Shock Interaction Research Articles

Numerical investigation on influence of different geometrical cavity flame holders and flame stabilization mechanism in hydrogen fueled multi-strut-based scramjet combustor

A detailed computational study was carried out to explore how the presence of a cavity and cavity angles influence the performance and combustion mechanisms within a scramjet combustor. This research utilized ANSYS Fluent 2021 to create hexahedral meshes and simulate the complex fluid dynamics in scramjet engines equipped with dual strut injectors. The simulations addressed various critical aspects of fluid mechanics, including the continuity, momentum (via the Navier-Stokes equations), and energy equations. These equations were used to model non-reacting flows, while the species transport equations were applied to analyse reacting flows. The turbulence within the combustor was modelled using the standard K-ε model, a common choice for such high-speed aerodynamic evaluations. The study's primary goal was to assess the effects of different geometrical configurations of cavity flame holders on the performance characteristics of an H2-based scramjet combustor overall efficiency, focusing on mixing efficiency, combustion performance, and total pressure loss. Both configurations with and without a cavity were examined. Key findings indicate that incorporating a cavity into the combustor design leads to developing a robust recirculation zone within the cavity area. This recirculation zone is pivotal in enhancing fuel-air mixing and combustion efficiency, with cavity-based combustors showing an earlier onset of combustion and achieving peak combustion efficiencies around 90–95%. The extent of the recirculation region is notably influenced by the proximity of the strut injector to the cavity's length. Additionally, the presence of a shear layer within the cavity generates a weak shock at the cavity's trailing edge. This shock interaction can adversely affect scramjet combustor performance, especially at higher cavity angles (α) and specific geometric configurations, such as an L/D ratio of 4 and α=30°. The L/D ratio of 4 and α=30° combustor model results in a 13.8% increase in turbulence intensity, which raises air-fuel mixing and there by improves mixing and combustion efficiency by 14.23% and 13.34%, compared to the other depth of the cavities. This advantage is critical, especially considering the compact length of the combustor, which is a desirable attribute in scramjet design.

Quasi-monoenergetic ion acceleration and neutron generation from laser-driven transverse collisionless shocks

Experiments using the OMEGA EP laser system were performed to study collisionless shock acceleration of ions driven by the interaction of a relativistically intense laser pulse with underdense plasma. The energy spectrum of accelerated ions in the direction transverse to laser propagation is measured to have several narrow-band peaks which are quasi-monoenergetic with a typical energy bandwidth of 3%. In deuterium plasmas, these ions generate a significant number of fast fusion neutrons. Particle-in-cell simulations confirm that these ions were accelerated by the interaction of transverse shocks and that the appearance of quasi-monoenergetic spectral features depends on the growth of an ion-electron two-stream instability during the interaction.

Explosion risks: Variety of deflagration-to-detonation transition scenarios in smooth tubes

In the framework of comprehensive assessment of explosion risks on board of spacecrafts and on the facilities of launch places, the paper is focused on the detailed analysis of particular scenarios of deflagration-to-detonation transition taking place in smooth tubes filled with acetylene-oxygen mixtures of different compositions. By means of precise numerical simulation it is demonstrated that various scenarios of detonation onset can take place depending on the mixture composition and its initial thermodynamic state. It is demonstrated that independent on the particular scenario always the basic mechanism of detonation onset via the formation of strong enough shock wave takes place. In more reactive mixtures the strong shock originates from the self-sustained process of joint pressure build up and reaction intensification exactly at the flame front. In less reactive mixtures the transient flow behavior leads to the shock waves generation and interaction. As a result, a brand new reaction kernel could arise in the area of shock waves interaction. In number of cases, that leads to the coupling between the shock wave and the newborn reaction front and results in the strong shock formation and further detonation onset.

