero-structural solver framework is developed and employed to examine both passive SCB and active surface morphing. To avoid unrealistic structural deformation in the transient or final states, the structural integrity is concurrently monitored so that the intermediate morphing solutions are restricted to achievable elastic deformations. The results indicate that transitional SWBLI can be controlled in this manner, to essentially inhibit transition and thus eliminate the separation and unsteadiness associated with the Görtler-like vortices. The mechanism modulates sharp increases in surface pressure at separation and shock-impingement locations encountered in uncontrolled SWBLI and results a lower specific entropy rise.

Dynamic resistivity of Pb(Zr0.95Ti0.05)O3 depolarized ferroelectric under shock wave compression

Explosive-driven ferroelectric generator (EDFEG) has important applications due to its excellent properties of high energy density and small volume. The output of EDFEG is based on the depolarization of ferroelectric during shock wave compression. In a normal mode configuration, a planar shock wave propagates in a direction perpendicular to the polarization axis. If the resulting depolarizing current passes through a large resistive load or a small capacitive load, high electric fields can be produced within the ferroelectric sample. In this case, a portion of the depolarizing charges are lost in the sample due to finite resistivity of shocked ferroelectrics during shock wave transit. But it is very difficult to accurately measure the resistivity of shocked ferroelectric during shock wave compression, due to high pressure and short duration time. In previous studies, the value of the resistivity of shocked Pb(Zr0.95Ti0.05)O3 (PZT95/5) ferroelectric was obtained from the experimental output charge difference for different large resistive loads or by fitting the experimental current histories. However, the current leakage was not observed directly in experiment in the past. Furthermore, the value of the resistivity obtained in each of all these studies was a time-averaged value. In the present work, a new experiment method is developed to investigate dynamic resistivity of PZT95/5 under shock wave compression, in which a pulse capacitor is used as an output load. The current leakage in shocked PZT95/5 is observed in the experiment at a shock stress of 3.5 GPa after the depolarization of all ferroelectrics. This current leakage is just related to the resistance of shocked PZT95/5 and the voltage applied. The experimental results show that the resistivity of shocked PZT95/5 continuously changes in a range of 2.2104 cm-3.5104 cm for time more than the shock transit time of the sample. Based on the experimental results, a dynamic resistance model is established to analyze the resistivity of depolarized PZT95/5 ferroelectric ceramic during shock wave transit in ferroelectric. The simulation results reveal dynamic characteristic of the resistivity of depolarized PZT95/5 ferroelectric ceramic under shock wave compression. The further analysis of experimental results shows that the resistivity continuously changes between 2.0104 cm and 8.0104 cm during shock transit in ferroelectrics. It is believed that dynamic characteristic of the resistivity of shocked PZT95/5 ferroelectric ceramic is related to pressure, electrical field applied and the defects in the material. The dynamic resistivity of shocked PZT95/5 obtained in this paper and its dynamic resistance model will be helpful for designing EDFEGs and their applications in the future.

Experimental investigation on dynamic lattice response by in-situ Xray diffraction method

Structure evolution under dynamic compression condition (high temperature, high pressure and high strain rate) is one of the most important problems in engineering and applied physics, which is vital for understanding the kinetic mechanism of shock-induced phase transition. In this work, an in-situ dynamic X-ray diffraction (DXRD) diagnostic method is established to probe the lattice response driven by shock waves. The geometry is suitable for the study of laser-shocked crystals. In order to eliminate the measurement error arising from the difference in experimental setup, the static and dynamic lattice diffraction signals are measured simultaneously in one shot by using a nanosecond burst of X-ray emitted from a laser-produced plasma. Experimental details in our investigation are as follows. 1) The laser driven shock wave transit time △ tShock and the shock pressure in sample are accurately determined from the shock-wave profile measurement by dual laser heterodyne velocimetry. 2) A laser pump-and-probe technique for adjusting the time-delay of DXRD diagnosis during △ tShock, with a series of repeated shock loadings is then employed to generate and measure the dynamic structure evolution. Using this method, the dynamic lattice response of[111] single-crystal iron is studied on Shenguang-Ⅱ facility. Single-shot diffraction patterns from both unshocked and shocked crystal are successfully obtained. An elastic-plastic transition process –elastic wave followed by a plastic wave– is observed in shocked[111] single-crystal iron on a lattice scale. The lattice compressibility values of the elastic wave and plastic wave are in agreement with those derived from the wave profiles. It is found that the Hugoniot elastic limit is measured to be about 6 GPa under nanosecond-pulsed laser shock compression. Such a high yield strength is consistent with recent laser ramp compression experimental results in polycrystalline Fe[Smith et al. 2011 J. Appl. Phys. 110 123515], suggesting that the peak pressure of elastic wave is dependent on the loading rate and the thickness of sample. Based on the analysis of diffraction patterns, the BCC phase is determined to be stable till 23.9 GPa, the highest pressure explored in this work, which might indicate that the phase transition strongly couples with the crystal orientation and loading rate. Some possible physical mechanisms remain to be further studied:whether the transition time hysteresis occurs or the metastable FCC phase exists in shocked[111] single crystal Fe, or the phase transition onset pressure increases under high strain-rate compression. Our DXRD results provide a primary experimental reference for the follow-up study on the phase kinetics.

