Showing papers on "Oblique shock published in 1991"

Morphology of standing oblique detonation waves

Abstract: Proposals have been made to utilize stabilized oblique detonation waves (ODWs) for the propulsion of hypersonic air-breathing vehicles and hypervelocity mass launchers. There exists hypersonic flight regimes where premixing of fuel and air may be desirable or unavoidable due to finite chemical induction times. Consequently, it is essential to understand under what conditions detonations may occur in order to design supersonic combustors to either avoid or utilize them efficiently. A theoretical analysis is made of supersonic flow of a combustible gas mixture past a wedge or an inclined wall, which shows that, for approach velocities roughly 25% or more greater than the Chapman-Jouguet velocity of the reactant mixture, there exists a usefully wide range of turning angles within which ODWs may be attached or stabilized. For smaller wedge angles, either an incomplete ODW, shock-induced combustion, or no combustion at all may ensue. For larger wedge angles, the wave will detach and form an overdriven normal detonation or normal shock-induced combustion wave immediately upstream of the stagnation point, decaying off axis to either a single oblique Chapman-Jouguet wave, or bifurcating to form an oblique shock followed by a shock-induced deflagration.

Hybrid simulation of the formation of a hot flow anomaly

Abstract: Results are presented from two-dimensional hybrid simulations of the interaction of a supercritical quasi-perpendicular collisionless shock wave with current sheets embedded in the upstream flow. The current sheet normals are perpendicular to the shock normal. When the motional electric field is directed toward the discontinuity, the interaction leads to the generation of a region with high ion temperature, low magnetic field, and low density, which has many properties of a class of events variously referred to as active current sheets, hot diamagnetic cavities, or hot flow anomalies (HFAs). The simulations demonstrate that the HFA results from the interaction of ions reflected at the shock with the current sheet. The interaction does not involve an instability and is not caused by the collision of the current sheet and the shock but rather is a property of a shock with an embedded current sheet. The simulations suggest that the HFAs formed in this way are always attached to the shock and extend into both the upstream and downstream regions.

Non-invasive determination of shock wave pressure generated by optical breakdown

Abstract: Shock waves generated by a laser-induced plasma were investigated using a pump-and-probe technique. Both 7-ns and 40-ps laser pulses at 1.06 μm were employed to initiate breakdown in water. Two He-Ne laser beams were used as a velocity probe, allowing the accurate measurement of the shock velocity around the plasma. The maximum shock pressure was determined from the measured shock velocities, the jump condition and the equation of state for water. The conservation of the total momentum of the shock front was used to derive expressions for the shock velocity, particle velocity and shock pressure vs. the distance (r) from the center of the plasma. For a shock wave of spherical symmetry, the shock pressure is proportional to 1/r2. Our work shows that the expanding plasma initially induces a shock wave; the shock wave dissipates rapidly becoming an acoustic wave within 300–500 μm.

Computational Study on the Interaction Between a Vortex and a Shock Wave

Abstract: A computational study of two-dimensional shock vortex interaction is discussed in this paper. A second order upwind finite volume method is used to solve the Euler equations in conservation form. In this method, the shock wave is captured rather than fitted so that the cases where shock vortex interaction may cause secondary shocks can also be investigated. The effects of vortex strength on the computed flow and acoustic field generated by the interaction are qualitatively evaluated.

Computation of shock wave diffraction at a ninety degrees convex edge

Abstract: This paper presents a numerical study of shock wave diffraction at a sharp ninety degrees edge, using an explicit second-order Godunov-type Euler scheme based upon the solution of a generalized Riemann problem (GRP). The Euler computations produce flow separation very close to the diffraction edge, leading to a realistic development of the separated shear layer and subsequent vortex roll-up. The diffracted shock wave, and the secondary shock wave, are both reproduced well. In addition a pair of vortex shocks are shown to form, extending well into the vortex core.

Structure of medium Mach number quasi-parallel shocks: Upstream and downstream waves

Abstract: The transition from steady low-Mach-number to unsteady high-Mach-number quasi-parallel shocks was investigated by performing large-scale 1D hybrid code simulations at increasing Mach numbers. It was found that only at very low Mach number shocks the steepening is limited by upstream phase-standing whistlers, as predicted by the classical theory (Tidman and Northrop, 1968). In the intermediate region of Mach numbers between 1.5 and 3.5, a very diverse behavior is observed. Backstreaming ions generate fast magnetosonic waves which dominate the upstream, with wavelengths longer than phase-standing whistlers. At increasing Mach numbers, the phase and group velocities of the dominant waves are reduced until they point back toward the shock; when there is sufficient energy flux in these waves, they lead to unsteady shock behavior and eventually to shock reformation.

