Browsing by Author "Leonardi, Stefano"
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Item Analysis of Model-Free Control of Wind Farms Using Large-Eddy Simulations(2019-08) Ciri, Umberto; 0000-0001-9953-6259 (Ciri, U); Leonardi, StefanoWind farms are clusters of wind turbines deployed over a relatively small area. During operations, the wake from upstream turbines may impinge on trailing turbines causing a decrease in power production. Wind farm control strategies aim at mitigating the effect of wake interactions. In this dissertation, model-free control strategies for wind farm power maximization have been evaluated using numerical simulations of the flow through wind farms. A model-free approach does not require a priori assumptions on the physical system, but learns on-line the system dynamics, avoiding modeling uncertainties. The control strategies are based on extremum-seeking control (ESC), a real-time gradient-based optimization algorithm. Either the turbine generator torque or the rotor yaw angle is used as the control parameter tuned by ESC to optimize the wind farm power production. The generator torque adjusts the turbine angular speed and the momentum deficit in the trailing wake, while the yaw angle serves to vary the direction of the wake and avoid trailing turbines. We first consider several implementations of ESC and assess their performances and practical feasibility. Both torque- and yaw-based ESC enhance power production, but the latter has a larger margin for improvement. For idealised turbine arrays, ESC achieves a potential power improvement of at least 7–8% compared to operations with design settings for an isolated turbine. After this calibration, we perform an optimization study for a real wind farm and obtain a quantitative evaluation of the impact of the control strategy in annual energy production. Large-eddy simulations with rotating actuator disk are used, in the first place, to provide a virtual wind farm to test the control algorithms. Additionally, the numerical data are investigated to gain a physical insight on the mechanisms underlying the performance improvement and broaden the impact of the optimization.Item Application of Microporous Coating in Passive Thermal Management Device(2022-08-01T05:00:00.000Z) Wang, Xiaomeng; You, Seung; Kiasaleh, Kamran; Leonardi, Stefano; Fadda, Dani; Xiong, GuopingDevelopments in electronic applications present numerous thermal management challenges for the dissipation of waste heat. The work presented in this dissertation is applicable to the dissipation of persistent heat from large areas and the spreading of waste heat from highly concentrated heat sources. Persistent uniform heat generation from a relatively large area is encountered in the use of batteries for electric vehicles among many other applications. In these applications, heat must be dissipated from a relatively large surface area, relative to the spacing between two adjacent heat sources. Highly concentrated heat sources are increasingly encountered due to the rapid development of electronic packaging techniques. Specifically, the overall system power densities continue to increase due to the size minimization trend of the electronic devices. These applications result in significant thermal management challenges. If not managed properly, increased temperatures can cause significant deterioration in a device’s performance and greatly reduce the product reliability. Traditional active cooling strategies, such as singlephase and two-phase active cooling systems, using pumps or compressors as auxiliary components, dissipate reasonably high heat fluxes and can be applied over large areas. However, the associated auxiliary devices increase the system’s complexity and decrease its reliability. Alternatively, passive cooling devices utilizing liquid-vapor phase change thermal systems, such as thermal ground plane (TGP), effectively dissipate or spread the heat. These can be wick type or wickless and rely on evaporation or boiling phase change. The wick type TGP or vapor chamber utilizes capillary forces to recirculate the evaporating liquid, the wicking structure plays an essential role in the overall heat dissipation effectiveness. For the wickless type TGP, which utilizes a bubble pumping mechanism to circulate the liquid instead of wicking, boiling heat transfer performance is dominant. In the current dissertation, firstly, the performance of an aluminum high-temperature, highconductive microporous coating, which can be used in wick-type thermal ground plane as the wicking material, is characterized through mass and heat transfer experiments utilizing water and highly wetting fluids for dissipating persistent heat from large areas. Secondly, the wettability effect on the nucleate boiling heat transfer performance of a copper high-temperature, thermally conductive, microporous coating is experimentally investigated. Thirdly, a wickless and orientation independent ultra-thin thermal ground plane is developed using this copper high-temperature, thermally conductive, microporous coating for spreading highly concentrated waste heat.Item Coarsening Droplet: Meniscus-mediated Spontaneous Droplet Climbing and Its Applications(2021-12-01T06:00:00.000Z) Guo, Zongqi; Dai, Xianming; Lee, Gil; You, Seung; Leonardi, Stefano; Qin, Zhenpeng; Xiong, GuopingInspired by nature, fundamental investigations of droplet directional movement grow explosively in recent years, aiming to generate energy, solve