Showing papers on "Oblique shock published in 2012"
TL;DR: In this paper, a study of the interaction between the pulsed-plasma jets and the shock/boundary layer interaction was performed in a time-resolved manner using 10 kHz schlieren imaging.
Abstract: A pulsed-plasma jet actuator is used to control the unsteady motion of the separation shock of a shock wave/boundary layer interaction formed by a compression ramp in a Mach 3 flow. The actuator is based on a plasma-generated synthetic jet and is configured as an array of three jets that can be injected normal to the cross-flow, pitched, or pitched and skewed. The typical peak jet exit velocity of the actuators is about 300 m/s and the pulsing frequencies are a few kilohertz. A study of the interaction between the pulsed-plasma jets and the shock/boundary layer interaction was performed in a time-resolved manner using 10 kHz schlieren imaging. When the actuator, pulsed at StL ≈ 0.04 (f = 2 kHz), was injected into the upstream boundary layer, the separation shock responded to the plasma jet by executing a rapid upstream motion followed by a gradual downstream recovery motion. Schlieren movies of the interaction showed that the separation shock unsteadiness was locked to the pulsing frequency of the actuator, with amplitude of about one boundary layer thickness. Wall-pressure measurements made under the intermittent region showed about a 30% decrease in the overall magnitude of the pressure fluctuations in the low-frequency band associated with unsteady large-scale motion of the separated flow. Furthermore, by increasing the pulsing frequency to 3.3 kHz, the amplitude of the separation shock oscillation was reduced to less than half the boundary layer thickness. Investigation into the effect of the actuator location on the shock wave/boundary layer interaction (SWBLI) showed qualitatively and quantitatively that the actuator placed upstream of the separation shock caused significant modification to the SWBLI unsteadiness, whereas injection from inside the separation bubble did not cause a noticeable effect.
137 citations
TL;DR: In this paper, the trajectories of flow particles are tracked and analyzed in detail based on the two-dimensional numerical simulation of continuously rotating detonations in an annular combustion chamber.
131 citations
TL;DR: In this paper, a high-order flux-vector-splitting code for Newtonian and special-relativistic hydrodynamics designed for direct numerical simulations of turbulent flows is presented.
Abstract: We present THC: a new high-order flux-vector-splitting code for Newtonian and special-relativistic hydrodynamics designed for direct numerical simulations of turbulent flows. Our code implements a variety of different reconstruction algorithms, such as the popular weighted essentially non oscillatory and monotonicity-preserving schemes, or the more specialised bandwidth-optimised WENO scheme that has been specifically designed for the study of compressible turbulence. We show the first systematic comparison of these schemes in Newtonian physics as well as for special-relativistic flows. In particular we will present the results obtained in simulations of grid-aligned and oblique shock waves and nonlinear, large-amplitude, smooth adiabatic waves. We will also discuss the results obtained in classical benchmarks such as the double-Mach shock reflection test in Newtonian physics or the linear and nonlinear development of the relativistic Kelvin-Helmholtz instability in two and three dimensions. Finally, we study the turbulent flow induced by the Kelvin-Helmholtz instability and we show that our code is able to obtain well-converged velocity spectra, from which we benchmark the effective resolution of the different schemes.
131 citations
TL;DR: In this article, the interaction of a planar shock wave with a dense particle curtain is investigated through modeling and experiments, and a physics-based model is developed to account for interphase coupling.
Abstract: The interaction of a planar shock wave with a dense particle curtain is investigated through modeling and experiments. The physics in the interaction between a shock wave with a dense gas-particle mixture is markedly differently from that with a dilute mixture. Following the passage of the shock wave, the dense particle curtain expands rapidly as it propagates downstream and pressures equilibrate throughout the flow field. In the simulations, the particles are viewed as point-particles and are traced in a Lagrangian framework. A physics-based model is then developed to account for interphase coupling. Compared to the standard drag law, four major improvements are made in the present interphase coupling model to take into account: (1) unsteady force contributions to particle force; (2) effect of compressibility on hydrodynamic forces; (3) effect of particle volume fraction on hydrodynamic forces; (4) effect of inter-particle collision. The complex behavior of the dense particle curtain is due to the interplay between two-way coupling, finite particle inertia, and unsteady forces. Incorporation of these effects through significant modeling improvements is essential for the simulation results to agree well with the experimental data. As a result of the large pressure gradient inside the particle curtain, the unsteady forces remain significant for a long time compared to the quasi-steady force and greatly influence the particle curtain motion.
