Showing papers on "Oblique shock published in 2005"

Control of edney IV interaction by pulsed laser energy deposition

Abstract: An experimental investigation was conducted to examine the effect of a pulsed Nd:YAG laser energy addition on the shock structures and surface pressure in a Mach 3.45 flow past a sphere. Two configurations were considered: 1) a sphere in a uniform freestream and 2) an Edney IV interaction generated by impingement of an oblique shock on the bow shock of the sphere

Receptivity of a supersonic boundary layer over a flat plate. Part 3. Effects of different types of free-stream disturbances

Abstract: Supersonic boundary-layer receptivity to different types of free-stream disturbance is studied for a Mach 4.5 boundary-layer flow over a flat plate by using the approaches of both direct numerical simulation and linear stability theory. This paper is Part 3 of a three-part study of the receptivity of supersonic boundary layers to free-stream disturbances. The present paper investigates receptivity to four types of different free-stream disturbances, i.e. slow and fast acoustic waves, entropy waves, and vorticity waves. A high-order shock-fitting scheme is used in the numerical simulation in order to accurately account for the effects of interactions between free-stream disturbance waves and the oblique shock wave. Numerical results on the generation of fast acoustic waves by free-stream entropy waves or vorticity waves are compared with those of a linear theory. Good agreement is obtained in both wave angles and amplitudes immediately behind the bow shock. It is found that the second-mode receptivity to free-stream slow acoustic waves is several times stronger than that to free-stream fast acoustic waves. This is because free-stream slow acoustic waves can directly induce and interact with the first and second Mack modes, while free-stream fast acoustic waves cannot. Instead, the free-stream fast acoustic waves can only induce and interact with stable mode I waves, which in turn induce unstable Mack modes. In the cases of receptivity to free-stream entropy waves and vorticity waves, it is found that the oblique shock wave created by the displacement of the boundary layer plays an important role because boundary-layer disturbances are mainly induced by fast acoustic waves generated behind the shock by free-stream forcing waves. As a result, mechanisms of the receptivity to free-stream entropy and vorticity waves are very similar to those of the receptivity to free-stream fast acoustic waves.

Transonic shock in a nozzle I: 2D case

Abstract: In this paper we establish the existence and uniqueness of a transonic shock for the steady flow through a general two-dimensional nozzle with variable sections. The flow is governed by the inviscid potential equation and is supersonic upstream, has no-flow boundary conditions on the nozzle walls, and an appropriate boundary condition at the exit of the exhaust section. The transonic shock is a free boundary dividing two regions of C 1,1-δ 0 flow in the nozzle. The potential equation is hyperbolic upstream where the flow is supersonic, and elliptic in the downstream subsonic region. In particular, our results show that there exists a solution to the corresponding free boundary problem such that the equation is always subsonic in the downstream region of the nozzle when the pressure in the exit of the exhaustion section is appropriately larger than that in the entry. This problem is motivated by the conjecture of Courant and Friedrichs on the transonic phenomena in a nozzle [10]. Furthermore, the stability of the transonic shock is also proven when the upstream supersonic flow is a small steady perturbation for the uniform supersonic flow and the corresponding pressure at the exit has a small perturbation. The main ingredients of our analysis are a generalized hodograph transformation and multiplier methods for elliptic equation with mixed boundary conditions and comer singularities.

Oblique shocks in rapid granular flows

Abstract: The interaction between rapid, free-surface granular flows and deflecting dams is investigated by laboratory experimentation and by the formulation and analysis of a shallow-layer model of the motion. It is found that uniform, downslope flows of grains are deflected to flow parallel to the barrier and that upstream of the barrier, the flow state undergoes an abrupt transition whereby its depth, velocity, and direction of motion change. These oblique shocks are investigated for a range of Froude numbers and for a range of angles between the deflector and the direction of steepest descent. The experimental results are found to be in good agreement with predictions from the shallow-layer theory. Experiments were also conducted with rapid, free-surface flows of water. They reveal not only similarities between the steady deflection patterns of the water and grain flows, but also some differences in the nature of their initial interaction. A simple interpretation for this is given in terms of the relatively high pressures that develop during the initial impact of the incompressible water with the impermeable barrier. Deflecting dams are deployed to defend against large-scale snow avalanches and these results are applied to this situation.