Shock wave effects on a turbulent flow

Abstract: A parametric study is done to investigate the change in a turbulent flow field caused by the passage of a shock wave. Two parameters are studied: the initial turbulent kinetic energy and the shock wave strength or density jump. A random or turbulent flow field is initiated within a two‐dimensional box. Euler’s equations are then solved using a second‐order accurate Godunov shock capturing method to calculate the change in turbulent structure and flow field parameters caused by the passage of a shock wave through the turbulent field. Two fields were analyzed, a random density field and a random velocity field. The passage of a shock through the random density field caused density and pressure variations that compare very well with experiments. Results of the shock passage through the random velocity field show that the shock causes an amplification in the turbulent kinetic energy of about 2 on a per unit mass basis. Furthermore, the length scale of the turbulent field behind the shock is smaller than that in front of the shock. Energy weighted wave numbers increase by as much as 30%. This change in length scales seems to be in disagreement with some experiments which seem to show larger time scales and larger length scales behind a shock, but in agreement with another experiment. For both results, fields containing strong shocks or weak turbulent fields yield the largest change in flow parameters. The shock wave is also affected by the turbulent field. Increasing the initial turbulent kinetic energy caused a straight shock wave to evolve into a shock containing curves and wrinkles of a size similar to the length scale of the unshocked turbulent field. These curves and wrinkles can lead to the generation of additional flow field oscillations.

Origin of diffuse superthermal ions at quasi‐parallel supercritical collisionless shocks

Abstract: Large-scale one-dimensional hybrid simulations of quasi-parallel shocks have been performed in order to study the acceleration of upstream energetic ions. In these self-consistent simulations a certain part of the incident ions is accelerated and constitute diffuse upstream particles, which are subject to further scattering in upstream magnetosonic waves of their own making. The number of superthermal particles close to the shock reaches a steady state within ≲30 Ωci−1. The ratio of diffuse upstream particles to solar wind particles decreases slightly with increasing shock Mach number and increases with decreasing angle ΘBn between the upstream magnetic field and the shock normal. The acceleration of thermal particles to superthermal energies occurs by a more or less coherent process: thermal ions of the incident distribution stay for an extended time period at the shock. Because of the large noncoplanarity magnetic field component they grad B drift within the coplanarity plane and gain energy by the tangential electric field. They also gain energy due to wave-particle scattering when they stay near the shock and when they finally leave the shock in the upstream direction during a shock re-formation cycle. Since the backstreaming particles excite the upstream waves by an ion/ion beam instability, they feed energy to the wave field. Therefore, in the shock frame a few of the backstreaming ions have an energy below the initial energy. The particle distribution is diffuse in velocity space and exhibits a spherical hole, which is approximately centered at the phase velocity of the upstream waves. This indicates that the particles are pitch angle scattered in the upstream wave field. The results show that superthermal upstream particles are an integral part of quasi-parallel collisionless shocks and that no particular seed particle population is necessary for shock acceleration. The shock structure and the first-order Fermi acceleration problem have therefore to be considered simultaneously.

Validity of Linearized Unsteady Euler Equations with Shock Capturing

Abstract: By examining the nature of shock movement, we first show that the unsteady lift due to the movement of a shock is linear with the magnitude of the shock movement. The argument that is presented holds true regardless of the shock structure, which is determined by the level of viscosity. This proof is the basis for showing that the linear perturbation equations can be used to determine not only the unsteadiness of the flowfield away from the shock but also the effect of the shock movement as well. Shock capturing is a computational technique that in effet adds a large amount of viscosity in the shock region, smearing shocks over several cells

The reflected pressure field in the interaction of weak shock waves with a compressible foam