water shortage, and protect the environment. To move the droplets, strategies were developed based on changing the property of the droplet/substrate, and the substrate geometry. However, it is challenging to remove submicrometer droplets from the surface, which limits the potential applications of water harvesting. In this dissertation, I present a new spontaneous droplet movement on the hydrophilic slippery liquidinfused porous surface (SLIPS), named coarsening droplet. The coarsening effect can rapidly remove droplets with a diameter less than 20 μm. As a mechanism, low interfacial tension of hydrophilic SLIPS enables droplet climbing, while a high oil surface tension provides the driven force. By applying the coarsening droplet for water harvesting, it re-evaluated the classical condensation model since 1973, which neglected droplet disappearance (e.g., removed from the surface) that can enhance the heat transfer. Our new model elucidates the comprehensive heat transfer process, giving rise to a clear guideline of surface design for dropwise condensation. To further apply our new condensation theory of rapid droplet removal, I present a vapor-liquid separation surface to further enhance the water harvesting. By separating the water vapor with condensed droplets, the surface is always fresh to new water nucleation and has a higher droplet disappearance frequency with smaller droplets. In this dissertation, the coarsening droplet is focused on the fundamental study to show the importance of droplet removal. As SLIPS is not durable for water harvesting due to the loss of lubricants, I present a quasi-liquid surface (QLS) by tethering flexible polymer on various solid substrates to solve the durability issue of SLIPS. QLS shows excellent durability during water harvesting experiments, which could be further applied to industrial applications. In this dissertation, Chapter 1, I introduced the fundamental and current progress of droplet directional removal. In Chapter 2, I reported the coarsening droplet and investigate the mechanism of the coarsening droplet as surface tension force. In Chapter 3, I apply the coarsening droplet for water harvesting. The self-propelled coarsening droplet on hydrophilic SLIPS shows rapid removal of condensed submicrometer droplets regardless of surface orientations, showing a promising approach in water harvesting. In Chapter 4, I re-evaluate the condensation model on a hydrophilic slippery liquid-infused surface. I propose a modified condensation model by considering the droplet coverage ratio and removal frequency, which can precisely predict the heat flux. In Chapter 5 I further enhance the water harvesting by achieving vapor-liquid separation on T-shape structures. In Chapter 6, a quasi-liquid surface (QLS) is investigated to solve the durability issue of SLIPS. In Chapter 7, I summarized all my contributions and novelties.Item Coupling of Mesoscale Weather Research and Forecasting Model to a High Fidelity Large Eddy Simulation(Institute of Physics Publishing) Santoni-Ortiz, Christian; Garcia-Cartagena, Edgardo Javier; Ciri, Umberto; Iungo, Giacomo V.; Leonardi, Stefano; 0000-0002-0990-8133 (Iungo, GV); 0000-0002-9809-7191 (Leonardi, S); Santoni-Ortiz, Christian; Garcia-Cartagena, Edgardo Javier; Ciri, Umberto; Iungo, Giacomo V.; Leonardi, StefanoNumerical simulations of the flow in a wind farm in north Texas have been performed with WRF (Weather Research and Forecasting model) and our in-house LES code. Five nested domains are solved with WRF to model the meso-scale variability while retaining a resolution of 50 meters in the wind farm region. The computational domain of our in-house LES code is nested into the inner most domain of the WRF simulation from where we get the inlet boundary conditions. The outlet boundary conditions are radiative and at this stage the coupling between the two codes is one-way. The turbines in WRF are mimicked using a modified Fitch approach, while in our in-house LES we have used a rotating actuator disk combined with immersed boundaries for tower and nacelle. Numerical results agree well with meteorological data from the met tower. The power production obtained numerically on each turbine compares well with SCADA data with an index of agreement ranging between 80% to 90%. The power production from the numerical results of our in-house LES code is slightly closer to SCADA data than that of WRF.Item Data-Driven RANS for Simulations of Large Wind Farms(Institute of Physics Publishing) Iungo, Giacomo V.; Viola, F.; Ciri, Umberto; Rotea, Mario A.; Leonardi, Stefano; 0000-0002-0990-8133 (Iungo, GV); 0000-0002-9809-7191 (Leonardi, S); Sorensen J.N.; Ivanell S.; Barney A.; Iungo, Giacomo V.; Ciri, Umberto; Rotea, Mario A.; Leonardi, StefanoIn the wind energy industry there is a growing need for real-time predictions of wind turbine wake flows in order to optimize power plant control and inhibit detrimental wake interactions. To this aim, a data-driven RANS approach is proposed in order to achieve very low computational costs and adequate accuracy through the data assimilation procedure. The RANS simulations are implemented with a classical Boussinesq hypothesis and a mixing length turbulence closure model, which is calibrated through the available data. High-fidelity LES simulations of a utility-scale wind turbine operating with different tip speed ratios are used as database. It is shown that the mixing length model for the RANS simulations can be calibrated