Previous studies have demonstrated that many existing communication systems can be formulated within a unified multirate filterbank transceiver model. A redundant block transmission system implemented via this unified multirate filterbank transceiver model is usually known as a multiple-input-multiple-output (MIMO) system in literature. This dissertation devises an optimum linear block-based precoder and the corresponding equalizer for MIMO systems over perfect reconstruction (PR) channels by exploiting the proposed oblique projection framework. Particularly, two main criteria of interest in a digital communication link with limited transmission power are investigated, namely, average bit error rate (BER) minimization and mutual information rate maximization. The study framework is developed as follows. For a block-based precoder, a received signal model is formulated for the two redundancy schemes, viz., trailing-zeros (TZ) and cyclic-prefix (CP). By exploiting the property of oblique projection, a cascaded equalizer for block transmission systems (i.e., MIMO systems) is proposed and implemented with a scheme, in which the inter-block interference (IBI) is completely eliminated by the oblique projection and followed by a matrix degree of freedom for inter-symbol interference (ISI) equalization. With the available channel state information at the transmitter side, the matrix for ISI equalization of the cascaded equalizer is utilized to design an optimum linear block-based precoder, such that the BER is minimized (or the mutual information rate is maximized), subject to the ISI-free and the transmission power constraints. Accordingly, the cascaded equalizer with the ISI-free constraint yields a cascaded ZF equalizer. Theoretical derivations and simulation results confirm that the proposed framework not only retains identical BER and information rate performances to previous works for cases with sufficient redundancy, but also allows their results to be extended to the cases of insufficient redundancy.

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Une méthode de calcul de forçage optimal a été employée afin d'analyser le caractère amplificateur sélectif de bruit d'écoulements compressibles pariétaux. Une telle approche inclut la prise en compte de croissances non-modales induites par la non-normalité des équations de Navier-Stokes linéarisées. La méthode numérique repose sur le calcul de la matrice résolvante globale et la résolution d'un problème aux valeurs propres associé à un problème d'optimisation. Les densités d'énergie des forçages et réponses optimaux calculés pour une couche limite supersonique ont pu être reliés à la courbe neutre expérimentale obtenue par Laufer et Vrebalovich, à condition de contraindre la localisation du forçage en amont de la branche inférieure. Par la suite, une étude paramétrique en nombre de Mach de la réceptivité 2D d'une interaction choc/couche limite laminaire a permis de caractériser le développement d'instabilités convectives de Kelvin-Helmholtz et Tollmien-Schlichting (TS) à haute fréquence. La réceptivité basse fréquence de ce système a été mise en relation avec la résonance d'un mode global stable. Par ailleurs, une extension de la méthode numérique 2D a été proposée pour le calcul de perturbations 3D. Son application au calcul du forçage optimal d'une couche limite à M=4.5 a permis de mettre en évidence la croissance non-modale 3D de streaks ainsi que le développement d'ondes TS obliques dont la croissance, en régime compressible, est favorisée par rapport à celle des ondes 2D. Cette étude a également permis d'observer la croissance du mode de Mack à plus haute fréquence. / Parietal compressible flows have been studied by means of optimal forcing computations in order to characterize the noise amplifier nature of these flows. This approach is able to take into account the non-modal growth of linear perturbations induced by the non-normality of the linearized Navier-Stokes equations. The numerical strategy is based on the computation of the global resolvent matrix and an eigenvalue problem stemming from an optimization problem. Optimal forcing and response energy densities of a supersonic boundary layer have been linked to the experimental neutral curve obtained by Laufer et Vrebalovich, provided that the forcing localization is constrained upstream from the lower branch. Afterwards, a parametric study with respect to the Mach number of the 2D receptivity of the laminar shock wave/boundary layer interaction flow has allowed to analyze the growth of Kelvin-Helmholtz and Tollmien-Schlichting instabilities (TS) occurring at high frequencies. At low frequencies, the receptivity of the system has been linked to the resonance of a stable global mode. Furthermore, the 2D numerical method has been extended to allow the computation of 3D perturbations. This approach has been applied to a supersonic boundary layer flow at M=4.5 in which the 3D non-modal growth of streaks has been identified, as well as the development of oblique TS waves, whose growth is larger than the one associated to 2D waves in compressible regime. This study has also allowed to detect the growth of Mack mode at higher frequencies.

