Showing papers on "Supersonic speed published in 1987"

Numerical simulation of compressible homogeneous flows in the turbulent regime

Abstract: Compressible flows with r.m.8. velocities of the order of the speed of sound are studied with direct numerical simulations using a pseudospectral method. We concentrate on turbulent homogeneous flows in the two-dimensional case. The fluid obeys the Navier-Stokes equations for a perfect gas, and viscous terms are included explicitly. No modelling of small scales is used. We show that the behaviour of the flow differs sharply at low compared with high r.m.9. Mach number Ma, with a transition at Mu = 0.3. In the large scales, temporal exchanges between longitudinal and solenoidal modes of energy retain an acoustical character; they lead to a slowing down of the decrease of the Mach number with time, which occurs with interspersed plateaux corresponding to quiescent periods. When the flow is initially supersonic, the small scales are dominated by shocks behind which vortices form. This vortex production is particularly prominent, when two strong shocks collide, with the onset of shear turbulence in the region downstream of the collision. However, at the resolutions reached by our code on a 256 x 256 uniform grid, this mechanism proves insufficient to bring vortices into equipartition with shocks in the small-scale tail of the energy spectrum.

The rapid expansion of a supersonic turbulent flow: role of bulk dilatation

Abstract: The rapid expansion of a turbulent boundary layer in supersonic flow is studied analytically and experimentally. Emphasis is placed on the effect of bulk dilatation on turbulent fluctuations. The hypotheses made in the analysis are similar to those in the rapid distortion theory and are used to simplify second-order closures. By assuming that the fluctuating velocity is solenoidal an extension of classical subsonic models is proposed. A new variable is defined, which takes into account the mean density variations, and behaves like the Reynolds stress tensor in subsonic flows with weak inhomogeneities and a weak dissipation rate. The results of the analysis are compared with turbulence measurements performed in a supersonic boundary layer subjected to an expansion fan. The proposed approximations describe correctly the evolution of turbulence intensities: bulk dilatation contributes predominantly to the Reynolds stress evolution. The boundary layer is ‘relaminarized’ by the expansion. Downstream of the latter, the layer returns to equilibrium. Measurements show that the turbulence decays slowly in the outer layer and increases rapidly in the inner layer.

Slit pulsed valve for generation of long‐path‐length supersonic expansions

Abstract: We describe a valve for production of jet‐cooled species in a pulsed, long‐path‐length (1.2‐cm) supersonic expansion. The valve produces 150–600‐μs‐duration pulses at repetition rates up to 60 Hz from a nozzle with variable slit width, and is suitable for use with corrosive gases and vapors.

Progress Toward Shock Enhancement of Supersonic Combustion Processes

Abstract: In air breathing propulsion systems for flight at Mach numbers 7 to 20, it is generally accepted that the combustion processes will be carried out at supersonic velocities with respect to the engine. The resulting brief residence time places a premium on rapid mixing of the fuel and air. To address this issue we are investigating a mechanism for enhancing the rate of mixing between air and hydrogen fuel over rates that are expected in shear layers and jets.

Review of Linear Compressible Stability Theory

Abstract: A review is given of some aspects of the linear compressible stability theory. Major attention is given to the inviscid theory and the additional solutions that arise when there is a region of supersonic flow relative to the phase velocity. For highly cooled flat-plate boundary layers at Mach number 5.8, the unstable region includes supersonic outgoing waves. A previously unknown neutral incoming wave has also been found. An example of viscous multiple solutions is given, along with calculations of higher viscous discrete modes and the compressible counterpart of the Squire mode.

Liquid-fueled supersonic combustion ramjets - A research perspective

Abstract: A review of past and current research applicable to liquid-fueled supersonic combustion ramjets is presented and discussed. An assessment of its strengths and shortcomings is made and, finally, a list of research opportunities that merit consideration in this rapidly expanding area of airbreathing propulsion is presented. Nomenclature = area Cf = average combustor wall skin friction coefficient D = isolator duct diameter d = injector diameter ER = fuel-air equivalence ratio / = fuel-air ratio h - penetration height, enthalpy Ah = driving enthalpy (Ref. 85) M -Mach number P = static pressure q = fuel-to-freestream dynamic pressure ratio, average q = combustor wall heat flux Ree = Reynolds number based on momentum thickness Sd = length of precombustion shock structure in supersonic combustion Ss = total length of precombustion shock structure T = temperature w =fuel jet lateral spreading w =mass flow rate x = axial distance downstream of injection 6 =fuel injection angle 6m = boundary-layer momentum thickness r/c =heat release combustion efficiency f = average combustor skin friction

