Engineering topics

Macaraeg, Michele G.

Publications and source records attributed to Macaraeg, Michele G..

13 records

NASA NTRS2002

Aeroacoustic Experiments in the NASA Langley Low-Turbulence Pressure Tunnel

A phased microphone array was used in the NASA Langley Low-Turbulence Pressure Tunnel to obtain acoustic data radiating from high-lift wing configurations. The data included noise localization plots and acoustic spectra. The tests were performed at Reynolds numbers based on the cruise-wing chord, ranging from 3.6 x 10(exp 6) to 19.2 x 10(exp 6). The effects of Reynolds number were small and monotonic for Reynolds numbers above 7.2 x 10(exp 6).

Choudhari, Meelan M.↗

NASA NTRS1999

In Search of the Physics: NASA's Approach to Airframe Noise

An extensive numerical and experimental study of airframe noise mechanisms associated with a subsonic high-lift system has been performed at NASA Langley Research Center (LaRC). Investigations involving both steady and unsteady computations and experiments on small-scale models with part-span flaps and full-span flaps are presented. Both surface (steady and unsteady pressure measurements, hot films, oil flows, pressure sensitive paint) and off-surface (5 holeprobe, particle-imaged velocimetry, laser velocimetry, laser light sheet measurements) were taken in the LaRC Quiet Flow Facility (QFF) and several hard-wall tunnels. Experiments in the Low Turbulence Pressure Tunnel (LTPT) included Reynolds number variations up to flight conditions. Successful microphone array measurements were also taken providing both acoustic source maps on the model, and quantitative spectra. Critical directivity measurements were obtained in the QFF. NASA Langley unstructured and structured Reynolds-Averaged Navier-Stokes codes modeled the steady aspects of the flows. Excellent comparisons with surface and off-surface experimental data were obtained. Subsequently, these meanflow calculations were utilized in both linear stability and direct numerical simulations of the flow fields to calculate unsteady surface pressures and farfield acoustic spectra. Accurate calculations were critical in obtaining not only noise source characteristics, but shear layer correction data as well. Techniques utilized in these investigations as well as brief overviews of the results are given.

Macaraeg, Michele G.↗

NASA NTRS1994

Ignition dynamics of a laminar diffusion flame in the field of a vortex embedded in a shear flow

The role of streamwise-spanwise vorticity interactions that occur in turbulent shear flows on flame/vortex interactions is examined by means of asymptotic analysis and numerical simulation in the limit of small Mach number. An idealized model is employed to describe the interaction process. The model consists of a one-step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a streamwise vortex embedded in a shear flow. It is found that the interaction of the streamwise vortex with shear gives rise to small-scale velocity oscillations which increase in magnitude with shear strength. These oscillations give rise to regions of strong temperature gradients via viscous heating, which can lead to multiple ignition points and substantially decrease ignition times. The evolution in time of the temperature and mass-fraction fields is followed, and emphasis is placed on the ignition time and structure as a function of vortex and shear strength.

Macaraeg, Michele G.↗

NASA NTRS1993

Role of acoustics in flame/vortex interactions

The role of acoustics in flame/vortex interactions is examined via asymptotic analysis and numerical simulation. The model consists of a one-step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are allowed to mix and react by convection and diffusion in the presence of an acoustic field or a time-varying pressure field of small amplitude. The main emphasis is on the influence of the acoustics on the ignition time and flame structure as a function of vortex Reynolds number and initial temperature differences of the reactants.

Jackson, T. L.↗

NASA NTRS1991

Ignition and structure of a laminar diffusion flame in the field of a vortex

The distortion of flames in flows with vortical motion is examined via asymptotic analysis and numerical simulation. The model consists of a constant density, one step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a vortex. The evolution in time of the temperature and mass fraction fields is followed. Emphasis is placed on the ignition time and location as a function of vortex Reynolds number and initial temperature differences of the reacting species. The study brings out the influence of the vortex on the chemical reaction. In all phases, good agreement is observed between asymptotic analysis and the full numerical solution of the model equations.

Macaraeg, Michele G.↗

NASA NTRS1991

Investigation of supersonic modes and three-dimensionality in bounded, free shear flows

It is found through two-dimensional temporal simulations of high-speed free shear layers that mean flow distortion is significantly increased when supersonic disturbances are introduced as initial conditions. The shear layer exhibits no subharmonic growth or roll-up, but rather a spectral broadening as energy is distributed into higher harmonics. Increasing the velocity of one side of the mixing layer (u2) to roughly 1/5 the speed of the high speed side (u1), allows a slight subharmonic growth at a very slow rate for two-dimensional modes. A first look at three-dimensional free shear flows is also presented for M = 2. No effect is seen for incompressible flow; however, stabilization is seen with respect to maximum temporal growth rates as the transverse velocity is increased. A much stronger, but similar effect is seen if u2 is increased. The wave direction of maximum growth for u2 is greater than 0.4 is found to be the direction of the faster stream (u1) over a broad range of transverse velocities.

