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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 163 records · Page 9
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NASA NTRS1987

A new approach to the effect of sound on vortex dynamics

Analytical results are presented on the effect of acoustic radiation on three-dimensional vortex motions in a homogeneous, slightly compressible, inviscid fluid. The flow is considered as linear and irrotational everywhere except inside a very thin cylindrical core region around the vortex filament. In the outside region, a velocity potential is introduced that must be multivalued, and it is shown how to compute this scalar potential if the motion of the vortex filament is prescribed. To find the motion of this singularity in an external potential flow, a variational principle involving a volume integral that must exclude the singular region is considered. A functional of the external potential and vortex filament position is obtained whose extrema give equations to determine the sought-after evolution. Thus, a generalization of the Biot-Savart law to flows with constant sound speed at low Mach number is obtained.

Lund, Fernando↗
NASA NTRS1987

Tidal estimation in the Pacific with application to SEASAT altimetry

The techniques for computing the eigenfunctions of the velocity potential (Proudman functions) set out in Sanchez, et al. (1986) in relation to the Atlantic-Indian Oceans are here applied to the Pacific Ocean, using a 6x6 degree grid of 510 points (455 points for the associated stream functions). Normal modes are computed from the first Proudman functions and have natural periods from 43.9h downward. Tidal syntheses are derived from these modes by direct application of the (frictionless) dynamic equations and by least-squares fitting of Proudman functions to the dynamically interpolated tide-gauge data of Schwiderski (1983). The modes contributing the most energy to the principal harmonic tidal constituents are different in the two computations: their natural periods are typically in the range of 9 to 16h for semidiurnal, and 14 to 43h for diurnal tides. The rms of fit for the Proudman functions is, in all cases, better than the corresponding value for the same number of spherical harmonics.

Sanchez, Braulio V.↗
NASA NTRS1988

The exact calculation of quadrupole sources for some incompressible flows

This paper is concerned with the application of the acoustic analogy of Lighthill to the acoustic and aerodynamic problems associated with moving bodies. The Ffowcs Williams-Hawkings equation, which is an interpretation of the acoustic analogy for sound generation by moving bodies, manipulates the source terms into surface and volume sources. Quite often in practice the volume sources, or quadrupoles, are neglected for various reasons. Recently, Farassat, Long and others have attempted to use the FW-H equation with the quadrupole source and neglected to solve for the surface pressure on the body. The purpose of this paper is to examine the contribution of the quadrupole source to the acoustic pressure and body surface pressure for some problems for which the exact solution is known. The inviscid, incompressible, 2-D flow, calculated using the velocity potential, is used to calculate the individual contributions of the various surface and volume source terms in the FW-H equation. The relative importance of each of the sources is then assessed.

Brentner, Kenneth S.↗
NASA NTRS1988

Prediction of the oscillating incompressible aerodynamics of a loaded airfoil cascade by a locally analytical method

A complete first order model is developed to predict the oscillating incompressible aerodynamics of an airfoil cascade, including the effects of steady loading. The cascade steady and unsteady flow fields are analyzed by considering a periodic flow channel. The velocity potential is separated into steady and unsteady harmonic components each of which is described by a Laplace equation. An analytical solution in individual grid elements of a body fitted computational grid is then determined, with the complete solution obtained by assembling these local solutions. The capabilities and validity of this model and solution technique are demonstrated by considering the steady and unsteady aerodynamics of both theoretical and experimental cascade configurations.

Chiang, Hsiao-Wei D.↗
NASA NTRS1987

A simplified model of the modified Hadley circulation

In this paper, a simplified model of modified Hadley circulation that is amenable to analysis is developed to find the low-latitude upper-tropospheric circulation which would exist in the absence of transient eddies. The model is developed as a perturbation of the ideal Hadley circulation solution due to zonally assymmetric thermal forcing and is solved semianalytically. The modified Hadley circulation, found using forcing derived from observed outgoing long-wave radiation, exhibits some similarities to observed features of the tropical and subtropical time-mean upper-tropospheric flow, particularly with regard to the velocity potential.

Schneider, Edwin K.↗
NASA NTRS1988

Prediction of oscillating thick cambered aerofoil aerodynamics by a locally analytic method

The effects of mean-flow incidence, airfoil camber, and airfoil thickness on the incompressible aerodynamics of an oscillating airfoil are investigated theoretically, developing and applying a first-order FEM based on locally analytical solutions (LASs). Laplace equations are used to describe the steady and unsteady harmonic velocity potentials; a body-fitted computational grid is employed; grid-element solutions for both potentials are determined using a numerical LAS method; and the LASs are then assembled to obtain a complete solution. Results for a series of flat-plate and Joukowski airfoils are presented in extensive graphs and discussed in detail.

