Geometric Ultrasound Localization Microscopy

Contrast-Enhanced Ultra-Sound (CEUS) has become a viable method for non-invasive, dynamic visualization in medical diagnostics, yet Ultrasound Localization Microscopy (ULM) has enabled a revolutionary breakthrough by offering ten times higher resolution. To date, Delay-And-Sum (DAS) beamformers are used to render ULM frames, ultimately determining the image resolution capability. To take full advantage of ULM, this study questions whether beamforming is the most effective processing step for ULM, suggesting an alternative approach that relies solely on Time-Difference-of-Arrival (TDoA) information. To this end, a novel geometric framework for microbubble localization via ellipse intersections is proposed to overcome existing beamforming limitations. We present a benchmark comparison based on a public dataset for which our geometric ULM outperforms existing baseline methods in terms of accuracy and robustness while only utilizing a portion of the available transducer data

Over-Tip Choking and Its Implications on Turbine Blade Tip Aerodynamic Performance

At engine representative flow conditions a significant portion of flow over a high pressure turbine blade tip is transonic. In the present work, the choking flow behavior and its implications on over-tip leakage flow loss generation are computationally analyzed. An extensively developed RANS code (HYDRA) is adopted. Firstly a high speed linear cascade validation case is introduced, and the computations are compared with the experimental data to identify and establish the capability of the code in predicting the aerodynamics losses for a transonic turbine blade tip. The computational studies are then carried out for the blading configuration at different flow conditions ranging from a nearly incompressible to a nominal transonic one, enabling to establish a qualitatively consistent trend of the tip leakage losses in relation to the exit Mach number conditions. The results clearly show that the local choking sets a limiter for the over tip leakage mass flow, leading to a different leakage flow structure compared to that in a low speed and/or unchoked condition. The existence of tip choking effectively blocks the influence of the suction surface side on the over-tip flow, and hence leads to a breakdown of the pressure-driven mechanism, conventionally used in tip treatment and designs. The decoupling between blade loading and over tip leakage mass flow is clearly identified and highlighted. Furthermore, the realization of the loading-leakage flow decoupling indicates a possibility of a high-load blading design with a relatively low tip leakage loss. A high load blading is generated and analyzed to demonstrate the feasibility of such designs with a reduced tip leakage loss

Investigation of Mechanisms Associated with Nucleate Boiling Under Microgravity Conditions

The focus of the present work is to experimentally study and to analytically/numerically model the mechanisms of growth of bubbles attached to, and sliding along, a heated surface. To control the location of the active cavities, the number, the spacing, and the nucleation superheat, artificial cavities will be formed on silicon wafers. In order to study the effect of magnitude of components of gravitational acceleration acting parallel to, and normal to the surface, experiments will be conducted on surfaces inclined at different angles including a downward facing surface. Information on the temperature field around bubbles, bubble shape and size, and bubble induced liquid velocities will be obtained through the use of holography, video/high speed photography and hydrogen bubble techniques, respectively. Analytical/numerical models will be developed to describe the heat transfer including that through the micro-macro layer underneath and around a bubble. In the micro layer model capillary and disjoining pressures will be included. Evolution of the interface along with induced liquid motion will be modelled. Subsequent to the world at normal gravity, experiments will be conducted in the KC-135 or the Lear jet especially to learn about bubble growth/detachment under low gravity conditions. Finally, an experiment will be defined to be conducted under long duration of microgravity conditions in the space shuttle. The experiment in the space shuttle will provide microgravity data on bubble growth and detachment and will lead to a validation of the nucleate boiling heat transfer model developed from the preceding studies performed at normal and low gravity (KC-135 or Lear jet) conditions

Acoustic imaging of natural gas seepage in the North Sea: Sensing bubbles under control of variable currents

Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo-spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship-born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long-lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data

