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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Juncture flow improvement for wing/pylon configurations by using CFD methodology

Transonic flow field around a fighter wing/pylon configuration was simulated by using an implicit upwinding Navier-Stokes flow solver (F3D) and overset grid technology (Chimera). Flow separation and local shocks near the wing/pylon junction were observed in flight and predicted by numerical calculations. A new pylon/fairing shape was proposed to improve the flow quality. Based on numerical results, the size of separation area is significantly reduced and the onset of separation is delayed farther downstream. A smoother pressure gradient is also obtained near the junction area. This paper demonstrates that computational fluid dynamics (CFD) methodology can be used as a practical tool for aircraft design.

Gea, Lie-Mine↗
NASA NTRS1993

A new flux splitting scheme

A new flux splitting scheme is proposed. The scheme is remarkably simple and yet its accuracy rivals and in some cases surpasses that of Roe's solver in the Euler and Navier-Stokes solutions performed in this study. The scheme is robust and converges as fast as the Roe splitting. An approximately defined cell-face advection Mach number is proposed using values from the two straddling cells via associated characteristic speeds. This interface Mach number is then used to determine the upwind extrapolation for the convective quantities. Accordingly, the name of the scheme is coined as Advection Upstream Splitting Method (AUSM). A new pressure splitting is introduced which is shown to behave successfully, yielding much smoother results than other existing pressure splittings. Of particular interest is the supersonic blunt body problem in which the Roe scheme gives anomalous solutions. The AUSM produces correct solutions without difficulty for a wide range of flow conditions as well as grids.

Liou, Meng-Sing↗
NASA NTRS1993

Impact of increased TOVS signal on the NMC global spectral model - A tropical-plume case study

Temperature and moisture signals are extracted from operationally available TOVS channel radiances for a tropical plume that developed over the North Pacific in January 1989, and are inserted into the NMC's operational medium-range forecast model. Several 48-h forecasts are made to test the impact of the additional information. It is shown, at least within a single tropical plume event, that there is still unutilized information within operational TOVS satellite observations. This information resides in the moisture channels and within small-amplitude, but horiontally coherent, thermal features. Perturbed upper-tropospheric relative humidity patterns are smoother and more consistent with satellite imagery than is the historical forecast.

Mcguirk, James P.↗
NASA NTRS1993

Evidence for two distinct morphological classes of gamma-ray bursts from their short time scale variability

We have analyzed the 241 bursts for which peak counts (C)max exist in the publicly available Burst and Transient Source Experiment (BATSE) catalog. Introducing peak counts in 1024 ms as a measure of burst brightness B and the ratio of peak counts in 64 and 1024 ms as a measure of short timescale variability V, we find a statistically significant correlation between the brightness and the short time scale variability of gamma-ray bursts. The bursts which are smoother on short time scales are both faint and bright, while the bursts which are variable on short time scales are faint only, suggesting the existence of two distinct morphological classes of bursts.

Lamb, D. Q.↗
NASA NTRS1993

Light scattering by size-shape distributions of randomly oriented axially symmetric particles of a size comparable to a wavelength

Rigorously light scattering by size-shape distributions of randomly oriented axially symmetric particles are calculated by the T-matrix method, as extended to randomly oriented scatterers. The computational scheme is described along with a newly developed convergence procedure that makes it possible to substantially reduce computer time and storage requirements. The elements of the Stokes scattering matrix for a power-law size distribution of randomly oriented moderately aspherical spheroids are shown to be much smoother than and differ substantially from those of equivalent monodisperse spheroids; averaging over orientations does not eliminate the necessity of averaging over particle sizes. The angular-scattering behavior of the ensembles of nonspherical particles is found to be significantly different from that of the equivalent polydisperse spheres.