accurately through the Reynolds stress of the axial and radial velocity components, and the gradient of the axial velocity in the radial direction. It is found that the mixing length is roughly invariant in the very near wake, then it increases linearly with the downstream distance in the diffusive region. The variation rate of the mixing length in the downstream direction is proposed as a criterion to detect the transition between near wake and transition region of a wind turbine wake. Finally, RANS simulations were performed with the calibrated mixing length model, and a good agreement with the LES simulations is observed.Item Data-Driven Reduced Order Model for Prediction of Wind Turbine Wakes(Institute of Physics Publishing) Iungo, Giacomo V.; Santoni-Ortiz, Christian; Abkar, M.; Porté-Agel, F.; Rotea, Mario A.; Leonardi, Stefano; Iungo, Giacomo V.; Santoni-Ortiz, Christian.; Rotea, Mario A.; Leonardi, StefanoIn this paper a new paradigm for prediction of wind turbine wakes is proposed, which is based on a reduced order model (ROM) embedded in a Kalman filter. The ROM is evaluated by means of dynamic mode decomposition performed on high fidelity LES numerical simulations of wind turbines operating under different operational regimes. The ROM enables to capture the main physical processes underpinning the downstream evolution and dynamics of wind turbine wakes. The ROM is then embedded within a Kalman filter in order to produce a time-marching algorithm for prediction of wind turbine wake flows. This data-driven algorithm enables data assimilation of new measurements simultaneously to the wake prediction, which leads to an improved accuracy and a dynamic update of the ROM in presence of emerging coherent wake dynamics observed from new available data. Thanks to its low computational cost, this numerical tool is particularly suitable for real-time applications, control and optimization of large wind farms.Item Data-enhanced Stochastic Dynamical Modeling for Wind Farms(December 2022) Bhatt, Aditya H. 1997-; Zare, Armin; Leonardi, Stefano; Rotea, Mario A.; Zhang, JieLow-fidelity analytical models of turbine wakes have traditionally been used to demonstrate the utility of advanced control algorithms in increasing the annual energy production of wind farms. In practice, however, it remains challenging to achieve significant performance improvements using closed-loop strategies that are based on conventional low-fidelity models. This is due to the over-simplified static nature of wake predictions from models that are agnostic to the complex aerodynamic interactions among turbines. In this thesis, we offer a stochastic dynamical modeling framework to improve the predictive capability of low-fidelity models while remaining amenable to control design. The framework is capable of capturing the effect of atmospheric turbulence on the thrust force and power generation as determined by the actuator disk concept. In this approach, we use stochastically forced linear models of the turbulent velocity field to augment the analytically computed wake velocity and achieve consistency with higher-fidelity models in capturing power and thrust force measurements. The power-spectral densities of our stochastic models are identified via convex optimization to ensure statistical consistency while preserving model parsimony. We demonstrate the utility of our approach in estimating the thrust force and power signals generated by large- eddy simulations of the flow over a cascade of turbines. We also evaluate the capability of our models in predicting turbulence intensities at the hub height of a multi-turbine wind farm.Item Direct Numerical Simulations of the Turbulent Flow over Super-Hydrophobic and Liquid Infused Surfaces: Effect of the Dynamics of the Interface on the Drag(2018-11-20) García Cartagena, Edgardo J.; Leonardi, StefanoSuperhydrophobic surfaces (SHS) and liquid infused surfaces (LIS) are studied through direct numerical simulations of a turbulent channel flow and a Couette flow. Either SHS or LIS are placed on the lower wall and modeled with the immersed boundary method. The dynamics of the interface between the two fluids is fully coupled to the Navier Stokes equations and solved with a Level Set method. A parametric study has been performed varying the viscosity ratio between the two fluids, substrate geometry, Reynolds number and Weber number. Different flow configurations are considered from ideal cases to more realistic, to assess the interface stability and drag of SHS and LIS. Because the viscosity ratio in LIS is closer to one, the interface is more stable. For SHS, even with very high interfacial tension, large deformations of the interface were observed. It is found that the dynamics of the interface has a detrimental effect on the drag reduction performance. A realistic configuration with random pinnacles as a substrate is studied in order to understand the effect that asperities in SHS and LIS have on drag. The form drag generated by the asperities showed to affect significantly the drag reducing properties of SHS and LIS. A roughness scale has been defined and proposed as a design criteria for drag reduction.Item Effect of the Turbine Scale on Yaw Control(John Wiley & Sons Ltd) Ciri, Umberto; Rotea, Mario A.; Leonardi, Stefano; 0000-0002-9809-7191 (Leonardi, S); Ciri, Umberto; Rotea, Mario A.; Leonardi, StefanoYaw misalignment between the incoming wind and the rotor of a turbine causes a lateral displacement