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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
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
A method based on phaselock techniques, for synchronizing the local oscillator signals in a proposed two-dish supersynthesis at 1420 MHz is presented. To demonstrate the feasibility of this method, the design and construction of a working system that provides phase-coherent, 1390 MHz signals at two sites, separated by a time-varying path length, is described. The phase accuracy of this system is ±5°. A provision for introducing a known phase difference between the two signals, in a manner that is suitable for interfacing with a digital computer, is included. Also, operation of the system over a frequency range greater than the expected range of doppler shift is possible, without the risk of locking to a wrong sideband. Transistor microwave oscillators at 1.4 GHz are used as voltage-controlled oscillators in this system. The performance of these devices is compared with that of the conventional voltage-controlled crystal oscillator/multiplier chain. Test results are given, which indicate that the system is suitable for use in an operational environment.
Data Types:
  • Physical Object
A mathematical model of a platform based flexible tethered satellite system in an arbitrary orbit, undergoing planar motion, is obtained using the Lagrangian procedure. The governing equations of motion account for the platform and tether pitch, longtitu-dinal tether oscillations, offset of the tether attachment point as well as deployment and retrieval of the tether. A numerical parametric study of the highly nonlinear, nonautonomous and coupled equations of motion gives considerable insight into the system dynamics useful in its design. Of particular interest are the interactions involving orbital eccentricity, system librations, tether flexibility and offset, retrieval maneuvers and initial disturbances. Results show that the offset strongly couples tether and platform dynamics, and the resulting responses show high frequency modulations corresponding to the longtitudinal tether oscillations. The system was found to be unstable during retrieval. The Linear Quadratic Regulator based offset control strategy, in conjunction with the platform mounted momentum gyros, is proposed to alleviate the situation. Results show that a strategy involving independent parallel control of low and high frequency responses can damp rather severe disturbances in a fraction of an orbit.
Data Types:
  • Physical Object
In experimental particle physics, researchers must often construct a mathematical model of the experiment that can be used in fits to extract parameter values. With very large data sets, the statistical precision of measurements improves, and the required level of detail of the model increases. It can be extremely difficult or impossible to write a sufficiently precise analytical model for modern particle physics experiments. To avoid this problem, we have developed a new method for estimating parameter values from experimental data, using a Maximum Likelihood fit which compares the data distribution with a “Monte Carlo Template”, rather than an analytical model. In this technique, we keep a large number of simulated events in computer memory, and for each iteration of the fit, we use the stored true event and the current guess at the parameters to re-weight the event based on the probability functions of the underlying physical models. The re-weighted Monte-Carlo (MC) events are then used to recalculate the template histogram, and the process is repeated until convergence is achieved. We use simple probability functions for the underlying physical processes, and the complicated experimental resolution is modeled by a highly detailed MC simulation, instead of trying to capture all the details in an analytical form. We derive and explain in detail the “Monte-Carlo Re-Weighting” (MCRW) fit technique, and then apply it to the problem of measuring the neutral B meson mixing frequency. In this thesis, the method is applied to simulated data, to demonstrate the technique, and to indicate the results that could be expected when this analysis is performed on real data in the future.
Data Types:
  • Physical Object
The influence of inertia, eccentricity and atmospheric forces on the attitude dynamics of gravity oriented, non-spinning, axi-symmetric satellites, executing general librational motion is investigated using analytical, numerical and analog techniques. The problem is studied in the increasing order of complexity. For the case of a circular orbit, the autonomous, conservative system represented by constant Hamiltonian yields zero-velocity curves and motion envelopes which identify regions of instability from conditional and guaranteed stable motion. The non-linear, coupled equations of motion are solved using approximate analytical techniques: Butenin’s variation of parameter method and invariant integral approach. A comparison with the numerical response, establishes their suitability in studies involving motion in the small. The invariant integral method maintains reasonable accuracy even for larger, predominantly planar, disturbances. However, for a general motion in the large, the analytical solutions provide only qualitative information and one is forced to resort to numerical, analogic or hybrid procedures. The analysis suggests strong dependence of system response on the in-plane disturbances and satellite inertia. The librational and orbital frequencies are of the same order of magnitude. It also shows that the stable solution, when represented in a three dimensional phase space may lead to 'regular', 'ergodic' or 'island' type regions. The limiting integral manifolds, given here for a few representative values of Hamiltonian, provide all possible combinations of initial conditions, which a satellite can withstand without tumbling. The results, for a range of satellite inertia, are condensed in the form of design plots, indicating allowable disturbances for stable motion. In general, the slender satellites exhibit better stability characteristics. The presence of aerodynamic torque destroys the symmetry properties of the integral manifolds. The stability of the equilibrium configuration, which now deviates from the local vertical, is established through Routh's as well as Liapunov's criteria. As the system is still autonomous and conservative, the Hamiltonian remains constant leading to the bounds of libration. Numerical analysis of the system response indicates increased sensitivity to planar disturbances. The distortion and contraction of the regular, ergodic and island type stability regions show the adverse effects of aerodynamic torque. The design plots suggest that the shorter satellites, normally not preferred from gravity-gradient considerations, could exhibit better stability characteristics in the presence of large aerodynamic torque. An alternate, economical approach to the dynamical analysis of the satellites is attempted using an analog computer. A comparison with the digital data establishes the suitability of the method for design purposes and real time simulation. As the regular surface represents the only usable stability region from design considerations, a detailed study to establish the bound between regular and ergodic type stability was undertaken. The periodic solutions, obtained numerically using variable secant iteration show their spinal character with the body of stability region built around them. Of particular significance is the fundamental periodic solution (two planar oscillations in one out-of- plane cycle) associated with the regular region, suitable for practical operation of a satellite. The remaining periodic solutions represent degeneration of the island-like areas surrounding the mainland. The results lead to a set of fundamental periodic solutions over a wide range of system parameters. Floquet's variational analysis is used to establish the critical disturbance [formula omitted], beyond which no stable motion can be expected. The periodic solutions together with the regular stability region are presented here as functions of Hamiltonian, satellite inertia and aerodynamic torque. The case study of GEOS-A satellite is also included. In elliptic orbit, the Butenin's analysis of coupled forced systems is found to give an approximate solution of good accuracy. However for this non-autonomous situation, where Hamiltonian is no longer a constant of the motion, the concept of integral manifold breaks down. Fortunately, the design plots can still be generated by direct utilization of the response characteristics. In general the stability region diminishes with increasing eccentricity and disappears completely for e > 0.35. The presence of atmosphere adds to the complex behaviour of this non-autonomous system, where even the equilibrium configuration now becomes periodic in character. The stability regions are further reduced with instabilities normally initiating in the planar degree of freedom. Finally, a possibility of using the atmospheric forces in attitude control is explored. The use of a set of horizontal flaps in conjunction with a semi-passive, velocity-sensitive controller appears to be promising. With a suitable choice of system parameters even a large disturbance can be damped in approximately two orbits.
Data Types:
  • Physical Object