In March 2007 five satellites were launched as part of the NASA mission THEMIS. The aim of the mission is to answer the unknown questions regarding the onset of substorms. THEMIS data has also been used within other research fields. Today many scientists aim to investigate wave phenomena, such as whistler waves, wave interactions in the radiation belts and general turbulence in the magnetosphere and the solar wind. These processes occur at intermediate frequencies (a few hundreds of Hertz). Correct and reliable results require good knowledge of the frequency response,  the so called transfer function, for the electric field instrument (EFI). Post-launch calibrations have given good knowledge of the instrument's response at high and low frequencies. However, at intermediate frequencies (50-3000 Hz) the transfer function has only been determined via calculations/simulations and not yet obtained from data collected in space. Moreover, the transfer function changes substantially in this range, as the instrument transitions from a resistive low-frequency coupling to a capacitive high-frequency coupling. The transition is known as the RC roll-off. In this thesis, data from different regions and with different electrical settings have been analyzed to estimate the EFI sensors' sheath impedance and transfer function. Data have been collected during July 2009 and March 2011. From the first period, I-V curves where extracted for four different regions (i.e. with different plasma conditions) and their associated sheath impedance calculated. I-V curves are graphical representations of how the voltage differs with the changed bias current. From the sheath impedance and the measured free-space capacitance the RC roll-off can been directly calculated. An experiment was also conducted in March 2011 where the instrument was run in a special mode designed to measure the relative transfer function with the probes run at different bias setting, yielding different sheath impedances. The analysis of the I-V curves and relative transfer function show similar results, which clearly differ from the earlier believed values. Values for the sheath impedance are lower (4-6 MΩ) than the expected (30 MΩ) and depend on the usher setting. The usher is an electronic device which should shield the sensor from the photoelectron produced by illumination of the preamplifier. This lower sheath resistance implies higher than expected RC roll-off frequency, a result which is confirmed by the results from the relative transfer function. The roll-off is between 2-3 kHz, compared to the 400-500 Hz assumed prior to this study based on the assumption of a sheath impedance of around 30 MΩ.

This thesis describes an experimental investigation of the flame transfer function between flow disturbances and heat release oscillations in lean, premixed combustors. This research effort was motivated by the fact that modern gas turbines, operating fuel-lean to minimize exhaust emissions, are susceptible to self-excited combustion oscillations. These instabilities generally occur when the unsteady combustion process couples with the acoustic modes of the combustion chamber. The resultant flow and structural vibrations can substantially reduce hot section part life. As such, avoiding operating regimes where high dynamics occur often requires operating at lower power outputs and/or higher pollutant emissions than the turbine is otherwise capable. This work demonstrated nonlinearities in the chemiluminescence response at large amplitude velocity oscillations in a turbulent, swirling flame. It is observed that the nonlinear flame response can exhibit a variety of behaviors, both in the shape of the response curve and the forcing amplitude at which nonlinearity is first observed depending on the operating conditions of the combustor. The phase between the flow oscillations and heat release is also seen to have substantial amplitude dependence. In addition, the interactions between the fundamental frequency and the higher and subharmonics of the measured signals can significantly influence the flame as well as the frequency response of the system. The nonlinear flame dynamics are governed by different mechanisms in different frequency and flowrate regimes. Three mechanisms, vortex rollup, unsteady flame liftoff, and parametric instability, are identified to influence the nonlinear flame response in these combustors. Analysis of the results shows that the mechanisms responsible for nonlinearity in the flame response are influenced by the Strouhal number, the mean velocity at the combustor dump plane, and the ratio of the oscillating velocity amplitude to the laminar flame speed.

The lenses are important elements in optical imaging systems. However, lenses are liable to defects such as dusts and thus deteriorate their imaging quality. The polluted lens can be verified equivalent to a polluted random screen set against a clean lens. In our model, the defects on random screen are assumed poison-distribution, overlapped and the transmittance effect of each defect is multiplicative. In this thesis, we will apply Kalman filter to estimate the optical transfer function for a defected imaging system. The experiments are set up by the instruments including the video camera, capture card, and personal computer. Kalman filter addresses an estimation problem defined by two models: the signal model and the observation model. Kalman filter was originally developed in the field of optimal estimation for application of controlling and tracking. Recently Kalman filter has been very often applied to the problems of image restoration. In this thesis, the signal model is obtained from a ratio of the defected and clean pictures in frequency domain. The observation model is built for an additive measurement noise from electronic sampling. Experimental results have demonstrated that the estimated optical transfer function is useful for image restoration.

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This research utilizes the imbibition experiments and X-ray tomography results for modeling fluid flow in naturally fractured reservoirs. Conventional dual porosity simulation requires large number of runs to quantify transfer function parameters for history matching purposes. In this study empirical transfer functions (ETF) are derived from imbibition experiments and this allows reduction in the uncertainness in modeling of transfer of fluids from the matrix to the fracture. The application of the ETF approach is applied in two phases. In the first phase, imbibition experiments are numerically solved using the diffusivity equation with different boundary conditions. Usually only the oil recovery in imbibition experiments is matched. But with the advent of X-ray CT, the spatial variation of the saturation can also be computed. The matching of this variation can lead to accurate reservoir characterization. In the second phase, the imbibition derived empirical transfer functions are used in developing a dual porosity reservoir simulator. The results from this study are compared with published results. The study reveals the impact of uncertainty in the transfer function parameters on the flow performance and reduces the computations to obtain transfer function required for dual porosity simulation.

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Thesis (M.S.)--Rochester Institute of Technology, 1989. / Includes bibliographical references (leaves 102-103).

Thesis (M.S.)--Rochester Institute of Technology, 1984. / Typescript. Includes bibliographical references (leaves 77-79).

Thesis (B.S.)--Rochester Institute of Technology, 1985. / Typescript. Includes bibliographical references (leaves 35-36).