Further understanding the small scale coherent structures which occur in high Reynolds number turbulence would be of enormous benefit. Therefore, the aim of the current project was to make well resolved three-dimensional flow measurements of the mixing flow between counter rotating impellers, using Tomographic Particle Image Velocimetry (TPIV).TPIV software was developed, with a novel approach permitting a significant reduction in processing time, and a series of numerical accuracy studies contributing to the fundamental understanding of this new technique. Basic flow characterisation determined the local isotropy, homogeneity and expected Reynolds number scaling. A favourable comparison between planar PIV and TPIV increased confidence in the latter, which was used to assess the dynamics and topology of the dissipation scale structures. In support of previous investigations similar topology, strain rate alignment, scale-invariance, and clustering behaviours are demonstrated. Correlated high enstrophy and dissipation regions occur in the periphery of larger structures, resulting in intermittency. Geometry characterisation indicates a predominance of tube-like structures, which are observed to form from larger ribbon-like structures through unsteady breakdown and vortex roll-up. Significant correlation between intermittent fields of dissipation and enstrophy describe the fine scales effects. These relationships should pave the way for more accurate models, capable of relating small scales and large scales during the prediction of dynamically important quantities.

The structure of turbulence in the subchannels of a large-scale 60 degree section of a CANDU 37-rod bundle was studied at Reynolds numbers equal to 50,000, 100,000 and 130,000. Measurements were conducted at roughly 33.81 rod diameters from the inlet of the rod bundle using single-point, two-component hot-wire anemometry. Analysis of the axial velocity signal indicated a weak effect of Reynolds number on the axial velocity distribution and a bulging of axial velocity contours toward the narrow gaps. The normalised normal Reynolds stresses and the normalised turbulent kinetic energy were found to decrease as the Reynolds number increased. The radial Reynolds shear stress varied linearly with radial distance from the rod, crossing zero at the location of local maximum of the axial velocity. This stress was symmetric about the central rod whereas the azimuthal Reynolds shear stress was anti-symmetric. The Reynolds number effect was weak but measurable on the integral length scales of the axial and radial velocity fluctuations but negligible on the integral length scale of the azimuthal velocity fluctuations, especially in the gap regions. The Taylor and Kolmogorov microscales increased from the wall toward the centre of the subchannel and decreased as the Reynolds number increased. The wall shear stress stress distribution around the central rod indicated no effect of Reynolds number, when normalized by the corresponding average. The wall shear stress reached local minima at rod-wall and rod-rod gaps and local maxima in the open flow regions. Vortex streets were generated within the subchannels very close to the inlet of the rod bundle. The convection speed and frequency of the vortex street were found to increase proportionately to Reynolds number, whereas the vortex spacing was not affected by the Reynolds number.

With the technological shift towards big data, internet of things (IoT), 5G applications and cloud computing, the demand for high capacity networks is dramatically increasing. To avoid congestion and saturation, content and service providers are re-designing their network (backbone, metro and data-centers interconnects) connectivity using gridless optical line systems along with programmable coherent transponders. The latter are expected to transmit data at different data rates up to 400 Gb/s. In 2008, the first coherent receiver was commercially available [1]. By means of high-speed analog to digital converters and adaptive digital signal processing (DSP) algorithms, such revolution in modern optical communication was possible. That allowed a better spectral efficiency using higher order modulation formats and further signal reach by means of compensating both linear and nonlinear impairments. Another key development was leveraging light polarization-diversity, that permits to double the data rate at the expense of receiver complexity. To further increase the capacity of fiber links, gridless DWDM networks are being developed for deployment in the next few years. The key idea is to allow variable bandwidth signals to be allocated on optical links and by performing the appropriate network layer optimization improved throughput can be achieved. These innovations are driving new types of challenges for routing and assignment methods, as well, DSP algorithms such as clock recovery and compensation of fiber non-linearity. This thesis is organized as a collection of contributions and composed of five major parts. The first part, consisting of chapters 2 and 3. Chapter 4 deals with tracking of fast state of polarization transient, i.e. dynamic aspect of optical channels, in presence of polarization dependent loss (PDL) and filtering effects due to reconfigurable optical add-drop multiplexers (ROADMs). Chapters 5 and 6 study the impact of filtering effects, quasi-static effects in optical links and transponders, represented by ROADMs in fixed-grid and Silicon Photonics (SiPh) modulators in flexible-grid networks, respectively. Chapters 7, 8 and 9, are related to clock recovery in digital coherent receivers. They cover mitigation of jitter in gridless applications, improving jitter when deploying phase interpolators (PI) and jitter injection as a test-mean to evaluate performance.

A very interesting effect in light propagation through a disordered system is Anderson localization of light, this phenomenon emerges as the result of multiple scattering of waves by electric inhomogeneities like spatial variations of index of refraction; as the amount of scattering is increased, light propagation is converted from quasi-diffusive to exponentially localized, with photons confined in a limited spatial region characterized by a fundamental quantity known as localization length. Light localization is strongly related to another interference phenomenon emerged from the multiple scattering effect: the coherent backscattering effect. In multiple scattering of waves, in fact, coherence is preserved in the backscattering direction and produces a reinforcement of the field flux originating an observable peak in the backscattered intensity, known as backscattering cone. The study of this peak provide quantitative information about the transport properties of light in the material. In this thesis we report a complete FDTD ab-initio study of light localization and coherent backscattering. In particular, we consider a supercontinuum pulse impinging on a sample composed of randomly positioned scatterers. We study coherent backscattering by averaging over several realizations of the sample properties. We study then the coherent backscattering cone properties as the relative permittivity of the sample is changed, relating the latter with the light localization inside the sample. We demonstrate important relationships between the width of the backscattering cone and the localization length, which shows a linear proportionality in the strong localization regime.