Modeling for Control of a Generic Airbreathing Hypersonic Vehicle

Abstract: The unique airframe-engine configuration of airbreathing hypersonic flight vehicles (AHFV) pose a significant challenge for design of controllers for these vehicles. The Airframe-engine configuration, the wide range of speed and the extreme flight conditions result in significant coupling among various dynamics and modeling uncertainties. There is almost a complete absence of models that adequately include and quantify the unique attributes for this class of vehicles. This paper describes a high-fidelity CFD-based model of a full scale generic airbreathing hypersonic flight vehicle under development at the Multidisciplinary Flight Dynamics and Control Laboratory (MFDCLab, www.calstatela.edu/centers/mfdclab) at California State University, Los Angeles (CSULA). The vehicle (CSULA-GHV), which has an integrated airframe-propulsion system configuration, resembles an actual test vehicle. The vehicle is specifically designed to study the challenges associated with modeling and control of airbreathing hypersonic vehicles and to investigate and quantify the couplings between the aerodynamics, the propulsion system, the structural dynamics, and the control system. The configuration of the vehicle and its dimensions are developed based on 2-D compressible flow theory, and a set of mission requirements broadly accepted for a hypersonic cruise vehicle intended for both space access and military applications. Analytical aerodynamic calculations are conducted assuming a cruising condition of Mach 10 at an altitude of 30 km. The 2-D oblique shock theory is used to predict the shock wave angles, the pressure on the frontal surface, and the Mach number at the engine inlet. The scramjet engine is simply modeled by a 1-D compressible flow with heating. The exit flow is modeled using 2-D expansion wave theory to predict the pressure on the rear surface. The unique aspect of this study is the use of coupled simulations using multi-physic software in conjunction with theory enabling quantification of the couplings which are broadly ignored in models used for control system design. Simulation results developed to date are presented.

Experimental study of Richtmyer-Meshkov instability induced by cylindrical shock waves

Abstract: The paper describes the results of holographic interferometric flow visualization of the Richtmyer-Meshkov instability induced by cylindrical shock waves propagating across cylindrical interfaces. Experiments were conducted in an annular coaxial vertical diaphragmless shock tube, which can produce converging cylindrical shock waves with minimum disturbances. The shock wave converged and interacted with a cylindrical soap bubble filled with He, Ne, air, Ar, Kr, Xe, or SF6. The soap bubble was placed coaxially in the test section. The effects of density variation on the Richtmyer-Meshkov instability for a wide range of Atwood numbers were determined. Pressure histories at different radii during the shock wave implosion and reflection from the center were measured. Double-exposure holographic interferometry was used and the motion of the converging shock wave and its interaction with the gaseous interface were visualized. The variation of the pressure at the center with interface Atwood number for constant i...

Macrostructure of collisionless bow shocks: 1. Scale lengths

Abstract: [1] The global structure of collisionless bow shocks is investigated using a 2.5-dimensional electromagnetic hybrid code. This allows us to study the macrostructure of the shock while accounting for microphysical processes at the shock. The study entails the interaction of solar wind with magnetic dipoles of varying strength. For very weak dipoles the interaction does not lead to formation of a shock since the obstacle is not strong enough for the flow to become subsonic. For lager dipole strengths, a bow shock/wave is formed due to the presence of a plasma stagnation region in front of the dipole. It is found that the quasi-perpendicular part of this boundary corresponds to a true shock wave, whereas the quasi-parallel side consists of a magnetosonic wave followed by a rotational discontinuity. The backstreaming ions in the foreshock of this interaction lead to the generation of parallel propagating sinusoidal waves. These waves result in beam scattering, however, do not affect the solar wind. The formation of quasi-parallel shock is tightly connected to the generation of oblique compressional waves. These waves are generated by backstreaming ions having a beam-ring distribution function and are an inherent part of the shock dissipation processes. The results demonstrate that the two classes of 30 s ULF waves observed in the ion foreshock are unrelated. The results also demonstrate that at least in 2.5-dimensional, plasma scales determine the nature of the bow shock to a large extent although system size can influence both particle acceleration and evolution of ULF waves.