Mishchenko, Michael I.↗
NASA NTRS1993

Plasma expansion and evolution of density perturbations in the polar wind - Comparison of semikinetic and transport models

Comparisons are made between transport and semikinetic models in a study of the time evolution of plasma density perturbations in the polar wind. The situations modeled include plasma expansion into a low-density region and time evolution of localized density enhancements and cavities. The results show that the semikinetic model generally yields smoother profiles in density, drift velocity, and ion temperature than the transport model, principally because of ion velocity dispersion. While shocks frequently develop in the results of the transport model, they do not occur in the semikinetic results. In addition, in the semikinetic results, two ion streams, or double-humped distributions, frequently develop. In the transport model results the bulk parameters, at a given time, often have a one-to-one correspondence in the locations of their local minima or maxima. This is a consequence of the coupling of the fluid equations. There is, however, no such relationship among the moments produced by the semikinetic model where the local moment maxima and minima are often shifted in altitude. In general, incorporation of enhanced heat fluxes in the transport model leads to somewhat improved agreement with the semikinetic results.

Ho, C. W.↗
NASA NTRS1992

Airborne gravimetry, altimetry, and GPS navigation errors

Proper interpretation of airborne gravimetry and altimetry requires good knowledge of aircraft trajectory. Recent advances in precise navigation with differential GPS have made it possible to measure gravity from the air with accuracies of a few milligals, and to obtain altimeter profiles of terrain or sea surface correct to one decimeter. These developments are opening otherwise inaccessible regions to detailed geophysical mapping. Navigation with GPS presents some problems that grow worse with increasing distance from a fixed receiver: the effect of errors in tropospheric refraction correction, GPS ephemerides, and the coordinates of the fixed receivers. Ionospheric refraction and orbit error complicate ambiguity resolution. Optimal navigation should treat all error sources as unknowns, together with the instantaneous vehicle position. To do so, fast and reliable numerical techniques are needed: efficient and stable Kalman filter-smoother algorithms, together with data compression and, sometimes, the use of simplified dynamics.

Colombo, Oscar L.↗
NASA NTRS1928

A Comparison of Propeller and Centrifugal Fans for Circulating the Air in a Wind Tunnel

The tests described in this paper afford a direct comparison of the efficiency and smoothness of flow obtained with propeller fan and multiblade centrifugal fan drives in the same wind tunnel. The propeller fan was found to be superior to the centrifugal fan in that the efficiency was about twice as great, and the flow much smoother.

Weick, Fred E↗
NASA NTRS1934

Increasing the Air Charge and Scavenging the Clearance Volume of a Compression-Ignition Engine

The object of the investigation presented in this report was to determine the effects of increasing the air charge and scavenging the clearance volume of a 4-stroke-cycle compression-ignition engine having a vertical-disk form combustion chamber. Boosting the inlet-air pressure with normal valve timing increased the indicated engine power in proportion to the additional air inducted and resulted in smoother engine operation with less combustion shock. Scavenging the clearance volume by using a valve overlap of 145 degrees and an inlet-air boost pressure of approximately 2 1/2 inches of mercury produced a net increase in performance for clear exhaust operation of 33 percent over that obtained with normal valve timing and the same boost pressure. The improved combustion characteristics result in lower specific fuel consumption, and a clearer exhaust.

Spanogle, J A↗
NASA NTRS1948

Full-Scale Investigation of Aerodynamic Characteristics of a Typical Single-Sotor Helicopter in Forward Flight

As part of the general helicopter research program being undertaken by the National Advisory Committee for Aeronautics to provide designers with fundamental rotor information, the forward-flight performance characteristics of a typical single-rotor helicopter, which is equipped with main and tail rotors, have been investigated in the Langley full-scale tunnel. The test conditions included operation of tip-speed ratios from 0.10 to 0.27 and at thrust coefficients from 0.0030 to 0.0060. Results obtained with production rotor were compared with those for an alternate set of blades having closer rib spacing and a smoother and more accurately contoured surface in order to evaluate the performance gains that are available by the use of rotor blades having an improved surface condition. The wind tunnel results are shown to be in fair agreement with the results of both flight tests and theoretical predictions.