of the wake. This effect can be exploited to avoid or mitigate wake interactions in wind farms, so that power losses are minimized. We performed large-eddy simulations to evaluate yaw control for a three-turbine wind farm. We used two different turbine models to assess how the size of the turbine rotor affects the farm efficiency and the effectiveness of the control strategy. A utility-scale wind turbine with rotor diameter of 126 m is compared with a scaled research wind turbine with rotor diameter of 27 m. In both cases, a model-free algorithm is used to determine the turbine yaw set point, which maximizes total power production. The algorithm is the nested extremum-seeking control (NESC), which allows for the coordinated optimization of the wind turbine operating points. The results achieved with NESC are validated by computing a static performance map for different yaw angles. NESC converges to optimal operating conditions, which are in good agreement with the static map benchmark. Numerical results show that a larger rotor diameter induces larger wake deflection, thus achieving higher power improvements. From the analysis of the turbine structural loads, an increase in damage equivalent load is observed for both the yawed turbine and the waked one. Present results suggest that there is a cost-effective trade-off between performance and loads for large turbines. © 2018 John Wiley & Sons, Ltd.Item Evaluation of Log-Of-Power Extremum Seeking Control for Wind Turbines Using Large Eddy Simulations(John Wiley & Sons Ltd) Ciri, Umberto; Leonardi, Stefano; Rotea, Mario A.; 0000-0002-9809-7191 (Leonardi, S); 0000-0002-4239-0591 (Rotea, MA); Ciri, Umberto; Leonardi, Stefano; Rotea, Mario A.The extremum seeking control (ESC) algorithm has been proposed to determine operating parameters that maximize power production below rated wind speeds (region II). This is usually done by measuring the turbine's power signal to determine optimal values for parameters of the control law or actuator settings. This paper shows that the standard ESC with power feedback is quite sensitive to variations in mean wind speed, with long convergence time at low wind speeds and aggressive transient response, possibly unstable, at high wind speeds. The paper also evaluates the performance, as measured by the dynamic and steady state response, of the ESC with feedback of the logarithm of the power signal (LP-ESC). Large eddy simulations (LES) demonstrate that the LP-ESC, calibrated at a given wind speed, exhibits consistent robust performance at all wind speeds in a typical region II. The LP-ESC is able to achieve the optimal set-point within a prescribed settling time, despite variations in the mean wind speed, turbulence, and shear. The LES have been conducted using realistic wind input profiles with shear and turbulence. The ESC and LP-ESC are implemented in the LES without assuming the availability of analytical gradients. ©2019 John Wiley & Sons, Ltd.Item Fluid Retention in Liquid Infused Surfaces: a Direct Numerical Simulation Study(2021-04-27) Garimella, Martand Mayukh Mayukh; Leonardi, StefanoNumerical and experimental studies using super-hydrophobic and liquid infused surfaces show that they offer promise in terms of reducing frictional drag. However, to utilize them in practical conditions, these surfaces must be designed to withstand the shear of the external flow. In studies done so far, texture geometries such as infinite longitudinal bars, transverse bars and staggered cubes have been used to support the infused fluid within the cavities. However, these geometries fall short in terms of retaining the infused fluid and sustaining drag reduction for long periods of time. Therefore, in this study, we have tried to address this issue by modeling texture geometries which can retain the infused fluid. We have designed surface geometries of mesh configurations having rectangular shaped cavities containing the infused fluid and restricting the infused fluid flow. While the pitch along the spanwise direction between longitudinal riblets was maintained constant, three pitch lengths of 11k,22k and 44k, where k is the height of roughness, were used between the transverse bars in the streamwise direction. These geometries have been modelled at the lower wall of a channel and turbulent channel flow has been simulated over them by direct numerical simulations (DNS). A viscosity ratio m = µ2/µ1 of 0.4 ( subscript 2 indicates the fluid in cavities and subscript 1 indicates external flowing fluid) mimicking the viscosity ratio of liquid infused surfaces (LIS) is considered. The first set of simulations was performed with W e = 0 to obtain perfect slip at the interface. The results agree well with those in literature for perfect slip conditions. A second set of simulations was performed at W e = 100 (W e+ = 3.6 × 10−4 ) to assess the deformation of the interface. A simulation with a streamwise pitch length of 11k and W e = 500 (W e+ = 2 × 10−3 ) was also performed to analyse the effect of large interface deformation on the external flow. This deformation of the interface is fully coupled with the Navier-Stokes equation and tracked in time using a Level set method. In comparison with the geometry having only longitudinal riblets, we observe an increase in drag and decrease in slip length. However, a minimum drag reduction of 5% can still be achieved under realistic conditions with finite surface tension (W e+ = 3.6 × 10−4 ) applied at the interface. However, when