The behavior of spheres in non-steady translational flow
has been studied experimentally for values of Reynolds number
from 0.2 to 3000. The aim of the work was to improve our
qualitative understanding of particle transport in turbulent
gaseous media, a process of extreme importance in power
plants and energy transfer mechanisms.
Particles, subjected to sinusoidal oscillations parallel
to the direction of steady translation, were found to have changes
in average drag coefficient depending upon their translational
Reynolds number, the density ratio, and the dimensionless
frequency and amplitude of the oscillations. When the Reynolds
number based on sphere diameter was less than 200, the
oscillation had negligible effect on the average particle drag.
For Reynolds numbers exceeding 300, the coefficient of
the mean drag was increased significantly in a particular
frequency range. For example, at a Reynolds number of
3000, a 25 per cent increase in drag coefficient can be produced
with an amplitude of oscillation of only 2 per cent of the sphere
diameter, providing the frequency is near the frequency at which
vortices would be shed in a steady flow at the mean speed. Flow
visualization shows that over a wide range of frequencies, the
vortex shedding frequency locks in to the oscillationfrequency.
Maximum effect at the natural frequency and lock-in show that a
non-linear interaction between wake vortex shedding and the
oscillation is responsible for the increase in drag.
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Measurements of some of the properties of high-degree solar p- and f- mode oscillations are presented. Using high-resolution velocity images from Big Bear Solar Observatory, we have measured mode frequencies, which provide information about the composition and internal structure of the Sun, and mode velocity amplitudes (corrected for the effects of atmospheric seeing), which tell us about the oscillation excitation and damping mechanisms.
We present a new and more accurate table of the Sun's acoustic vibration frequencies, [...], as a function of radial order n and spherical harmonic degree l. These frequencies are averages over azimuthal order m and approximate the normal mode frequencies of a nonrotating, spherically symmetric Sun near solar minimum. The frequencies presented here are for solar p- and f- modes with [...], [...], and [...]. The uncertainties, [...] , in the frequencies are as low as 3.1 pHz. The theoretically expected f-mode frequencies are given by [...], where g is the gravitational acceleration at the surface, [...] is the horizontal component of the wave vector, and [...] is the radius of the Sun. We find that the observed frequencies are significantly less than expected for l > 1000, for which we have no explanation.
Observations of high-degree oscillations, which have very small spatial features, suffer from the effects of atmospheric image blurring and image motion (or "seeing"), thereby reducing the amplitudes of their spatial-frequency components. In an attempt to correct the velocity amplitudes for these effects, we have simultaneously measured the atmospheric modulation transfer function (MTF) by looking at the effects of seeing on the solar limb. We are able to correct the velocity amplitudes using the MTF out to [...]. We find that the frequency of the peak velocity power (as a function of l) increases with l. We also find that the mode energy is approximately constant out to [...], at which point it begins to decrease. Mode energy is expected to be constant as a function of f if the modes are excited by stochastic interactions with convective turbulence in the solar convection zone. Finally, we discuss the accuracy of the seeing correction and a test of the correction using the 1989 March 7 partial solar eclipse.
A theory is presented for the calculation of the velocity potential of a harmonically oscillating delta wing having subsonic leading edges in a supersonic flow. The velocity potential is expanded in a power series in powers of the reduced frequency. Two modes of oscillation, plunging and pitching, are considered. For both modes the analysis is carried through the term linear in reduced frequency, this being generally sufficient for dynamic stability analyses. The results thus obtained for the pitching mode verify those of Miles (Ref. 9) obtained by an integral transformation of the steady-state solution. In addition, the term that is quadratic in the reduced frequency is presented for the plunging mode to illustrate the general procedure.
Lift and pitching moment coefficients are calculated from the velocity potential and numerical results valid for low frequencyoscillations are presented.
This report covers an experimental investigation of the relationship between the vortex shedding frequency and self excited torsional oscillationfrequency for a thin airfoil. The work consisted of measurements of velocity fluctuations in the airstream in the vicinity of a wing model mounted in a wind tunnel so that it could oscillate about the wing axis. The velocity fluctuation measurements were made with the Wing restrained and with the wing oscillating at various angles of attack and wind velocities.
Two distinct types of oscillations were found. One type was self sustaining and increased in amplitude with increasing wind velocity while the other type stopped for velocities beyond some critical value.
I. Plasma Oscillations and Radio Noise from the Disturbed Sun.
Many investigators have suggested that plasma oscillations in the solar corona may be the source of large bursts of radio noise in the meter wavelength region. Two aspects of this problem are considered in this report: (a) the excitation of plasma oscillations by directed beams of charged particles, and (b) the conversion of energy in the longitudinal plasma oscillations to transverse electromagnetic waves by means of random inhomogeneities in electron density.
It appears unlikely that charged particles whose velocity is much less than the r.m.s. thermal velocity of the coronal electrons will excite plasma oscillations. Charged particles whose velocity is much greater than the r.m.s. thermal velocity excite oscillations in a band of frequencies, including frequencies above the local plasma frequency. However, qualitative arguments indicate that the noise should be concentrated in a narrow band of frequencies slightly below the local plasma frequency. Thus it is impossible to explain the Type II (slow) bursts in the manner assumed and unlikely that the Type III (fast) bursts are explainable in this manner. The transfer of energy is studied in detail and it is shown that only waves whose phase velocity is less than the directed beam of charged particles receive energy from the beam.
It is shown that plasma oscillations radiate a small fraction of their energy if the electron density is not uniform. In particular, random fluctuations in density, of the amount expected in thermal equilibrium, cause about 10[superscript -5] of the plasma-oscillation energy to be radiated; the remainder is dissipated by short-range collisions. Larger fluctuations than this are likely, and hence more energy should be radiated.
II. A Field Analysis of the M Type Backward Wave Oscillator.
A field theory of electron beams focused by crossed electric and magnetic fields is given. The theory is basic to the understanding of the small signal behavior of crossed field electron devices. It is applied to explain the slipping stream, or diocotron, effect as a coupling of two surface waves of the electron beam, and to derive the start oscillation conditions of the M-type backward wave oscillator. It is found that the slipping stream effect can reduce the starting current by an appreciable factor. The results are compared with the thin beam theory which neglects space charge effects.
An analysis of a loaded strip transmission line is given, from which a method of representing space harmonic slow wave circuits by a surface admittance boundary condition is obtained. Forward and backward space harmonic interaction may be treated equally well.
Damped free oscillations of the magnetization have been clearly
observed at the completion of 180° flux reversal along both the easy
and the hard axis in Ni-Fe thin films. The flux component perpendicular
to the applied pulse field was observed using a single turn pickup
loop around the film. The frequency of the oscillation was studied
as a function of applied pulse field and compared with the results obtained
by ferromagnetic resonance. The frequency of the damped free
oscillation agreed quite well with that obtained by resonance when the
frequency was measured after the oscillation had damped to small amplitude.
The damping constant obtained from the decay of the oscillation
agreed quite well with that obtained from the half-power line-width
of the resonance curve.
The Landau-Lifshitz equation proposed for the coherent rotation,
using the value of the damping constant obtained by resonance, could
describe the initial part of the magnetization reversal and the damped
free oscillation in the films with low angular dispersion. Agreement
between the experimental and the calculated transverse flux change for
the entire waveform could not be obtained by using the value of damping
constant obtained by resonance. The agreement was better at both
higher applied field or lower anisotropy dispersion. The effect of
eddy currents was negligible on the flux reversal but appeared as a
slight increase of the damping constant obtained by resonance