Chiang, Hsiao-Wei D.↗
NASA NTRS1989

Development and application of a program to calculate transonic flow around an oscillating three-dimensional wing using finite difference procedures

A finite difference method for solving the unsteady transonic flow about harmonically oscillating wings is investigated. The procedure is based on separating the velocity potential into steady and unsteady parts and linearizing the resulting unsteady differential equation for small disturbances. The differential equation for the unsteady potential is linear with spatially varying coefficients and with the time variable eliminated by assuming harmonic motion. Difference equations are derived for harmonic transonic flow to include a coordinate transformation for swept and tapered planforms. A pilot program is developed for three-dimensional planar lifting surface configurations (including thickness) for the CRAY-XMP at Boeing Commercial Airplanes and for the CYBER VPS-32 at the NASA Langley Research Center. An investigation is made of the effect of the location of the outer boundaries on accuracy for very small reduced frequencies. Finally, the pilot program is applied to the flutter analysis of a rectangular wing.

Weatherill, Warren H.↗
NASA NTRS1988

Prediction of incidence effects on oscillating airfoil aerodynamics by a locally analytical method

A complete mathematical model is formulated to analyze the effects of mean-flow incidence angle on the unsteady aerodynamics of an oscillating airfoil in an incompressible flow field. A velocity potential formulation is utilized. The steady flow is independent of the unsteady flow field but coupled to it through the boundary conditions on the oscillating airfoil. The numerical solution technique for both the steady and unsteady flow fields is based on a locally analytical method. The flow model and solution method are then verified through the excellent correlation obtained with the Theodorsen oscillating-flat-plate and Sears transverse-gust classical solutions. The effects of mean flow incidence on the steady and oscillating airfoil aerodynamics are then investigated.

Chiang, Hsiao-Wei D.↗
NASA NTRS1988

Color graphics workstations used for simultaneous display of satellite and model data sets

Model-measurement display methods for studying actual or synthetic satellite measurements of cloudiness are developed. Visualization and animation techniques are employed to analyze the data. Two types of animation sequences, one based on Insat IR geosynchronous satellite imagery combined with upper level velocity potentials calculations and isoline depictions of the high amplitude centers of low frequency cloudiness activity, and the other combines a mesoscale cloud model simulation of a severe hail storm and synthetic microwave radiation signatures, are described.

Cooper, Harry J.↗
NASA NTRS1990

Relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1, FGGE

FGGE Level III-b analyses, produced by the Goddard Laboratory for Atmospheres, NASA, are used to investigate the relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1 (January 5 - March 4, 1979). The mean state of two 15-day periods is examined, as well as day-to-day variations for the entire 59-day period. In Period 1 (January 6-20), the central South Pacific was extremely active convectively, while in Period 2 (February 3-17), convective activity over the western Indian Ocean was enhanced. Episodes of strong outflow in the tropics, as measured by the upper tropospheric velocity potential, were found to be well correlated with the strengthening and propagation of westerly wind maxima in the subtropics. The average location of the westerly maximum over the South Pacific and Indian oceans was about 16 deg latitudes south, and slightly east, of its corresponding heat source. For a cyclone case study which is presented, however, this distance was considerably less. The response time between the upper level tropical outflow and subtropical westerly enhancement appears to be less than 12 hours; however, an exact temporal scale was difficult to identify.

Hurrel, James W.↗
NASA NTRS1990

Cascade aerodynamic gust response including steady loading effects

To predict the unsteady convected gust aerodynamic response of a cascade comprised of arbitrary thick and cambered aerofoils in an incompressible, inviscid, flow field, a complete first-order model is formulated. The flow is analyzed by considering a periodic flow channel. The velocity potential is separated into steady and unsteady harmonic components, each described by a Laplace equation. The strong dependence of the unsteady aerodynamics on the steady effects of aerofoil and cascade geometry and incidence angle is manifested in the coupling of the unsteady and steady flow fields through the unsteady boundary conditions. Analytical solutions in individual grid elements of a body-fitted computational grid are then determined, with the complete solution obtained by assembly of these local solutions. The validity and capabilities of this model and solution technique are then demonstrated by analyzing the steady and unsteady aerodynamics of both theoretical and experimental cascade configurations.

Chiang, Hsiao-Wei D.↗
NASA NTRS1991

Finite element solution of transient fluid-structure interaction problems

A finite element approach using NASTRAN is developed for solving time-dependent fluid-structure interaction problems, with emphasis on the transient scattering of acoustic waves from submerged elastic structures. Finite elements are used for modeling both structure and fluid domains to facilitate the graphical display of the wave motion through both media. For the liquid, the use of velocity potential as the fundamental unknown results in a symmetric matrix equation. The approach is illustrated for the problem of transient scattering from a submerged elastic spherical shell subjected to an incident tone burst. The use of an analogy between the equations of elasticity and the wave equation of acoustics, a necessary ingredient to the procedure, is summarized.