Theoretical investigation of a 2 kA DC nitrogen arc in a supersonic nozzle

Abstract: The arc conservation equations based on laminar flow and on the boundary layer assumption have been solved for a 2 kA DC nitrogen arc burning in a supersonic nozzle at a stagnation pressure of 23 atm. An approximate radiation transport model is used to account for the emission and absorption of radiation. The computed results are in good agreement with experiments. A hybrid approach (i.e. a combination of integral and differential methods) is used to predict the pressure in the nozzle in the presence of the arc. It has been found that isothermal external flow provides the best agreement between the calculated and the measured pressures. The relative importance of radiation transport, convection (both enthalpy and kinetic energy) and thermal conduction is discussed. It has also been found that in the region upstream of the nozzle throat there is a radial inflow heated up by strong radiation absorption while in the supersonic region this flow changes its direction. The outward radial flow provides the energy source for the arc to expand axially in the thermal layer. Viscous stresses play a very small role in momentum balance and axial velocity profiles are typical of those of inviscid fluid flows.

Spectra of noise and amplified turbulence emanating from shock-turbulence interaction

Abstract: This work is a small extension of NACA studies of the early fifties that predicted amplification of turbulence on passing through a shock wave (observed for turbulent boundary layers), as well as the generation of intense noise (observed for supersonic jets). The first solved the basic gasdynamics problem of the interaction of an infinite planar shock with a single three-dimensional spectrum component of turbulence (an oblique sinusoidal shear wave). The second developed the comprehensive 3D spectrum analysis necessary to generalize the scenario to the interaction of a shock wave with convected homogeneous turbulence. Numerical calculations were carried out to yield curves (vs. Mach number) of rms sound pressure, temperature fluctuation, and two components of turbulent velocity downstream of the shock, for two cases of preshock turbulence. The present numerical study reproduces these for one case and provides in addition their one-dimensional power spectra (vs. wavenumber of frequency). Ratios of the several postshock spectra to the longitudinal preshock turbulence spectrum (4D) have been computed for a wide range of Mach numbers; curves vs. wavenumber are presented for two scenarios of preshock turbulence: isotropy and axisymmetry, both based on the von Karman 3D spectrum.

Coherent large-scale structures in high Reynolds number supersonic jets

Abstract: The flow structure of a 508 mm (2 in) diameter jet operated at a full expanded Mach number of 137, with Reynolds numbers in the range 17 to 235 million, was examined for the first 20 jet diameters To facilitate the study of the large scale structure, and determine any coherence, a discrete tone acoustic excitation method was used Phase locked flow visualization as well as laser velocimeter quantitative measurements were made The main conclusions derived from this study are: (1) large scale coherent like turbulence structures do exist in large Reynolds number supersonic jets, and they prevail even beyond the potential core; (2) the most preferential Strouhal number for these structures is in the vicinity of 04; and (3) quantitatively, the peak amplitudes of these structures are rather low, and are about 1% of the jet exit velocity Finally, since a number of unique problems related to LV measurements in supersonic jets were encountered, a summary of these problems and lessons learned therefrom are also reported

Measurements of temperature, density, pressure, and their fluctuations in supersonic turbulence using laser-induced fluorescence

Abstract: A laser-induced fluorescence method has been developed that provides simultaneous measurements of temperature, density, and their fluctuations owing to turbulence in unheated compressible flows. Pressure and its fluctuations are also deduced using the equation of state. Fluorescence is induced in nitric oxide that has been seeded into a nitrogen flow in concentrations of 100 ppm. Measurements are obtained from each laser pulse, with a spatial resolution of 1 mm and a temporal resolution of 125 ns. The method was applied to a supersonic, turbulent, boundary-layer flow with a free-stream Mach number of 2. For stream conditions in the range from 150-300 K and 0.3-1 atm, temperature is measured with an uncertainty of approximately 1 percent rms, while density and pressure uncertainties are approximately 2 percent rms.