Land Supply and Money Growth in China

China has experienced several episodes of inflation in recent years. Popular arguments attribute these episodes to relatively high growth rates of money, which were then primarily explained by China’s accumulation of foreign exchange reserves and the undervaluation of RMB. We attempt to explain China’s high monetary growth rates through the supply of land. Under China’s land system, the supply of land is controlled by the government and can be viewed as exogenous to the monetary system. An increase in the money supply stimulates bank loans and thereby monetary growth. Both an error correction model and a simultaneous equations model are developed to explore the effect of the land supply on monetary growth. The empirical results show that the effect of the land supply on the money supply is significantly positive and even exceeds that of foreign exchange reserves. The significance for monetary policy is that, under China’s existing political economy, both the central bank and local governments should be responsible for monetary policy and price levels

An acoustic water tank disdrometer

Microwave engineers and geomorphologists require rainfall data with a much greater temporal resolution and a better representation of the numbers of large raindrops than is available from current commercial instruments. This thesis describes the development of an acoustic instrument that determines rain parameters from the sound of raindrops falling into a tank of water. It is known as the acoustic water tank disdrometer (AWTD).There is a direct relationship between the kinetic energy of a raindrop and the acoustic energy generated upon impact. Rain kinetic energy flux density (KE) is estimated from measurements of the sound field in the tank and these have been compared to measurements from a co-sited commercial disdrometer.Furthermore, using an array of hydrophones it is possible to determine the drop size and impact position of each raindrop falling into the tank. Accumulating the information from many impacts allows a drop size distribution (DSD) to be calculated.Eight months of data have been collected in the eastern UK. The two methods of parameter estimation are developed and analysed to show that the acoustic instrument can produce rain KE measurements with a one-second integration times and DSDs with accurate large drop-size tails