the surface tension was reduced, a drag increase was observed. As these modeled texture geometries retain the infused fluid, they offer a long term gain over the ideal geometries mentioned before. Compared to flat channel flow, there is a decrease in turbulent Reynolds stresses and kinetic energy production showing promise for further studies.Item Investigation on the Organization of Turbulence for High Reynolds-number Boundary-layers Through LiDAR Experiments(December 2022) Puccioni, Matteo 1991-; Nourani, Mehrdad; Iungo, Giacomo Valerio; Anderson, William; Leonardi, Stefano; Jin, YaqingLight Detection And Ranging (LiDAR) technology has gained growing attention for research in the realm of atmospheric turbulence due to its capability to probe the atmospheric boundary layer (ABL) with high spatial and temporal resolution within a volume with height and horizontal extent comparable to the ABL thickness. In this work, several high Reynolds- number turbulent flows, such as ABL for onshore and marine environment, wakes generated by utility-scale wind turbines, have been probed with the LiDAR anemometry with the aim of investigating the variability of the mean kinetic energy, the spatial and spectral heterogeneity of the streamwise momentum resulting from the dynamics of various turbulent eddies. In the first part of this dissertation, the LiDAR spatial averaging process, which is the source for a reduced turbulence intensity measured though a wind LiDAR, has been systematically corrected through a novel data-driven procedure based on the quantification of the energy damping for the streamwise velocity spectra at high-frequencies owing to the inertial sub-range. This correction method has enabled reverting the low-pass filtering operated by the LiDAR measuring system on the turbulent velocity fluctuations, and obtain a corrected estimate of the second-order statistical moment of the streamwise velocity. Subsequently, LiDAR measurements collected for the ABL evolving over a very flat and homogeneous terrain are interrogated to investigate the distribution of the streamwise turbulence intensity associated with wall-attached eddies and larger coherent structures as a function of height. This work has culminated with the proposition of an analytical model for the prediction of the linear coherence spectrum, which is based on the Townsend’s attached eddy hypothesis. Finally, LiDAR experiments performed for marine ABL, ABL interacting with utility-scale wind turbines, and coupling LiDAR with snow particle image velocimetry to investigate atmospheric turbulence are presented.Item Large-Eddy Simulations of Two In-Line Turbines in a Wind Tunnel with Different Inflow Conditions(MDPI AG) Ciri, Umberto; Petrolo, Giovandomenico; Salvetti, Maria Vittoria; Leonardi, Stefano; Ciri, Umberto; Leonardi, StefanoNumerical simulations reproducing a wind tunnel experiment on two in-line wind turbines have been performed. The flow features and the array performances have been evaluated in different inflow conditions. Following the experimental setup, different inlet conditions are obtained by simulating two grids upstream of the array. The increased turbulence intensity due to the grids improves the wake recovery and the efficiency of the second turbine. However, the inlet grid induces off-design operation on the first turbine, decreasing the efficiency and increasing fatigue loads. Typical grid flow patterns are observed past the rotor of the first turbine, up to the near wake. Further downstream, the signature of the grid on the flow is quite limited. An assessment of numerical modeling aspects (subgrid scale tensor and rotor parameterization) has been performed by comparison with the experimental measurements.Item Microwave Plasma Pyrolysis of Biomass: Process and Applications(December 2022) Stein, Benjamin Emmanuel 1986-; Voit, Walter; Leonardi, Stefano; Auciello, Orlando; Bleris, Leonidas; Cogan, Stuart; Prasad, ShaliniIn order to meet the growing demands of an increasing global population without irreversibly depleting the planet’s resources in the face of a rapidly changing climate a sustainable circular economy will need to be adopted, one based on renewable biologically derived materials. The Microwave Plasma Pyrolysis (MPP) process detailed in this dissertation is intended to help enable this transition by establishing a standard methodology for the conversion of renewable biological material feedstocks like hemp and fungi into high-value end products capable of replacing non-renewable or petroleum-based products in a scalable and efficient manner. To this end a custom MPP system is designed and built around a common kitchen microwave oven and tested with commercially available hemp-based canvas fabrics as well as fungal samples collected from the Dallas ecosystem. It is found that the MPP process converts biomass into a matrix of carbon phases including graphite and a form of diamond called Ultrananocrystalline Diamond (UNCD) while retaining the original sample’s superstructure. The pyrolyzed biomass is investigated via a variety of complementary physical and chemical characterization techniques and is shown to possess hierarchical porosity at the macro, micro, and nanoscales. Being that this is the first time it has been reported that diamond can be produced from a biological source in such a rapid and efficient manner, a mechanism is discussed to explain this unique transformation, with comparisons to conventional methods