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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16

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NASA NTRS1983

Aerodynamic features of turbulent flames

The paper provides an interpretation for a variety of turbulent flame phenomena relating them to the aerodynamic properties of the flow field. This includes the classical notion of the breakdown of laminar flames propagation in ducts, the characteristic features of turbulent flame-fronts recorded by schlieren photography, as well as flame instabilities leading to flashback occurring in a combustion chamber behind a bluff-body flame holder. The rudimentary nature of turbulent flame propagation is shown to be associated with (1) vortex motion of a large-scale turbulent eddy, combined with (2) self-advancement of the flame at the appropriate normal burning speed and (3) the concomitant action of distributed sources of specific volume. The results furnish an insight into the macroscopic properties of the mechanism of turbulent flame propagation, rationalizing in particular its capability to sustain high flow velocities at relatively low normal burning speeds.

Oppenheim, A. K.↗

NASA NTRS1983

Computational technique for three-dimensional compressible flow past wings at high angles of attack

A computational technique, based on the integral solution of the full potential equation, has been developed for the solution of three-dimensional high-subsonic flows past wings at high angles of attack. The problem includes two sources of nonlinearities: a boundary oriented nonlinearity (separated flow roll up) and a region oriented nonlinearity (flow compressibility). The former is represented by a nonlinear vortex lattice while the latter is represented by a source distribution inside a finite volume. The solution is obtained by using double iteration cycles; a separated flow (wake) iteration cycle and a compressibility iteration cycle. The computational technique is applied to a delta wing. The results show that the technique is very promising and efficient.

Kandil, O. A.↗

NASA NTRS1984

Analysis of axisymmetric body effects on rotor aerodynamics using modified slender body theory

A modified slender body theory was used in modelling the flowfield around an axisymmetric body at zero angle of attack. The model was defined in an attempt to characterize body-induced effects on helicopter rotor performance and loading using efficient computation methods. The streamfunction for an axial source distribution was applied in the body model, which was then adapted to the helicopter analysis code (CAMRAD). The body-induced velocities were calculated for separate locations on the rotor disk. Three different rotor models in nonuniform inflow with a free wake geometry were considered. Comparison of predictions with data on ellipsoidal and axisymmetric bodies showed good agreement for moderately thick ellipsoidal bodies. Oscillatory blade bending moments were increased by the presence of the body, but overall performance impact was negligible. Computational efficiency was 10-20 percent better than with a potential flow panel code.

Yamauchi, G. K.↗

NASA NTRS1985

Hexagonal uniformly redundant arrays for coded-aperture imaging

Uniformly redundant arrays are used in coded-aperture imaging, a technique for forming images without mirrors or lenses. This technique is especially important for the high energy X-ray and gamma-ray region above 20 kev. In this technique, a mask consisting of opaque (closed) and trasparent (open) areas is placed between the photon sources to be imaged and a position sensitive detector or a detector array. Each source casts a shadow pattern of the mask or aperture onto the detector. This shadow pattern may be viewed as an encoded signal for that source direction. If each possible source code is unique, the detected composite of overlapping shadow patterns may be decoded to produce an image of the source distribution.

Finger, M. H.↗

NASA NTRS1985

Explanation of the secondary to primary ratio within the continuous Fermi accelerator model

The secondary to primary ratio in galactic cosmic radiation at relativistic momenta is calculated in a model, where the primaries are continuously accelerated from the thermal galactic background medium by 1st and 2nd order Fermi acceleration. It is shown that the measured decrease with momentum does not exclude that cosmic rays are accelerated in the interstellar medium as a whole. Once a momentum dependence of the mean lifetime and the different spatial source distributions are adequately taken into account, the measured decreasing ratio can be explained.