Comprehensive Evaluation of the Massively Parallel Direct Simulation Monte Carlo Kernel “Stochastic Parallel Rarefied-Gas Time-Accurate Analyzer” in Rarefied Hypersonic Flows—Part A: Fundamentals

The Direct Simulation Monte Carlo (DSMC) method, introduced by Graeme Bird over five decades ago, has become a crucial statistical particle-based technique for simulating low-density gas flows. Its widespread acceptance stems from rigorous validation against experimental data. This study focuses on four validation test cases known for their complex shock–boundary and shock–shock interactions: (a) a flat plate in hypersonic flow, (b) a Mach 20.2 flow over a 70-degree interplanetary probe, (c) a hypersonic flow around a flared cylinder, and (d) a hypersonic flow around a biconic. Part A of this paper covers the first two cases, while Part B will discuss the remaining cases. These scenarios have been extensively used by researchers to validate prominent parallel DSMC solvers, due to the challenging nature of the flow features involved. The validation requires meticulous selection of simulation parameters, including particle count, grid density, and time steps. This work evaluates the SPARTA (Stochastic Parallel Rarefied-gas Time-Accurate Analyzer) kernel’s accuracy against these test cases, highlighting its parallel processing capability via domain decomposition and MPI communication. This method promises substantial improvements in computational efficiency and accuracy for complex hypersonic vehicle simulations.

Role of different cavity flame holders on the performance characteristics of supersonic combustor

Abstract The study’s primary goal was to assess the effects of different geometrical configurations of cavity flame holders on the performance characteristics of an H2-based scramjet combustor overall efficiency, focusing on mixing efficiency, combustion performance, and total pressure loss. Key findings indicate that incorporating a cavity into the combustor design leads to developing a robust recirculation zone within the cavity area. This recirculation zone is pivotal in enhancing fuel-air mixing and combustion efficiency, with cavity-based combustors showing an earlier onset of combustion and achieving peak combustion efficiencies around 90–95 %. The extent of the recirculation region is notably influenced by the proximity of the strut injector to the cavity’s length. This shock interaction can adversely affect scramjet combustor performance, especially at higher cavity angles (α) and specific geometric configurations, such as an L/D ratio of 4 and α = 30°. This advantage is critical, especially considering the compact length of the combustor, which is a desirable attribute in scramjet design.

Microphysics of shock-grain interaction for inertial confinement fusion ablators in a fluid approach.

Ablator materials used for inertial confinement fusion, such as high-density carbon (HDC) and beryllium, have grain structure which may lead to small-scale density nonuniformity and the generation of perturbations when the materials are shocked and compressed. Here, we use a combination of a linear theory of shock interaction with density nonuniformity [Velikovich et al., Phys. Plasmas 14, 072706 (2007)10.1063/1.2745809] and numerical simulations to study shock interaction with a model representation of HDC grains. While the shock-grain interaction is nonlinear, the linear theory shows some key features of the shock-grain interaction, which also hold for the (nonlinear) simulations. The postshock perturbations are made up of sonic reflections off of grain boundaries and vorticity deposition along them, with the latter dominating the perturbed energy content. The mean (per mass) postshock perturbed kinetic energy decreases with increasing grain size, but energy will be deposited at increasing spatial scale. From the perspective of the postshock perturbed energy, the detailed linear theory largely supports a proposed method [S. Davidovits et al., Phys. Plasmas 29, 112708 (2022)1070-664X10.1063/5.0107534] for deresolving the grains (in a similar grains model) that treats the grains statistically. Our simulation results highlight the influence of thermal conduction on the perturbation dynamics at grain scales.

Stereoscopic cells of three-dimensional detonation waves propagating in square ducts

The present study delves into the examination of the stereoscopic cells and wavefront structures characterizing the propagation of three-dimensional detonation waves within square ducts. Leveraging numerical solutions derived from three-dimensional reactive Euler equations, incorporating an induction-exothermic reaction kinetic model, this work reveals the distinct classification of three modes of detonation waves based on the direction of propagation and the phase characteristics of transverse shock waves on the wavefront. This paper delineates the presence of two different types of phenomena: duct wall slapping waves due to shock–wall collisions and internal slapping waves resulting from shock interactions. Furthermore, this investigation exposes the existence of two distinct types of triple-wave lines on the wavefront: the first comprising a strong Mach disk, a weak Mach disk, and a transverse shock wave; the second characterized by a weak Mach disk, an incident shock wave, and a transverse shock wave. Notably, the pressure behind the first type of triple-wave line is observed to be the highest. It elucidates the transition from two- to three-dimensional detonation waves, revealing that the prevalence of transverse shock waves on the wavefront in the rectangular and diagonal modes is twofold and quadruple, respectively, when compared to their two-dimensional counterparts within identical ducts/channels.

