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At least 739 records · Page 41

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NASA NTRS1983

Calculation of boundary layers near the stagnation point of an oscillating airfoil

The results of an investigation of boundary layers close to the stagnation point of an oscillating airfoil are reported. Two procedures for generating initial conditions, the characteristics box scheme and a quasi-static approach, were investigated, and the quasi-static approach was shown to be appropriate provided the initial region was far from any flow separation. With initial conditions generated in this way, the unsteady boundary layer equations were solved for the flow in the leading edge region of a NACA 0012 airfoil oscillating from 0 to 5 deg. Results were obtained for both laminar and turbulent flow, and, in the latter case, the effect of transition was assessed by specifying its occurrence at different locations. The results demonstrate the validity of the numerical scheme and suggest that the procedures should be applied to calculation of the entire flow around oscillating airfoils.

Cebeci, T.↗

NASA NTRS1983

A tunable dual frequency dye laser - dual frequency oscillator design

The pulsed dye laser offers a tunable oscillator, followed by three amplifiers. It is pumped by a dual frequency Nd:YAG laser. Tuning and spectral width are controlled by a holographic network connected to a high power telescope. The modified two wavelength dye laser allows for absorption lidar techniques for remote sensing of the atmosphere. Line switching is achieved by electrooptical commutation. A feasibility experiment was performed with the original oscillator. A model was then built, and tested with different dyes. After a few modifications were made to improve the conversion efficiency, this oscillator was inserted in the laser to check whether the amplifier stages were correctly adjusted.

Abury, Y.↗

NASA NTRS1983

Solar oscillations observed in the total irradiance

An analysis of the total solar irradiance measurements made by the Solar Maximum Mission's active cavity radiometer, for evidence of global oscillations, has indicated that the most energetic low degree p-mode oscillations in the five-minute band have amplitudes of a few parts per million of the total irradiance. A positive detection is made of modes with l values of zero, 1 and 2. The distribution differs from that of the velocity spectrum, with relatively more power at the lower values of l. The 160-min oscillation is not detected in the power spectrum, and an upper limit of 5 parts per million is placed with 99% confidence on its amplitude.

Woodard, M.↗

NASA NTRS1983

Transonic pressure distributions on a rectangular supercritical wing oscillating in pitch

Steady and unsteady aerodynamic data were measured on a rectangular wing with a 12 percent thick supercritical airfoil mounted in the NASA Langley Transonic Dynamics Tunnel. The wing was oscillated in pitch to generate the unsteady aerodynamic data. The purpose of the wind-tunnel test was to measure data for use in the development and assessment of transonic analytical codes. The effects on the wing pressure distributions of Mach number, mean angle of attack, and oscillation frequency and amplitude were measured. Results from the newly-developed XTRAN3S program (a nonlinear transonic small disturbance code) and from the RHOIV program (a linear lifting surface kernel function code) were compared to measured data for a Mach number of 0.7 and for oscillation frequencies ranging from 0 to 20 Hz. The XTRAN3S steady and unsteady results agreed fairly well with the measured results. The RHOIV unsteady-result agreement was fair but, of course, did not predict shock effects.

Ricketts, R. H.↗

NASA NTRS1982

Wake interference for a heated oscillating cylinder

Penney and Jefferson (1966) have studied heat transfer from an oscillating, horizontal wire. The present investigation has the objective to determine the governing parameters which indicate when interaction between an oscillating circular cylinder and its wake will reduce the apparent heat transfer coefficient in quasi-steady conditions, taking into account, if possible, also the determination of the approximate magnitude of the reduction. A definition is provided of a nondimensional vertical mass flux, representing the induced flow due to heating of the stagnant fluid. It is hypothesized that the flux is related to a natural convection parameter which describes the heating of the wake. For oscillation of a circular cylinder in air under the conditions studied, it is found that the application of a cross-flow correlation in a quasi-steady, transient analysis predicts heat transfer parameters in close agreement with experiment over a certain range, provided interaction with the heated wake is avoided.

Mceligot, D. M.↗

NASA NTRS1982

Damping of thermoacoustic oscillations

A commonly encountered and troublesome problem in cryogenic systems is related to the occurrence of thermoacoustic oscillations (TAO). The oscillations are accompanied by large heat fluxes which can cause large increases in the boiloff from dewars. Such a boiloff can lead to a serious degradation in performance. It appears, therefore, highly advisable to incorporate mechanisms for damping TAO in those parts of the system where there oscillations might occur. The present investigation is concerned with the criteria for the design of such damping mechanisms. Attention is given to the theory regrading the suppression of TAO, a damper consisting of a capillary with a surge tank, and porous solid dampers.