122 citations
TL;DR: In this article, the difference in the oblique shock and detonation angles is proposed as the criterion to determine the transition pattern with the aid of shock-polar analysis, and it is found that the smooth transition will appear when the angle difference is small, while the abrupt transition will occur when the difference is large.
Abstract: Oblique detonation waves are simulated to study the evolution of their morphology as gasdynamic and chemical parameters are varied. Although two kinds of transition pattern have previously been observed, specifically an abrupt transition and a smooth one, the determining factors for the transition pattern are still unclear. Numerical results show that the transition pattern is influenced by the inflow Mach number, chemical activation energy and heat release. Despite the fact that these parameters were known to influence the detonation instability, the transition pattern variation cannot be predicted according to the instability criterion. In this study, the difference in the oblique shock and detonation angles is proposed as the criterion to determine the transition pattern with the aid of shock-polar analysis. It is found that the smooth transition will appear when the angle difference is small, while the abrupt transition will occur when the difference is large. The shift from the smooth transition to the abrupt transition occurs when the angle difference is about 15 degrees-18 degrees. The previously proposed criterion using the characteristic time ratio is also examined and compared with the present angle difference criterion, and the latter is proved to provide better results.
117 citations
TL;DR: In this article, the background flow field of a scramjet isolator that accommodates a shock train contains complex compression and expansion waves, referred as background waves, causing large streamwise and transverse parameter gradients upstream of the shock train.
Abstract: The background flow field of a scramjet isolator that accommodates a shock train contains complex compression and expansion waves, referred as “background waves,” causing large streamwise and transverse parameter gradients upstream of the shock train. Therefore, the available results of shock train research obtained by direct-connect methods might be not applicable for real scramjet isolators. Special tests are therefore performed for an inlet/isolator model. Close coupling is found between the shock train and the background shocks. The pointing direction of the leading shock switches upwards and downwards repeatedly during the upstream propagation of the shock train. Three unstable stages with substantial oscillations are also observed, interlaced with four stable stages. In addition, the interference of the background shock waves increases the sustainable back-pressure ratio and decreases the length of the shock train. However, this does not mean that the background waves in the isolator should be intensified intentionally.
114 citations
TL;DR: In this article, a study of the shock buffet onset and instability mechanism via Reynolds-averaged Navier-Stokes simulations on several airfoils is presented, and the numerical setup and the AUmaras turbulence closure are validated based on wind-tunnel data from NACA 0012 and RA16SC1.
Abstract: A study of shock-buffet onset and instability mechanism via Reynolds-averaged Navier―Stokes simulations on several airfoils is presented. The numerical setup and the Spalart―AUmaras turbulence closure are validated based on wind-tunnel data from NACA 0012 and RA16SC1 airfoils. The paper presents simulations of the flow past three • airfoils: the subsonic NACA 0012, the supercritical RA16SC1, and the thin, transonic/supersonic NACA 64A204, at pre- and postbuffet conditions, and within a cycle of developed shock buffet. The shock-buffet cycle is found to be »■• similar in nature for all airfoils, originating in unstable interaction of the shock and the separation bubble. Simulation results support the notion that buffet onset is not related to the bursting of the separation bubble behind the shock. Shock-buffet categorizing is posited as a transonic prestall instability phenomenon that depends on the shock strength and location. Shock-buffet onset conditions occur when the shock position is behind and sufficiently close to the upper-surface maximum curvature location. Additionally, it is suggested that offset conditions are when the shock is at an upstream location and the flow aft of it is fully separated.
109 citations
TL;DR: In this paper, a multiphase shock tube has been constructed to drive a planar shock wave into a dense gas-solid field of particles, which results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 20%.