Ultrafast plane-wave ultrasound imaging offers very high frame rates (exceeding thousands of frames per second) but entails large volumes of backscattered data collected by a sensor array over multiple plane-wave emissions at different angles. We propose a simple method for reducing the total amount of sampled data. First, we acquire the zero-angle data in full, and then we perform deterministic subsampling of the remaining nonzero-angle data. Our subsampling patterns are angle-specific and derived based on the zero-angle data using a Fourier-domain migration technique. We use two experimental datasets to evaluate the benefits and drawbacks of our proposed method in terms of spatial resolution and contrast-to-noise ratio, observed in the resulting B-mode images. / Graduate

In this thesis I address the problem of the commentators' interpretations regarding Hume's philosophy of mind. Precisely what theory of mind Hume articulates is the subject of much debate among the commentators although a definitive view has, as yet, not been attributed to Hume. The issue Is, of course, complicated by the fact that Hume says so many things about mind, and these numerous and often conflictlng assertions entirely elude the commentators' endeavours to systematize them into a coherent theory. Through and examination of what I take to be three representative readings, viz., those of R.F. Anderson, C.V. Salmon, and John Bricke, I show that each interpretation cannot but fail as a systematization of Hume's views about mind into a theory, since a positive reading of Hume's philosophy of mind cannot be accomplished without either abandoning aspects of Hume's metaphysics, acknowledging that a preferred interpretation Is unable to account for certain views that Hume propounds, alleging a deeply rooted inconsistency, or all of these possibilities together. Why Hume's work eludes the commentators In this way is a problem that is examined by considering his intentions in A Treatise of Human Nature (and the Enquiries); that is, more specifically, the aim of his sceptical critique of metaphysical concepts and doctrines, his conviction as an epistemologist that our knowledqe claims are confined to the realm of experience, and his project, viz., to set the sciences on a new foundation. The whole discussion calls attention to the difficulty that is encountered in the interpretation of Hume's views about mind and its importance, for Hume scholarship, lies precisely in the endeavour to bring these problems to light via a critique of the accepted readings. / Thesis / Master of Arts (MA)

<p> Jakeman's random walk model with step number fluctuations describes the amplitude scattered from a rough medium in terms as the coherent summation of (independent) individual scatterers' contributions. For a population following a birthdeath- immigration (BDI) model, the resulting statistics are k-distributed and the multiplicative representation of the amplitude as a Gaussian speckle modulated by a Gamma radar cross-section (RCS) is recovered. The main objective of the present thesis is to discuss techniques for the inference of the RCS in local time in order to facilitate anomaly detection. We first show how the Pearson class of diffusions, which we derive on the basis of a discrete population model analogous to the BDI, encompasses this Gamma texture as well as other texture models studied in the literature. Next we recall how Field & Tough derived, in an Ito calculus framework, the dynamics and the auto-correlation function of the scattered amplitude from the random walk model. In particular, they showed how the RCS was observable through the intensity-weighted squared fluctuations of the phase. Thanks to a discussion of the sources of discrepancy arising during this process, we derive an analytical expression for the inference error based on its asymptotic behaviours, together with a condition to minimize it. Our results are then extended to the Pearson class of diffusions whose importance for radar clutters is described. Next, we consider an experimental caveat, namely the presence of an additional white noise. The finite impulse response Wiener filter enables the design of the optimal filter to retrieve the scattered amplitude when it lies in superposition with thermal noise, thus enabling the usage of our inference technique. Finally, we consider weak scattering when a coherent signal lies in superposition with the aforementioned (strongly) scattered amplitude. Strong and weak scattering patterns differ regarding the correlation structure of their radial and angular fluctuations. Investigating these geometric characteristics yields two distinct procedures to infer the scattering cross-section from the phase and intensity fluctuations of the weakly scattered amplitude, thus generalizing the results obtained in the strong scattering case. </p> / Thesis / Doctor of Philosophy (PhD)

Ultrashort pulsed lasers have become critical to understanding light-matter interactions in new regimes such as generation of attosecond pulses, laser filamentation, and intense relativistic processes. Development of more powerful and energetic ultrafast lasers is required for advancing these fields of study. Several petawatt class systems now exist with more in development to further scale peak power and extend the frontier of ultrafast laser technology. Another relevant solution to the scaling of energy and power of ultrashort pulses is coherent beam combining (CBC). CBC is useful for not only scaling of laser parameters but also to mitigate parasitic nonlinear processes associated with high-intensity ultrashort pulses. In addition CBC is flexible and can be implemented as part of other techniques for ultrashort pulse amplification such as optical-parametric chirped-pulse amplification (OPCPA). In this thesis, CBC of ultrashort laser pulses is investigated based upon the method known as divided-pulse amplification (DPA). Active, passive and hybrid DPA have been achieved in a flashlamp-pumped Nd:YAG laser seeded from a Ti:sapphire mode-locked laser. Picosecond pulses at a repetition rate of 2.5 Hz were amplified and combined to record energy of 216 mJ with a combination efficiency of 80%. Engineering of the Nd:YAG amplifier chain for high-efficiency energy extraction is presented. In addition, phasing of actively divided pulses with a CW pilot laser co-propagating with the pulsed beam is also demonstrated. Analysis of multiple DPA configurations shows the viability of the method for a variety of different laser architectures including discussion of design restrictions.

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