Macaraeg, Michele G.↗

NASA NTRS1990

Bounded free shear flows - Linear and nonlinear growth

The physics of bounded free shear flows is studied at supersonic Mach numbers. Wall locations are at 1500 momentum thicknesses away from the shear layer center line. Intrinsic to this flow are high multiplicities of supersonic modes not present in the unbounded case. It is seen that these supersonic modes are resistant to subharmonic growth, a necessary precursor for efficient mixing. Previous results (Eberhardt et al., 1988) with wall locations at less than one half momentum thickness away from the shear layer center indicate that modes resembling channel modes (maximum amplification at the walls) did not roll up in numerical simulations. The similarity between these two cases is pointed out.

Macaraeg, Michele G.↗

NASA NTRS1989

New instability modes for bounded, free shear flows

A class of highly amplified supersonic disturbances are found for high-speed, bounded mixing layers at high values of streamwise wavenumber. Their amplification is an order of magnitude greater than the most amplified modes, which occur at 60-65 deg at low streamwise wavenumber. These disturbances are stabilized by increasing Mach number, viscosity, and sweep; however, the effect of sweep on the most amplified mode is not significant until the wave propagation angle reaches 30 deg. The maximum growth rate of the unstable disturbances decreases as the temperature of the higher Mach number stream is increased. The structure of these disturbances is such that the phase speed with respect to the mean flow is subsonic in a small region in the center of the shear layer, and supersonic on either side of this region.

Macaraeg, Michele G.↗

NASA NTRS1989

Analysis of artificial viscosity effects on reacting flows using a spectral multidomain technique

Standard techniques used to model chemically-reacting flows require an artificial viscosity for stability in the presence of strong shocks. The resulting shock is smeared over at least three computational cells, so that the thickness of the shock is dictated by the structure of the overall mesh and not the shock physics. A gas passing through a strong shock is thrown into a nonequilibrium state and subsequently relaxes down over some finite distance to an equilibrium end state. The artificial smearing of the shock envelops this relaxation zone which causes the chemical kinetics of the flow to be altered. A method is presented which can investigate these issues by following the chemical kinetics and flow kinetics of a gas passing through a fully resolved shock wave at hypersonic Mach numbers. A nonequilibrium chemistry model for air is incorporated into a spectral multidomain Navier-Stokes solution method. Since no artificial viscosity is needed for stability of the multidomain technique, the precise effect of this artifice on the chemical kinetics and relevant flow features can be determined.

Macaraeg, Michele G.↗

NASA NTRS1989

A spectral multi-domain technique applied to high-speed chemically reacting flows

The first applications of a spectral multidomain method for viscous compressible flow is presented. The method imposes a global flux balance condition at the interface so that high-order continuity of the solution is preserved. The global flux balance is imposed in terms of a spectral integral of the discrete equations across adjoining domains. Since the discretized equations interior to each domain solved are uncoupled from each other, and since the interface relation has a block structure, the solution scheme can be adapted to the particular requirements of each subdomain. The spectral multidomain technique presented is well-suited for the multiple scales associated with the chemically reacting and transition flows in hypersonic research. A nonstaggered multidomain discretization is used for the chemically reacting flow calculation, and the first implementation of a staggered multidomain mesh is presented for accurately solving the stability equation for a viscous compressible fluid.

Macaraeg, Michele G.↗

NASA NTRS1988

A spectral collocation solution to the compressible stability eigenvalue problem

A newly developed spectral compressible linear stability code (SPECLS) (staggered pressure mesh) is presented for analysis of shear flow stability, and applied to high speed boundary layers and free shear flows. The formulation utilizes the first application of a staggered mesh for a compressible flow analysis by a spectral technique. An order of magnitude less number of points is needed for equivalent accuracy of growth rates compared to those calculated by a finite difference formulation. Supersonic disturbances which are found to have oscillatory structures were resolved by a spectral multi-domain discretization, which requires a factor of three fewer points than the single domain spectral stability code. It is indicated, as expected, that stability of mixing layers is enhanced by viscosity and increasing Mach number. The mean flow involves a jet being injected into a quiescent gas. Higher temperatures of the injected gas is also found to enhance stability characteristics of the free shear layer.

Macaraeg, Michele G.↗

NASA NTRS1987

An analysis of artificial viscosity effects on reacting flows using a spectral multi-domain technique

Analytical and experimental techniques for modeling the aerothermodynamics of hypersonic flight are assessed, together with the problems which will be encountered in developing reusable hypersonic vehicles. Emphasis is placed on a numerical coupling between a nonequilibrium chemistry model and hypersonic flow kinematics. Finite difference and finite element descriptions of flow fields in which molecules encounter a shock wave and undergo various motion (and thereby energy) transformations are discussed. The effects of artificial smearing of the shock wave are considered in terms of the resulting effects on the distribution of the energies and chemical composition of the transition region. Results are provided from schlieren photographs of shock-tube experiments, Navier-Stokes calculations of axisymmetric flow over a conical body, and calculations using a spectral multidomain approach for chemically reacting flows.

Macaraeg, Michele G.↗

NASA NTRS1987

A spectral multi-domain technique for analysis of chemically reacting flows

A technique is presented which models the chemical kinetics and flow kinematics of a nonionized air mixture passing through a fully resolved shock wave, thus alleviating the need for artificial viscosity. The quasi-one-dimensional Navier-Stokes equations and the species conversation equation are the governing equations. The validity of the multidomain Navier-Stokes algorithm is demonstrated via comparison with experiment.

Macaraeg, Michele G.↗