Implicit CFD methods for transitional shock wave – boundary layer interaction

Purpose This paper aims to explore the potential of transition prediction methods for modelling transitional shock wave/boundary layer interactions. The study is fuelled by the strong interest of researchers and airframe manufacturers in reducing the drag of vehicles flying at transonic speeds. The principle of drag reduction via flow laminarity is valid, provided there is no need for the flow to sustain large pressure gradients or shocks. This is true, as laminar boundary layers are less resistant to flow separation. Design/methodology/approach It is, therefore, worthwhile to assess the performance of CFD methods in modelling laminar boundary layers that can be tripped to turbulent just before an interaction with a shock. In this work, the CFD solver of Liverpool University is used. The method is strongly implicit, and, for this reason, the implementation of intermittency-based models requires special attention. The Navier–Stokes equations, the transport equations of the kinetic energy of turbulence and the turbulent frequency are inverted at the same time as the transport equations for the flow intermittency and the momentum thickness Reynolds number. Findings The result is stable and robust convergence even for complex three-dimensional flow cases. The method is demonstrated for the flow around the V2C section of the TFAST EU, F7 project. The results suggest that the intermittency-based model captures the fundamental physics of the interaction, but verification and validation are needed to ensure that accurate results can be obtained. For this reason, comparisons with the TFAST experiments is put forward as a means of establishing confidence in the transition prediction tools used for shock/boundary layer interaction simulation. Research limitations/implications At the moment, experimental data for transonic transitional buffet are not yet available, although this will change in the near future. Practical implications The required CPU time is neither insignificant not prohibitive for routine computations. Social implications Reducing aircraft drag without compromising on stall characteristics will result in lower fuel consumption and contribute to a greener and more economic flight for passengers. Originality/value To the authors’ knowledge, this is the first time that transitional buffet has been addressed.

High-resolution PIV measurements of a transitional shock wave–boundary layer interaction

This study investigates the effects of boundary layer transition on an oblique shock wave reflection. The Mach number was 1.7, the unit Reynolds number was 35 × 106 m?1, and the pressure ratio over the interaction was 1.35. Particle image velocimetry is used as the main flow diagnostics tool, supported by oil-flow and Schlieren visualizations. At these conditions, the thickness of the laminar boundary layer is only 0.2 mm, and seeding proved to be problematic as practically no seeding was recorded in the lower 40 % of the boundary layer. The top 60 % could, however, still be resolved with good accuracy and is found to be in good agreement with the compressible Blasius solution. Due to the effects of turbulent mixing, the near-wall seeding deficiency disappears when the boundary layer transitions to a turbulent state. This allowed the seeding distribution to be used as an indicator for the state of the boundary layer, permitting to obtain an approximate intermittency distribution for the boundary layer transition region. This knowledge was then used for positioning the oblique shock wave in the laminar, transitional (50 % intermittency) or turbulent region of the boundary layer. Separation is only recorded for the laminar and transitional interactions. For the laminar interaction, a large separation bubble is found, with a streamwise length of 96 ??i,0 . The incoming boundary layer is lifted over the separation bubble and remains in a laminar state up to the impingement point of the shock wave. After the shock, transition starts and a turbulent profile is reached approximately 80–90 ??i,0 downstream of the shock. Under the same shock conditions, the transitional interaction displays a smaller separation bubble (43 ??i,0 ), and transition is found to be accelerated over the separation bubble.