Abstract: Currently there is a substantial lack of data for interactions of shock waves with particle fields having volume fractions residing between the dilute and granular regimes. To close this gap, a novel multiphase shock tube has been constructed to drive a planar shock wave into a dense gas–solid field of particles. A nearly spatially isotropic field of particles is generated in the test section by a gravity-fed method that results in a spanwise curtain of spherical 100-micron particles having a volume fraction of about 20%. Interactions with incident shock Mach numbers of 1.66, 1.92, and 2.02 are reported. High-speed schlieren imaging simultaneous with high-frequency wall pressure measurements are used to reveal the complex wave structure associated with the interaction. Following incident shock impingement, transmitted and reflected shocks are observed, which lead to differences in particle drag across the streamwise dimension of the curtain. Shortly thereafter, the particle field begins to propagate downstream and spread. For all three Mach numbers tested, the energy and momentum fluxes in the induced flow far downstream are reduced about 30–40% by the presence of the particle field.
105 citations
TL;DR: In this article, the authors examine the mechanisms behind the coupling between corner separation and separation away from the corner when holding a high-Mach-number normal shock in a rectangular channel and show that the alteration of the -region, which occurs in the supersonic portion of the SBLI interaction (SBLI), is more important than the generation of any blockage in the subsonic region downstream of the shock wave.
Abstract: Experiments are conducted to examine the mechanisms behind the coupling between corner separation and separation away from the corner when holding a high-Mach-number normal shock in a rectangular channel. The ensuing shock wave interaction with the boundary layer on the wind tunnel floor and in the corners was studied using laser Doppler anemometry, Pitot probe traverses, pressure sensitive paint and flow visualization. The primary mechanism explaining the link between the corner separation size and the other areas of separation appears to be the generation of compression waves at the corner, which act to smear the adverse pressure gradient imposed upon other parts of the flow. Experimental results indicate that the alteration of the -region, which occurs in the supersonic portion of the shock wave/boundary layer interaction (SBLI), is more important than the generation of any blockage in the subsonic region downstream of the shock wave.
93 citations
TL;DR: In this article, a series of sensors are mounted on a cylindrical specimen normal to the flow direction to measure dynamic pressures of the blast, and the results indicate that the blast wave loading is significantly different for locations inside and outside of the shock tube.
Abstract: Shock tubes have been extensively used in the study of blast traumatic brain injury due to increased incidence of blast-induced neurotrauma in Iraq and Afghanistan conflicts. One of the important aspects in these studies is how to best replicate the field conditions in the laboratory which relies on reproducing blast wave profiles. Evolution of the blast wave profiles along the length of the compression-driven air shock tube is studied using experiments and numerical simulations with emphasis on the shape and magnitude of pressure time profiles. In order to measure dynamic pressures of the blast, a series of sensors are mounted on a cylindrical specimen normal to the flow direction. Our results indicate that the blast wave loading is significantly different for locations inside and outside of the shock tube. Pressure profiles inside the shock tube follow the Friedlander waveform fairly well. Upon approaching exit of the shock tube, an expansion wave released from the shock tube edges significantly degrades the pressure profiles. For tests outside the shock tube, peak pressure and total impulse reduce drastically as we move away from the exit and majority of loading is in the form of subsonic jet wind. In addition, the planarity of the blast wave degrades as blast wave evolves three dimensionally. Numerical results visually and quantitatively confirm the presence of vortices, jet wind and three-dimensional expansion of the planar blast wave near the exit. Pressure profiles at 90° orientation show flow separation. When cylinder is placed inside, this flow separation is not sustained, but when placed outside the shock tube this flow separation is sustained which causes tensile loading on the sides of the cylinder. Friedlander waves formed due to field explosives in the intermediate-to far-field ranges are replicated in a narrow test region located deep inside the shock tube.
92 citations
TL;DR: In this paper, the natural gas flows in diffusers were numerically calculated using the navier-stokes equations with the RSM (reynolds stress model) and the behavior of gas dynamic parameters was analyzed under conditions of shock waves and boundary layers.
TL;DR: In this article, the dynamics of a flexible two-dimensional panel subjected to an impinging oblique shock is considered numerically in the inviscid regime, and it is shown that for a sufficiently strong shock self-excited oscillations arise in the coupled system and either supercritical or subcritical bifurcations emerge at a value of dynamic pressure which can be considerably lower than that corresponding to standard panel flutter.
TL;DR: In this paper, a large-eddy simulation is used to investigate unsteady mechanisms in supersonic flows, which reveal the existence of complex mechanisms, which need to be carefully investigated for efficient design of propulsion systems.
