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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Perpendicular blade vortex interaction and its implications for helicopter noise prediction: Wave-number frequency spectra in a trailing vortex for BWI noise prediction

Perpendicular blade vortex interactions are a common occurrence in helicopter rotor flows. Under certain conditions they produce a substantial proportion of the acoustic noise. However, the mechanism of noise generation is not well understood. Specifically, turbulence associated with the trailing vortices shed from the blade tips appears insufficient to account for the noise generated. The hypothesis that the first perpendicular interaction experienced by a trailing vortex alters its turbulence structure in such a way as to increase the acoustic noise generated by subsequent interactions is examined. To investigate this hypothesis a two-part investigation was carried out. In the first part, experiments were performed to examine the behavior of a streamwise vortex as it passed over and downstream of a spanwise blade in incompressible flow. Blade vortex separations between +/- one eighth chord were studied for at a chord Reynolds number of 200,000. Three-component velocity and turbulence measurements were made in the flow from 4 chord lengths upstream to 15 chordlengths downstream of the blade using miniature 4-sensor hot wire probes. These measurements show that the interaction of the vortex with the blade and its wake causes the vortex core to loose circulation and diffuse much more rapidly than it otherwise would. Core radius increases and peak tangential velocity decreases with distance downstream of the blade. True turbulence levels within the core are much larger downstream than upstream of the blade. The net result is a much larger and more intense region of turbulent flow than that presented by the original vortex and thus, by implication, a greater potential for generating acoustic noise. In the second part, the turbulence measurements described above were used to derive the necessary inputs to a Blade Wake Interaction (BWI) noise prediction scheme. This resulted in significantly improved agreement between measurements and calculations of the BWI noise spectrum especially for the spectral peak at low frequencies, which previously was poorly predicted.

Devenport, William J.↗
NASA NTRS1979

Low-speed wind-tunnel investigation of wing fins as trailing-vortex-alleviation devices on a transport airplane model

The trailing-vortex-alleviation effectiveness of both a one- and a two-fin configuration (semicircular with a radius of 0.043 semispan) on a jumbo-jet transport airplane model in its landing configuration was investigated in the Langley V/STOL tunnel, by the trailing-wing sensor technique. The fins were located on the upper surface of the transport model wing along the 30-percent-chord line. The fin configurations were effective in reducing the vortex-induced rolling moment, by amounts varying from 28 to 60 percent, on the trailing wing model located at a distance of 7.8 transport model wing spans downstream of the transport model. The flow over the fins and over the transport airplane model wing downstream of the fins was observed to be separated and turbulent. All fin configurations caused a reduction in maximum lift coefficient, a positive increment in drag coefficient, and an increment in nose-up pitching-moment coefficient on the transport airplane model.

Croom, D. R.↗
NASA NTRS1992

Viscous and inviscid instabilities of a trailing vortex

The linear stability of the trailing line vortex model of Batchelor (1964) is studied using a spectral collocation and matrix eigenvalue method. The entire unstable region in the swirl/axial wavenumber parameter space is mapped out for various azimuthal wavenumbers for both the inviscid and viscous stability problem. The results of the study provide a direct numerical validation of the large-azimuthal-wavenumber asymptotic analysis of Leibovich and Stewartson (1983). It is shown that accurate results are obtained up to azimuthal wavenumbers of 10,000 and greater, and the agreement with the asymptotic theory is excellent.

Mayer, Ernst W.↗
NASA NTRS1976

Low-speed wind-tunnel investigation of various segments of flight spoilers as trailing-vortex-alleviation devices on a transport aircraft model

An investigation was made in the Langley V/STOL tunnel to determine, by the trailing wing sensor technique, the effectiveness of various segments of the existing flight spoilers on a transport aircraft model when they were deflected as trailing-vortex-alleviation devices. The four combinations of flight-spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 40 to 50 percent at distances behind the transport model of from about 7 to 27 transport wing spans, 27 spans being the downstream limit of distances used in this investigation. Results obtained at about 7 wing spans downstream for two of the spoiler configurations indicate that essentially all the reduction in induced rolling moment on the trailing wing model was realized at a spoiler deflection of about 30 deg.

Croom, D. R.↗
NASA NTRS1972

Systems simulation for an airport trailing vortex warning system

The approach, development, and limited system studies associated with a system simulation for an Airport Trailing Vortex Warning System are documented. The usefulness is shown of a systems engineering approach to the problem of developing a system, as dictated by aircraft vortices, which will increase air-traffic flow in the takeoff/landing corridors of busy airports while maintaining the required safety factor for each operation. The simulation program has been developed in a modular form which permits new, more sophisticated component models, when they become available and are required, to be incorporated into the program with a minimum of program modifications. This report documents a limited system study that has been performed using this Total System Simulation Model. The resulting preliminary system requirements, conclusions, and recommendations are given.