Supersonic vortex rings

Abstract: Experiments by Phan & Stollery (In Proc . 14 th International Symposium on Shock Tubes and Waves , pp. 519-526 (1983)) indicated the presence of a supersonic vortex ring with an embedded rearward-facing shock in the unsteady flow that follows the emergence of a normal shock from a circular tube. The vortex flow is supersonic in the sense that the on-axis flow is supersonic in the frame of reference of the vortex ring. In the present work this flow phenomenon is studied by using differential interferometry and a detailed description of the flow-starting process is given. A simplified quasi-steady calculation is performed and a comparison is made with previously measured pressures in the flow field. These results are in agreement with the physical description of the flow development.

Euler solver for three-dimensional supersonic flows with subsonic pockets

Abstract: A new finite-difference scheme has been developed to solve efficiently the unsteady Euler equations for three-dimensional inviscid supersonic flows with subsonic pockets The technique utilizes planar Gauss-Seidel relaxation in the marching direction and approximate factorization n the crossflow plane An 'infinitely large' time step is used in parts of the flowfield where the component of velocity in the marching direction is supersonic - here the Gauss-Seidel sweeps are restricted to the forward direction only, and the procedure reduces to simple space-marching; a finite time step is used in parts of the flowfield where the marching component of velocity is subsonic - here, backward and forward Gauss-Seidel sweeps are employed to allow for upstream and downstream propagation of signals, and a time-asymptotic steady state is obtained The discretization formulas are based on finite-volume implementation of high accuracy (up to third-order) total variation diminishing formulations Numerical solutions are obtained for an analytically defined forebody, a realistic fighter configuration, and the Space Shuttle The results are in very good agreement with available experimental data and numerical solutions of the full-potential equation

Computational and experimental investigation of cavity flowfields

Abstract: This paper presents a computational and experimental investigation of supersonic flow past a cavity in a flat plate. The source of the particular interest in this problem is the ongoing study of the aerodynamic interference effects between a separating store and its bay in the parent body. An upwind relaxation scheme, utilizing flux vector splitting and line-Gauss-Seidel iterations, is used to solve Reynolds-averaged Navier-Stokes equations. Spatial discretizations of this two-dimensional analysis are based on implicit and finite-volume methods. Turbulence is modeled and shocks are captured. The flowfield of the symmetry plane at the half-width is computationally visualized and all flow properties are computed. Experimental tests are conducted in the Langley Unitary Plan Wind Tunnel to measure wall pressures and to capture schlieren photographs. Qualitative as well as quantitative data of computations and experiments agree very well. These two vehicles of investigation are merged to show open, closed and transitional cavity flow behaviors.

Prediction of hypersonic shock-wave/turbulent-boundary-layer interaction flows

Abstract: Solutions of the Reynolds-averaged Navier-Stokes equations are presented and compared with experimental surface data for a series of hypersonic shock-wave/turbulent-boundary-layer interaction flows. The turbulence models used include algebraic and two-equation eddy-viscosity models developed for transonic and supersonic flows. Also several additional modifications to the two-equation model to account for compressibility effects are developed and used. Although the modifications improve the agreement with the experimental data, no single model or modification correctly predicts all the test cases.

Prediction and control of transition in hypersonic boundary layers

Abstract: In this paper, the role of compressible linear stability theory in prediction of boundary layer transition at supersonic and hypersonic speeds is investigated. Computations for sharp cones, using the e exp N method with N = 10, show that the first oblique Tollmien-Schlichting mode is responsible for transition at adiabatic wall conditions for freestream Mach numbers up to 7. For cold walls, the two-dimensional second mode dominates the transition process at lower hypersonic Mach numbers due to the well-known destabilizing effect of cooling on the second mode. It is shown that pressure gradient and suction may be used to stabilize this mode. Some results on the real gas effects on hypersonic boundary-layer stability are presented.

A numerical model for supersonic reacting mixing layers

Abstract: A current research effort is underway at the NASA Langley Research Center to achieve a detailed understanding of important phenomena present when a supersonic flow undergoes a chemical reaction. A computer program has been developed to study the details of such flows. The program has been constructed to consider the multicomponent diffusion and convection of important species, the finite-rate reaction of these species, and the resulting interaction between the fluid mechanics and chemistry. Code results from the analysis of a spatially developing and reacting mixing layer are presented, and conclusions are drawn regarding the structure of the evolving layer and its associated flame.

Compressed air nozzle

Abstract: A compressed air nozzle for accelerating the flow of air from a compressor to supersonic speed has an axial passage which comprises a converging portion and a diverging portion interconnected by an elongated throat. The nozzle is useful in tools for dislodging earth for excavation.