Abstract: This work considers numerical simulations of supersonic flows when shock/turbulent boundary layer interaction occurs. Such flows reveal the existence of complex mechanisms, which need to be carefully investigated for efficient design of propulsion systems. In this study, large-eddy simulation is used to investigate unsteady mechanisms. Since a shock-capturing scheme is used, a hybrid numerical scheme has been developed to reduce its dissipative properties. The issue of the generation of coherent turbulent inlet boundary conditions is also addressed. To avoid introducing artificial low-frequency modes that could affect the interaction, a method based on a digital-filter approach originally developed by Klein et al. (2003) and modified by Xie & Castro (2008) and Touber & Sandham (2009) is used to provide a synthetic-inflow condition over a relatively short distance. The obtained results are analyzed and discussed in terms of mean and turbulent quantities. Excellent agreement between LES and experimental data is obtained for both the undisturbed boundary layer and the shock impingement region. In the latter case, oscillations of the reflected shock occurring at low frequencies are observed, in agreement with previous numerical and experimental findings. Moreover, simulations reveal the presence of such frequencies mainly near the shock foot and within the recirculation bubble. This point gives credit to the hypothesis that the instabilities of the reflected shock are due to the intrinsic low-frequency movement of the shock/bubble acting dynamically as a coupled system.
TL;DR: In this article, a high-order flux-vector-splitting code for Newtonian and special-relativistic hydrodynamics designed for direct numerical simulations of turbulent flows is presented.
Abstract: We present THC: a new high-order flux-vector-splitting code for Newtonian and special-relativistic hydrodynamics designed for direct numerical simulations of turbulent flows. Our code implements a variety of different reconstruction algorithms, such as the popular weighted essentially non oscillatory and monotonicity-preserving schemes, or the more specialised bandwidth-optimised WENO scheme that has been specifically designed for the study of compressible turbulence. We show the first systematic comparison of these schemes in Newtonian physics as well as for special-relativistic flows. In particular we will present the results obtained in simulations of grid-aligned and oblique shock waves and nonlinear, large-amplitude, smooth adiabatic waves. We will also discuss the results obtained in classical benchmarks such as the double-Mach shock reflection test in Newtonian physics or the linear and nonlinear development of the relativistic Kelvin-Helmholtz instability in two and three dimensions. Finally, we study the turbulent flow induced by the Kelvin-Helmholtz instability and we show that our code is able to obtain well-converged velocity spectra, from which we benchmark the effective resolution of the different schemes.
TL;DR: In this paper, the main features of a shock reflection on a turbulent boundary layer are described and the main space-time properties of the leading shock motions are described together with their links with the other regions of the flow.
Abstract: This paper aims at describing the main features of a shock reflection on a turbulent boundary layer. The data used for this analysis are the results of large-eddy simulations of the interaction carried out with three different shock intensities: from incipient to fully separated cases. Computational results are validated vs experiments obtained for the same interaction geometries. The main space-time properties of the leading shock motions are described together with their links with the other regions of the flow. In particular, information about the origin of the shock motion is derived from the correlations between shock motion and unsteady pressure fields. It is shown that the shock motion reveals the flow unsteadiness found in the interaction region.
TL;DR: In this paper, the authors studied particle acceleration at an oblique shock by extending their earlier particle acceleration and transport in heliosphere (PATH) code to include shocks with arbitrary BN, where BN is the angle between the upstream magnetic field and the shock normal.
TL;DR: In this article, two vortex generators are mounted in front of the shock wave region on the upper surface of an OAT15A supercritical airfoil at Mach number of 0.73 and angle of attack of 3.5 degrees.
Abstract: In the present paper, extremely unsteady shock wave buffet induced by strong shock wave/boundary-layer interactions (SWBLI) on the upper surface of an OAT15A supercritical airfoil at Mach number of 0.73 and angle of attack of 3.5 degrees is first numerically simulated by IDDES, one of the most advanced RANS/LES hybrid methods. The results imply that conventional URANS methods are unable to effectively predict the buffet phenomenon on the wing surface; IDDES, which involves more flow physics, predicted buffet phenomenon. Some complex flow phenomena are predicted and demonstrated, such as periodical oscillations of shock wave in the streamwise direction, strong shear layer detached from the shock wave due to SWBLI and plenty of small scale structures broken down by the shear layer instability and in the wake. The root mean square (RMS) of fluctuating pressure coefficients and streamwise range of shock wave oscillation reasonably agree with experimental data. Then, two vortex generators (VG) both with an inclination angle of 30 degrees to the main flow directions are mounted in front of the shock wave region on the upper surface to suppress shock wave buffet. The results show that shock wave buffet can be significantly suppressed by VGs, the RMS level of pressure in the buffet region is effectively reduced, and averaged shock wave position is obviously pushed downstream, resulting in increased total lift.