of diamond growth and production. The application demonstrated in this dissertation as an air filtration membrane is influenced by the global COVID-19 viral pandemic, which exposed serious failure modes in conventional facemask systems based on non-woven electro-spun or melt-blown polypropylene fibers. Since a fully sized facemask derived from pyrolyzed fungal mycelium or hemp canvas was not feasible during this dissertation research, a unique methodology derived from TEM grid particle sampling research was developed utilizing additive manufacturing and tested in collaboration with an industry standard FDA aerosol mask testing setup using electrostatically neutral salt particles. Further functionalization and enhancement of the plasma pyrolyzed biomass is demonstrated through Microwave Plasma Chemical Vapor Deposition (MPCVD), used to deposit additional layers of diamond and other nanocarbons including graphite and graphene. Prior work in the literature investigating applications of conventionally pyrolyzed biomass have included energy storage in the form of high surface area nanocarbons for battery electrodes and as performance enhancing additives in composites.Item Modeling and Analysis of Stochastic Base Flow Uncertainties in Wall-bounded Shear Flows(2022-12-01T06:00:00.000Z) Hewawaduge, Dhanushki Buddhika 1990-; Zare, Armin; Griffith, Todd; Leonardi, Stefano; Koeln, JustinSpatially distributed dynamical systems arise in a variety of science and engineering problems and are typically described by Partial Integro-Differential (P(I)DEs) equations. Important examples of such systems include the wave equations, Maxwell equations, Burgers equations, Schrodinger equations, and the Navier-Stokes equations. An appropriate way to study and control such systems often involves the spatio-temporal analysis of linearized forms of these equations around base profiles, which either describe a steady-state solution or a long-time averaged mean of a simulation- or experiment-based field. In addition, deterministic or stochastic forcing is commonly used to compensate for the neglected nonlinear terms and evaluate the input-output features of the linearized dynamics. However, uncertainty in both the base profile and nature of the inputs challenge the effectiveness of linearized models for analysis and control design. Motivated by applications in the analysis and control of complex fluid flows, this thesis demonstrates how modeling sources of stochastic base flow uncertainty can enable physical discovery and statistical modeling of quantities of interest. We provide an input-output framework to analyze the effect of base flow perturbations on the stability and receptivity properties of transitional and turbulent channel flows. Such base flow variations are modeled as persistent white-in-time stochastic excitations that enter the linearized dynamics as multiplicative sources of uncertainty that can alter the stability of the linearized dynamics and their receptivity to exogenous excitation. We provide verifiable conditions for mean-square stability and study the frequency response of the flow subject to additive and multiplicative sources of uncertainty using the solution to the generalized Lyapunov equation. Our approach does not rely on costly stochastic simulations or adjointbased sensitivity analyses. We use our framework to uncover the Reynolds number scaling of critically destabilizing variance levels of the base flow uncertainty, study the reliability of numerically estimated mean velocity profiles in turbulent channel flows, and the robust performance of a typical boundary control strategy for turbulence suppression in the wake of parametric uncertainties. For small-amplitude base flow perturbations, we adopt a perturbation analysis to provide a computationally efficient method for computing the variance amplification of velocity fluctuations around the uncertain base. Moreover, we study the flow structures that are extracted from a modal decomposition of the resulting velocity covariance matrix at energetically dominant locations of wall-parallel wavenumbers. In the final part of this thesis, we use the developed input-output framework to evaluate the robust performance transverse lower-wall oscillations as a flow control strategy when oscillations are subject to imperfections in amplitude and phase. These imperfections, cause the nominally harmonic flow control strategy to resemble a random oscillatory pattern.Item Numerical Simulations for Turbulent Drag Reduction Using Liquid Infused Surfaces(2017-12) Arenas-Navarro, Isnardo; 0000-0002-4570-2255 (Leonardi, S); Leonardi, Stefano; Minkoff, Susan ENumerical simulations of the turbulent flow over Super Hydrophobic and Liquid Infused Surfaces have been performed in this work. Three different textured surfaces have been considered: longitudinal square bars, transversal square bars and staggered cubes. The numerical code combines an immersed boundary method to mimic the substrate and a level set method to track the interface. Liquid Infused Surfaces reduce the drag by locking a lubricant within structured roughness to facilitate a slip velocity at the surface interface. The conceptual idea is similar to Super Hydrophobic Surfaces, which rely on a lubricant air layer, whereas liquid-infused surfaces use a preferentially wetting liquid lubricant to create a fluid-fluid interface. This slipping interface has been shown to be an effective method of passively reducing skin friction