Dingeldein, Richard C↗
NASA NTRS1994

Low-Noise Spiral Bevel Gears

Modified spiral bevel gears that generate relatively little noise and vibration designed and fabricated for use in U.S. Army OH-58D helicopter. Noise reduced by 12 to 19 dB. Similar low-noise, low-vibration spiral bevel gears used in other helicopters, with consequent benefits in comfort and health of pilots and passengers, enhancement of pilots' performance and safety through reduction of audible distraction, and reduction in cost and weight of helicopters through reduction in amount of sound-proofing material. Low-noise, low-vibration spiral bevel gears also used in drive axles of cars and trucks for smoother, quieter rides.

Lewicki, David G.↗
NASA NTRS1993

Venus gravity

The anomalous gravity field of Venus shows high correlation with surface features revealed by radar. We extract gravity models from the Doppler tracking data from the Pioneer Venus Orbiter (PVO) by means of a two-step process. In the first step, we solve the nonlinear spacecraft state estimation problem using a Kalman filter-smoother. The Kalman filter was evaluated through simulations. This evaluation and some unusual features of the filter are discussed. In the second step, we perform a geophysical inversion using a linear Bayesian estimator. To allow an unbiased comparison between gravity and topography, we use a simulation technique to smooth and distort the radar topographic data so as to yield maps having the same characteristics as our gravity maps. The maps presented cover 2/3 of the surface of Venus and display the strong topography-gravity correlation previously reported. The topography-gravity scatter plots show two distinct trends.

Reasenberg, Robert D.↗
NASA NTRS1993

On the stability analysis of approximate factorization methods for 3D Euler and Navier-Stokes equations

The convergence characteristics of various approximate factorizations for the 3D Euler and Navier-Stokes equations are examined using the von-Neumann stability analysis method. Three upwind-difference based factorizations and several central-difference based factorizations are considered for the Euler equations. In the upwind factorizations both the flux-vector splitting methods of Steger and Warming and van Leer are considered. Analysis of the Navier-Stokes equations is performed only on the Beam and Warming central-difference scheme. The range of CFL numbers over which each factorization is stable is presented for one-, two-, and three-dimensional flow. Also presented for each factorization is the CFL number at which the maximum eigenvalue is minimized, for all Fourier components, as well as for the high frequency range only. The latter is useful for predicting the effectiveness of multigrid procedures with these schemes as smoothers. Further, local mode analysis is performed to test the suitability of using a uniform flow field in the stability analysis. Some inconsistencies in the results from previous analyses are resolved.

Demuren, A. O.↗
NASA NTRS1992

Preliminary results of polarization signatures for glacial moraines in the Mono Basin, Eastern Sierra Nevada

The valleys of the Mono Basin contain several sets of lateral and terminal moraines representing multiple stages of glaciation. The semi-arid climate with slow weathering rates preserved sequences of nested younger moraines within older ones. There is a well established relative chronology and recently exposure dating provided a new set of numerical dates. The moraines span the late Wisconsin (11-25 ka) to the Illinoian (130-190 ka) glaciations. The Mono Basin area was used as a 'calibration site' to establish remote dating techniques for eventual transfer to the more inaccessible but geomorphically and climatically similar moraines of the South American Andes Mountains. Planned polarimetric synthetic aperture radar (SAR) imagery acquired by JPL AIRSAR (South American Campaign) and SIR-C (Andes super-site) are analyzed to establish chronologies of previously undated moraine sequences in a study of Pleistocene climatic change in the Southern Hemisphere. The dry climate and sparse vegetation is also favorable for correlation of ground surface roughness with radar polarization signature. The slow weathering processes acting over thousands of years reduce the size, frequency, and angularity of surface boulders while increasing soil development on the moraines. Field observations based on this hypothesis result in relative ages consistent with those inferred from nested position within the valley. Younger moraines, therefore, will appear rougher than the older smoother moraines at scales measurable at AIRSAR wavelengths. Previously documented effects of ground surface roughness on polarization signatures suggest that analysis of moraine polarization signatures can be useful for relative dating. The technique may be extended to predict numerical ages. The data set reported were acquired on 8 Sep. 1989 with the JPL Airborne SAR (AIRSAR) collecting polarimetric imagery at C- (5.6 cm), L- (24 cm), and P-band (68 cm) with a flight-line parallel to the strike of the mountains. Phase calibration was performed on the analyzed scene by setting the co-phase of a smooth lake to zero as described. Absolute amplitude calibration was not possible because corner reflectors were not deployed.

