Acoustic resolution photoacoustic Doppler velocimetry in blood-mimicking fluids

Photoacoustic Doppler velocimetry provides a major opportunity to overcome limitations of existing blood flow measuring methods. By enabling measurements with high spatial resolution several millimetres deep in tissue, it could probe microvascular blood flow abnormalities characteristic of many different diseases. Although previous work has demonstrated feasibility in solid phantoms, measurements in blood have proved significantly more challenging. This difficulty is commonly attributed to the requirement that the absorber spatial distribution is heterogeneous relative to the minimum detectable acoustic wavelength. By undertaking a rigorous study using blood-mimicking fluid suspensions of 3 μm absorbing microspheres, it was discovered that the perceived heterogeneity is not only limited by the intrinsic detector bandwidth; in addition, bandlimiting due to spatial averaging within the detector field-of-view also reduces perceived heterogeneity and compromises velocity measurement accuracy. These detrimental effects were found to be mitigated by high-pass filtering to select photoacoustic signal components associated with high heterogeneity. Measurement under-reading due to limited light penetration into the flow vessel was also observed. Accurate average velocity measurements were recovered using "range-gating", which furthermore maps the cross-sectional velocity profile. These insights may help pave the way to deep-tissue non-invasive mapping of microvascular blood flow using photoacoustic methods

High accuracy ultrasonic degradation monitoring

This thesis is concerned with maximising the precision of permanently installed ultrasonic time of flight sensors. Numerous sources of uncertainty affecting the measurement precision were considered and a measurement protocol was suggested to minimise variability. The repeatability that can be achieved with the described measurement protocol was verified in simulations and in laboratory corrosion experiments as well as various other experiments. One of the most significant and complex problems affecting the precision, inner wall surface roughness, was also investigated and a signal processing method was proposed to improve the accuracy of estimated wall thickness loss rates by an order of magnitude compared to standard methods. It was found that the error associated with temperature effects is the most significant among typical experimental sources of uncertainty (e.g. coherent noise and coupling stability). By implementing temperature compensation, it was shown in laboratory experiments that wall thickness can be estimated with a standard deviation of less than 20 nm when temperature is stable (within 0.1 C) using the signal processing protocol described in this thesis. In more realistic corrosion experiments, where temperature changes were of the order of 4 C), it was shown that a wall thickness loss of 1 micron can be detected reliably by applying the same measurement protocol. Another major issue affecting both accuracy and precision is changing inner wall surface morphology. Ultrasonic wave reflections from rough inner surfaces result in distorted signals. These distortions significantly affect the accuracy of wall thickness estimates. A new signal processing method, Adaptive Cross-Correlation (AXC), was described to mitigate the effects of such distortions. It was shown that AXC reduces measurement errors of wall thickness loss rates by an order of magnitude compared to standard signal processing methods so that mean wall loss can be accurately determined. When wall thickness loss is random and spatially uniform, 90% of wall thickness rates measured using AXC lie within 7.5 ± 18% of the actual slope. This means that with mean corrosion rates of 1 mm/year, the wall thickness estimate with AXC would be of the order of 0.75-1.1 mm/year. In addition, the feasibility of increasing the accuracy of wall thickness loss rate measurements even further was demonstrated using multiple sensors for measuring a single wall thickness loss rate. It was shown that measurement errors can be decreased to 30% of the variability of a single sensor. The main findings of this thesis have led to 1) a solid understanding of the numerous factors that affect accuracy and precision of wall thickness loss monitoring, 2) a robust signal acquisition protocol as well as 3) AXC, a post processing technique that improves the monitoring accuracy by an order of magnitude. This will benefit corrosion mitigation around the world, which is estimated to cost a developed nation in excess of 2-5% of its GDP. The presented techniques help to reduce response times to detect industrially actionable corrosion rates of 0.1 mm/year to a few days. They therefore help to minimise the risk of process fluid leakage and increase overall confidence in asset management.Open Acces

Measurement of web tension distribution by point source pulse excitation

A web is a material that is produced as a continuous sheet and stored in wound roll form. Mechanics of web material handling in production, coating or conditioning, and winding operations affect web uniformity and the material stress/strain state, thus affecting roll quality. In an effort to improve all aspects of web handling procedures, much attention has been focused on acquisition and utilization of on-line web handling process information such as web tension. Tension is a quantity basic to web production and processing yet historically has been difficult to measure except in an average sense. Improvements in on-line tension measurement accuracy have foreseeable application to automated tension control systems and winder maintenance used in modern day web production/processing facilities. This paper describes a new means of noncontacting, local web tension measurement through use of a point source pneumatic excitation coupled to signal acquisition and processing schemes. Advantages of this new system include variable web excitation rate, variable system tuning for different applications, high lateral tension distribution resolution, and compact, easily serviceable transducer head assembly. This work was sponsored through the Web Handling Research Center (WHRC), an NSF funded research facility located at Oklahoma State University in Stillwater, Oklahoma.Mechanical and Aerospace Engineerin

Development of a Time-Resolved Laser-Induced Fluorescence Technique for Nonperiodic Oscillations