drag in turbulent flows. Details are given on the effect of the viscosity ratio between the two fluids and the dynamics of the interface on drag reduction. An attempt has been made to reconcile Super-Hydrophobic, Liquid Infused and rough wall under the same framework by correlating the drag to the wall normal velocity fluctuations.Item Numerical simulations of structural and fluid dynamics for aerodynamic performance improvement(2021-04-26) Yu, Haoliang; Malik, Arif; Leonardi, StefanoThe present research aims to understand and improve the aerodynamic performance of airfoils in unmanned aerial vehicle (UAV) and wind energy applications using numerical approaches. Specifically, the research applications include: 1) the flexibility tailoring of passively induced airfoil shapes for thin UAV wings, and 2) the aerodynamic performance evaluation of wind turbine blade airfoils that include idealized leading edge (LE) damage patterns aimed at emulating erosion. In both applications, fundamental insights that motivate subsequent optimum design configurations are sought through the use of computational tools of varying efficiency and fidelity. In regard to the first airfoil type studied, UAVs have attracted special attention in recent decades due to their unique and adaptable functionality for both military and civilian applications. Among fixed-wing UAVs, those with flexible passively-deforming wings have been shown to achieve extended aerodynamic endurance, reduced power consumption, and beneficial stability characteristics. Since neither excessively flexible nor excessively rigid wings maximize aerodynamic performance, flexibility tailoring for such membrane wings is still of significant interest. However, the numerical and experimental studies to date have been mostly limited to 2D studies, specifically to chordwise flexibility. To gain insights into furv ther design improvements, such as enabling extended aerodynamic endurance, more complex 3D geometric flexibilities, as are investigated and described in this work. Emulating a bioinspired flexible UAV wing design, a novel topology optimization using a genetic algorithm with an efficient fluid structure interaction (FSI) model produces a wing frame configuration with optimal flexibility distribution. The decoupled effects of the induced camber and spanwise bending deformation are analyzed to understand their contributions to performance improvements. Regarding the second airfoil type studied, designing wind turbine blades to achieve both extended service life and high operating efficiency is of great interest. Leading edge erosion, which poses significant problems to efficiency, necessitates research into the understanding of the underlying fluid dynamics. However, strong three-dimensionality of flow and relatively small scale of erosion poses great challenges to understanding and predicting the flow behavior numerically in terms of fidelity and computational time. Presented is a reduced order model (ROM) proposed for efficient drag prediction on a streamlined body with surface imperfections that emulate leading-edge roughness or erosion-induced damage. It requires as input only the geometric description of damage. Satisfactory performance is demonstrated via comparison with direct numerical simulations. Insights into the flow physics influencing both form and friction contributions to total drag are presented, a preferable damage mode from an engineering design aspect is revealed. In summary, the described work addresses the research gaps through applying a set of numerical tools with varying fidelity and efficiency to conduct investigations from the aspects of aerodynamic performance, geometric design, and optimization. The results of the research provide new understanding in how to improve aerodynamic performance in both airfoil application types.Item Theoretical Modelling of Transition Metal Oxide Compounds for Application in Electronic and Electrochemical Systems(2022-05-01T05:00:00.000Z) Conlin, Patrick; Cho, Kyeongjae; Leonardi, Stefano; Kim, Jiyoung; Young, Chadwin D.; Fischetti, Massimo V.Density functional theory (DFT) is the predominant methodology for predictive theoretical calculation of material properties used today. It is extensively applied in solid-state physics, chemistry, and materials science to model a wide range of systems on the atomic scale. The popularity of DFT is due in large part to the high-degree of accuracy provided by the methodology, coupled with a relatively low computational cost compared to alternatives like all-electron models. While DFT has been very successful at predicting diverse sets of material properties, there are crucial areas where DFT remains deficient. One of the most notable of these deficiencies is the inability of DFT to accurately describe transition metal compounds. Transition metal compounds are a huge material space with many technological applications. A comprehensive understanding of the physics underlying commonly used computational methods is required in order to best to correct these methods for a given class of compounds. This work begins with a survey of available methodologies for modelling transition metal oxides. Subsequent sections detail the properties of compounds investigated for specific technological application. Particular attention is given to high-mobility p-type semiconductors, solid-electrolytes for Li-ion batteries, and properties of amorphous phases.Item Wall Modeling for Turbulent Flow over