Abstract: This paper deals with the waves that are reflected from slabs of porous compressible foam attached to a rigid wall when impacted by a weak shock wave. The interest is in establishing possible attenuation of the pressure field after a shock or blast wave has struck the surface. Foam densities from 14 to 38 kg/m3 were tested over a range of shock wave Mach numbers less than 1.4. It is shown that the initial reflected shock wave strength is accurately predicted by the pseudo-gas model of Gelfand et al. (1983), with a pressure ratio of approximately 80% of the value for reflection off a rigid wall. Evidence is presented of gas entering the foam during the early stages of the process. A second wave emerges from the foam at a later stage and is separated from the first by a region of constant velocity and pressure. This second wave is not a shock wave but a compression front of significant thickness, which emerges from the foam earlier than predicted by the pseudo-gas model. Analysis of the origin of this wave points to much more complex flows within the foam than previously assumed, particularly in an apparent decrease in average wave front speed as the foam is compressed. It is shown that the pressure ratio across both these waves taken together is slightly higher than that for reflection off a rigid wall. In some cases this compression wave train is followed by a weak expansion wave.

Electron acceleration at nearly perpendicular collisionless shocks: 2. Reflection at curved shocks

Abstract: Test particle simulations by Krauss-Varban et al. (1989), carried out for plane shocks, have confirmed previous results of Wu (1984) and Leroy and Mangeney (1984) that electrons can be effectively accelerated at nearly perpendicular shocks. This paper investigates the reflection and acceleration of electrons at a nearly perpendicular shock, using two-dimensional test-particle calculations which account for the effect of shock curvature. The computations show that reflected electron fluxes are of the order of observed fluxes. For several reasons, the combined effects of shock curvature are far less severe than anticipated.

Effect of periodic blowing on attached and separated supersonic turbulent boundary layers

Abstract: Periodic blowing at frequencies up to 5 kHz was used to control the unsteadiness of two-dimensional shockwave/turbulent boundary-layer interactions. Two separate experiments were performed. In the first case, periodic blowing was introduced through a spanwise slot in the wall to produce an unsteady shock-wave/boundarylayer interaction boundary layer on the tunnel wall. In the second case, periodic blowing was introduced into the shock-induced separation bubble formed by a 24-deg compression corner interaction. The incoming flow conditions for both experiments were My. = 2.84, Rejl = 6.5 x 10 7/m, and 80 = 26 mm. Measurements of the fluctuating mass flux and wall pressure were made, and the unsteady flowfield was visualized through stroboscopic schlieren videography. The measurements were conditionally sampled based on the phase of the blowing cycle. The results suggest that (at least in this case) the naturally unsteady shock motion in the compression ramp interaction does not contribute significantly to the turbulence amplification, as had been previously thought. Instead, there is strong evidence to suggest that large-scale motions associated with the maxima in the angular momentum profiles in the downstream boundary layer are responsible for the large mixing observed.

An Investigation of Contoured Wall Injectors for Hypervelocity Mixing Augmentation

Abstract: A parametric study of a class of contoured wall fuel injectors is presented. The injectors were aimed at enabling shock-enhanced mixing for the supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Short combustor residence time, a requirement for fuel injection parallel to the freestream, and strong sensitivity of overall vehicle performance to combustion efficiency motivated the investigation. Several salient parametric dependencies were investigated. Injector performance was evaluated in terms of mixing, losses, jet penetration and heating considerations. A large portion of the research involved a series of tests conducted at the NASA Langley High - Reynolds Number Mach 6 Wind-Tunnel. Helium was used as an injectant gas to simulate hydrogen fuel. The parameters investigated include injector spacing, boundary layer height, and injectant to freestream pressure and velocity ratios. Conclusions concerning injector performance and parameter dependencies are supported by extensive three-dimensional flow field surveys as well as data from a variety of flow visualization techniques including Rayleigh scattering, Schlieren, spark-shadowgraph, and surface oil flow. As an adjunct to these experiments, a three-dimensional Navier-Stokes solver was used to conduct a parametric study which closely tracked the experimental effort. The results of these investigations strongly complemented the experimental work. Use of the code also allowed research beyond the fairly rigid bounds of the experimental test matrix. These studies included both basic investigations of shock-enhanced mixing on generic injectors, and applied efforts such as combining film-cooling with the contoured wall injectors. Location of an oblique shock at the base of the injection plane was found to be a loss-effective method for enhancing hypervelocity mixing through baroclinic generation of vorticity and subsequent convection and diffusion. Injector performance was strongly dependent on the displacement effect of the hypersonic boundary layer which acted to modify the effective wall geometry. Strong dependence on injectant to freestream pressure ratio was also displayed. Mixing enhancement related to interaction of the unsteady component of the boundary layer with both steady and unsteady components of the flow field was found to be secondary, as were effects due to variation in mean shear between the injectant and the freestream in the exit plane.