TL;DR: In this paper, a new wind-tunnel flowfield has been proposed that captures much of the key shock boundary-layer interaction physics of supersonic external compression inlets.
Abstract: T HE interaction of a shock wavewith a turbulent boundary layer constitutes a fundamental problem of high-speed fluid mechanics. A detailed survey of past work on high-speed interactions has been carried out by Settles and Dolling [1] and Smits and Dussauge [2]. The shock interaction problem is particularly germane to the design of supersonic inlets. In such supersonic inlets, deceleration of the flow is achieved through a succession of oblique shock waves followed by a terminal normal shock. Boundary layers form on the inlet surfaces and interact with the shock system, giving rise to various shock/boundary-layer interactions (SBLIs). Each interaction of oblique/normal shock waves with the boundary layer causes stagnation pressure losses and downstream spatial distortions seen by the engine. An inlet must be carefully designed to minimize these losses and distortions during the compression process since they affect overall propulsion performance. In mixed-compression inlets, shock-induced separation can lead to engine unstart, which requires that the entire propulsion system undergo a restart sequence during flight. In external compression inlets, specifically axisymmetric configurations, a thick hubside boundary layer increases blockage and can decrease compressor performance. Thus, successfully controlling SBLIs has the potential to significantly improve supersonic inlet performance. As will be discussed in the following, various techniques of flow control for SBLIs have been proposed. However, it is often difficult to interpret the results because the flowfield may be too specific to an individual inlet configuration or too basic such that a relationship to inlet performance is not clear. To address this issue, a newwind-tunnel flowfield has been proposed [3] that captures much of the key shock boundary-layer interaction physics of supersonic external compression inlets. Thisflowfieldwill be used to study the novel flow control methods introduced herein. The conventionalflow control technique for SBLI conditions in an engine inlet employs a bleed of the boundary layer [4,5]. This bleed Presented as Paper 2010-4464 at the 40th AIAA Fluid Dynamics Conference and Exhibit, Chicago, IL, 28 June–1 July 2010; received 31 January 2011; revision received 26 July 2011; accepted for publication 11 August 2011. Copyright©2011 by theAmerican Institute ofAeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0748-4658/12 and $10.00 in correspondence with the CCC. Ph.D. Candidate, Aerospace Engineering, 104 S. Wright Street. Member AIAA. Professor of Aerodynamics, Department of Engineering, Trumpington Street. Associate Fellow AIAA. Professor, Mechanical and Aerospace Engineering, 122 Engineer’s Way. Associate Fellow AIAA. JOURNAL OF PROPULSION AND POWER Vol. 28, No. 1, January–February 2012
TL;DR: Observations lead to the conclusion that the shock ramp structure of supercritical high Mach number shocks is formed as a balance of dispersion and nonlinearity.
Abstract: Whistler wave trains are observed in the foot region of high Mach number quasiperpendicular shocks. The waves are oblique with respect to the ambient magnetic field as well as the shock normal. The Poynting flux of the waves is directed upstream in the shock normal frame starting from the ramp of the shock. This suggests that the waves are an integral part of the shock structure with the dispersive shock as the source of the waves. These observations lead to the conclusion that the shock ramp structure of supercritical high Mach number shocks is formed as a balance of dispersion and nonlinearity.
TL;DR: In this article, the authors investigate the operation condition of a fluidic thrust vector using injection of the control flow tangential to the main jet direction; co-flow injection is used to analyze the dynamic characteristics of fluidic control of jet vectoring up-and downward from the nozzle axis, so that the response time of jet deflection to control flow injection and the pressure dispersion on the nozzle wall were investigated.