Multistage interaction of a shock wave and a strong vortex

Abstract: The interaction between a shock wave and a strong vortex is simulated systematically through solving the two-dimensional, unsteady compressible Navier–Stokes equations using a fifth-order weighted essentially nonoscillatory finite difference scheme. Our main purpose in this study is to characterize the flow structure and the generation of sound waves of the shock–strong vortex interaction. The simulations show that the interaction of a shock wave and a strong vortex has a multistage feature. It contains the interaction of the shock wave and the initial vortex, of the reflected shock wave and the deformed vortex and of the shocklets and the deformed vortex. The shocklets are generated by the secondary interaction. Due to the complex reflected shock structure, there exist interactions between the reflected shock waves and the sound waves. Many pressure waves are embedded in the second and third sound waves.

Influence of shock-wave profile shape on dynamically induced damage in high-purity copper

Abstract: Studies of the creation of damaged regions leading to failure are conducted using flat-top and triangular shock waves generated in gas-gun experiments as well as quasi-isentropic ramp waves. Shock waves are used to generate release waves, both behind the shock and on reflection at the free surface. It is the interaction of these release waves that places the material in a state of tension which can ultimately result in damage and possibly complete failure. The peak tensile stress and its location in the material are determined by the wave shape. Damage evolution processes and localized behavior are studied under flat-top, triangular, and ramp wave loading∕unloading using time-resolved free-surface velocity interferometry and post-experiment metallurgical analysis of the soft recovered samples.

Interactions Between Shock and Acoustic Waves in a Supersonic Inlet Diffuser

Abstract: The interactions between shock and acoustic waves in a supersonic inlet diffuser are investigated numerically. The model treats the viscous flowfield in an axisymmetric, mixed-compression inlet operating under supercritical conditions. It is solved by means of a finite-volume approach using a four-stage Runge-Kutta scheme for temporal derivatives and the Harten-Yee upwind total-variation-diminishing scheme for spatial terms. Various distinct flow structures, including shock/boundary-layer and shock/shock interactions, are studied under the effects of externally imposed pressure oscillations at the diffuser exit over a wide range of forcing frequencies and amplitudes. As a result of the terminal shock oscillation induced by the impressed disturbances and the cyclic variation of the oblique/normal shock intersection, large vorticity fluctuations are produced in the radial direction. The characteristics of the shock/boundary-layer interactions (such as the size of the separation bubble, the terminal shock configuration, and the vorticity intensity) are also greatly influenced by the acoustic-driven shock oscillation. The overall response of the inlet aerodynamics to acoustic waves can be characterized by the mass-transfer and acoustic-admittance functions at the diffuser exit. Their magnitudes decrease with increasing frequency. A supersonic inlet acts as an effective acoustic damper, absorbing disturbances arising downstream. Severe flow distortion, however, may arise from shock oscillation and subsequently degrade the combustor performance.

X-ray synchrotron emission from the oblique shock in the jet of the powerful radio galaxy 3C 346

Abstract: We report the first detection, with Chandra, of X-ray emission from the jet of the powerful narrow-line radio galaxy 3C 346. X-rays are detected from the bright radio and optical knot at which the jet apparently bends by approximately 70°. The Chandra observation also reveals a bright galaxy-scale atmosphere within the previously known cluster and provides a good X-ray spectrum for the bright core of 3C 346. The X-ray emission from the knot is synchrotron radiation, as seen in lower-power sources. In common with these sources, there is evidence of morphological differences between the radio/optical and X-ray structures, and the spectrum is inconsistent with a one-component continuous-injection model. We suggest that the X-ray-bright knot is associated with a strong oblique shock in a moderately relativistic, light jet, at ∼ 20° to the line of sight, and that this shock is caused by the jet interacting with the wake in the cluster medium behind the companion galaxy of 3C 346. The general jet curvature can result from pressure gradients in the cluster atmosphere.