Forster, Richard R.↗
NASA NTRS1992

Detecting surface roughness effects on the atmospheric boundary layer via AIRSAR data: A field experiment in Death Valley, California

The part of the troposphere influenced by the surface of the earth is termed the atmospheric boundary layer. Flow within this layer is influenced by the roughness of the surface; rougher surfaces induce more turbulence than smoother surfaces and, hence, higher atmospheric transfer rates across the surface. Roughness elements also shield erodible particles, thus decreasing the transport of windblown particles. Therefore, the aerodynamic roughness length (z(sub 0)) is an important parameter in aeolian and atmospheric boundary layer processes as it describes the aerodynamic properties of the underlying surface. z(sub 0) is assumed to be independent of wind velocity or height, and dependent only on the surface topography. It is determined using in situ measurements of the wind speed distribution as a function of height. For dry, unvegetated soils the intensity of the radar backscatter (sigma(sup 0)) is affected primarily by surface roughness at a scale comparable with the radar wavelength. Thus, both wind and radar respond to surface roughness variations on a scale of a few meters or less. Greeley showed the existence of a correlation between z(sub 0) and sigma(sup 0). This correlation was based on measurements over lava flows, alluvial fans, and playas in the southwest deserts of the United States. It is shown that the two parameters behave similarly also when there are small changes over a relatively homogeneous surface.

Blumberg, Dan G.↗
NASA NTRS1993

Computational techniques for high-speed flows with viscous and chemical effects

Algorithms for solving the Euler and the Navier-Stokes equations in conjunction with chemical kinetic equations are presented. The convective flux is estimated from a quasi one dimensional interpolation procedure. Shock, contact, and expansion waves and thermochemical nonequilibrium phenomena are captured by the Lax-Friedrichs technique. Relaxation techniques were developed to enhance their effectiveness in dealing with spatial and temporal stiffness associated with the physical problems. Both explicit and implicit smoothers were implemented into the standard multigrid time stepping method. Unsteady and steady scalar problems are discussed. A perfect gas and equilibrium air shock tube problem is investigated. Numerical schemes and techniques are compared for the problems of shock and boundary layer interaction and three dimensional viscous, nonequilibrium flow encompassing an aerobrake. The results are comparable in accuracy against other high order non-oscillatory techniques. The multigrid methods are assessed using a Mach 8 flow over a complete planar body, a sphere, and a blunt delta wing at 20 deg incidence. Applying an implicit multigrid method on a nested grid of 128 by 64 nodes the reduction factor is 0.25. The central processing unit reduction factor is 2.2 after both the single and multigrid Runge-Kutta solutions converged to machine zero on a grid of 37 by 41 by 73 nodes.

Li, C. P.↗
NASA NTRS1993

A multigrid method for steady Euler equations on unstructured adaptive grids

A flux-difference splitting type algorithm is formulated for the steady Euler equations on unstructured grids. The polynomial flux-difference splitting technique is used. A vertex-centered finite volume method is employed on a triangular mesh. The multigrid method is in defect-correction form. A relaxation procedure with a first order accurate inner iteration and a second-order correction performed only on the finest grid, is used. A multi-stage Jacobi relaxation method is employed as a smoother. Since the grid is unstructured a Jacobi type is chosen. The multi-staging is necessary to provide sufficient smoothing properties. The domain is discretized using a Delaunay triangular mesh generator. Three grids with more or less uniform distribution of nodes but with different resolution are generated by successive refinement of the coarsest grid. Nodes of coarser grids appear in the finer grids. The multigrid method is started on these grids. As soon as the residual drops below a threshold value, an adaptive refinement is started. The solution on the adaptively refined grid is accelerated by a multigrid procedure. The coarser multigrid grids are generated by successive coarsening through point removement. The adaption cycle is repeated a few times. Results are given for the transonic flow over a NACA-0012 airfoil.

Riemslagh, Kris↗