Numerical studies of electron dynamics in oblique quasi-perpendicular collisionless shock waves

Abstract: Linear and nonlinear electron damping of the whistler precursor wave train to low Mach number quasi-perpendicular oblique shocks is studied using a one-dimensional electromagnetic plasma simulation code with particle electrons and ions. In some parameter regimes, electrons are observed to trap along the magnetic field lines in the potential of the whistler precursor wave train. This trapping can lead to significant electron heating in front of the shock for low beta(e). Use of a 64-processor hypercube concurrent computer has enabled long runs using realistic mass ratios in the full particle in-cell code and thus simulate shock parameter regimes and phenomena not previously studied numerically.

Instability in oblique C-type shocks

Abstract: A sequence of steady, oblique, C-type shock waves differing in the angle between the ambient magnetic field and the direction of shock propagation, θ s , are examined for stability to small perturbations in the plane containing the fluid velocities and the magnetic field. As θ s is reduced the steady shocks become stronger and the growth rate and wavenumber of the fastest growing mode increase

Characteristic-based flux limiters of an essentially third-order flux-splitting method for hyperbolic conservation laws

Abstract: A flux limiter based on characteristic variables is extended by a control volume flux formulation to approximate the convection term at the cell interface for an essentially third-order-accurate scheme. The basic algorithm uses implicit MUSCL-type flux splitting and the approximate factorization method. It is applied to three test problems: (i) a one-dimensional shock tube problem; (ii) a two-dimensional problem of an oblique shock step with Mach numbers 3 and 10 and a shock angle of 59°;(iii) a two-dimensional problem of transonic inviscid flow past an NACA0012 aerofoil with Mach number 0·8 at zero angle of attack. The computational results by the new flux limiter function are compared with the results of direct applications of the SMART algorithm, Leonard's SHARP algorithm, the third-order Van Leer flux-splitting method with a smooth limiter, Harten's second-order unwind-biased TVD scheme, Chakravarthy's third-order MUSCL-type TVD scheme and the exact solution. The comparison shows that the present method gives the most accurate and least oscillatory results with a rapid rate of convergence.

Fast acceleration of ions at quasi‐perpendicular shocks

Abstract: Acceleration of low-energy protons by quasi-perpendicular shocks is investigated. The pitch angle distribution of ions in the energy range 35 keV to 1 MeV has been determined across the interplanetary shock that passed through the ISEE 3 spacecraft on November 30, 1979. Upstream of the shock a bidirectional angular distribution was observed. It is suggested that multiple crossings of the field line with the surface of the shock, forming a “magnetic bottle”, may account for such an unusual angular distribution. The shock event was modeled by integrating particle trajectories numerically. Qualitative agreement between observations and simulations supports the idea of magnetic bottle field line formation. A detailed numerical study of particle acceleration has shown that in the bottle topology the particle flux is enhanced close to the shock front, contrary to the original scatter-free model, i.e., assuming homogeneous magnetic fields on both sides of the shock. It is suggested that multiple crossings of the field line with the shock may explain “shock spike” events.

Reflection of shock wave from a compression corner in a particle-laden gas region

Abstract: We investigated in this paper the progression of a shock-wave reflected from a compression corner in a particle-laden gas medium using a TVD class numerical technique and a MacCormack scheme. For a gas-only flow, the numerical results agreed well with the existing experimental data, suggesting that the gas phase is correctively solved. The effect of particle size and mass fraction ratio is investigated for a dilute gas-particle flow. It has been shown that the shock-wave diffraction and the flow configuration after the shock can become remarkably different from the gas-only flow depending on the particle parameters. Relaxation phenomenon due to the momentum drag and the heat exchange between the gas and the particle phases is explained.