Shock formation at the magnetic collimation of relativistic jets

Abstract: If the observed relativistic plasma outflows in astrophysical jets are magnetically collimated and a single-component model is adopted, consisting of a wind-type outflow from a central object, then a problem arises with the inefficiency of magnetic self-collimation to collimate a sizeable portion of the mass and magnetic fluxes in the relativistic outflow from the central object. To solve this dilemma, we have applied the mechanism of magnetic collimation to a two-component model consisting of a relativistic wind-type outflow from a central source and a non-relativistic wind from a surrounding disc. By employing a numerical code for a direct numerical solution of the steady-state problem in the zone of super-fast magnetized flow, which allows us to perform a determination of the flow with shocks, it is shown that in this two-component model it is possible to collimate into cylindrical jets all the mass and magnetic fluxes that are available from the central source. In addition, it is shown that the collimation of the plasma in this system is usually accompanied by the formation of oblique shock fronts. The non-relativistic disc-wind not only plays the role of the jet collimator, but it also induces the formation of shocks as it collides with the initially radial inner relativistic wind and also as the outflow is reflected by the system axis. Another interesting feature of this process of magnetic collimation is a sequence of damped oscillations in the width of the jet.

Preliminary Design of a 2D Supersonic Inlet to Maximize Total Pressure Recovery

Abstract: *† This paper provides a method of preliminary design for a two-dimensional, mixed compression, two-ramp supersonic inlet to maximize total pressure recovery and match the mass flow demand of the engine. For an on-design condition, the total pressure recovery is maximized according to the optimization criterion, and the dimensions of the inlet in terms of ratios to the engine face diameter are calculated. The optimization criterion is defined such that in a system of (n-1) oblique shocks and one normal shock in two dimensions, the maximum shock pressure recovery is obtained when the shocks are of equal strength. This paper also provides a method to estimate the total pressure recovery for an off-design condition for the specified inlet configuration. For an off-design condition, conservative estimation of the total pressure recovery is given so that performance of the engine at the off-design condition can be estimated. To match the mass flow demand of the engine, the second ramp angle is adjusted and the open/close schedule of a bypass door is determined. The effects of boundary layer are not considered for the supersonic part of the inlet, however friction and expansion losses are considered for the subsonic diffuser. Nomenclature α = Angle of attack j β = The installation angle of the j th ramp γ = The ratio of specific heats j δ = The flow deflection angle of the j th shock (j th ramp half angle) d θ = The half expansion angle of the subsonic diffuser j θ = The shock wave angle of the j th shock * A = The cross section area of flow tube at throat where the flow is sonic j A = The cross section area of flow at j th station point 54 AR = The ratio of inlet cross section areas at station points 5 and 4 5 d = The engine diameter at station point 5 (engine face) 6 d = The engine diameter at station point 6 (fan face) H = Flight altitude c h , 0 h = The captured freestream flow tube height i h = The height of inlet at the entry, measured perpendicular to the flight direction j h = The height of j

Separated Shock-Boundary-Layer Interaction Control Using Streamwise Slots

Abstract: Experiments have been performed to assess the potential of discretely placed arrays of streamwise slots to control a separated normal shock wave‐turbulent boundary-layer interaction. The supersonic blowdown wind tunnel was operated at a Mach number of 1.5 and a freestream Reynolds number of 26 × × 10 6 m −1 .A tM = 1.5 slot control bifurcated the shock to give a λ shock structure that was significantly larger than that seen without control. The effect on the shock was fairly two-dimensional and persisted in the region between control devices, showing that three-dimensional control devices can have a global effect on the shock structure. In addition, slot control altered the nature of the separated boundary layer from a two-dimensional separation bubble to give highly three-dimensional regions of attached and separated flow. There is evidence that slot control also introduced streamwise vortices, which may help delay or prevent downstream separation.

