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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Vortices induced in a stagnation region by wakes - Their incipient formation and effects on heat transfer from cylinders

Horseshoe-like vortices, induced by wakes in the stagnation region of bluff bodies, are proposed as an efficient mechanism for augmentation of convective heat transfer. The vortex 'flow module' induced by single or multiple wakes, which had not been observed previously, was first documented and the resulting flow field was studied using various visualization techniques and hot-wire anemometry. In an attempt to understand the driving force behind this flow module, the conditions at which incipient formation of the vortices occurs were investigated. Existence of such a threshold is essential and was hitherto an open question in analytical studies of stability of flow in stagnation region. Finally, effects of the flow module on heat transfer from a cylinder were measured.

Nagib, H. M.↗
NASA NTRS1977

Viscous effects in aircraft trailing vortices

The mechanism of merging of like-signed aircraft vortices leading to a rapid redistribution of trailed vorticity in a wake through both convective and turbulent processes was investigated. Research was done experimentally in a small wind tunnel and analytically through the use of a code which computes turbulent transport using a second-order closure turbulent model. Computations are reported which demonstrate the merging phenomenon, and comparisons are made with experimental results. The usefulness of point vortex computations in predicting merging was explored. Limited computations showed that jet exhaust does not appreciably alter the merging phenomenon. The effect of ambient atmospheric turbulence on the aging of an aircraft wake was investigated at a constant turbulent dissipation rate. It was shown that under stable atmospheric conditions, when atmospheric macroscales are less than or equal to the vortex spacing, misleading results may be obtained.

Bilanin, A. J.↗
NASA NTRS1977

The instability of the thin vortex ring of constant vorticity

A theoretical study indicating that vortex rings at moderate Reynolds numbers are unstable to azimuthal bending waves is presented. Only the case of a thin vortex ring with a core of constant vorticity in an inviscid flow is examined. The disturbance flow and the mean flow of the vortex ring are derived as asymptotic solutions near the core; the stability analysis is developed completely for a certain class of bending waves that are unstable on a line filament in the presence of strain. The vortex ring is found to be always unstable for at least two wavenumbers for which waves on a line filament of the same vorticity distribution would not rotate. Published experimental results are cited to support these conclusions.

Widnall, S. E.↗
NASA NTRS1978

Computational studies of turbulent merger of corotational vortices

A computational study has been conducted to gain a better understanding of the criteria governing the merger of corotating vortices. The zero-equation and one-equation turbulent-flow models are used to model the interaction of two rolled-up, corotational vortices. A model appropriate for the swirling flow under consideration has been proposed to prescribe the algebraic variation of mixing-length in the flow field. The unsteady, two-dimensional equations in a cross-plane are solved for primitive variables using an implicit finite-difference code. The computed downstream merging distances are in good agreement with available wind tunnel data.

Raj, P.↗
NASA NTRS1979

Observations on streamwise vortices in laminar and turbulent boundary layers

The frequent but often unsuspected presence of streamwise vortices in nominally two dimensional laminar and turbulent boundary layers and some of their consequences are described. Since there is no body of systematic information on streamwise vortices imbedded in boundary layers, a number of issues concerning their occurrence and behavior are discussed in the form of a set of succinct observations. Desirable experimental and numerical research to remedy the current lack of knowledge is recommended.

Morkovin, M. V.↗
NASA NTRS1979

Longitudinal vortices in a transitioning boundary layer

Naturally occurring spanwise variations of the streamwise velocity component, characteristic of longitudinal vortices embedded in a transitioning boundary layer were explored using hot-wire anemometers. A vibrating ribbon introduced stable or unstable Tollmien-Schlichting waves into the laminar boundary layer. These damped or growing disturbances always developed a strong three dimensional pattern even though no spanwise perturbations were artificially induced. Changing the radius of the leading edge and other modifications to the flat plate, wind tunnel and boundary layer did not alter the spanwise wavelength of the vortices.