Jeffreys, H. B.↗
NASA NTRS1955

Theoretical study of the tunnel-boundary lift interference due to slotted walls in the presence of the trailing-vortex system of a lifting model

The equations presented in this report give the interference on the trailing-vortex system of a uniformly loaded finite-span wing in a circular tunnel containing partly open and partly closed walls, with special reference to symmetrical arrangements of the open and closed portions. Methods are given for extending the equations to include tunnel shapes other than circular. The rectangular tunnel is used to demonstrate these methods. The equations are also extended to nonuniformly loaded wings.

Matthews, Clarence W↗
NASA NTRS1991

Velocity measurements in a turbulent trailing vortex and their application to BWI noise prediction

The objectives were to observe the turbulence structure and spectral characteristics of the trailing vortex shed by a rectangular NACA 0012 wing over a range of conditions and to incorporate these observations into the blade-wake interaction (BWI) noise-prediction method of Glegg (1989). The following sections are presented: (1) measurements performed during the first year of this two year investigation; (2) presentation and discussion of a representative sample of the results; (3) implications for the BWI noise prediction method; and (4) re-evaluation of work planned for the second year.

Devenport, William J.↗
NASA NTRS1976

Low-speed wind-tunnel investigation of flight spoilers as trailing-vortex-alleviation devices on a medium range wide-body tri-jet airplane model

An investigation was made in the V/STOL tunnel to determine, by the trailing wing sensor technique, the effectiveness of various segments of the existing flight spoilers on a medium range wide body tri-jet transport airplane model when they were deflected as trailing vortex alleviation devices. The four combinations of flight spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 15 to 60 percent at distances behind the transport model of from 3.9 to 19.6 transport wing spans, 19.6 spans being the downstream limit of distances used. Essentially all of the reduction in induced rolling moment on the trailing wing model was realized at a spoiler deflection of about 45 deg.

Croom, D. R.↗
NASA NTRS1974

Correlation of turbulent trailing vortex decay data

A correlation function, derived on the basis of self similar variable eddy viscosity decay, is introduced and utilized to correlate aircraft trailing vortex velocity data from ground and flight experiments. The correlation function collapses maximum tangential velocity data from scale model and flight tests to a single curve. The resulting curve clearly shows both the inviscid plateau and the downstream decay regions. A comparison between experimental data and numerical solution shows closer agreement with the variable eddy viscosity solution than the constant viscosity analytical solution.

Iversen, J. D.↗
NASA NTRS1976

Low speed wind tunnel investigation of flight spoilers as trailing-vortex-alleviation devices on an extended-range wide body tri-jet airplane model

An investigation was made in the Langley V/STOL tunnel to determine, by the trailing wing sensor technique, the effectiveness of various segments of the existing flight spoilers on an extended-range wide-body tri-jet transport airplane model when they were deflected as trailing-vortex-alleviation devices. On the transport model with the approach flap configuration, the four combinations of flight-spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 25 to 45 percent at downstream distances behind the transport model of 9.2 and 18.4 transport wing spans. On the transport airplane model with the landing flap configuration, the four combinations of flight-spoiler segments investigated were effective in reducing the induced rolling moment on the trailing wing model by as much as 35 to 60 percent at distances behind the transport model of from 3.7 to 18.4 transport wing spans, 18.4 spans being the downstream limit of distances used.

Croom, D. R.↗
NASA NTRS1976

Correlation of turbulent trailing vortex decay data

A correlation function, derived on the basis of self-similar variable eddy-viscosity decay, is introduced and utilized to correlate aircraft trailing vortex velocity data from ground and flight experiments. The correlation function collapses maximum tangential velocity data from scale-model and flight tests to a single curve. The resulting curve clearly shows both the inviscid plateau and the downstream decay regions. A comparison between experimental data and numerical solution shows closer agreement with the variable eddy viscosity solution than the constant viscosity analytical solution.