Dynamic calibration of fast-response probes in low-pressure shock tubes

Abstract: Shock tube flows resulting from the incomplete burst of the diaphragm are investigated in connection with the dynamic calibration of fast-response pressure probes. As a result of the partial opening of the diaphragm, pressure disturbances are observed past the shock wave and the measured total pressure profile deviates from the envisaged step signal required by the calibration process. Pressure oscillations are generated as the initially normal shock wave diffracts from the diaphragm's orifice and reflects on the shock tube walls, with the lowest local frequency roughly equal to the ratio of the sound speed in the perturbed region to the shock tube diameter. The energy integral of the perturbations decreases with increasing distance from the diaphragm, as the diffracted leading shock and downwind reflections coalesce into a single normal shock. A procedure is proposed to calibrate fast-response pressure probes downwind of a partially opened shock tube diaphragm.

Three-Dimensional Normal Shock-Wave/Boundary-Layer Interaction in a Rectangular Duct

Abstract: The three-dimensional flow structure induced by normal shock-wave/turbulent boundary-layer interaction in a constant-area rectangular duct is investigated by a laser-induced-fluorescence method This diagnostic system uses an argon-ion laser as a light source, and the target gas is dry nitrogen with iodine seeded as a fluorescence material The Mach-number distributions in the duct are obtained from the measured fluorescence intensity, and the threedimensional flow pattern in the expansion region downstream of the initial shock wave is clarified In addition to this, the region having locally higher Mach number near the duct corners is observed immediately behind the shock wave, and the three-dimensional shape of the boundary layers is found These flow characteristics are reproduced by solving the Navier‐Stokes equations numerically The calculated result reveals that the complicated shock-wave configuration is formed at the duct corner because of the interaction of two bifurcated shock waves developed on the two perpendicularly adjacent walls The simple flow model is also constructed by considering this interaction This model can explain very well the three-dimensional flow characteristics

Effect of strong thermalization on shock dynamical behavior

Abstract: [1] The dynamics of the perpendicular shock front is examined under various plasma parameters by using particle-in-cell numerical simulation. As widely accepted, above the critical Mach number (∼3) the front of (quasi-)perpendicular shocks show nonstationary behavior due to the shock self-reformation. In much higher Mach number regime (MA > 20), we find that dynamics of the shock front self-reformation can be modified. Nonlinear evolution of microinstabilities in the shock transition region results turbulent profiles in a microscopic view (≤c/ωpe), while, from a macroscopic view (>several c/ωpe) because of rapid, strong thermalization in the shock transition region, the localized accumulation of the plasma due to ion dynamics is smeared out in both of the velocity phase space and real space. As a result, the shock self-reformation is realized within a reduced time and space. We can say there is a possibility that rapid, strong dissipation helps to stabilize the macroscopic shock front dynamics; the shock self-reformation still persists, though. The strong thermalization is caused by the nonlinear evolution of two-stream instability between the electron and the reflected/incident ion components and following ion-acoustic instability. We think that the modification of the shock self-reformation process observed in high Mach number regime indicates an important role of electron kinetics and heating in the macroscopic shock front behavior.

On Particle Acceleration around Shocks. II. A Fully General Method for Arbitrary Shock Velocities and Scattering Media

Abstract: The probability that a particle, crossing the shock along a given direction, is reflected backward along another direction was shown to be the key element in determining the spectrum of nonthermal particles accelerated via the Fermi mechanism around a plane-parallel shock in the test-particle limit. Here an explicit equation for this probability distribution is given for both the upstream and downstream sections. Although analytically intractable, this equation is solved numerically, allowing the determination of the spectrum in full generality, without limitation to shock speed or scattering properties. A number of cases are then computed, making contact with previous numerical work, in all regimes: Newtonian, transrelativistic, and fully relativistic.

Direct Calculation of Wave Implosion for Detonation Initiation

Abstract: Numerical simulations of imploding shock waves for detonation initiation are reported. We solve the one-dimensional Navier-Stokes equations and two-dimensional Euler equations for chemically reactive flows by the space-time conservation element and solution element method. One-dimensional results in cylindrical coordinates show that a converging shock produced by breaking a diaphragm for an initial pressure ratio of 1:0.2 atm is able to successfully initiate a detonation in an argon-diluted hydrogen/oxygen mixture (0.2H 2 + 0.1O 2 + 0.7Ar) initially at 300 K. The result also shows a two-shock implosion system caused by the interaction between the reflected primary shock and the imploding contact discontinuity. Two-dimensional solutions focus on imploding polygonal shock fronts. In each polygonal section, the imploding shock is analogous to a planar shock wave entering a channel with converging walls leading to complex wave reflections. Similar to that in the one-dimensional results, pressure histories in the focal region show multiple implosions.