Time-resolved measurements of ion dynamics could be key to understanding the physics of instabilities, electron transport, and erosion in Hall thrusters. Traditional measurements of the ion velocity distribution in Hall thrusters using laser-induced fluorescence (LIF) are time-averaged since lock-in amplifiers must average over a long time constant for a reasonable signal-to-noise ratio. Over about the past decade, at least four other time-resolved LIF techniques have been developed and applied to Hall thrusters or similar plasma devices. One limitation of these techniques is the implicit assumption of periodic oscillations in the averaging scheme. There is a need for a more general technique since Hall thrusters can operate with nonperiodic oscillations that vary unpredictably. This dissertation presents the development of a time-resolved LIF (TRLIF) technique that addresses this need. This system averages the signal using a combination of electronic filtering, phase-sensitive detection, and Fourier analysis. A transfer function is measured to map an input signal (such as discharge current) to an output signal (TRLIF signal). The implicit assumption of this technique is that the input is related to the output by a time-invariant linear system, a more general assumption than periodicity. The system was validated using a hollow cathode with both periodic and random discharge current oscillations. A series of benchmark tests was developed to validate the signal by verifying that it satisfies theoretical expectations. The first campaign with the H6 Hall thruster demonstrated signal recovery in both periodic and nonperiodic modes. Measurements of the evolution of the ion flow downstream show that kinematic compression explains the width of the ion velocity distribution only at certain phases of the oscillation. A distinct change in ion dynamics was detected as the magnetic field magnitude increased: a high-amplitude, relatively periodic oscillation in the ion velocity distribution gave way to a low-amplitude, chaotic oscillation. High amplitude oscillations of the mean ion velocity suggest that the bimodal distributions detected at many operating conditions (with time-averaged measurements) are the result of oscillations.PhDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133454/1/durot_1.pd

Measuring degradation in cable insulation material under realistic operation conditions

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In reality the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances, such as oxygen and water, are blocked from entering into the insulation. Thus, in order to generate results that are practically valid, these conditions must be replicated in laboratory experiments.This contribution presents a measuring system developed for performing ageing experiments in a hermetically sealed environment. The material degradation is evaluated through measurements of changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is primarily accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle-plane type and the needle injection process is largely automated to secure a large degree of repeatability in specimen preparation.\ua0Initial measurements utilising four synchronized but independent data streams show that the electrical tree inception voltage can be accurately detected using the developed specimen capsule. The impedance change of the specimen during the test shows to be a particularly useful measure.\ua0In order to further validate the methodology as well as contribute knowledge on the material’s resistance to degradation caused by its exposure to enhanced electrical stress, 40 specimens were prepared and used in an experiment that explores whether multiple joint failures along an HVDC-cable may have any effect on the condition of the cable’s the insulation material. The results indicate that the impact seen in the electrical tree inception voltage is minor and that the insulation has withstood the enhanced stress with negligible consequences

Repetitive Operation of the University of Saskatchewan Compact Torus Injector

Development of fueling technologies for modern and future tokamak reactors is essential for their implementation in a commercial energy production setting. Compared to the presently available fueling technologies, gas or cryogenic pellet injection, compact torus injection presents an effective and efficient method for directly fueling the central core of tokamak plasmas. Fueling of the central core of a tokamak plasma is pivotal for providing efficient energy production. The central core plasma of a reactor contains the greatest density of fusion processes. For consistent and continuous fueling of tokamak fusion reactors, compact torus injectors must be operated in a repetitive mode. The goal of this thesis was to study the feasibility of firing the University of Saskatchewan Compact Torus Injector (USCTI) in a repetitive mode. In order to enable USCTI to fire repetitively, modifications were made to its electrical system, control system and data acquisition system. These consisted primarily of the addition of new power supplies, to enable fast charging of the many capacitor banks used to form and accelerate the plasma. The maximum firing rate achieved on USCTI was 0.33 Hz, an increase from the previous maximum firing rate of 0.2 Hz achieved at UC Davis. Firing USCTI in repetitive modes has been successful. It has been shown that the CTs produced in any given repetitive series are properly formed and repeatable. This is made evident through analysis of data collected from the CTs' magnetic fields and densities as they traveled along the injector barrel. The shots from each experiment were compared to the series' mean data and were shown to be consistent over time. Calculations of their correlations show that there are only minimal deviations from shot to shot in any given series

Formulation and Experimental Verification of Alternate Calibration Techniques of the Temperature Dependent Response of Phosphor Fluorescence

The overall goal of this work is to provide an alternative approach to the thermographic phosphor (TP) time constant calibration method for temperature recovery. In this work two techniques are proposed that retain the pulsed source input used in the standard TP time constant calibration approach but reinterpret the phosphor response taken a fixed distance such that the single-exponential decay assumption is removed. The methods do not require knowledge of key parameters prior to data processing, nor do they involve complicated numerical schemes that attempt to fit data in the low signal-to-noise region of the phosphor response. The approaches do involve integrating the full phosphor response signal to arrive at a single value related to the integrated intensity trajectory. This value can be calibrated to temperature. One method uses the slope of the integrated intensity in the rise portion of the phosphor emission as the calibration parameter, while the second technique uses the total integral of the emission as the calibration parameter. Both techniques are validated as an effective means of TP calibration by experimental data. First, the phosphor emission response is recorded at different steady-state temperatures in order to form a calibration curve. Different regression models are investigated to determine the functional relationship that best fits the observed calibration data. Second, the phosphor is heated under transient conditions and both calibration techniques are applied to resolve the temporal temperature history of the test sample. From the experimental results, it is found that a bi-exponential based calibration curve or a rational function based calibration curve accurately predict the temperature measurements of the transient tests for both calibration procedures. However, it is suggested that the total integrated intensity method is more reliable compared to the slope calibration method since smaller error estimates are observed using the total integral in the transient sense. Another attractive feature of the integral method is that the only numerical manipulation of the raw physical experimental data to resolve the calibration parameter involves integrating the signal. The outcome of this work is highly encouraging and indicates that these techniques could be found useful in certain applications