Iversen, J. D.↗
NASA NTRS1976

Flight investigation of the response of a helicopter to the trailing vortex of a fixed-wing aircraft

A flight investigation was conducted to quantitatively determine the response of a medium-weight helicopter to the trailing-vortex system of a fixed-wing aircraft. Flight tests and analytical tools were both utilized in the investigation. The flight tests involved an extensively instrumented UH-1H helicopter and a C-54 aircraft. Penetrations of the vortex system by the UH-1H were made at the following nominal conditions: the C-54 flew at 5500 feet altitude at a nominal gross weight of 58,000 pounds and an indicated airspeed of 115 knots in a cruise configuration. The UH-1H, nominally 7200 pounds gross weight, flew at 60 knots indicated airspeed during the penetrations at separation distances of 6.64 nautical miles to 0.42 nautical mile between aircraft. In general, the data analyzed for the above tests indicated that no unsafe penetration occurred. Further, penetrating vehicle attitude changes and structural loads were nominal. In addition, the response of the helicopter did not change appreciably with decreased separation distance.

Mantay, W. R.↗
NASA NTRS1993

Nonlinear vortex trail dynamics. II - Analytic solutions

Spatially periodic large amplitude solutions of the von Karman model are obtained in the neighborhood of singularities. These singularities correspond to vortex clusters in the physical plane. The quasi-periodic and unbounded solutions found analytically confirm earlier numerical work and show qualitative agreement with experimental observations of large-scale phenomena of vortex trails. Separatrices or heteroclinic orbits were explicitly found for an integrable approximate equation, which indicate that the von Karman model itself supports chaotic solutions.

Lim, Chjan C.↗
NASA NTRS1973

The application of laser Doppler velocimetry to trailing vortex definition and alleviation

A laser Doppler velocimeter whose focal volume can be rapidly traversed through a flowfield has been used to overcome the problem introduced by excursions of the central vortex filament within a wind tunnel test section. The basic concepts of operation of the instrument are reviewed and data are presented which accurately define the trailing vortex from a square-tipped rectangular wing. Measured axial and tangential velocity distributions are given, both with and without a vortex dissipator panel installed at the wing tip. From the experimental data, circulation and vorticity distributions are obtained and the effect of turbulence injection into the vortex structure is discussed.

Orloff, K. L.↗
NASA NTRS1971

Investigation of an aircraft trailing vortex using a tuft grid

With the increasing capacity of airport terminal areas, and the use of the new large jet transports, it has become important to understand the turbulent wake created by these aircraft. A study of the trailing vortex of a wing has been made using a tuft grid in a 6 foot wind tunnel. The study included an investigation of the use of mass injection at the wing tip as a means of destroying the vortex. Test results show that a fully developed, stable, vortex exists at least a distance of thirty chord lengths downstream of the wing, and that the swirl of the vortex can be reduced or eliminated by mass injection at the wing tip.

Mason, W. H.↗
NASA NTRS1996

Recent Laboratory and Numerical Trailing Vortex Studies

Results from two laboratory studies and two numerical studies are presented. In the first laboratory study, measurements of the strength of vortices from a three-dimensional (3-D) model wing are presented. The measurements follow the vortices as they evolve in time from a two-dimensional (2-D) line vortex pair to the development and migration of 3-D vortex rings. It is shown that the resulting vortex rings can contain up to 40 percent of the initial vortex circulation. Thus, the formation of vortex rings may not necessarily signal the end of the wake hazard to following aircraft. In the second laboratory study, we present the results of an experiment which shows how the spanwise drag distribution affects wake-vortex evolution. In this experiment, we modified the spanwise drag distribution on a model wing while keeping the total lift and drag constant. The results show that adding drag on or near the centerline of the wing has a larger effect than adding drag at or near the wingtips. These measurements complement the results of NASA studies in the 1970s. In the first numerical study, results of 3-D numerical calculations are presented which show that the vortex Reynolds number has a significant influence on the evolution and migration of wake vortices. When the Reynolds number is large, 3-D vortex rings evolve from the initially 2-D line vortex pairs. These vortex rings then migrate vertically. When the Reynolds number is lower, the transition of vorticity from 2-D to 3-D is delayed. When the Reynolds number is very low, the vortices never transition to 3-D, and the vertical migration is significantly reduced. It is suggested that this effect may have been important in previous laboratory wake-evolution studies. A second numerical study shows the influence that vertical wind shear can have on trailing vortex evolution.

Delisi, Donald P.↗
NASA NTRS1972

Wind tunnel test data for wing trailing vortex flow survey

Data are presented on velocity measurements made with hot wire anemometers in the wake of a model of the CV-990 aircraft and a rectangular wing in the NASA-Ames 2.14x3.05 meter (7- by 10-foot) wind tunnels. Variables included angle of attack, tunnel speed and axial distance up to 12 chord lengths downstream from the wing trailing edge. Effects of deflecting trailing edge flaps and small spoiler panels are shown.

Chigier, N. A.↗