Abstract: The purpose of this research is to investigate the operation condition of fluidic thrust vector using injection of the control flow tangential to the main jet direction; co-flow injection. The physical model of concern includes a chamber and a supersonic nozzle for supersonic main jet injection, and two chambers with slots for control flow injection. Steadystate numerical and experimental studies were conducted to investigate operating parameters; detailed flow structures, jet deflection angles, and shock effects were observed near the nozzle exit. An unsteady numerical calculation was conducted to analyze the dynamic characteristics of fluidic control of jet vectoring up- and downward from the nozzle axis, so that the response time of jet deflection to control flow injection and the pressure dispersion on the nozzle wall were investigated. Internal nozzle performance was predicted for total pressure range of the jet from 300 kPa to 1000 kPa to the control flow pressure from 120 to 200 kPa. To take into account the important features of high-speed flows, including shock-boundary layer interactions, a low Reynolds number k-e turbulence model with compressible-dissipation and pressure-dilatation effects was applied.
TL;DR: In this paper, the formation and evolution of a vortex ring generated at the open end of a short driver-section shock tube has been simulated numerically for pressure ratios (PR) of 3 and 7.
Abstract: Formation and evolution of a compressible vortex ring generated at the open end of a short driver section shock tube has been simulated numerically for pressure ratios (PR) of 3 and 7 in the present study. Numerical study of compressible vortex rings is essential to understand the complicated flow structure and acoustic characteristics of many high Mach number impulsive jets where simultaneously velocity, density and pressure fields are needed. The flow development, incident shock formation, shock diffraction, vortex ring formation and its evolution are simulated using the AUSM+ scheme. The main focus of the present study is to evaluate the time resolved vorticity field of the vortex ring and the shock/expansion waves in the starting jet for short driver section shock tubes—a scenario where little data are available in existing literature. An embedded shock and a vortex induced shock are observed for PR = 7. However the vortex ring remains shock free, compact and unaffected by the trailing jet for PR = 3. Numerical shadowgraph shows the evolution of embedded shock and shock/expansion waves along with their interactions. The velocity and vorticity fields obtained from simulation are validated with the particle image velocimetry results and these data match closely. The translational velocity of the vortex ring, velocity across the vortex and the centre line velocity of the jet obtained from simulation also agree well with the experimental results.
TL;DR: In this paper, the authors used time-resolved stereo particle-image velocimetry (TR-SPIV) and unsteady pressure measurements of transonic flow in a supercritical DRA-2303 airfoil to analyze the origin and nature of the dynamic shock wave-boundary layer interaction.
Abstract: Time-resolved stereo particle-image velocimetry (TR-SPIV) and unsteady pressure measurements are used to analyze the unsteady flow over a supercritical DRA-2303 airfoil in transonic flow. The dynamic shock wave–boundary layer interaction is one of the most essential features of this unsteady flow causing a distinct oscillation of the flow field. Results from wind-tunnel experiments with a variation of the freestream Mach number at Reynolds numbers ranging from 2.55 to 2.79 × 106 are analyzed regarding the origin and nature of the unsteady shock–boundary layer interaction. Therefore, the TR-SPIV results are analyzed for three buffet flows. One flow exhibits a sinusoidal streamwise oscillation of the shock wave only due to an acoustic feedback loop formed by the shock wave and the trailing-edge noise. The other two buffet flows have been intentionally influenced by an artificial acoustic source installed downstream of the test section to investigate the behavior of the interaction to upstream-propagating disturbances generated by a defined source of noise. The results show that such upstream-propagating disturbances could be identified to be responsible for the upstream displacement of the shock wave and that the feedback loop is formed by a pulsating separation of the boundary layer dependent on the shock position and the sound pressure level at the shock position. Thereby, the pulsation of the separation could be determined to be a reaction to the shock motion and not vice versa.
TL;DR: In this article, the cross-shock electrostatic potential at the front of collisionless shocks plays a key role in the distribution of energy at the shock front, and a statistical study of the crossshock potential calculated for around 50 crossings of the terrestrial bow shock is presented.
Abstract: [1] The cross-shock electrostatic potential at the front of collision-less shocks plays a key role in the distribution of energy at the shock front. Multipoint measurements such as those provided by the Cluster II mission provide an ideal framework for the study of the cross-shock potential because of their ability to distinguish between temporal and spacial variations at the shock front. We present a statistical study of the cross-shock potential calculated for around 50 crossings of the terrestrial bow shock. The statistical dependency of the normalized (with resect to upstream ion kinetic energy) cross-shock potential (ΦK) on the upstream Alfven Mach number is in good agreement with analytical results that predict decrease of Φk with increasing Mach number.