Richtmyer–Meshkov instability for elastic–plastic solids in converging geometries

We present a detailed study of the interface instability that develops at the boundary between a shell of elastic–plastic material and a cylindrical core of confined gas during the inbound implosive motion generated by a shock-wave. The main instability in this configuration is the so-called Richtmyer–Meshkov instability that arises when the shock wave crosses the material interface. Secondary instabilities, such as Rayleigh–Taylor, due to the acceleration of the interface, and Kelvin–Helmholtz, due to slip between solid and fluid, arise as the motion progresses. The reflection of the shock wave at the axis and its second interaction with the material interface as the shock moves outbound, commonly known as re-shock, results in a second Richtmyer–Meshkov instability that potentially increases the growth rate of interface perturbations, resulting in the formation of a mixing zone typical of fluid–fluid configurations and the loss of the initial perturbation length scales. The study of this problem is of interest for achieving stable inertial confinement fusion reactions but its complexity and the material conditions produced by the implosion close to the axis prove to be challenging for both experimental and numerical approaches. In this paper, we attempt to circumvent some of the difficulties associated with a classical numerical treatment of this problem, such as element inversion in Lagrangian methods or failure to maintain the relationship between the determinant of the deformation tensor and the density in Eulerian approaches, and to provide a description of the different events that occur during the motion of the interface. For this purpose, a multi-material numerical solver for evolving in time the equations of motion for solid and fluid media in an Eulerian formalism has been implemented in a Cartesian grid. Equations of state are derived using thermodynamically consistent hyperelastic relations between internal energy and stresses. The resolution required for capturing the state of solid and fluid materials close to the origin is achieved by making use of adaptive mesh refinement techniques. Rigid-body rotations contained in the deformation tensor have been shown to have a negative effect on the accuracy of the method in extreme compression conditions and are removed by transforming the deformation tensor into a stretch tensor at each time step. With this methodology, the evolution of the interface can be tracked up to a point at which numerical convergence cannot be achieved due to the inception of numerical Kelvin–Helmholtz instabilities caused by slip between materials. From that point, only qualitative conclusions can be extracted from this analysis. The influence of different geometrical parameters, initial conditions, and material properties on the motion of the interface are investigated. Some major differences are found with respect to the better understood fluid–fluid case. For example, increasing the wave number of the interface perturbations leads to a second phase reversal of the interface (i.e., the first phase reversal of the interface naturally occurs due to the initial negative growth-rate of the instability as the shock wave transitions from the high-density material to the low-density one). This phenomenon is caused by the compressive effect of the converging geometry and the low density of the gas with respect to the solid, which allows for the formation of an incipient spike in the center of an already existing bubble. Multiple solid–gas density ratios are also considered. Results show that the motion of the interface asymptotically converges to the solid–vacuum case. When a higher initial density for the gas is considered, the growth rate of interface perturbations decreases and, in some situations, its sign may reverse, as the fluid becomes more dense than the solid due to having higher compressibility. Finally, the influence of the Mach number of the driving shock and the yield stress on the mixing-zone is examined. We find that the width of the mixing zone produced after the re-shock increases in proportion to the strength of the incident shock. An increased yield stress in the solid material makes the interface less unstable due to vorticity being carried away from the interface by shear waves and limits the generation of smaller length scales after the re-shock.

High-velocity shock compression of SiC via molecular dynamics simulation

Molecular dynamics (MD) simulation on high-velocity shock compression of cubic SiC was performed using a Tersoff potential. With particle velocities in a wide range from 0.1km/s to 20km/s, the propagations of shock waves were described in detail. The evolution processes from elastic wave to phase transition wave were revealed. It was found that the shock compression of cubic SiC shows four distinct regimes. With particle velocities below 2km/s or above 16km/s, only a single elastic shock wave or single phase transition shock wave generates. With particle velocities in the narrow range from 2km/s to 3km/s, local plastic deformation is shown in SiC materials. When particle velocities are between 3km/s and 16km/s, a well-defined two-wave structure generates, consisting of an elastic wave followed by a phase transition wave. The phenomenon of splitting shock waves is similar with the separate silicon and diamond systems. The results showed that the critical pressure for the generation of phase transition wave is 140GPa. The analysis of the simulation results showed that the theory of stress waves in macroscopic systems is suitable for microscopic systems.