Everstine, Gordon C.↗
NASA NTRS1991

Intraseasonal oscillation of convective activity in the tropical Southern Hemisphere - May 1984-April 1986

This paper studies the intraseasonal oscillation of convection and related variables in the Southern Hemisphere tropics utilizing European Centre for Medium-range Weather Forecasts analyses of 1984 to 1986. One aspect of the research was that the original, unfiltered, time series of various variables was first analyzed to determine if a statistically significant signal occurred on the intraseasonal time scale. The analysis showed that two variables, outgoing longwave radiation and velocity potential at 200 hPa, provided the best evidence of an intraseasonal oscillation. The oscillation propagated eastward, and the convective activity for both years was more intense over the Indian Ocean-Indonesia-western Pacific region than elsewhere.

Vincent, Dayton G.↗
NASA NTRS1991

Nonlinear oscillations of inviscid free drops

The present analysis of free liquid drops' inviscid oscillations proceeds through solution of Bernoulli's equation to obtain the free surface shape and of Laplace's equation for the velocity potential field. Results thus obtained encompass drop-shape sequences, pressure distributions, particle paths, and the temporal evolution of kinetic and surface energies; accuracy is verified by the near-constant drop volume and total energy, as well as the diminutiveness of mass and momentum fluxes across drop surfaces. Further insight into the nature of oscillations is provided by Fourier power spectrum analyses of mode interactions and frequency shifts.

Patzek, T. W.↗
NASA NTRS1993

A case study of the intraseasonal oscillation traversing the TOGA-COARE LSD

The paper presents examination of tree intraseasonal (30-60 day) oscillations (ISOs) that occurred during the southern summer season (December 1, 1985 - February 28, 1986) traversing the Large-Scale Domain (LSD) TOGA-COARE, the region which also plays an important role in ENSO, Australian monsoon, and extratropical circulations. Data presented include Hovmoeller diagrams of 5-day running means of 250-mb velocity potential anomalies and OLR anomalies; graphs of five-day running means of OLR in precipitable water (W) per sq m, averaged over 10 x 10 deg boxes centered on 5 S and (1) 145 E, (2) 155 E, (3) 165 E, and (4) 165 D, indicating the midpoint of each ISO; and vertical profiles of zonal wind in m/s averaged over the time period that each ISO spends in the 10 x 10 deg box centered at 5 S, and 175 E and 145 E.

Vincent, Dayton G.↗
NASA NTRS1956

On slender-body theory and the area rule at transonic speeds

The basic ideas of the slender-body approximation have been applied to the nonlinear transonic-flow equation for the velocity potential in order to obtain some of the essential features of slender-body theory at transonic speeds. The results of the investigation are presented from a unified point of view which demonstrates the similarity of slender-body solutions in the various Mach number ranges. The transonic area rule and some conditions concerning its validity follow from the analysis.

FLOW, SUPERSONIC↗
NASA NTRS1950

Linearized compressible-flow theory for sonic flight speeds

The partial differential equation for the perturbation velocity potential is examined for free-stream Mach numbers close to and equal to one. It is found that, under the assumptions of linearized theory, solutions can be found consistent with the theory for lifting-surface problems both in stationary three-dimensional flow and in unsteady two-dimensional flow. Several examples are solved including a three dimensional swept-back wing and two dimensional harmonically-oscillating wing, both for a free stream Mach number equal to one. Momentum relations for the evaluation of wave and vortex drag are also discussed. (author)

Heaslet, Max A↗
NASA NTRS1947

Theoretical study of air forces on an oscillating or steady thin wing in a supersonic main stream

A theoretical study, based on the linearized equations of motion for small disturbance, is made of the air forces on wings of general plan forms moving forward at a constant supersonic speed. The boundary problem is set up for both the harmonically oscillating and the steady conditions. Two types of boundary conditions are distinguished, which are designated "purely supersonic" and "mixed supersonic." the method is illustrated by applications to a number of examples for both the steady and the oscillating conditions. The purely supersonic case involves independence of action of the upper and lower surfaces of the airfoil and present analysis is mainly concerned with this case. A discussion is first given of the fundamental or elementary solution corresponding to a moving source. The solutions for the velocity potential are then synthesized by means of integration of the fundamental solution for the moving source. The method is illustrated by applications to a number of examples for both the steady and the oscillating cases and for various plan forms, including swept wings and rectangular and triangular plan forms. The special results of a number of authors are shown to be included in the analysis.

Garrick, I E↗