Anders, J. B.↗
NASA NTRS1978

Unsteady vortical and entropic distortions of potential flows round arbitrary obstacles

The analysis concerns the alterations produced when small amplitude disturbances, including entropy and vorticity disturbances, are imposed on steady potential flows. For the most general nonacoustic incident distortion field that can be imposed on the uniform upstream flow, it is shown that the perturbation velocity at any point of the resulting unsteady compressible and vortical flow consists of a part that is a known function of the imposed upstream distortion field and the mean flow variables and a potential part that can be found by solving a linear inhomogeneous wave equation with a dipole-type source term whose strength is a known function of the imposed upstream distortion field. The theory is applied to the unsteady flow past a corner, and a closed-form analytical solution is found.

Goldstein, M. E.↗
NASA NTRS1979

An observational test of the quasi-geostrophic relation between eddy momentum flux convergence and eddy fluxes of potential vorticity and potential temperature

The quasi-geostrophic relation between the fluxes of momentum, potential vorticity and potential temperature is tested with atmospheric data by computing the convergence of momentum flux as a residual of the potential temperature and potential vorticity flux and comparing it to the momentum flux convergence computed directly. It is shown that in the troposphere between 18 and 74 deg N the observed momentum flux convergence differs from the quasi-geostrophic convergence by 25-60% depending on tropospheric level and season.

Heck, W. J.↗
NASA NTRS1979

Effect of subjectively enhancing the initial vorticity field in a short-range numerical forecast

The paper tests a simple method to incorporate subsynoptic-scale information into the initial conditions of a forecast model. The amplitude, orientation and vertical structure of the vorticity field associated with the subsynoptic-scale disturbance are estimated subjectively; this is done using a combination of climatological and physical models together with all available data. The initial values of geopotential are then obtained in the vicinity of the subsynoptic-scale disturbance, as over the rest of the domain, through the balance equation. Two sets of initial conditions are tested in 12-hr forecasts using a six-layer version of a primitive equation forecast model (Anthes and Warner, 1978). The analysis schemes consisted of (1) a careful subjective analysis of radiosonde data with a high-resolution objective interpolation, and (2) the same preceding analysis but including data derived from the enhanced vorticity fields in the vicinity of the subsynoptic-scale disturbance. The results are discussed in some detail.

Fried, G. D.↗
NASA NTRS1980

Full scale visualization of the wing tip vortices generated by a typical agricultural aircraft

The trajectories of the wing tip vortices of a typical agricultural aircraft were experimentally determined by flight test. A flow visualization method, similar to the vapor screen method used in wind tunnels, was used to obtain trajectory data for a range of flight speeds, airplane configurations, and wing loadings. Detailed measurements of the spanwise surface pressure distribution were made for all test points. Further, a powered 1/8 scale model of the aircraft was designed, built, and used to obtain tip vortex trajectory data under conditions similar to that of the full scale test. The effects of light wind on the vortices were demonstrated, and the interaction of the flap vortex and the tip vortex was clearly shown in photographs and plotted trajectory data.

Cross, E. J., Jr.↗
NASA NTRS1980

Full scale visualization of the wing tip vortices generated by a typical agricultural aircraft

The trajectories of the wing tip vortices of a typical agricultural aircraft were experimentally determined by flight test. A flow visualization method, similar to the vapor screen method used in wind tunnels, was used to obtain trajectory data for a range of flight speeds, airplane configurations, and wing loadings. Detailed measurements of the spanwise surface pressure distribution were made for all test points. Further, a powered 1/8 scale model of the aircraft was designed, built, and used to obtain tip vortex trajectory data under conditions similar to that of the full-scale test. The effects of light wind on the vortices were demonstrated, and the interaction of the flap vortex and the tip vortex was clearly shown in photographs and plotted trajectory data.

Cross, E. J., Jr.↗
NASA NTRS1980

Experiments on multiple vortices in a side dump gas generator ramjet

The role of multiple vortices in the flame stabilization process of a 4 inlet side dump ramjet was determined. Very rich propane air mixtures were injected at the head to simulate a gas generator ramjet. Swirl was introduced by canting the inlets at the dump station. The inlet angle, swirl angle, and the distance of the head from the dump station were varied over a wide range to determine the importance of the vortices located at the dump stations and at the head. It is shown that with proper design, the vortex at the head can be made to perform as a pilot burner allowing a great deal of versatility of operation and it appears to provide all the criteria of pollution free operation.

