Developments in Holographic Imaging for Near-wall Flow Measurement, Spray Diagnostics and Insect Behavior Monitoring

Abstract

This thesis is separated into three parts. In part one, I develop a new holographic imaging technique for near wall flow measurements. The viscous sublayer is a region of a wall bounded turbulent flow located next to the wall, spanning anywhere from the millimeter to the micrometer scale, where viscous forces dominate and the wall shear stress (or drag) originate. The presence of flow modifications within the sublayer, in the form of roughness structures have shown great promise towards drag reduction, the underlying mechanisms for which are not fully understood. By performing high resolution flow measurements within the sublayer, we can identify the direct impact of these structures on the flow, which will aid in engineering surfaces for optimal drag reduction. However, performing such high resolution 3D flow measurements within the sublayer is challenging with existing flow diagnostic techniques. As part of this work, I invented Digital Fresnel Reflection Holography (DFRH), a modification of digital inline holography, to enables quantitative high resolution 3D measurements of flows next to the wall, at resolutions sufficient to resolve the viscous sublayer. I will first cover some background information including (i) a review of relevant literature on near wall flow measurement techniques, highlighting some of their key limitations towards the study of the viscous sublayer and (ii) an overview of holographic imaging principle in the introduction. Next the working principle of the technique is presented including a detailed experimental setup and numerical processing algorithms for extracting information. A proof-of-concept experiment is used to showcase the validity of this technique for near wall flow measurements, followed by a calibration experiment to identify measurement accuracy. Next, I include two specific studies that investigate the impact of particle seeding density and laser source coherence on the performance of the measurement. Finally, the technique is applied towards measurement of a laminar flow before being applied on a turbulent channel, to investigate the viscous sublayer. I conclude with a summary and a discussion of some interesting avenues for future work. In part two of the thesis, I present developments of digital inline holography for the behavior monitoring of fruit flies ($D. Melanogaster$) for the study of neurodegenerative diseases. Fruit flies are widely used as model organisms to study genetic diseases but current behavior monitoring systems are unable to provide both low resolution statistical behaviors of motion (e.g., walking, flying, resting) as well as high resolution complex behaviors (e.g., wing flaps, leg motions etc.) using the same system. In this study, I propose a holographic behavior observatory which can monitor a group of flies placed in a large volume and provide both low resolution statistics of motion as well as high resolution complex behaviors. As part of this work, I review some of the current solutions used for monitoring flies in their natural environments (or \emph{in-situ}) following which a full description of the experimental setup is provided. Next, I discuss the numerical algorithm used for processing which employs a spectral focus metric to speed up computations. A detailed validation of the numerical algorithm as well as the calibration of the position and velocity measurement are provided through independent experiments. Finally, the long term statistics of motion including fly trajectories, motion ethograms and probability distributions are presented along with some of the complex behaviors identifying and differentiating a collision avoidance response from a landing responses. Finally, in part three, I discuss the development of a holographic system for the characterization of droplets generated by agricultural sprays for mitigation of spray drift. Droplet generation by atomization is a complex fluid dynamic process forming a wide range of sizes. The need to limit spray drift, where smaller sized droplets are carried away by the wind, is vital for precision farming applications and requires accurate sizing of droplets produced by the nozzles over its operation range. Droplet sizes are routinely characterized using laser diffraction which, apart from being an integral measurement, often assumes droplets to be spherical. Such an assumption is highly restrictive, especially for agricultural nozzles where a wide range of sizes and shapes are produced. In this work, I develop a holographic droplet diagnostic system which can automatically measure the size as well as shape of the droplet (defined as its eccentricity) at high resolution. A complete description of the experimental system is first provided, including the wind tunnel spray generation system and the accompanying holographic imaging system used for the measurements. A calibration is performed with monodisperse droplets to establish the sizing precision and accuracy of the technique, before measuring the spray generated droplets. Apart from a cumulative and logarithmic size distribution functions for the droplets, we also obtain the size-shape joint probability distribution function quantifying droplet shape.