Computational analysis of re-ignition and re-initiation mechanisms of quenched detonation waves behind a backward facing step

Re-ignition and re-initiation mechanisms of quenched detonation waves passing over a backward facing step are investigated using a high-resolution discontinuous Galerkin method, in which the reaction chemistry is described using a detailed chemical model. A stoichiometric Hydrogen/Oxygen mixture with different diluents (Argon and Nitrogen) is considered to assess effects of the mixture composition and reactivity on the initiation process. It is found that the mixture reactivity and the spatio-temporal evolution of the diffracted detonation wave are critical in controlling the development of the ignition kernels, and the possible transition to detonation. Due to curvature effects and the increasing shock angle, the incident shock wave transitions from a regular reflection to a Mach reflection. Through parametric studies, it is shown that for more reactive Argon-diluted mixtures, ignition first appears behind the regular shock reflection, which is followed by spontaneous re-initiation through the SWACER (Shock Wave Amplification by Coherent Energy Release) mechanism. By replacing Argon with Nitrogen, the reactivity reduces and re-ignition through Mach reflection is observed. The subsequent re-initiation is primarily controlled by flame-acoustic and wave-wave interactions. Hot-spot re-initiation behind the Mach stem is not found due to the short ignition window that exists over the limited range of shock angles. A theoretical model is developed to confirm these findings and to quantify the coupling among the incident shock angle, the thermodynamic state of the shock-compressed mixture, and the ignition delay. Effects of viscous-diffusive transport are also assessed, and it is shown that the viscous dissipation at the wall promotes ignition and transition to re-initiation.

Dynamic transition from Mach to regular reflection of shock waves in a steady flow

AbstractThe steady, two-dimensional transition criteria between regular and Mach reflection are well established. Little has been done on the dynamic effect on transition due to a rapidly rotating wedge. Results from experiments and computations done on steady and unsteady shock wave transition from Mach reflection to regular reflection, MR $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\rightarrow $ RR, are described. The measured motion and the initial shock incidence was used to mimic the experiment with a two-dimensional numerical code. The maximum rotation speed achieved at transition was approximately $2500^\circ \ {\mathrm{s}}^{-1}$. Rapid wedge rotation was shown to have a significant measurable effect on transition. The code was also applied to the dependence of dynamic MR $\rightarrow $ RR transition on other variables in the parameter space. These include rotation about the leading or trailing edge, initial incidence and rotation speed at two free-stream conditions. Impulsively started rotation in these cases was used with the rotation specified by $M_E = \omega c/a_{\infty }$ where $\omega $ is constant angular velocity (negative anticlockwise), $c$ the distance from the edge considered to the pivot point and $a_{\infty }$ the free-stream sound speed. For the Mach numbers and range of rotation speeds tested, both the wedge and shock angle at transition decreased with increased rotation speed. The sensitivity of the transition angle to changing the rotation point from the trailing edge to the experimental model pivot point was investigated briefly at a free-stream Mach number of $M = 2.98$ with $M_E = -0.1$. The wedge angle at transition increased by 1.5° and the shock angle at transition decreased by 1.5°, a significant variation. The effect of the initial incidence was also investigated. By reducing the initial wedge angle from 24.5 to 23.5° for these initial conditions the shock angle at transition decreased by approximately 1.8°, also a marked sensitivity. The flow field development for impulsive rotation about the wedge trailing and leading edges at $M = 1.93$ for $M_E = -0.075$ was analysed in some detail. The flow field development is very sensitive to the rotation centre, more especially at large rotation rates. Four phases of the Mach stem development were identified in both cases. For rotation about the wedge leading edge the Mach stem height remains constant until the expansion waves arrive at the triple point. This is followed by an increase in Mach stem height, which then remains constant for a short period after which it decreases until transition to RR. For rotation about the wedge trailing edge the impulsive start generates a disturbance on the incident wave which propagates down the wave, through the triple point and down the Mach stem. The stem height is constant until the arrival of the incident wave disturbance. This causes a sudden decrease in Mach stem height. Subsequently, the Mach stem height remains constant for a short time, before it decreases until transition to RR. Similar effects in the variation of stem height with wedge angle occur at the higher Mach number of 2.98. It was demonstrated that MR can be maintained for a while at zero wedge incidence with a sufficiently large rotation rate of $M_E = -0.1$, with $M=1.93$, for both leading and trailing edge pivot points.

Experiments on transitional shock wave--boundary layer interactions at Mach 5

An experimental campaign was carried out to investigate transitional shock wave--boundary layer interactions (SWBLI) at Mach and unit Reynolds numbers of 5 and 15.9 × 106 1/m, respectively. An impinging shock that generates 7° flow deflection resulted in separated SWBLI flowfield on axisymmetric centrebody. Various flow diagnostics were utilised such as schlieren photography, quantitative infrared thermography, shear sensitive liquid crystals, pressure sensitive paints and particle image velocimetry (PIV) to provide a complete time-averaged experimental data set. One nominally laminar case (with triggered transition due to SWBLI) and four natural transition cases with varying intermittency were tested. Heat transfer and shear stress peaks occurred around the reattachment point. For nominally laminar case, the separation induces transition, and thus, heat transfer and pressure peaks were found to be the highest. For the cases with natural transition with different intermittency levels, where incoming boundary layer is in state of transition, the magnitude of pressure and heat transfer peaks initially started to increase reaching a maximum and afterwards decreased towards the highest intermittency case. The presence of streamwise vortices was apparent for laminar case. Pressure peaks were found to occur slightly downstream of heat flux/shear stress peaks. PIV results (for laminar case only) showed high levels of turbulence above the separation region, proving triggered transition behaviour.