Flow Separation in Rocket Nozzles, a Simple Criteria

Abstract: Cold and hot flow tests were conducted to investigate the flow separation in rocket nozzles. The results are presented. A separatio n data base including a wide range of literature data is established to evaluate the influence of propellant combination and nozzle design on flow separation. As a result a simple separation criteria is suggested. Nomenclature p = pressure cf = friction coefficient Ma = wall Mach number Mades = design Mach number κ = adiabatic exponent θ = deflection angle σ = oblique shock angle u,U = velocity δ = boundary layer thickness δ *

The Origin of Complex Behavior of Linearly Polarized Components in Parsec-Scale Jets

Abstract: Evidence that the magnetic fields of extragalactic jets have a significant fraction of their energy in a random component is briefly summarized, and a detailed model of evolving, jet polarization structures is constructed, based on this picture. The evolving magnetic field structure of an oblique shock complex that forms in a relativistic jet simulation is explored by using velocity data from the hydrodynamical simulation to advect an initially random magnetic field distribution. Radiative transfer calculations reveal that emission from a propagating region of magnetic field, ‘ordered’ by the shock, and lying approximately transverse to the flow direction, merges with that from an evolving sheared region at the flow periphery. If such a flow were barely resolved, observation would suggest evolution from a somewhat oblique, to a more longitudinal, magnetic field structure with respect to the flow axis, while higher resolution observations would infer a component following a non-linear trajectory, and with a magnetic field orientation that rotates during evolution. This result highlights the ambiguity in interpreting VLBP data, and illustrates the importance of simulations in providing a framework for proper interpretation of such data. Subject headings: galaxies: jets — hydrodynamics — magnetic fields — polarization — radiative transfer — relativity — shock waves

Shock Wave Induced Separation Control by Streamwise Vortices

Abstract: Control of shock wave and boundary layer interaction finds still a lot of attention. Methods of this interaction control have been especially investigated in recent decade. This research was mostly concerned with flows without separation. However, in many applications shock waves induce separation often leads to strong unsteady effects. In this context it is proposed to use streamwise vortices for the interaction control. The results of experimental investigations are presented here. The very promising results were obtained, meaning that the incipient separation was postponed and the separation size was reduced for the higher Mach numbers. The decrease of the RMS of average shock wave oscillation was also achieved.

Three-dimensional reactive shock bifurcations

Abstract: We model interactions of a premixed flame with incident and reflected shocks in a rectangular shock tube using three-dimensional (3D) reactive Navier–Stokes numerical simulations. Shock-flame interactions occur in the presence of boundary layers that cause the reflected shock to bifurcate and form a reactive shock bifurcation (RSB), which contains a flame in the recirculation zone behind the oblique shock. The recirculation zone acts as a flame holder thus attaching the flame to the shock in the vicinity of the wall, and providing a mechanism for a detonationless supersonic flame spread. The accelerated burning induced by an RSB, and Mach stems that may result from RSB–RSB interactions, promote hot-spot formation, and eventually accelerate deflagration-to-detonation transition. Schlieren-type images generated from the simulation results show that the 3D structure of an RSB may not always be easily recognized in experiments if the RSB is attached to the surface of the observation window. The main 3D effect observed in the simulations is caused by the presence of the second no-slip wall in a 3D rectangular channel. Two RSBs that form at adjacent walls interact with each other and produce an oblique Mach stem between two oblique shocks. The oblique Mach stems then interacts with a central Mach stem that forms near symmetry plane, and this interaction creates a hot-spot that leads to a detonation initiation.