Lerche, I.↗

NASA NTRS1985

Detection of galactic Al-26 gamma radiation by the SMM spectrometer

The Solar Maximum Mission satellite's gamma-ray spectrometer has detected a line near 1.81 MeV in each of the three years (1980-1982) over which the Galactic center traversed the broad aperture of that instrument. No significant intensity variation is noted over this period. The Galactic center/anticenter intensity ratio is greater than 2.5, and the center of the emission is noted to be consistent with the location of the Galactic center. For an assumed source distribution which follows the more than 100 MeV Galactic gamma radiation, the total flux in the direction of the Galactic center and the measured energy of the line are consistent with the detection of a narrow gamma-ray line from interstellar Al-26 by HEAO 3 in 1979-1980.

Share, G. H.↗

NASA NTRS1985

Lift due to thickness for low aspect ratio wings in incompressible flow

The problem under consideration is a numerical study of the effects of thickness on lift for low aspect ratio wings in steady incompressible inviscid flow at moderate angles of attack. At these angles of attack the flow separates along the leading edge giving rise to a lift substantially higher than that computed by classical attached flow potential theory. The problem is treated as a perturbation expansion in a small thickness parameter. The lifting elements of the flow are modeled using a nonlinear vortex lattice method which replaces the leading and trailing edge vortex sheets by segmented straight vortex filaments. The thickness elements of the flow are modeled with a mean plane source distribution and a modification to the wing boundary conditions. Results are obtained for wings with biconvex and NACA 0012 sections which compare well with available experimental data. The important observation that the effect of thickness is to decrease the lift is made.

Dodbele, S. S.↗

NASA NTRS1985

Gamma-ray burst statistics - Guidance or deception?

An evaluation is made of efforts to statistically ascertain the distance scale of gamma-ray bursts. The Venera 11 and 12 KONUS experiment peak count flux P sampling is noted to be incomplete for the longest and shortest bursts. This may be due to the effect being either a real physical limit to the burst population, or an artifact. Current data favor the latter explanation. Where N is the annual rate of bursts with a fluence exceeding S ergs/sq cm, it is shown that an unbounded, isotropic, homogeneous burst source distribution must have d log N/d log P of less than -3/2, for all P independent of any intrinsic P distribution or the existence and relative distance of any separate burst classes. The KONUS data are consistent with earlier conclusions that bursts are distributed on a large galactic scale.

Jennings, M. C.↗

NASA NTRS1986

Studies of acoustic emission from point and extended sources

The use of simulated and controlled acoustic emission signals forms the basis of a powerful tool for the detailed study of various deformation and wave interaction processes in materials. The results of experiments and signal analyses of acoustic emission resulting from point sources such as various types of indentation-produced cracks in brittle materials and the growth of fatigue cracks in 7075-T6 aluminum panels are discussed. Recent work dealing with the modeling and subsequent signal processing of an extended source of emission in a material is reviewed. Results of the forward problem and the inverse problem are presented with the example of a source distributed through the interior of a specimen.

Sachse, W.↗

NASA NTRS1986

Consideration of some factors affecting low-frequency fuselage noise transmission for propeller aircraft

Possible reasons for disagreement between measured and predicted trends of sidewall noise transmission at low frequency are investigated using simplified analysis methods. An analytical model combining incident plane acoustic waves with an infinite flat panel is used to study the effects of sound incidence angle, plate structural properties, frequency, absorption, and the difference between noise reduction and transmission loss. Analysis shows that these factors have significant effects on noise transmission but they do not account for the differences between measured and predicted trends at low frequencies. An analytical model combining an infinite flat plate with a normally incident acoustic wave having exponentially decaying magnitude along one coordinate is used to study the effect of a localized source distribution such as is associated with propeller noise. Results show that the localization brings the predicted low-frequency trend of noise transmission into better agreement with measured propeller results. This effect is independent of low-frequency stiffness effects that have been previously reported to be associated with boundary conditions.