Experimental study on the propagation mechanism of acetylene-air detonation waves in a unilaterally intermittently constrained channel

This study experimentally explores the propagation mechanisms of acetylene/air detonation waves within a channel intermittently constrained on one side, utilizing soot foil and high-speed schlieren photography to capture the cellular structure and shock-flame evolution. The experiments revealed that the detonation waves traverse the semi-enclosed channel with various discrete wall configurations on the side in three distinct propagation modes: (I) periodic detonation failure and re-initiation; (II) single extinction and re-initiation; (III) non-extinction. Mode I occurs exclusively when the open area ratio exceeds 0.85, while detonation tends to favour Mode II when the gaps between discrete walls exceed three times the cell size; otherwise, it tends towards Modes III. The re-initiation mechanism of detonation involves curvature shocks inducing local explosions of reactive mixtures through multiple Mach reflections off the discrete walls. The self-sustained propagation mechanism of the detonation wave is maintained by the interaction of strong transverse shocks reflected from discrete walls with the inherent transverse waves within the detonation structure, sustaining the instability of the cellular detonation.

Particle acceleration, escape, and non-thermal emission from core-collapse supernovae inside non-identical wind-blown bubbles

Context. In the core-collapse scenario, supernova remnants (SNRs) evolve inside complex wind-blown bubbles structured by massive progenitors during their lifetime. Therefore, particle acceleration and the emissions from these SNRs can carry the fingerprints of the evolutionary sequences of the progenitor stars. Aims. We investigate the impact of the ambient environment of core-collapse SNRs on particle spectra and emissions for two progenitors with different evolutionary tracks while accounting for the spatial transport of cosmic rays (CRs) and the magnetic turbulence that scatters CRs. Methods. We used the RATPaC code to model the particle acceleration at the SNRs with progenitors having zero-age main sequence (ZAMS) masses of 20 M⊙ and 60 M⊙. We constructed the pre-supernova circumstellar medium (CSM) by solving the hydrodynamic equations for the lifetime of the progenitor stars. Then, the transport equation for cosmic rays, the magnetic turbulence in test-particle approximation, and the induction equation for the evolution of a large-scale magnetic field were solved simultaneously with the hydro-dynamic equations for the expansion of SNRs inside the pre-supernova CSM in 1-D spherical symmetry. Results. The profiles of gas density and temperature of the wind bubbles along with the magnetic field and the scattering turbulence regulate the spectra of accelerated particles for both of the SNRs. For the 60 M⊙ progenitor, the spectral index reaches 2.4, even below 10 GeV, during the propagation of the SNR shock inside the hot shocked wind. In contrast, we did not observe a persistent soft spectra at earlier evolutionary stages of the SNR with the 20 M⊙ progenitor, for which the spectral index becomes 2.2 only for a brief period during the interaction of SNR shock with the dense shell of red supergiant (RSG) wind material. At later stages of evolution, the spectra become soft above ~10 GeV for both SNRs, as weak driving of turbulence permits the escape of high-energy particles from the remnants. The emission morphology of the SNRs strongly depends on the type of progenitors. For instance, the radio morphology of the SNR with the 20 M⊙ progenitor is centre-filled at early stages, whereas that of the more massive progenitor is shell-like.

Optimization and data mining for shock-induced mixing enhancement inside scramjet using stochastic deep-learning flowfield prediction