Self-similar Evolution of Relativistic Shock Waves Emerging from Plane-parallel Atmospheres

Abstract: We study the evolution of the ultrarelativistic shock wave in a plane-parallel atmosphere adjacent to a vacuum and the subsequent breakout phenomenon. When the density distribution is a power law in distance from the surface, there is a self-similar motion of the fluid before and after the shock emergence. The time evolution of the Lorentz factor of the shock front is assumed to follow a power law when the time is measured from the moment at which the shock front reaches the surface. The power index is found to be determined by the condition for the flow to extend through a critical point. The energy spectrum of the ejected matter as a result of the shock breakout is derived, and its dependence on the strength of the explosion is also deduced. The results are compared with the self-similar solution for the same problem with nonrelativistic treatment.

Implosion of a spherical shock wave reflected from a spherical wall

Abstract: The paper describes results of experiments of a converging spherical shock wave reflected from a spherical wall. In order to visualize the motion and the flow field behind the shock waves, an aspheric lens-shaped transparent test section made of acrylic PMMA (polymethyl methacrylate) with an inner spherical cavity was designed and constructed. This test section made optical flow visualization with collimated object beams possible. Spherical shock waves were produced at the centre of the spherical cavity by explosion of silver azide pellets ranging from 1.0 to 10.0 mg with corresponding energies of 1.9 to 19 J. The charges were ignited by irradiation of a pulsed Nd:YAG laser beam. Pressures were also measured at two points with pressure transducers mounted flush at the inner wall of the test section. The pellet was simultaneously ignited on two sides or was shaped to produce a uniform diverging spherical shock wave. This spherically diverging shock wave was reflected from the spherical inner wall of the test section to form a converging spherical shock wave. We visualized the shock-wave motion by using double exposure holographic interferometry and time-resolved high-speed video recording. The sequence of diverging and converging spherical shock-wave propagations and their interaction with gaseous explosion products were observed. The convergence, acceleration and stability of the imploding shock wave in the test section were studied.

Experimental evidence for Plotkin model of shock unsteadiness in separated flow

Abstract: Experimental evidence is presented in support of a model of separation shock unsteadiness developed by Plotkin [AIAA J. 13, 1036 (1975)]. Under this model, the position of the separation shock follows linearly damped Brownian motion. The model describes the manner in which relatively broad-band perturbations in the incoming flow lead to relatively low-frequency motion of the separation shock. Close agreement was found between the predictions of the model and the autospectra and autocorrelations of wall pressure fluctuations and shock position fluctuations for several blunt fin flows at Mach 3 and Mach 5. Given the similarity of the power spectra of wall-pressure fluctuations for a variety of separated, supersonic flows, this description may have broad applicability.

Inflow and shock formation in supersonic, rarefied plasma expansions

Abstract: In this paper the physics of plasma expansion in the rarefied regime is reviewed. Densities, temperatures, and velocity distributions in argon, hydrogen, and nitrogen expansions that have been measured using laser scattering and fluorescence techniques are compared. The velocity distributions in the region of the expansion where the density is below the background density show a bimodal character. It is interpreted in terms of a component expanding from the source and a component flowing into the plasma expansion from the periphery. Also in the shock of the expansion, bimodal velocity distributions are encountered. These distributions show the gradual change in the flow from supersonic to subsonic—the formation of the shock. From a comparison of the three expansions, a general view of the shock formation is derived. This new insight leads to a better understanding of how the chemical reactivity of the usually impenetrable, supersonic plasma can be used most efficiently.

Attenuation of shock waves propagating over arrayed baffle plates

Abstract: The paper reports results of shock tube experiments of the attenuation of shock waves propagating over arrayed baffle plates, which is motivated to simulate shock wave attenuation created accidentally at the acoustic delay line in synchrotron radiation factory upon the rupture of a metal membrane separating the acceleration ring at high vacuum and atmospheric test chambers. Experiments were carried out, by using double exposure holographic interferometry with double path arrangement, in a 100 mm×180 mm shock tube equipped with a test section of 180 mm×1100 mm view field. Two baffle plate arrangements were tested: Oblique and staggered baffle plates; and vertical symmetric ones. Pressures were measured along the shock tube sidewall at individual compartments for shock Mach numbers ranging from 1.2 to 3.0 in air. The results were compared with a numerical simulation. The rate of shock attenuation over these baffle plates was compared for vertical and oblique baffle plates. Shock wave attenuation is more pronounced in the oblique baffle plate arrangements than in the vertical ones.