Choudhury, P. R.↗
NASA NTRS1981

Correlative study of thermospheric gravity waves and tropospheric vorticity area index

Based on the occurrence frequency of wave-like structures in 23,691 individual measurements of thermospheric density, it is demonstrated that there is no significant hemispherical correlation between satellite-measured thermospheric wave-like structures and the tropospheric vorticity area index in the epoch 1974-1976. However, in the northern polar zone, (60-90 deg) N, wave occurrence frequency is greater for large values of the 500 mb vorticity area index.

Chiu, Y. T.↗
NASA NTRS1981

Influence of velocity gradients on measurements of velocity and streamwise vorticity with hot-wire X-array probes

An analysis and measurement of the effects of the streamwise velocity gradients partial derivative of U with respect to y and partial derivative of U with respect to z, on the velocity components, U, v, and w, and the streamwise vorticity component, omega sub x measured in turbulent flow with a pair of orthogonal hot-wire X arrays, is presented. It is shown that these gradients, which can have the same order of magnitude instantaneously as the mean shear stress at the wall, cause extremely large errors in the measured instantaneous cross-stream velocity and streamwise vorticity components.

Vukoslavcevic, P.↗
NASA NTRS1983

On issues concerning flow separation and vortical flows in 3 dimensions

This review provides an illustrated introduction laying the knowledge base for vortical flows about three dimensional configurations that are of typical interest to aerodynamicists and researchers in fluid mechanics. The paper then compiles a list of ten issues, again in illustrative format, that the authors deem important to the understanding of complex vortical flows. These issues and our responses to them provide, it is hoped, a skeletal framework on which to hang the ensuing conference proceedings.

Peake, D. J.↗
NASA NTRS1982

Development of secondary flow and vorticity in curved ducts, cascades, and rotors, including effects of viscosity and rotation

This paper is concerned with the numerical solution of the secondary vorticity equations in curved ducts, cascades, and rotors. The classical approach of splitting the flow into primary and secondary flow fields is employed and extended to include effects of viscosity, rotation, and density stratification. All elliptical effects are neglected and the Crank-Nicolson method is used to solve the secondary vorticity equation. The secondary flow field is obtained by solving a Poisson equation using a successive over-relaxation method. The results are compared with theoretical and experimental data from stationary ducts, compressor and turbine cascades. The agreement is good for most of the cases.

Pouagare, M.↗
NASA NTRS1983

Numerical studies of the formation and destruction of vortices in a motored four-stroke piston-cylinder configuration

A finite-difference procedure which solves the conservation equations of mass, momentum, and energy is used to investigate the effects of the compression ratio, engine speed, bore-to-stroke ratio, and air intake flow angle on the turbulent flow field within an axisymmetric piston-cylinder configuration. It is shown that in a four-stroke piston-cylinder configuration, the intake stroke is characterized by the formation of a piston vortex. The piston vortex is stretched during the intake stroke, and the head vortex has an almost constant diameter. For a 0-deg air intake flow angle, both vortices disappear by the end of the compression stroke; for an air intake flow angle of 45 deg, the flow field within the cylinder shows three elongated vortices which persist into the compression stroke and then break up and merge. It is also shown that larger bore-to-stroke ratios give rise to lower turbulent levels than smaller bore-to-stroke ratios and that the turbulent intensity is almost independent of the rpm.

Schock, H. J.↗
NASA NTRS1982

Vortices in turbulence

Theoretical and experimental results of studies to determine the flow parameters and structures of plane mixing layers are reported. Smoke visualization, combined with hot-wire anemometry, sheets of laser light, and photography were used to gather data from the wall flow. The behavior of vortex rods was examined, noting that the rods persisted only if new vortex energy was supplied from the sublayer. A power spectral density was defined for the velocity fluctuations, as was a hierarchy of velocity scales for geometrically similar vortices. The length scale grows linearly with downstream distance, where the flow structures are fed by longitudinal vortices. A model is developed for vortex pairing in sequential order from the bottom of the mixing layer outward in a repetitive process involving vortex stretching. The model is actually a migration strategy that satisfies the flow self-preservation constraints.

Perry, A. E.↗