The characteristics of several types of heated sensors used for measuring oceanic turbulence have been examined. The author measured the steady heat flux from glass coated ellipsoidal microbead thermistors, computed numerically the unsteady heat flux from an ideal constant temperature flat plate, and analyzed the steady flow calibration data of a paralene-c coated plate thermistor as well as the steady and unsteady calibration data of two conical constant temperature hot film anemometers. In order to understand the behaviour of probes alone, thermal models of these sensors have been developed. The models incorporate a Nusselt number governed steady heat flux from the wetted surface and the thermal effects of a surface coating and a supportive substrate. Derived functional relationships between the steady heat flux and the flow rate agree favourably with the available calibration data. The quasi-steady sensitivity of these probes when used as anemometers or thermometers as well as their signal contamination by temperature or velocity are calculated using the functional heat flux relationships. The substrate and the coating reduce the sensitivity to temperature and to speed as well as the ratio of speed-to-temperature sensitivity. The response of sensors is not governed by the Nusselt number when the boundary layer is unsteady. The unsteady response of a sensor to velocity oscillations is governed by its unsteady viscous boundary layer and may increase with increasing frequency over some frequency ranges. The response bandwidth is wider for velocity than for temperature. The ratio of unsteady temperature-to-velocity sensitivity is highest at zero frequency. Frequency response calibrations methods must realistically simulate the sensor's unsteady viscous boundary layer. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

An optical technique was developed to make possible a study of the instantaneous structure of the turbulent wall-pressure field. The approach involved the use of real-time laser-holographic-moiré interferometry. A moiré fringe pattern generated by the holographic method was superimposed on the surface of a specially-fabricated compliant pipe wall. The compliant surface, in response to wall-pressure changes, introduces optical path length changes which are manifested by distortions in the fringe field. The fringe distortions, observed during flow, were recorded (framed area, 11 mm x 34 mm) by means of medium-speed motion photography. The amplitude of fringe distortion provides a measure of the pressure magnitude at the wall. A 26.3 mm ID horizontal glass pipline (7.0 m long) supplied with distilled water from a constant head reservoir was used in the study. Photographs taken of the fringe patterns observed at a flow velocity (U) of 0.47 m/sec (Re[sub=d] = 12,300) were analysed. Results show that the wall-pressure field consists of a positive and negative pressure region. A statistical analysis reveals that the wall-pressure distribution is asymmetrical (Skewness = -0.29). From an analysis of the pressure patterns, a relationship between the generation of wall-pressure fluctuations and known wall-layer flow characteristics is inferred. A flow model is proposed to explain some aspects of the wall region dynamics and a mechanism for particle detachment from a wall, during turbulent flow, is also presented. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

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The operation of a grating lateral shear heterodyne interferometer as a wavefront sensor for atmospherically perturbed wavefronts is analyzed. A novel wavefront sensor design is given and its feasibility is proven by laboratory experiments. The applications in mind are closed-loop active optical systems for compensating atmospheric perturbations and open-loop atmospheric wavefront measuring device. The optical properties of the turbulent atmosphere are summed up and the resulting wavefront sensor requirements are given. Among them are the property of sell-referencing, high white light efficiency, independence of scintillation effects, and high spatial and temporal sampling rates. Then the general heterodyne grating shearing interferometer is introduced. A description of the phase measurement by the heterodyne process in the frequency domain has been derived. The heterodyne process is interpreted as convolutions of the signal with a pair of filter functions, which isolate a particular harmonic term of the signal and provide its phase. The representation of the convolutions in the frequency domain provides an elegant way to analyse the systematic errors of the heterodyning with general, non-sinusoidal signals. Also the random phase errors of the heterodyne process have been determined using Gaussian error propagation. An algorithm is derived to carry out the wavefront reconstructions from the measured differences on a square array of discrete data points. It is based on a modal expansion in complex exponentials, leading to a simple filtering operation in the spatial frequency domain. The algorithm provides unbiased reconstructions over the finite data set. It has minimal error propagation in a least squares sense. It is computationally efficient in that the number of operations required for a reconstruction is approximately proportional to the number of wavefront points, if a Fast-Fourier-Transform algorithm is used. Finally, a compact wavefront sensor design is described fulfilling the requirements posed by the turbulent atmosphere. It determines wavefronts at 24 by 24 points at a sampling rate of 60 Hz. A rms-wavefront error of better than λ/20 can be achieved with astronomical light sources of sixth stellar magnitude. Laboratory experiments proved the feasibility of the design.

Large eddy simulations are considered for wind plants with varied spanwise and streamwise spacing. Data from five different configurations of staggered and aligned LES wind turbine arrays with a neutrally stratified atmospheric boundary layer are employed for analysis. Fields are analyzed by evaluating the anisotropy stress invariants based on the Reynolds shear stresses and dispersive stress tensor. The relationship between quantities are drawn as a function of the wind plant packing. Reynolds stresses and dispersive stresses are investigated alongside a domain altered version of the second and third scalar invariants, ξ, η, as well as the combination of the two invariants described by the function F = 1−27η 2 + 54ξ 3. F is a measure of the approach to either a two-component turbulence (F=1) or an isotropic turbulence (F=0). The invariant η describes the degree of anisotropy while ξ describes the characteristic shape. For the purposes of this study, the LES data is analyzed to understand the effects of canopy density on anisotropy and dispersive stresses, adding further insight and detail for future modeling techniques.

Graduation date: 2000

Master of Science