In Situ Scanning Probe Techniques for Evaluating Electrochemical Systems

Falling technology costs are allowing renewable sources of energy to become increasingly more competitive with fossil fuel-based sources. However, challenges still remain in the widespread deployment of sources like wind and solar due to their intermittent nature and cost-prohibitive storage options. An attractive solution to address these issues is by using renewably derived energy to drive electrolysis reactions that generate useful chemicals and fuels. In order to do this effectively and economically, efficient and durable electrocatalysts are needed for the reactions of interest, such as hydrogen production from water electrolysis. Presently, the best catalysts for this process are noble metals such as platinum, which are expensive and in limited supply. The discovery and mechanistic understanding of earth abundant materials that can also efficiently catalyze these reactions remains a current research focus. Scanning probe microscopy (SPM) techniques can be used to aid in the discovery of these materials, as they are able to investigate catalyst surfaces in situ and at a higher resolution than conventional 3-electrode electroanalytical methods. This dissertation explores the use of two in situ SPM techniques, scanning electrochemical microscopy (SECM) and scanning photocurrent microscopy (SPCM), for evaluating both photocatalytic and electrocatalytic electrochemical systems. Three different studies that use these two techniques were carried out over the duration of my thesis work and are presented in Chapters 2 through 4. After providing an overview of solar fuels and SPM techniques in Chapter 1, Chapter 2 describes the design considerations, implementation and demonstration of a home-built SECM instrument for use with nonlocal continuous line probes (CLPs) that can achieve high areal imaging rates with compressed sensing (CS) image reconstruction. The CLP consists of an electroactive band electrode sandwiched between two insulating layers, where one of the insulating layers needs to be on the same length scale as the band electrode because it determines the average separation distance from the band electrode to the substrate. Similarly, the spatial resolution of the CLP is determined by the thickness of the band and the realizable imaging rate is determined by its width and linear scan rate. Like conventional SECM systems, a combination of linear motors and a bipotentiostat is needed. However, for the CLP-SECM system both linear and rotational motors are needed to scan at different substrate angles to obtain the necessary raw signal to reconstruct the target electrochemical image with CS algorithms. Detailed descriptions of the microscope design, CLP fabrication, and the procedures necessary to carry out the CLP-SECM imaging are given in this chapter. Measurements with this novel CLP-SECM microscope are done with flat platinum disk electrode samples of varying sizes. A substrate-generation-probe-collection mode is used during the SECM linescan measurements to illustrate procedures for position calibration of the system, CLP and substrate cleaning, as well as verifying the sensitivity along the length of the CLP. Finally, linescans over a three disk platinum sample were taken and CS image reconstruction was done, with as few as three linescans, to demonstrate the order of magnitude time advantage of this approach over conventional SECM scanning methods. In Chapter 3, colorimetric imaging studies are done using a pH dye indicator to visualize the plume of electroactive species that is generated during in situ SECM measurements for both conventional and CLP-SECM systems. In SECM, the signal recorded by the probe is facilitated by transport of electroactive species and not by direct contact between the probe and the substrate, which is typical of many scanning probe microscopy (SPM) techniques. One of the complexities with SECM is being able to fully understand the interaction between the electroactive species generated at the substrate and the probe. Thus in order to understand this further, a pH indicator dye is used to visualize pH gradients associated with the hydrogen product plume generated by water electrolysis during in situ SECM measurements. The in situ colorimetric experiments are then used to inform assumptions about the system and validate simulations using finite element modeling software. From this study, we are able to develop quantitative relationships to describe how the plume of electroactive species influences the recorded current at the probe for different probe geometries. Finally, we use this initial study as groundwork for investigating the influence of higher probe scan speeds where convection starts to play a role on the distortion of the signal and plume dynamics, and how it can be corrected using CS post-processing methods. Lastly, SPCM is employed in Chapter 4 to study the optical efficiency losses due to varying size bubbles on a photoelectrode surface. Individual single hydrogen bubbles ranging from 100 µm to 1000 µm were generated on a photoelectrode surface and a laser was used to scan over single isolated bubbles to create localized optical efficiency maps based on photocurrent and external quantum efficiency (EQE). Moreover, a ray-tracing model based on Snell’s law was also constructed to compare to experimental SPCM linescans. This model showed very good agreement to the experimental SPCM linescan results. This investigation showed that larger bubbles lead to higher optical efficiency losses, not only due to higher inactive electrochemically active surface areas (ECSAs) but also due to a larger region of total internal reflection of light from the edge regions of bubbles. A macroscale study over a large photoelectrode surface was also done where the images of the surface were taken while the “sawtooth” was measured under AM1.5 illumination. Consequently, a predictive current−time profile was generated from the single bubble SPCM empirical relationship between bubble size and optical losses and was compared to the experimental measurement. Understanding how bubbles can impact the efficiency of the overall system is important, as bubbles in a system and on an electrode surface increases ohmic resistances, optical losses, and kinetic losses. Overall, this study can be used as a starting point for designing systems, electrolyte, and catalyst surfaces to improve one or more of the aforementioned losses

Bubble Plumes and their use for Sound Mitigation

In this work we study bubble plumes in the light of their recent use for sound mitigation of offshore pile driving. We start the thesis by the hydrodynamics of bubble plumes and how to measure the relevant properties. We developed a conductivity based measurement method for void fraction and bubble size measurements. This method is combined with an underwater camera allowing for in-situ calibration of the system. The resulting measurements are presented, and show a different maintained growing angle for different bubble generation manifolds. Further measurements in a 10x31x40 m3 basin are used to calibrate a recently developed model. Based on these results a complete model describing both the integral quantities, such as the void fraction and the width of the plume, as well as the local bubble size distribution is extensively explained. We noted that the slip velocity of the bubbles, required for good agreement between the measurements and the model, needed to be significanlty higher than the slip velocity of a single bubble. We attribute this to the collective effects of the bubbles. Finally we looked at the resulting acoustic performance of bubble curtains. Here we found that splitting the airflow between two manifolds significanlty increases the performance while using the same amount of compressed air. This can be explained by the region of interest of low frequencies, where reflection is the dominant mechanism for sound mitigation. Halving the air flow rate per manifold hardly influences the reflective properties for an individaul bubble curtain. Splitting the airflow rate between two manifolds thus leverages this property. Finally we comment on the applicability of equivalent fluid models in the modelling of the sound mitigation by bubble curtains