A generalisation of the theory of geometrical shock dynamics

Abstract: The study of the propagation of a shock down a tube of slowly varying cross sectional area has proved to be most valuable in understanding the dynamics of shocks. A particular culmination of this work has been the theory of geometrical shock dynamics due to Whitham (1957, 1959). In this theory the motion of a shock may be approximately computed independently of a determination of the flow field behind the shock. In this paper the propagation of a shock down such a tube is reconsidered. It is found that the motion of the shock is described by an infinite sequence of ordinary differential equations. Each equation is coupled to all of its predecessors but only to its immediate successor, a feature which allows the system to be closed by truncation. Of particular relevance is the demonstration that truncation at the first equation in the sequence yields the A-M relation that is the basis for Whitham's highly successful theory. Truncation at the second equation yields the next level of approximation. The equations so obtained are investigated with analytic solutions being found in the strong shock limit for the propagation of cylindrical and spherical shock waves. Implementation of the theory in the numerical scheme of geometrical shock dynamics allows the computation of shock motion in more general geometries. In particular, investigation of shock diffraction by convex corners of large angular deviation successfully yields the observed inflection point in the shock shape near the wall. The theory developed allows account to be taken of non-uniform flow conditions behind the shock. This feature is of particular interest in consideration of underwater blast waves in which case the flow behind the shock decays approximately exponentially. Application of the ideas developed here provides an excellent description of this phenomenon.

Ion injection simulations of quasi‐parallel shock re‐formation

Abstract: One-dimensional hybrid simulations are used to investigate the process of quasi-parallel shock reformation and to examine the coupling of a beam of ions reflected at the shock to the incoming solar wind. A simple simulation configuration is constructed that makes it possible to control the properties of the background plasma and of the reflected ions. The length and time scales for the coupling of the reflected ions to the background plasma are investigated as functions of the upstream magnetic field direction, beam density, and beam temperature. The coupling length and time scales are found to vary systematically with the upstream magnetic field direction. The coupling occurs at roughly the time and location where the injected ions become deflected transverse to the shock normal direction.

Whistler waves associated with the Uranian bow shock: Outbound observations

Abstract: High-resolution magnetic field measurements from the first outbound crossing of the Uranian bowshock by the Voyager 2 spacecraft between January 27 and 30, 1986, are examined. Evidence is found of enhanced whistler wave activity in the vicinity of three shock crossings but little or no evidence of such activity elsewhere. Two wave events display two separate and simultaneous wave enhancements each. From an investigation of these events using high-resolution field data, it is concluded that they are analogous to those whistler waves upstream of the earth's bow shock that are driven by beams of electrons. An instability analysis is presented to show that a single electron beam with reasonable parameters can penetrate both of the upstream and downstream of a shock crossing. This event displays only one relatively broad spectral enhancement in the same frequency regime and is left-hand polarized in the spacecraft frame. It is argued that this event is the result of a gyrating proton distribution associated with the oblique shock.

Three-dimensional shock wave-turbulent boundary layer interactions generated by a sharp fin at Mach 4

Abstract: This paper describes a combined experimental and theoretical study of three-dimensional swept shock wave-turbulent boundary layer interactions at Mach 4 generated by a sharp fin of angles alpha equals 16 and 20 degrees. The theoretical model is the three-dimensional compressible Reynolds-averaged Navier-Stokes equations with turbulence incorporated through the algebraic eddy viscosity model of Baldwin and Lomax. Previous computations have been performed by Horstman using the Baldwin-Lomax, Cebeci-Smith and Jones Launder models. Computed results for the surface pressure, skin friction and streamline angles are compared with experiment and previous numerical results. The present results display good agreement with experimental data for surface pressure and surface flow direction. All turbulence models fail to accurately predict the peak skin friction. The computed flowfields are in agreement with many of the features of the quasi-conical flowfield model of Settles.

On the quasi-conical flowfield structure of the swept shock wave-turbulent boundary layer interaction

Abstract: The swept oblique shock-wave/turbulent-boundary-layer interaction generated by a 20-deg sharp fin at Mach 4 and Reynolds number 21,000 is investigated via a series of computations using both conical and three-dimensional Reynolds-averaged Navier-Stokes equations with turbulence incorporated through the algebraic turbulent eddy viscosity model of Baldwin-Lomax. Results are compared with known experimental data, and it is concluded that the computed three-dimensional flowfield is quasi-conical (in agreement with the experimental data), the computed three-dimensional and conical surface pressure and surface flow direction are in good agreement with the experiment, and the three-dimensional and conical flows significantly underpredict the peak experimental skin friction. It is pointed out that most of the features of the conical flowfield model in the experiment are observed in the conical computation which also describes the complete conical streamline pattern not included in the model of the experiment.