One of the significant challenges in supersonic combustion ramjet engines lies in effective mixing of the fuel, primarily due to the high momentum of supersonic inflow at the combustor. Among the various fluid phenomena associated with fuel mixing, it is well recognized that the interaction between the fuel-injection jet plume and oblique shock waves can significantly enhance mixing efficiency. However, the most suitable interaction for optimal mixing enhancement remains yet to be clarified. The present study conducts model-based optimization and data mining for shock-induced mixing enhancement of angled-slot injection in a two-dimensional scramjet combustor. Stochastic deep-learning flowfield prediction has been utilized to enable fast and reliable evaluations of a substantial number of designs. Prediction errors can be estimated without requiring correct data owing to uncertainty quantification techniques. Data mining and sensitivity analysis, coupled with flowfield prediction, have revealed the optimal shock interaction with the fuel jet plume characterized by a pronounced downstream recirculation region. The mechanism that drives mixing enhancement through this recirculation region has been discussed based on the results of optimization and sensitivity analysis. This study has yielded valuable insights for the future design of scramjet injectors. Furthermore, the effectiveness of the model-based design and analysis has been demonstrated through the present study, showcasing its potential for guiding future developments in scramjet technology.

Drop breakup in bag regime under the impulsive condition

This study experimentally investigates the spatiotemporal evolution and associated local instabilities of a drop subjected to a weak shock wave. The front and side views of the drop are captured to understand its three-dimensional evolution and breakup. The interaction of shock causes the windward side of the drop to compress and generate a surface wave over it. Its temporal amplification is found to be governed by Kelvin–Helmholtz instability. The core of the deformed drop expands in a stream-wise direction, forming a Rayleigh–Taylor instability-driven bag structure. Consistent pressure gradients across the bag cause its continuous elongation until the pressure gradient overcomes the surface tension. This continuous elongation leads the sheet to undergo kinematic thinning, which causes the sheet to destabilize and nucleate the hole. This hole recedes and gathers liquid from upstream to thicken its interface, called the bag rim. The accelerating receding motion of the bag rim triggers Rayleigh–Taylor instability, and the corrugation that forms over it grows into ligaments and destabilizes to shed droplets through end pinching and ligament merging. Additionally, the accelerating rim undergoes radial expansion, with its further destabilization governed by the coupled effect of Rayleigh–Taylor and Rayleigh–Plateau instabilities, as well as the collision of the receding bag rim. This leads to the formation of corrugations, which grow into ligaments and further destabilize to shed drops via end pinching. Nonlinear effects dominate ligament dynamics and increase with the Weber number. The asymmetric ejection of the daughter drop from the rim causes it to evolve into a bag, undergoing tertiary breakup.

Observations of Locally Excited Waves in the Low Solar Atmosphere Using the Daniel K. Inouye Solar Telescope

We present an interpretation of the recent Daniel K. Inouye Solar Telescope (DKIST) observations of propagating wave fronts in the lower solar atmosphere. Using MPS/University of Chicago MHD radiative magnetohydrodynamic simulations spanning the solar photosphere, the overshoot region, and the lower chromosphere, we identify three acoustic-wave source mechanisms, each occur at a different atmospheric height. We synthesize the DKIST Visible Broadband Imager G-band, blue-continuum, and Ca ii K signatures of these waves at high spatial and temporal resolution, and conclude that the wave fronts observed by DKIST likely originate from acoustic sources at the top of the solar photosphere overshoot region and in the chromosphere proper. The overall importance of these local sources to the atmospheric energy and momentum budget of the solar atmosphere is unknown, but one of the excitation mechanisms identified (upward propagating shock interaction with down-welling chromospheric plasma resulting in acoustic radiation) may be an important shock dissipation mechanism. Additionally, the observed wave fronts may prove useful for ultralocal helioseismological inversions and promise to play an important diagnostic role at multiple atmospheric heights.

Misaligned precessing jets are choked by the accretion disc wind

ABSTRACT We analytically and numerically study the hydrodynamic propagation of a precessing jet in the context of tidal disruption events (TDEs) where the star’s angular momentum is misaligned with the black hole spin. We assume that a geometrically thick accretion disc undergoes Lense–Thirring precession around the black hole spin axis and that the jet is aligned with the instantaneous disc angular momentum. At large spin-orbit misalignment angles $\theta _{\it LS}$, the duty cycle along a given angle that the jet sweeps across is much smaller than unity. The faster jet and slower disc wind alternately fill a given angular region, which leads to strong shock interactions between the two. We show that precessing jets can only break out of the wind confinement if $\theta _{\it LS}$ is less than a few times the jet opening angle $\theta _{\rm j}$. The very small event rate of observed jetted TDEs is then explained by the condition of double alignment: both the stellar angular momentum and the observer’s line of sight are nearly aligned with the black hole spin. In most TDEs with $\theta _{\it LS}\gg \theta _{\rm j}$, the jets are initially choked by the disc wind and may only break out later when the disc eventually aligns itself with the spin axis due to the viscous damping of the precession. Such late-time jets may produce delayed radio rebrightening as seen in many optically bright TDEs. Our model is also applicable to jets associated with (stellar mass) black hole-neutron star mergers where the black hole’s spin is misaligned with the orbital angular momentum.