Immersed boundary methods are a class of techniques in computational fluid dynamics where the Navier-Stokes equations are simulated on a computational grid that does not conform to the interfaces in the domain of interest. This facilitates the simulation of flows with complex moving and deforming geometries without considerable effort wasted in generating the mesh. The first part of this dissertation is concerned with the aerodynamics of the cross-section of a species of flying snake, Chrysopelea paradisi (paradise tree snake). Past experiments have shown that the unique cross-section of this snake, which can be described as a lifting bluff body, produces an unusual lift curve--with a pronounced peak in lift coefficient at an angle of attack of 35 degrees for Reynolds numbers 9000 and beyond. We studied the aerodynamics of the cross-section using a 2-D immersed boundary method code. We were able to qualitatively reproduce the spike in the lift coefficient at the same angle of attack for flows beyond a Reynolds number of 2000. This phenomenon was associated with flow separation at the leading edge of the body that did not result in a stall. This produced a stronger vortex and an associated reduction in pressure on the dorsal surface of the snake cross-section, which resulted in higher lift. The second part of this work deals with the analysis of the direct forcing method, which is a popular immersed boundary method for flows with rigid boundaries. We begin with the fully discretized Navier-Stokes equations along with the appropriate boundary conditions applied at the solid boundary, and derive the fractional step method as an approximate block LU decomposition of this system. This results in an alternate formulation of the direct forcing method that takes into consideration mass conservation at the immersed boundaries and also handles the pressure boundary conditions more consistently. We demonstrate that this method is between first and second-order accurate in space when linear interpolation is used to enforce the boundary conditions on velocity. We then develop a theory for the order of accuracy of the direct forcing method with linear interpolation. For a simple 1-D case, we show that the method can converge at a range of rates for different locations of the solid body with respect to the mesh. But this effect averages out in higher dimensions and results in a scheme that has the same order of accuracy as the expected order of accuracy of the interpolation at the boundary. The discrete direct forcing method for the Navier-Stokes equations exhibits an order of accuracy between 1 and 2 because the velocities at the boundary are linearly interpolated, but the resulting boundary conditions on the pressure gradient turn out to be only first-order accurate. We recommend linearly interpolating the pressure gradient as well to make the method fully second-order accurate. We have also developed two open source codes in the course of these studies. The first, cuIBM, is a two-dimensional immersed boundary method code that runs on a single GPU. It can simulate incompressible flow around rigid bodies with prescribed motion. It is based on the general idea of a fractional step method as an approximate block LU decomposition, and can incorporate any type of immersed boundary method that can be made to fit within this framework. The second code, PetIBM, can simulate both two and three-dimensional incompressible flow and runs in parallel on multiple CPUs. Both codes have been validated using well-known test cases.

abstract: Current policies subsidizing or accelerating deployment of photovoltaics (PV) are typically motivated by claims of environmental benefit, such as the reduction of CO2 emissions generated by the fossil-fuel fired power plants that PV is intended to displace. Existing practice is to assess these environmental benefits on a net life-cycle basis, where CO2 benefits occurring during use of the PV panels is found to exceed emissions generated during the PV manufacturing phase including materials extraction and manufacture of the PV panels prior to installation. However, this approach neglects to recognize that the environmental costs of CO2 release during manufacture are incurred early, while environmental benefits accrue later. Thus, where specific policy targets suggest meeting CO2 reduction targets established by a certain date, rapid PV deployment may have counter-intuitive, albeit temporary, undesired consequences. Thus, on a cumulative radiative forcing (CRF) basis, the environmental improvements attributable to PV might be realized much later than is currently understood. This phenomenon is particularly acute when PV manufacture occurs in areas using CO2 intensive energy sources (e.g., coal), but deployment occurs in areas with less CO2 intensive electricity sources (e.g., hydro). This thesis builds a dynamic Cumulative Radiative Forcing (CRF) model to examine the inter-temporal warming impacts of PV deployments in three locations: California, Wyoming and Arizona. The model includes the following factors that impact CRF: PV deployment rate, choice of PV technology, pace of PV technology improvements, and CO2 intensity in the electricity mix at manufacturing and deployment locations. Wyoming and California show the highest and lowest CRF benefits as they have the most and least CO2 intensive grids, respectively. CRF payback times are longer than CO2 payback times in all cases. Thin film, CdTe PV technologies have the lowest manufacturing CO2 emissions and therefore the shortest CRF payback times. This model can inform policies intended to fulfill time-sensitive CO2 mitigation goals while minimizing short term radiative forcing. / Dissertation/Thesis / M.S. Civil and Environmental Engineering 2013