TL;DR: In this paper, the interaction of a shock train with a normal suction slot is presented and the structure and pressure recovery within the shock train is analyzed by means of Schlieren images and wall pressure measurements.
Abstract: The interaction of a shock train with a normal suction slot is presented. It was found that when the pressure in the suction slot is smaller or equal to the static pressure of the incoming supersonic flow, the pressure gradient across the primary shock is sufficient to push some part of the near wall boundary layer through the suction slot. Due to the suction stabilized primary shock foot, the back pressure of the shock train can be increased until the shock train gradually changes into a single normal shock. During the experiments, the total pressure and therewith the Reynolds number of the flow were varied. The structure and pressure recovery within the shock train is analysed by means of Schlieren images and wall pressure measurements. Because the boundary layer is most important for the formation of a shock train, it has been measured by a Pitot probe. Additionally, computational fluid dynamics is used to investigate the shock boundary-layer interaction. Based on the experimental and numerical results, a simplified flow model is derived which explains the phenomenology of the transition of a shock train into a single shock and derives distinct criteria to maintain a suction enhanced normal shock. This flow model also yields the required suction mass flow in order to obtain a single normal shock in a viscous nozzle flow. Furthermore, it allows computation of the total pressure losses across a normal shock under the influence of boundary-layer suction.
TL;DR: In this paper, a method based on Riemann interactions is proposed for the analysis of two different nozzle geometries and supersonic flow, which is ideal for conceptual design, control design, or control evaluation studies.
Abstract: interaction between operating condition and plume shape complicates the analysis of such nozzles compared to traditional bell nozzles. A method that is based on Riemann interactions is proposed for the analysis of two such nozzle geometries. The method assumes two-dimensional geometries and supersonic flow. Unlike the method of characteristics, this method accounts explicitly for the presence of oblique shocks and curved shear layers. Comparisons to both experiment and computational fluid dynamics are shown. The solution method requires no grid generation and typically runs in less than a minute on a single desktop computer, which is ideal for conceptual design, control design, or control evaluation studies. It includes high-temperature gas modeling and finite-rate chemistry. Nomenclature A = area c = specific heat Ex = momentum conservation error H = height or length scale M = Mach number nexp = number of discrete waves in expansion nsp = number of species p = pressure r = length of characteristic R = normalized gas constant T = temperature u = magnitude of flow velocity W = molecular weight x, y = spatial coordinates Y = mass fraction = angle between wave and upstream flow = ratio of specific heats = deflection angle across a wave = angle of deflection caused by boundary layer = flowpath angle = momentum thickness " = ratio of static pressures = Mach angle = Prandtl-Meyer angle = streamwise coordinate = density = angle between wave and x-axis ˙ ! = molar rate of production
TL;DR: In this article, a large scale one-dimensional hybrid simulation is performed to investigate the generation mechanism of diffuse superthermal ions at a high Mach number quasi-parallel collisionless shock.
Abstract: [1] A large scale one-dimensional hybrid simulation is performed to investigate the generation mechanism of diffuse superthermal ions at a high Mach number quasi-parallel collisionless shock. The shock exhibits a cyclic behavior and reforms periodically. The generation of the diffuse ions is associated with the reformation of the shock. At the beginning of the reformation cycle, a part of ions are reflected by the shock due to the existence of the cross shock potential. At the same time, an upstream wave is brought back by the upstream plasma and interacts with the shock. The upstream wave begins to steepen as it approaches the shock, and then a new shock front is formed. The reflected ions are trapped between the new and old shock fronts. They are accelerated every time they are reflected by the new shock front until the reformation cycle of the shock is finished and the particles escape from the shock. In this way, diffuse superthermal ions are generated in the quasi-parallel shock, which may be further accelerated to higher energy due to shock diffusive acceleration.
TL;DR: In this paper, self-similar solutions are obtained for unsteady, one-dimensional isothermal flow behind a shock wave in a rotational axisymmetric non-ideal gas in the presence of an azimuthal magnetic field.