Investigation of Reynolds number effects on flow stability of a transonic compressor using a dynamic mode decomposition method

With the continuous increase in aircraft flight altitude, low Reynolds number effects in high-altitude environments significantly impact the stability of compressor operation. In this study, dynamic mode decomposition (DMD) of the flow field at various time instances near stall conditions in a transonic compressor was performed to investigate the influence of Reynolds number (Re) effects on critical flow characteristics and instability mechanisms. The results indicate that under different Reynolds number index (RNI) conditions near stall conditions, the frequency of tip leakage flow varies. Under high RNI condition, the influence of tip leakage flow is concentrated mainly from the leading edge to the mid-chord. The stall mode is primarily caused by the interaction of shock with the Tip Leakage Vortex, resulting in the breakdown of the vortex. The development of vortex structures inside the passage leads to a delay in the flow field response to blade passing. Under low RNI condition, the tip clearance leakage flow is stronger, with a relatively reduced influence from the leading edge to the mid-chord and an increased impact from the mid-chord to the trailing edge. The main cause of the stall mode is the circumferential movement of vortex structures, causing secondary leakage over adjacent blade tip clearances. The blockage formed by vortex structures within the passage is weaker, and the flow field response to blade passing remains consistent with the blade passing frequency. However, as stall develops further, the flow field response significantly decreases.

Kinetic effects on the interaction of counter-propagating plasma shocks inside an ICF hohlraum

The interaction and interpenetration of two counter-propagating plasma shocks are investigated via hybrid fluid-PIC (particle-in-cell) simulations. This study seeks to probe the kinetic effects and ion collisions on the structure of colliding plasma shocks in complex multi-ion-species plasma, in particular, the presence of the expansion of high-Z plasma bubbles against the low-Z filled gas inside an ICF hohlraum. The superposition of shock wave results in a wave-like electric field in the downstream region. The electric field can further reduce the kinetic energy of the incoming particles, and modulate the ion density profile. It finally generates a new downstream platform of high temperature and high density. However, on the hundred-ps time scale, cumulative ion collisions can still significantly alter the structure of the shock wave and the reflection of ions by the shock front. This study will help to improve the predictions of hohlraum plasma states and the understanding of the shock wave interactions.

Simulating shock interaction with a cavity-embedded cylinder/droplet using a real-fluid hybrid scheme at near-critical conditions

Simulating shock interaction with a cavity-embedded cylinder/droplet using a real-fluid hybrid scheme at near-critical conditions

Experimental studies of confined detonations of plasticized high explosives in inert and reactive atmospheres

When explosives detonate in a confined space, repeated boundary reflections result in complex shock interactions and the formation of a uniform quasi-static pressure (QSP). For fuel-rich explosives, mixing of partially oxidized detonation products with an oxygen-rich atmosphere results in a further energy release through rapid secondary combustion or ‘afterburn’. While empirical formulae and thermochemical modelling approaches have been developed to predict QSP, a lack of high-fidelity experimental data means questions remain around the deterministic quality of confined explosions, and the magnitude and mechanisms of afterburn reactions. This article presents experimental data for RDX- and PETN-based plastic explosives, demonstrating the high repeatability of the QSP generated in a sealed chamber using pressure transducers and high-speed infrared thermometry. Detonations in air, nitrogen and argon atmospheres are used to identify the contribution of afterburn to total QSP, to estimate the duration of afterburn reactions and to speculate on the flame temperature associated with this mechanism. Computational fluid dynamic modelling of the experiments was also able to accurately predict these effects. Understanding and quantifying explosions in complex environments are critical for the design of effective protective structures: the mechanisms described here provide a significant step towards the development of fast-running engineering models for internal blast events.