Un code de résolution des équations de Navier-Stokes pour un fluide incompressible a été développé en utilisant une approche mixte spectral/différences finies, compatible avec une mise en oeuvre dans un environnement massivement parallèle. On procède, grâce à ce nouvel outil, à des simulations directes de la turbulence dans une configuration où l'agitation est synthétisée à l'aide d'un forçage aléatoire. La production de turbulence est confinée dans une couche centrale du domaine et s'auto-diffuse en direction d'une surface libre ou d'une paroi adhérente. Dans cette configuration on obtient un état statistiquement stationnaire où le cisaillement moyen, généralement à l'origine de la production de la turbulence, est nul. Ces conditions permettent de mieux comprendre l'origine du transfert intercomposantes, caractéristique de la partie lente du terme de corrélation pression-déformation dans les équations-bilan des tensions de Reynolds. L'accent est mis sur l'analyse de ce transfert lorsqu'il s'effectue sous l'influence de l'effet de blocage au voisinage d'une surface. Les résultats obtenus permettront de mieux appréhender la modélisation des termes de corrélation pression-déformation au voisinage d'une paroi dans les modèles de fermeture au second ordre. / A Navier-Stokes solver for incompressible flow has been developed using a mixed spectral/finite-difference approach, while being compatible with a massively parallel environment. We use it to perform direct numerical simulations in a situation where the turbulent agitation is synthesized under the action of a random forcing. The turbulence production is confined in a central layer and self-diffuses towards a free-slip or no-slip surface. With this set-up, we obtain a statistical steady state in which the mean shear, usually associated with the turbulence production, is zero. These conditions allow a better understanding of the intercomponent energy transfer, induced by the slow part of the pressure-strain correlation in the Reynolds tensor budget. We focus on this transfer when it occurs in combination with the blocking effect, in the vicinity of the surface. The results will help to model the pressure-strain correlation in a second- order-closure context.

Cette thèse consiste en l'étude expérimentale et numérique des processus de formation des particules de suie au sein des flammes laminaires non-pré-mélangées et partiellement prémélangées sous l'influence d'un champ magnétique ou d'une stimulation acoustique. Dans une premiére étape, la capacité du code CIAO à prédire la fraction volumique de suie dans une flamme axisymétrique est étudiée. Par la suite, deux flammes subissant une stimulation acoustique ont été étudiées. Les résultats peuvent être utilisés pour améliorer les modèles de suie futurs, en particulier concernant les différentes échelles temporelles de la chimie en phase gazeuse, et la formation d'hydrocarbures polyaromatiques (PAH) et de suie couplée avec des flux transitoires. Pour étudier la formation des particules de suie sous l'influence de gradients de champ magnétique, un brûleur de type Santoro est utilisé. Les techniques de mesure utilisées dans le cadre de cette thèse sont l'imagerie directe à haute cadence, la technique Background Oriented Schlieren (BOS) et la méthode d'Absorption/Emission Modulée (MAE). Une augmentation de la fraction volumique de suie intégrée a été mise en évidence lorsque le gradient de champ magnétique est ascendant. Une analyse de stabilité linéaire locale appliquée à l'écoulement non-visqueux est présentée pour une flamme sous l'influence de la perturbation magnétique envisagée. Le gradient de champ magnétique provoque alors une réduction du taux d'amplification. De fait, l'étude est complété par l'identification d'un domaine où les flammes qui oscillent naturellement peuvent être stabilisées et contrôlées par des gradients de champ magnétique. / In this thesis light is shed on the soot formation processes in laminar coflow flames influenced by magnetic field gradients and acoustic forcing. Both influences have been assessed experimentally and numerically. First, the CIAO in-house code's ability to predict soot volume fraction fields in a steady coflow flame is studied. Then, two acoustically forced cases were studied. These findings are used to improve future soot models, especially, concerning the different time scales of gas phase chemistry and the formation of polycyclic aromatic hydrocarbons (PAH) and soot coupled with unsteady flows. To investigate soot formation under magnetic field gradients, a Santoro type burner is used. The measurement techniques applied in the course of this thesis are high-speed luminosity measurements, Background Oriented Schlieren (BOS) and one- and two-color Modulated Absorption/Emission (MAE) techniques. The magnetic field impact on soot formation was first studied experimentally in steady laminar flames. A scaling of soot production similar to the increased integrated soot volume fraction with increased oxygen content in the coflow was documented. A local inviscid stability analysis is presented for an ethylene coflow flame to investigate the flame's response to small perturbations of the mean velocity, temperature, fuel, and oxygen massfraction under magnetic field exposure. The magnetic field is found to reduce the perturbations' growth rate. The magnetic field study is completed by identifying a domain where naturally oscillating flames can be stabilized and controlled by magnetic field gradients.

