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The determination of the amplitude-response characteristic is an important means of checking a network design. However, this measurement is usually a time-consuming procedure and at best does not yield a continuous curve. A device which would produce a continuous curve corresponding to the amplitude-response characteristic would be extremely useful in network design and development. It is shown that the system response to a frequency-modulated signal can be made to approximate the amplitude-response characteristic if the frequency is varied slowly enough so that the "quasi-stationary" conditions exist. The physical realization of this slowly varying frequency requires an oscillator with an extremely large frequency range, controllable by one circuit parameter. The greatest difficulties involved in the design of this oscillator were the development of a simple and stable subtractor and the synthesis of the frequency-determining networks. A mathematical analysis was made to determine the characteristics of the network necessary to produce a logarithmic relation between the oscillator frequency and the control position. The audio-frequency sweep generator was constructed using networks designed to approximate the required characteristics and when tested proved to have a satisfactory output waveform. Any improvement in the oscillator performance would require a better approximation to the specified network characteristics.
Data Types:
  • Document
A program of research has been undertaken to examine the interaction between vortex shedding and the galloping type oscillation which square cylinders are subject to when immersed in a water stream. It is possible that the fluctuating force from the vortex shedding could quench the galloping oscillation if it acts as a forced oscillation (independent of cylinder motion). An experiment was designed where a square cylinder with one degree of freedom could oscillate transversely to the water flow. The amplitude and frequency of the cylinder oscillation were measured. By using a hot film anemometer spectra of the fluctuating velocity in the wake were taken to determine what frequencies vortex shedding occurred at. The results show that for velocities greater than the resonant velocity the galloping oscillation is dominant and the cylinder motion controls the frequencies of the wake. For velocities less than the resonant velocity no galloping occurs and the vortex shedding seems to control any cylinder motion which occurs. To explain this type of response a mathematical model has been constructed. The model is a set of two coupled self excited oscillators} one with the characteristics of the galloping oscillation and the other with the characteristics of the fluctuating lift force from the vortex shedding. Using the model some aspects of the observed interaction are explained.
Data Types:
  • Document
The Néel temperatures of both the hydrated and 96% deuterated single crystal specimen of antiferromagnetic manganese chloride, have been compared. The sample was placed in the tank circuit of a radio-frequency oscillator; the inductance of the coil and hence the frequency of oscillation thus depended on the susceptibility of the specimen. As the crystal in the liquid helium bath was warmed through the Neel temperature, the oscillation frequency was monitored by a frequency counter. By this method, the Neel temperature could be deduced to change by -2.3% when the crystal was 96% deuterated. This result is comparable to the measurements on CoCl₂.6H₂O and CuCl₂.2H₂O. A semi-quantitative explanation proposes an intimate relationship between the superexchange interaction and the hydrogen bond strength.
Data Types:
  • Physical Object
An investigation into various methods of calculation of the high frequency performance parameter f[formula omitted] for microwave heterojunction bipolar transistors is presented. Two high frequency representations of the device are developed: equivalent circuits consisting entirely of lumped circuit elements, and a two-port network based on drift-diffusion equations. Proper account is taken in these representations of the phase delay associated with carrier transit time through the base and base-collector space charge region. Also included are the charging time effects due to the various parasitic circuit elements associated with actual devices. A single-sided isolated structure is used in simulations and it is found that both representations yield remarkably similar characteristics for the behavior of unilateral gain U with frequency. For devices in which the dominant factors limiting high frequency performance are the parasitic resistances and capacitances, it is found that U rolls off at 6 dB/octave through the region where U = 1 and the values predicted for f[formula omitted] via these two methods, as well as via the widely-used analytical expression involving f[formula omitted] and (R[formula omitted]C[formula omitted])[formula omitted], are in agreement. However, when the periods of the oscillations are on the order of the carrier transit times, and the device parasitics are sufficiently low so as to not limit performance, resonance effects occur in U in the region where U = 1 and the prediction of f[formula omitted] obtained via the two equivalent circuit approaches deviates markedly from the predictions of the analytical expression.