Model and full scale study of twin supersonic plume resonance

Abstract: This paper examines the effect of both nozzle geometry and scale on the twin supersonic plume resonance phenomenon associated with aircraft having engine nozzle center-to-center spacings less than two diameters. Exit plane near field dynamic pressures were measured for both single and dual nozzle operation in 4.7 percent model and full scale under static conditions. The frequencies associated with this phenomenon were predicted to within 5 percent for a full scale F-15 aircraft. Amplitude levels associated with this phenomenon were found to dominate the dynamic pressure fluctuations in the inter-nozzle region, and reach a level near the structural design limit for this aircraft. The model scale studies, which involved both axisymmetric and rectangular geometry, indicated that amplitude levels could be expected to be much higher in flight. High amplitude levels would likely occur in the overexpanded region for axisymmetric geometry, and in the underexpanded region for rectangular geometry.

Constant pressure panel method for supersonic unsteady airload analysis

Abstract: Calcul des forces aerodynamiques generalisees sur des surfaces portantes dans un ecoulement non visqueux isentrope

Aerodynamic sensitivities from subsonic, sonic and supersonic unsteady, nonplanar lifting-surface theory

Abstract: The technique of implicit differentiation has been used in combination with linearized lifting-surface theory to derive analytical expressions for aerodynamic sensitivities (i.e., rates of change of lifting pressures with respect to general changes in aircraft geometry, including planform variations) for steady or oscillating planar or nonplanar lifting surfaces in subsonic, sonic, or supersonic flow. The geometric perturbation is defined in terms of a single variable, and the user need only provide simple expressions or similar means for defining the continuous or discontinuous global or local perturbation of interest. Example expressions are given for perturbations of the sweep, taper, and aspect ratio of a wing with trapezoidal semispan planform. In addition to direct computational use, the analytical method presented here should provide benchmark criteria for assessing the accuracy of aerodynamic sensitivities obtained by approximate methods such as finite geometry perturbation and differencing. The present process appears to be readily adaptable to more general surface-panel methods.

Design and fabrication requirements for low-noise supersonic/hypersonic wind tunnels

Abstract: A schematic diagram of the new proposed Supersonic Low Disturbance Tunnel (SLDT) is shown. Large width two dimensional rapid expansion nozzles guarantee wide quiet test cores that are well suited for testing models at large angle of attack and for swept wings. Hence, this type of nozzle will be operated first in the new proposed large scale SLDT. Test results indicate that the surface finish of pilot nozzles is critical. The local roughness Reynolds number criteria R sub k is approx. = 10 will be used to specify allowable roughness on new pilot nozzles and the new proposed tunnel. Experimental data and calculations for M = 3.0, 3.5, and 5.0 nozzles give N-factors from 6 to 10 for transition caused by Goertler vortices. The use of N is approx. = 9.0 for the Goertler instability predicts quiet test cores in the new M = 3.5 and M = 6.0 axisymmetric long pilot nozzles that are 3 to 4 times longer than observed in the test nozzles to date. The new nozzles utilize a region of radial flow which moves the inflection point far downstream and delays the onset and amplification of the Goertler vortices.

Piloting igniter for supersonic combustor

Abstract: A piloting fuel injector assembly for a supersonic ramjet engine has a cowl 24, 70 confining a pilot air portion of the airstream. A conical member 34, 74 produces a subsonic recirculation zone 30, 76 and a shock wave falling within the cowl. Pilot fuel injected 40, 82 upstream of the recirculation zone burns stoichiometrically. The exhaust 38, 86 from the cowl is sonic, producing a shock wave which facilitates ignition of the main fuel injected 46, 84 upstream of the cowl exhaust.

Numerical simulations of adiabatic axisymmetric accretion flow. I. A new mechanism for the formation of jets

Abstract: Numerical simulations of the uniform axisymmetric flow past a gravitating sphere have been studied. It is found that the structure of the flow is extremely sensitive to the boundary condition at the surface of the gravitating object. For the case in which the boundary is totally absorbing, a steady state flow is reached. However, for a boundary which is not totally absorbing, steady state flows are not obtained. The morphology of the flow is also sensitive to the Mach number at infinity and to the ratio of the free-fall velocity at the surface of the gravitating object to the flow velocity at inifinity. A new mechanism for the formation of jets is identified in which a fraction of the accretion energy is tapped to drive an anisotropic supersonic outflow with collimation provided by a combination of the inertia of matter which surrounds the beam and the development of multiple shock structures.