Complex Roughness(2018-08) Zhu, Xiaowei; Anderson, William; Leonardi, Stefano; Lu, Hongbing; Minkoff, Susan E.; Qin, ZhenpengTurbulent flow over complex rough surfaces is crucial in both engineering and boundary layer meteorology science. The surface morphology has significant effects on the flow, but it is computationally expensive to solve all the turbulent scales especially when the air flows over complex roughness. Large-eddy simulation (LES) with wall model is therefore employed in this situation. This dissertation focuses on the wall modeling of turbulent flow over complex roughness such as urban topography. As the wall effects can be accurately represented by the equilibrium logarithmic law via roughness length, z0, this dissertation aims to parameterize z0 over complex roughness. In this dissertation, two types of complex roughness are discussed: spatially heterogenous urban-like topography (Chapter 3) and multiscale fractal urban-like topography (Chapter 4). For the spatially heterogeneous urban-like topography, a priori prediction method for z0 based on the statistical moments of surface height is proposed especially for the boundarylayer turbulent flow. Using a posteriori LES results, we demonstrate that the skewness of surface height (as measures of the presence of the extreme value, or the “heavy tail” events) has non-negligible effects, which received less attention as topographic parameters in the past. This finding is reconciled with a model recently proposed by Flack and Schultz (2010) who demonstrate that z0 can be modeled with standard deviation and skewness, and two empirical coefficients (one for each moment). We find that the empirical coefficient related to skewness is not constant but exhibits a dependence on standard deviation over certain ranges. For idealized, quasi-uniform cubic topographies and complex, fully random urbanlike topographies, we demonstrate robust performance of the generalized Flack and Schultz model against contemporary roughness correlations. The multiscale fractal-like topographies pose a particular challenge to numerical simulation schemes since the large-scale elements are resolved, but the small-scale descendant elements cannot be resolved on the computational mesh grid. A local wall model representing the effects of unresolved sub-generation roughness is needed in such scenario. By virtue of selfsimilarity among scales, we develop a methodology and a roughness model for the unresolved scales by learning from the large-scale momentum fluxes. And then the roughness model for the unresolved scales via the equilibrium logarithmic law is established. The research shows that aerodynamic stress associated with descendant, sub-generation scale elements can be parameterized, thus that the turbulent flow over fractal-like geometry can be simulated with only the large generations resolved on the computational mesh grid. The key questions we ask in this dissertation are: How does the spatial heterogeneity affect the transport of the turbulent flow? How to model the sub-generation scales which are smaller than the mesh grid for a fractal topography? The results, and the modeling framework developed herein, have practical implications for the operation of numerical weather prediction models and the initialization of high-resolution solutions.Item Wind Farm Flow and Power Capture: Optimal Design of LiDAR Experiments, Flow Physics, and Mid-fidelity Modeling(2021-12-01T06:00:00.000Z) Letizia, Stefano; Iungo, Giacomo Valerio; Hamlen, Kevin; Griffith, Todd; Jin, Yaqing; Leonardi, StefanoNowadays there is an urgent need for wind farm flow models with increased accuracy and low computational costs for the prediction of turbine performances and wakes. An improvement of current standards of wind farm simulations can be achieved only through a better understanding and modeling of the complex physical mechanisms governing the wind farm aerodynamics. Low computational requirements are necessary to enable large amount of simulations needed for the optimal design, real-time monitoring and online control of wind power plants. To this aim, a holistic research project has been conceived and implemented, which is the focus of this Ph.D. thesis. The adopted research strategy includes three main tasks: i) optimal design and execution of field experiments for monitoring wind-farm operations through scanning LiDAR, meteorological and SCADA data; ii) statistical analysis of LiDAR field measurements for probing wake evolution, wake interactions, effects of atmospheric stability, and flow distortions due to topography; iii) development of a data-driven RANS model for accurate and low-computational-cost simulations of wind farm operations. This research project has enabled quantifying and modeling effects on wind farm operations connected with the turbine aerodynamics and the atmospheric stability regime, and detecting the occurrence of topography wakes, which are flow regions with reduced wind speed and enhanced turbulence intensity being detrimental for wind turbines installed on complex terrains. The main deliverables of this project are the LiDAR Statistical Barnes Objective Analysis (LiSBOA), a tool for the optimal collection and statistical characterization of LiDAR measurements, and the Pseudo-2D RANS (P2D-RANS) wind farm model, which has been recently distributed among several industrial partners and with the intended uses of simulating and monitoring the operations of several wind power plants.