Experimental study on hysteresis phenomena of shock wave structure in an over-expanded axisymmetric jet

In recent studies on two-dimensional supersonic jets, it is reported that the hysteresis phenomenon for the reflection type of shock waves in the jet flow field is occurred under the quasi-steady flow condition and this phenomenon is affected by the transitional pressure ratio between the regular reflection and Mach reflection. However, so far, there are few researches on the hysteresis phenomenon for the transition of shock waves between regular and Mach reflection in over-expanded supersonic jets and the phenomenon has not been investigated satisfactorily. Therefore, the purpose of this study is to clarify the hysteresis phenomena for the reflection type of shock wave in the over-expanded axi-symmetric supersonic jet experimentally, and to discuss the relationship between hysteresis phenomenon and rate of the change of pressure ratio with time. Furthermore, the effect of Mach number at the nozzle exit on hysteresis loop was investigated for two kinds of nozzle.

Numerical investigation on the shock wave transition in a three-dimensional scramjet isolator

The scramjet isolator, which is used to prevent the hypersonic inlet from disturbances that arise from the pressure rise in the scramjet combustor due to the intense turbulent combustion, is one of the most critical components in hypersonic airbreathing propulsion systems. Any engineering error that is possible in the design and manufacturing procedure of the experimental model, and the intense heat release in the scramjet combustor, may cause the performance of the isolator to decrease, leading to its lack of capability in supporting the back pressure. The coupled implicit Reynolds Averaged Navier–Stokes (RANS) equations and the two-equation standard k− ε turbulent model have been employed to numerically simulate the flow fields in a three-dimensional scramjet isolator. The effects of the divergent angle and the back pressure on the shock wave transition and the location of the leading edge of the shock wave train have been estimated and discussed. The obtained results show that the present numerical results are in very good agreement with the available experimental shadow-pictures, and the numerical method is more suitable for capturing the shock wave train and predicting the location of the leading edge of the shock wave train in the scramjet isolator than the present two-dimensional numerical methods. This is due to the small width-to-height ratio of the isolator and the intense three-dimensional flow structures. On increasing the divergent angle of the scramjet isolator, the static pressure along the central symmetrical line of the isolator decreases sharply. This is due to the strong expansion wave generated at the entrance of the isolator, and when the divergent angle of the isolator is sufficiently large, namely 1.5°, a zone of negative pressure is formed just ahead of the leading edge of the shock wave train. At the same time, the shock wave train varies from being oblique to being normal, and then back to oblique. With an increase in the prescribed back pressure at the exit of the scramjet isolator, the leading edge of the shock wave train moves forward towards the entrance of the isolator, and when the back pressure is sufficiently large, unstart conditions in the hypersonic inlet can take place if the shock train reaches the inlet.

Detonation wave initiated by explosive condensation of supersaturated carbon vapor

An experimental study of the influence of condensation of supersaturated carbon vapor formed behind reflected shock waves on the process of propagation of a shock wave and formation of a detonation wave of condensation is carried out. Highly supersaturated carbon vapor was formed from thermal decay of unstable carbon suboxide C3O2 → C + 2CO behind a shock wave in mixtures containing 10–30% C3O2 in Ar. This reaction was followed by fast growth of condensed carbon particles, accompanied by heat release. Experiments have shown a considerable temperature and pressure increase in the narrow zone behind the wave front, resulting in shock wave amplification and transition to a detonation-like regime. An analysis of the kinetics and heat release in the given conditions and calculations based upon one-dimensional detonation theory have shown that in a mixture of 10% C3O2 + Ar, insufficient heat release resulted in a regime of “overdriven detonation”. In a mixture of 20% C3O2 + Ar a very good coincidence of measured values of pressure and wave velocity with calculated Chapman–Jouguet parameters is observed. In a 30% C3O2 + Ar mixture, an excess heat release caused a slow down of the effective condensation rate and a regime of “underdriven detonation” is observed.