Wavelike spiral asymmetries are an intriguing aspect of Tropical Cyclone dynamics. Previous work hypothesized that some of them are Vortex Rossby Waves propagating on the radial gradient of mean–flow relative vorticity. In the Intermittently Forced Vortex Rossby Wave theory, intermittent convection near the eyewall wind maximum excites them so that they propagate wave energy outward and converge angular momentum inward. The waves’ energy is absorbed as the perturbation vorticity becomes filamented near the outer critical radii where their Doppler–shifted frequencies and radial group velocities approaches zero. This process may initiate outer wind maxima by weakening the mean–flow just inward from the critical radius. The waves are confined to a relatively narrow annular waveguide because of their slow tangential phase velocity and the narrow interval between the Rossby wave cut–off frequency, where the radial wavenumber is locally zero, and the zero frequency, where it is locally infinite.

abstract: The National Oceanic and Atmospheric Administration (NOAA)’s National Water Model (NWM) will provide the next generation of operational streamflow forecasts at different lead times across United States using the Weather Research and Forecasting (WRF)-Hydro hydrologic system. These forecasts are crucial for flood protection agencies and water utilities, including the Salt River Project (SRP). The main goal of this study is to calibrate WRF-Hydro in the Oak Creek Basin (OCB; ~820 km2), an unregulated mountain sub-watershed of the Salt and Verde River basins in Central Arizona, whose water resources are managed by SRP and crucial for the Phoenix Metropolitan area. As in the NWM, WRF-Hydro was set up at 1-km (250-m) resolution for the computation of the rainfall-runoff (routing) processes. Model forcings were obtained by bias correcting meteorological data from the North American Land Data Assimilation System-2 (NLDAS-2). A manual calibration approach was designed that targets, in sequence, the sets of model parameters controlling four main processes responsible for streamflow and flood generation in the OCB. After a first calibration effort, it was found that WRF-Hydro is able to simulate runoff generated after snowmelt and baseflow, as well as magnitude and timing of flood peaks due to winter storms. However, the model underestimates the magnitude of flood peaks caused by summer thunderstorms, likely because these storms are not captured by NLDAS-2. To circumvent this, a seasonal modification of soil parameters was adopted. When doing so, acceptable model performances were obtained during calibration (2008-2011) and validation (2012-2017) periods (NSE > 0.62 and RMSE = ~2.5 m3/s at the daily time scale). The process-based calibration strategy utilized in this work provides a new approach to identify areas of structural improvement for WRF-Hydro and the NWM. / Dissertation/Thesis / Masters Thesis Civil, Environmental and Sustainable Engineering 2020

Dust-climate interaction over the Middle East and North Africa (MENA) has long been studied, as it is the "dustiest" region on earth. However, the quantitative and qualitative understanding of the role of dust direct radiative effect on MENA climate is still rudimentary. The present dissertation investigates dust direct radiative effect on MENA climate during summer with a special emphasis on the sensitivity of climate response to dust shortwave absorption, which is one of the most uncertain components of dust direct radiative effect. Simulations are conducted with and without dust radiative effect, to differentiate the effect of dust on climate. To elucidate the sensitivity of climate response to dust shortwave absorption, simulations with dust assume three different cases of dust shortwave absorption, representing dust as a very efficient, standard and inefficient shortwave absorber. The non-uniformly distributed dust perturb circulations at various scales. Therefore, the present study takes advantage of the high spatial resolution capabilities of an Atmospheric General Circulation Model (AGCM), High Resolution Atmospheric Model (HiRAM), which incorporates global and regional circulations. AMIP-style global high-resolution simulations are conducted at a spatial resolution of 25 km. A significant response in the strength and position of the local Hadley circulation is predicted in response to meridionally asymmetric distribution of dust and the corresponding radiative effects. Significant responses are also found in regional circulation features such as African Easterly Jet and West African Monsoon circulation. Consistent with these dynamic responses at various scales, the tropical rainbelt across MENA strengthens and shifts northward. Similarly, the temperature under rainbelt cools and that over subtropical deserts warms. Inter-comparison of various dust shortwave absorption cases shows that the response of the MENA tropical rainbelt is extremely sensitive to the strength of shortwave absorption. Further analyses reveal that the sensitivity of the rainbelt stems from the sensitivity of the multi-scale circulations that define the rainbelt. Importantly, the summer precipitation over the semi-arid strip south of Sahara, including Sahel, increases in response to dust radiative effect. The maximum response and sensitivity are predicted over this region. The sensitivity of the responses over Sahel, especially that of precipitation, is comparable to the mean state. Locally, the precipitation increase reaches up to 50% of the mean, while dust is assumed to be a very efficient absorber. As the region is characterized by the "Sahel drought", the predicted precipitation sensitivity to the dust loading over this region has a wide-range of socioeconomic implications. The present study, therefore, suggests the importance of reducing uncertainty in dust shortwave absorption for a better simulation and interpretation of the MENA climate in general, and of Sahel in particular.