Tward, E.↗

NASA NTRS1983

Solar oscillations with 13-day period

Reference is made to the solar observations made by Claverie et al. (1982) over a three-month period in the summer of 1981 which show oscillatory velocity with a period of 13.1 days and amplitude of 6.6 m/s. These investigators reject the possibility that they see the Doppler shift from a radial oscillation, because the amplitude is implausibly large. They also do not believe that their signal was induced by solar magnetic fields, since typical mean solar fields are too small. Photo-electric drift-scan measurements of the solar diameter and full-disk magnetograms taken at Kitt Peak National Observatory are examined here for evidence of variations corresponding to the velocity oscillations of the 13.1-day period. An upper limit on radius variations is reported which is a factor of six below the amplitude needed to explain the velocity observations as a radial oscillation. Attention is also given to the possible role of the rotation of large-scale surface magnetic features.

Duvall, T. L., Jr.↗

NASA NTRS1983

The effect of a leading-edge slat on the dynamic stall of an oscillating airfoil

The dynamic stall characteristics of a slatted airfoil were investigated experimentally on a 2-ft-chord airfoil oscillating in pitch at M = 0.2 for a range of reduced frequency and mean angle of oscillation. The slat produced a flow that remained attached to the airfoil for angles well above those normally attained by the retreating blade of a helicopter during high speed flight. The dynamic stall vortex usually associated with these flight conditions was completely eliminated for all angles under 30 deg. Instantaneous surface pressure, lift, and pitching moment data are presented as a function of incidence throughout the oscillation cycle; a detailed analysis of instantaneous boundary-layer flow behavior for the various test conditions is included.

Carr, L. W.↗

NASA NTRS1983

On the stability of the unsteady boundary layer on a cylinder oscillating transversely in a viscous fluid

The stability of the two-dimensional flow induced by the tranverse oscillation of a cylinder in a viscous fluid is investigated in both the linear and weakly nonlinear regime. The major assumption that is made to simplify the problem is that the oscillation frequency is large in which case an unsteady boundary layer is set up on the cylinder. Results are given for cylinders of elliptic cross section and it is found that for any given eccentricity the most dangerous configuration is when the cylinder oscillates parallel to its minor axis. Some discussion of nonlinear effects is also given and for the circular cylinder it is shown that the steady streaming boundary layer of the basic flow is significantly altered by the instability.

Hall, P.↗

NASA NTRS1984

Quasi-periodic oscillations in a symmetric general circulation model

Observational evidence has been presented for the existence of quasi-periodic fluctuations of the tropical circulation with periods around two weeks and around 40 days. It is expected that an understanding of the mechanisms of these quasi-periodic oscillations in the tropical atmosphere will improve the predictability of the short range climate fluctuations in the tropics. The present study evolved as an outgrowth of an investigation conducted by Goswami et al. (1984). In this investigation remarkable oscillations of the Hadlay circulation for an ocean covered earth were observed. In the current study evidence is presented regarding the episodic behavior of the tropical circulation in general, and the propagation characteristics of these oscillations in the lower atmosphere. Attention is given to the results of six different experiments.

Goswami, B. N.↗

NASA NTRS1984

ISEE 1 and 2 observations of an oscillating outward moving current sheet near midnight

The ISEE 1 and 2 magnetometers are used to examine a current sheet which is moving outward with a velocity of about 17 km/sec, and whose current is flowing into the ionosphere. A wave is traveling along the current sheet, from midnight toward the east, at 400 km/sec. The plasma oscillations normal to the current sheet associated with this wave are sufficient for explaining the amplitude of the electric field oscillations in the plane of the current sheet observed by the University of California's electric field detector. The present oscillating current sheet model explains both magnetic and electric field observations.

Kelly, T. J.↗

NASA NTRS1984

Umbral oscillations in sunspots

Contrary to the claim by von Uexkuell et al. (1983), their observations of upward phase propagation of umbral oscillations in the chromosphere are in agreement with the photospheric resonance theory of Thomas and Scheuer (1982) and in contradiction to the chromospheric resonance theory of Zhugzhda et al. (1983). Other observational evidence also indicates that the fundamental 3-min umbral oscillation is due to a photospheric resonance, although the closely-spaced multiple peaks sometimes seen in the power spectrum of chromospheric oscillations may well be due to chromospheric resonances.