Mixson, J. S.↗

NASA NTRS1987

Loss of lift due to thickness for low-aspect-ratio wings in incompressible flow

The problem under consideration is a numerical study of the effects of thickness on lift for low-aspect-ratio wings in steady incompressible inviscid flow at moderate angles of attack. At these angles of attack the flow separates along the leading edge giving rise to a lift substantially higher than that computed by classical attached-flow potential theory. The problem is treated as a perturbation expansion in a small thickness parameter. The lifting elements of the flow are modeled using a nonlinear vortex-lattice method which replaces the leading and trailing-edge vortex sheets by segmented straight vortex filaments. The thickness elements of the flow are modeled with a mean-plane source distribution and a modification to the wing boundary conditions. Results are obtained for wings with biconvex and NACA 0012 sections which compare well with available experimental data. The important observation that the effect of thickness is to decrease the lift is made.

Dodbele, S. S.↗

NASA NTRS1987

Operating envelopes of particle sizing instrumentation used for icing research

The Forward Scattering Spectrometer Probe and the Optical Array Probe are analyzed in terms of their ability to make accurate determinations of water droplet size distributions. Sources of counting and sizing errors are explained. The paper describes ways of identifying these errors and how they can affect measurement.

Hovenac, Edward A.↗

NASA NTRS1987

Rotor noise measurement using a directional microphone array

A directional array of microphones was used to measure the noise from a 40 percent scale model rotor in a large aeroacoustic wind tunnel. The development and design of this directional array is described. A design goal was that the array focus on a constant sensing area over a broad frequency range. The implementation of the array design is presented, followed by sample results for several different rotor test conditions. The directional array spectral results are compared with predictions of broadband self noise, and with total rotor noise measurements obtained from individual microphones of the array. The directional array is demonstrated to be a useful tool in examining noise source distributions.

Marcolini, Michael A.↗

NASA NTRS1987

Redesign of a supercritical wing in the presence of an engine nacelle

If an obstacle, such as an engine nacelle, is placed in the flow near a virtually shockless wing, shocks of substantial magnitude may appear on the wing. A source distribution method is presently employed in the modeling of an engine nacelle in compressible flow, through incorporation into a free-boundary supercritical wing design code. Code calculation results are presented which show the transonic shock to appear, on average, 10 percent forward of its position on the clean wing. Attention is also given to an associated method for the calculation of wave drag.

Ross, David S.↗

NASA NTRS1988

Operating envelopes of particle sizing instrumentation used for icing research

Hovenac, Edward A.↗

NASA NTRS1988

Computation of the velocity field of an excited shear layer

An infinitely thin shear layer emanating from a semi-infinite flat plate subjected to acoustic excitation is considered. The flow field outside the excited shear layer is computed employing a source distribution approach. Results are given for the region of the velocity field that cannot easily be obtained by analytical approximations.

Nallasamy, M.↗

NASA NTRS1988

QSO absorption lines and the universe at redshifts between 1 and 4

Observations of QSO absorption lines at redshifts between 1 and 4 are discussed, focusing on the study of Ly-alpha clouds. The temperature, size, spatial distribution, source of ionization, origin, and physical characteristics of Ly-alpha clouds are examined. Also, consideration is given to the possibility of dust obscuration occurring when the line-or-sight to a distant QSO passes through the disk of a spiral galaxies.

Ostriker, Jeremiah P.↗

NASA NTRS1989

Asymptotic/numerical analysis of supersonic propeller noise

An asymptotic analysis based on the Mach surface structure of the field of a supersonic helical source distribution is applied to predict thickness and loading noise radiated by high speed propeller blades. The theory utilizes an integral representation of the Ffowcs-Williams Hawkings equation in a fully linearized form. The asymptotic results are used for chordwise strips of the blade, while required spanwise integrations are performed numerically. The form of the analysis enables predicted waveforms to be interpreted in terms of Mach surface propagation. A computer code developed to implement the theory is described and found to yield results in close agreement with more exact computations.

Myers, M. K.↗