Characteristics and optimization of multi-body separation in supersonic ground effects

The electromagnetic launch of aerospace vehicles is an option for reusable earth/orbit space transportation in the future. In contrast to open space, complex ground effects emerge in supersonic near-ground multi-body separations. Here, the separation processes between a supersonic aerospace vehicle (traveling at Mach 1.5) and an electromagnetic sled were studied with a focus on the characteristics of the flow evolution and aerodynamic interference. The results show that the evolution of the supersonic near-ground separation can be divided into three substages: choked flows in the narrow gap, multi-body shock interactions and independent ground effect interference. The aerodynamic loads of the vehicle strongly correspond to the separation stages. The high-pressure region at the leading edge of the electromagnetic sled continuously sweeps the vehicle abdomen and causes significant fluctuations in the aerodynamic lift and moment, which play a dominant role in the separation trajectory. The altitude of the vehicle that flies away from the electromagnetic sled is relatively stable when the preset attack angle is 0° Following this separation, the vehicle has a pitch angle of -0.25° and tends toward a negative angle of attack. A vertical distance of 4.2 m is insufficient for optimal separation. Thus, the separation characteristics were optimized by combining the preset rudder deflection angle of 0° and inclined track with a slope angle of 1.5° Following this separation, the pitch angle of the vehicle is 3.4°, and the vertical distance increases to 14.8 m, which significantly improves the separation safety.

Large Eddy Simulation of Laminar and Turbulent Shock/Boundary Layer Interactions in a Transonic Passage

Abstract Shock/boundary layer interactions (SBLI) are a fundamental fluid mechanics problem relevant in a wide range of applications including transonic rotors in turbomachinery. This paper uses wall-resolved large eddy simulation (LES) to examine the interaction of normal shocks with laminar and turbulent inflow boundary layers in transonic flow. The calculations were performed using GENESIS, a high-order, unstructured LES solver. The geometry created for this study is a transonic passage with a convergent-divergent nozzle that expands the flow to the desired Mach number upstream of the shock and then introduces constant radius curvature to simulate local airfoil camber. The Mach numbers in the divergent section of the transonic passage simulate single stage commercial fan blades. The results predicted with the LES calculations show significant differences between laminar and turbulent SBLI in terms of shock structure, boundary layer separation and transition, and aerodynamic losses. For laminar flow into the shock, significant flow separation and low-frequency unsteadiness occur, while for turbulent flow into the shock, both the boundary layer loss and the low-frequency unsteadiness are reduced. The time period of the unsteadiness was hypothesized to align with the time it takes for turbulent structures to convect from the shock to the trailing edge and acoustic disturbances to travel back from the trailing edge to the shock, and this is supported by the LES results.

Shock–shock interaction on the Reusability Flight Experiment (ReFEx)

AbstractThe Reusability Flight Experiment (ReFEx) is currently under development at the German Aerospace Center (DLR). A coupled experimental and numerical campaign was carried out to investigate the surface heating on the payload geometry during return consisting of a forebody and canard. In this way, numerical tools for a post-flight analysis can be preemptively improved where required. Experiments were undertaken at the High Enthalpy Shock Tunnel Göttingen (HEG) on a 1:4 scale model with the use of temperature sensitive paint on the payload geometry to obtain surface heat flux. The model configuration was varied in angle of attack and canard deflection. Laminar and turbulence-model solvers in the DLR-TAU code were used for the numerical simulations. This investigation focussed on the shock–shock interaction of the nose bow shock with the leading-edge shock of the canards showing significant surface heat flux along the canard. Larger surface heat fluxes were measured in the experiments downstream of the shock interaction on the canard, than obtained from the laminar CFD calculations. This was attributed to transition of the boundary layer within the interaction regions, in the presence of significant adverse pressure gradients. Other flow features along the forebody in the vicinity of the canard were qualitatively matched better by the fully-turbulent numerical solutions than the laminar ones. This work aims to demonstrate the extent to which the numerical and experimental tools assist useful insights into flow phenomena during the return stages of the ReFEx payload geometry, and the aspects for which improvements are required.