Data Types:
  • Document
A pressure transducer, sensitive to acoustic level pressures, was designed and used to measure amplitude, frequency and phase of fluctuating pressure on the surface of a three inch diameter circular cylinder at rest and exhibiting large-amplitude vortex-excited oscillation in a uniform incident wind flow. The phase of the fluctuating pressure relative to the cylinder motion and the cylinder amplitude and frequency were recorded. A disc probe connected to the pressure transducer was used in wake surveys for the stationary and oscillating cylinder. Measurements, made in the Reynolds number range 1.5(10⁴)
Data Types:
  • Physical Object
The flow around a circular cylinder exhibiting vortex-induced oscillation is modelled by 2 potential vortices in a 2-dimensional, inviscid and irrotational flow. The lift on the cylinder is obtained from the general form of the Blasius equation. Pressure distribution is obtained from the pressure equation in a moving frame of reference. The lift expression is coupled to the dynamic equation of the cylinder. The phase and amplitude of oscillation are determined by the method of equivalent linearization. A relationship between amplitude of oscillation and strength of the vortices is proposed. Boot mean square pressure distribution at the Strouhal frequency on the surface of the oscillating cylinder is determined.
Data Types:
  • Physical Object
In the development of a pulse position modulation analogue computer the requirement arose to control accurately the frequency of a magnetic drum. A control system was designed which compares the drum frequency to a 1000 cps tuning fork oscillator. Any deviation in phase produces a corresponding change in torque on the drum drive motor which then corrects the phase error. As part of the over-all project a precision power supply was developed. The requirements were met by modifying existing designs.
Data Types:
  • Document
Coupling of the flexural, longitudinal, and torsional vibration modes of a column subjected to periodic axial loading was analytically and experimentally investigated. The initial crookedness of the column and longitudinal inertia give rise to coupled flexural-longitudinal vibrations. Further, the Weber effect and longitudinal inertia result in coupling between longitudinal and torsional oscillations. To assess the validity of the theory, an experimental apparatus was set up to axially excite a column using a vibration control generator and an electromagnetic shaker. The experimental results were in good agreement with the theoretical predictions. Coupled longitudinal vibrations exhibiting a frequency ratio of 1:2 were observed. Coupled flexural oscillations were also observed, though a frequency ratio was not established. Further, the experimental results suggest that coupled vibrations other than those theoretically expected were present. In particular, a longitudinal coupled vibration with a frequency ratio of 1:3 was observed, and a corresponding coupled flexural oscillation was present. A coupled torsional mode was experimentally observed when the applied frequency was twice the fundamental torsional frequency. A second coupled torsional mode appeared when the excitation frequency was three times the fundamental torsional frequency. The phase relationship between the coupled vibrations was observed. The resonant coupled vibrations were found to be significant at certain frequencies.
Data Types:
  • Document
A series of Q-spoiled neodymium glass oscillators has been constructed to produce radiation which, when frequency doubled to 5300 A, can be used in a optical scattering experiment on a plasma. Though unexpectedly the required light power was never obtained, an exhaustive theoretical analysis has revealed that this failure was due to superfluorescence depopulation of the inversion. When the problem is corrected by means of a samarium doped glass cladding for the neodymium glass rods, the power necessary to observe the electron satellite feature can easily be obtained.
Data Types:
  • Document
Performance of a set of torus-shaped nutation dampers, suitable for arresting relatively low frequency oscillations, is studied experimentally using a simple test facility. More important parameters affecting the damper performance are established and their influence assessed for a variety of configurations with the aim to arrive at more promising geometries. Results suggest damping characteristics to be particularly sensitive to physical properties of the liquid used, its height in the torus, damper geometry, and dynamical parameters representing amplitude and frequency. Among the configurations studied, dampers with perforated inside tubes, baffles, horizontal layers and floating rectangular pieces of wood in flow showed most favorable performance in terms of energy dissipated per unit volume. Such nutation dampers are likely to be suitable in tackling a variety of vibrational problems of industrial aerodynamics, earthquake engineering and off-shore structures.
Data Types:
  • Document