Thomas, J. H.↗

NASA NTRS1984

Electromagnetic damping of neutron star oscillations

A simple model of magnetic field perturbations driven by neutron star oscillations is used to estimate the electromagnetic power radiated by g-modes and torsional oscillations. The calculation assumes that the neutron star has a frozen-in magnetic field which is perturbed by the oscillatory motions of the surface. The disturbances propagate into the vacuum as outgoing electromagnetic waves. The relative effectiveness of Joule heating of the neutron star crust by pulsation-induced electric currents is estimated. It is concluded that electromagnetic damping is the dominant energy dissipation mechanism for quadrupole g-mode oscillations of neutron stars. For dipole spheroidal modes, both electromagnetic radiation and Joule heating are important, and there is no gravitational radiation emitted by these modes.

Mcdermott, P. N.↗

NASA NTRS1984

Stability of multifrequency negative-resistance oscillators

A general criterion is derived for the stability of a negative-resistance oscillator with respect to small perturbations in the operating point. The derivation applies when the oscillator output consists of an arbitrary number of related frequency components, including possible nonharmonic components. Examples are given of the application of the stability criterion to coaxial IMPATT oscillator circuits, with experimental verification of the frequency and output power at theoretically determined stable operating points.

Bates, B. D.↗

NASA NTRS1984

Overview of Solar Seismology: Oscillations as Probes of Internal Structure and Dynamics in the Sun

The physical nature of solar oscillations is reviewed. The nomenclature of the subject and the techniques used to interpret the oscillations are discussed. Many of the acoustic and gravity waves that can be observed in the atmosphere of the Sun are actually resonant or standing modes of the interior; precise measurements of the frequencies of such modes allow deductions of the internal structure and dynamics of this star. The scientific objectives of such studies of solar seismic disturbances, or of solar seismology, are outlined. The reasons why it would be very beneficial to carry out further observations of solar oscillations both from ground based networks and from space will be discussed.

Toomre, J.↗

NASA NTRS1984

Atomic hydrogen maser active oscillator cavity and bulb design optimization

The performance characteristics and reliability of the active oscillator atomic hydrogen maser depend upon oscillation parameters which characterize the interaction region of the maser, the resonant cavity and atom storage bulb assembly. With particular attention to use of the cavity frequency switching servo (1) to reduce cavity pulling, it is important to maintain high oscillation level, high atomic beam flux utilization efficiency, small spin exchange parameter and high cavity quality factor. It is also desirable to have a small and rigid cavity and bulb structure and to minimize the cavity temperature sensitivity. Curves for a novel hydrogen maser cavity configuration which is partially loaded with a quartz dielectric cylinder and show the relationships between cavity length, cavity diameter, bulb size, dielectric thickness, cavity quality factor, filling factor and cavity frequency temperature coefficient are presented. The results are discussed in terms of improvement in maser performance resulting from particular design choices.

Peters, H. E.↗

NASA NTRS1984

On the stability of the unsteady boundary layer on a cylinder oscillating transversely in a viscous fluid

The stability of the two-dimensional flow induced by the transverse oscillation of a cylinder in a viscous fluid is investigated in both the linear and weakly nonlinear regime. The major assumption that is made to simplify the problem is that the oscillation frequency is large in which case an unsteady boundary layer is set up on the cylinder. Results are given for cylinders of elliptic cross section and it is found that for any given eccentricity the most dangerous configuration is when the cylinder oscillates parallel to its minor axis. Some discussion of nonlinear effects is also given and for the circular cylinder it is shown that the steady streaming boundary layer of the basic flow is significantly altered by the instability.

Hall, P.↗

NASA NTRS1984

Dynamic characteristics of a hydrostatic gas bearing driven by oscillating exhaust pressure

Vibration of a statically loaded, inherently compensated hydrostatic journal bearing due to oscillating exhaust pressure is investigated. Both angular and radial vibration modes are analyzed. The time-dependent Reynolds equation governing the pressure distribution between the oscillating journal and sleeve is solved together with the journal equation of motion to obtain the response characteristics of the bearing. The Reynolds equation and the equation of motion are simplified by applying regular perturbation theory for small displacements. The numerical solutions of the perturbation equations are obtained by discretizing the pressure field using finite-difference aproximations with a discrete, nonuniform line-source model which excludes effects due to feeding hole volume. An iterative scheme is used to simultaneously satisfy the equations of motion for the journal. The results presented include Bode plots of bearing-oscillation gain and phase for a particular bearing configuration for various combinations of parameters over a range of frequencies, including the resonant frequency.

Watkins, C. B.↗