Shock waves produced by reflected detonations

Abstract: Confined detonations produce a shock wave that repeatedly reflects within the container, producing an unsteady, turbulent flowfield and slowly decaying as it interacts with its wake. We report experimental and numerical studies of this phenomenon in planar, cylindrical, and spherical geometries. Comparison of one-dimensional numerical simulations and the experimental results suggest that the effective dissipation rates are up to an order of magnitude larger than quasisteady turbulent channel flow mechanisms would predict. While the wave amplitude decay rates cannot be accurately predicted, most of the qualitative features of the measured pressure waveforms are faithfully reproduced in the numerical simulations. Further experimentation with more ideal vessels and multidimensional simulations including turbulence models are probably required to significantly improve the present estimates.

Analytical and experimental investigations of the oblique detonation wave engine concept

Abstract: Wave combustors, which include the Oblique Detonation Wave Engine (ODWE), are attractive propulsion concepts for hypersonic flight. These engines utilize oblique shock or detonation waves to rapidly mix, ignite, and combust the air-fuel mixture in thin zones in the combustion chamber. Benefits of these combustion systems include shorter and lighter engines which will require less cooling and can provide thrust at higher Mach numbers than conventional scramjets. The wave combustor's ability to operate at lower combustor inlet pressures may allow the vehicle to operate at lower dynamic pressures which could lessen the heating loads on the airframe. The research program at NASA-Ames includes analytical studies of the ODWE combustor using CFD codes which fully couple finite rate chemistry with fluid dynamics. In addition, experimental proof-of-concept studies are being carried out in an arc heated hypersonic wind tunnel. Several fuel injection designs were studied analytically and experimentally. In-stream strut fuel injectors were chosen to provide good mixing with minimal stagnation pressure losses. Measurements of flow field properties behind the oblique wave are compared to analytical predictions.

Mixing enhancement of reacting parallel fuel jets in a supersonic combustor

Abstract: Pursuant to a NASA-Langley development program for a scramjet HST propulsion system entailing the optimization of the scramjet combustor's fuel-air mixing and reaction characteristics, a numerical study has been conducted of the candidate parallel fuel injectors. Attention is given to a method for flow mixing-process and combustion-efficiency enhancement in which a supersonic circular hydrogen jet coflows with a supersonic air stream. When enhanced by a planar oblique shock, the injector configuration exhibited a substantial degree of induced vorticity in the fuel stream which increased mixing and chemical reaction rates, relative to the unshocked configuration. The resulting heat release was effective in breaking down the stable hydrogen vortex pair that had inhibited more extensive fuel-air mixing.

A numerical study of shock wave/boundary layer interaction in nonequilibrium chemically reacting air - The effects of catalytic walls

Abstract: This paper presents a numerical study that investigate the effects of nonequilibrium chemistry, and in particular, wall catalycity on the separated flow region generated by an oblique shock wave impinging upon a flat plate boundary layer. To obtain a solution to this problem, the full two dimensional Navier-Stokes equations were solved using MacCormack's predictor-corrector time dependent technique on a rectangular grid. Nonequilibrium chemistry was included by utilizing the 5 species, 17 reaction modified Dunn-Kang chemical kinetics model. Separate results were obtained for: calorically perfect, chemically reacting - noncatalytic wall, and chemically reacting - fully catalytic wall cases, for a given set of flow conditions. A direct comparison of all three cases revealed a slight decrease in the peak heat transfer for the noncatalytic wall case, as compared to the calorically perfect case. On the other hand, the fully catalytic wall case had a tremendous increase in the peak surface heat transfer. It is concluded that, for the particular conditons treated here (nearly frozen flow in the free stream), the effects of the nonequilibrium chemically reacting flow on the shock-wave/boundary-layer interaction depend critically on the catalycity of the wall, having virtually no effect for the case of a noncatalytic wall, and exerting a tremendous effect for a fully catalytic wall.

Crossing shock wave-turbulent boundary layer interactions

Abstract: Three-dimensional interactions between crossing shock waves generated by symmetric sharp fins and a turbulent boundary layer on a flat plate are investigated experimentally and theoretically at Mach number 2.95 and freestream unit Reynolds number 1.96 x 10 to the 7th/ft. The incoming boundary layer has a thickness of 4 mm at the location of the fin leading edges. A comparison of experimental and computational results for two sets of fin angles (11 x 11 and 9 x 9 deg) shows general agreement with regard to surface pressure measurements and surface streamline patterns. The principal feature of the streamline structure is a collision of counterrotating vortical structures emanating from near the fin leading edges and meeting at the geometric centerline of the interaction.