TL;DR: In this paper, two-dimensional test particle simulations based on strictly perpendicular shock profiles chosen at a fixed time in 2D Particle-in-Cell (PIC) simulations, are performed in order to investigate the impact of the shock front ripples on incident ion (H+) dynamics.
Abstract: [1] Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the front of a supercritical quasi-perpendicular shock can be rippled. Recent two-dimensional simulations have focused on two different types of shock front rippling: (1) one characterized by a small spatial scale along the front is supported by lower hybrid wave activity, (2) the other characterized by a large spatial scale along the front is supported by the emission of large amplitude nonlinear whistler waves. These two rippled shock fronts are self-consistently observed when the static magnetic field is perpendicular to (so called “B0-OUT” case) or within (so called “B0-IN” case) the simulation plane, respectively. On the other hand, several studies have been made on the reflection and energization of incoming ions with a shock but most have been restricted to a one dimensional shock profile only (no rippling effects). Herein, two-dimensional test particle simulations based on strictly perpendicular shock profiles chosen at a fixed time in two-dimensional Particle-in-cell (PIC) simulations, are performed in order to investigate the impact of the shock front ripples on incident ion (H+) dynamics. The acceleration mechanisms and energy spectra of the test-ions (described by shell distributions with different initial kinetic energy) interacting with a rippled shock front are analyzed in detail. Both “B0-OUT” and “B0-IN” cases are considered separately; in each case, y-averaged (front rippling excluded) and non-averaged (front rippling included) profiles will be analyzed. Present results show that: (1) the incident ions suffer both shock drift acceleration (SDA) and shock surfing acceleration (SSA) mechanisms. Moreover, a striking feature is that SSA ions not only are identified at the ramp but also within the foot which confirms previous 1-D simulation results; (2) the percentage of SSA ions increases with initial kinetic energy, a feature which persists well with a rippled shock front; (3) furthermore, the ripples increase the porosity of the shock front, and more directly transmitted (DT) ions are produced; these strongly affect the relative percentage of the different identified classes of ions (SSA, SDA and DT ions), their average kinetic energy and their relative contribution to the resulting downstream energy spectra; (4) one key impact of the ripples is a strong diffusion of ions (in particular through the frontiers of their injection angle domains and in phase space which are blurred out) which leads to a mixing of the different ion classes. This diffusion increases with the size of the spatial scale of the front ripples; (5) through this diffusion, an ion belonging to a given category (SSA, SDA, or DT) in y-averaged case changes class in non-averaged case without one-to-one correspondence.
TL;DR: In this article, a further study is carried out to assess the influence of several parameters including the converging angle θ 0, the incident planar shock Mach number M0, and the shock tube height h on the wall profile and the convergence wave.
Abstract: In our previous work, the technique of generating cylindrical converging shock waves based on shock dynamics theory was proposed. In the present work, a further study is carried out to assess the influence of several parameters including the converging angle θ0, the incident planar shock Mach number M0, and the shock tube height h on the wall profile and the converging shock wave. Combining the high-speed schlieren photography and the numerical simulation with the shock dynamics theory, the characteristics of wall profiles, cylindrical converging shock waves, and thermodynamic properties for different controllable parameters are analyzed. It is found that these parameters have great effects on shapes of the wall profile and experimental investigation favors large values of M0 and h and moderate θ0. The experimental sequences of schlieren images indicate that the shocks moving in the converging part are of circular shapes, which further verifies the method in our previous work. In addition, the changes of ...
TL;DR: In this paper, the formation and structure of oblique detonation waves initiated by semi-infinite wedges and cones are presented, and it is shown that, for such low wedge or cone angles, a planar oblique Chapman-Jouguet detonation is eventually initiated following an induction process.
Abstract: The formation and structure of oblique detonation waves initiated by semi-infinite wedges and cones are presented. For wedge or cone angles less than the deflection angle required for an oblique Chapman–Jouguet (CJ) detonation, different wave structures have been previously reported. Using the method of characteristics and numerical simulations, it is shown that, for such low wedge or cone angles, a CJ oblique detonation is eventually initiated following an induction process. It its thus demonstrated that shock-induced combustion with the reaction front remaining uncoupled to the oblique shock in the far field is not a valid solution. Simulations with semi-infinite cones reveal that the effect of the front curvature around the cone axis allows oblique detonations to be formed at angles lower than that of a planar CJ oblique detonation.