Browsing by Author "Griffith, D. Todd"
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Item An Efficient 3D Mathematical Model to Predict Structural Dynamics and Chatter in Cold Rolling Mills(2022-05-01T05:00:00.000Z) Patel, Akash; Lee, Jeong-Bong; Malik, Arif; Griffith, D. Todd; Park, Wooram; Qian, Dong; Zipf, MarkThe research described in this dissertation aims to provide a highly efficient predictive computational tool to improve the dimensional quality of cold rolled metal strip, particularly for high-value, thin specialty alloys. The corresponding objectives of this work are to (1) understand the transfer of high-fidelity roll grinding errors that may generate complex geometric defects on the strip, and to investigate a novel method for correction of such defects; and (2) develop a highly efficient 3D dynamic predictive model for the time-history of mill and strip transient behavior, including highly damaging chatter vibrations. First, a novel approach that can potentially correct for high-fidelity geometric defects in cold rolled strip is proposed. High- fidelity flatness defects in thin cold-rolled strip that arise from highly localized thickness strain variations present an ongoing challenge to the metals industry. A primary cause of such defects, based on rolling practice, but for which the effects have not been rigorously investigated, may be the transfer of localized diameter deviations from the work rolls that arise from roll grinding errors due to grinding performance inaccuracies. The proposed research addresses the effects of high-fidelity roll diameter deviation transfer to the strip, as well as their correction. Parametric case studies are first undertaken using a 4-high mill to investigate the influences that roll diameter, strip reduction, strip width, and material strength have on the 3D transfer of high- fidelity work roll diameter deviations to the rolled sheet. The studies are conducted with an efficient 3D mathematical roll-stack model that predicts the associated high-fidelity strip thickness profile deviations using the simplified-mixed finite element method (SM-FEM). Reduction deviations, which strongly correlate to strip flatness/shape defects, are first quantified and analyzed to understand the transfer characteristics of localized work-roll grinding deviations relative to benchmarked perfectly smooth work rolls. Results of the study reveal that the high- fidelity transfer depends not only on the specific roll grinding deviation amplitude and mill loading, but also on the location of the roll diameter deviations along the roll face length due to non-negligible 3D bulk roll-stack deformations, as well as the effective stiffness ratio between the work roll and the strip. The inability of conventional flatness control devices to correct for high-fidelity roll diameter deviations is also demonstrated. Based on this work, suggested is a novel corrective approach to identify customized work roll grinding profiles that are tailored to strip with specific pre-existing high-fidelity defect patterns generated in previous rolling passes. Using the described SM-FEM modeling technique, high-fidelity “corrective” roll diameter profiles could eventually be applied in-situ during rolling, whereby the profiles are “engineered” to account for the predicted 3D mill deflections, contact force distributions, and coupled micro/macro scale deformation mechanics. Following investigation of the SM-FEM formulation to high-fidelity static problems involving the transfer of roll grinding error to the strip, the SM- FEM method is adapted to create a general purpose 3D structural dynamics model capable of predicting the transient behavior of the mill components and strip thickness profile geometry. Over the last three decades, computational models have been developed and employed in effort to understand dynamic disturbances in the rolling operation. Such disturbances can potentially lead to self-excitation (chatter vibrations) and result in significant gauge variations in the exit strip, as well as strip rupture and/or damage to the mill in extreme cases. Numerous challenges exist, however, in adequately modeling the 3D dynamic behavior. For instance, highly coupled relationships exist between several rolling process parameters, including the rolling force/torque, strip entry/exit tensions, rolling speed, roll gap profile, friction, neutral point, etc. In addition, both “hard” and “soft” nonlinearities are present, including continuous changes in the roll/strip contact conditions, elastic-plastic deformation of the strip, and nonlinear elastic flattening the rolls. These factors make it very difficult to effectively and efficiently model the rolling operation even under a quasi-static (or steady-state) assumption. Moreover, the existing models that account for structural dynamics of the rolling process exploit many simplifications, such as modeling the mill structure as linear lumped parameter system, and symmetry in the motions of rolls, among other assumptions. Even with these assumptions, the current state-of-the-art models are generally not capable of accommodating conventional strip thickness profile/flatness control mechanisms, such as roll bending, roll shifting, or non-uniform machined roll profiles, which severely restricts the flexibility/applicability to accommodate complex mill structures. A 3D general purpose dynamic model that can incorporate profile/flatness control mechanisms in addition to complex mill configurations (like 12-high or 20-high cluster mills) can provide significant new insights into rolling dynamics and chatter investigations. Accordingly, the presented research combines the static SM-FEM mathematical formulation with a Newmark- Beta time integration technique to develop a highly-efficient, stable, high-fidelity global-stiffness based transient structural dynamics model. Case studies are carried out to demonstrate the features and capabilities of the presented model in addressing the aforementioned research gaps and challenges. Based on the results, the presented structural dynamics model is able to efficiently capture time histories due to discrete disturbances on both vertical and cluster-type mill configurations. For chatter investigation, however, an appropriate roll-bite model to capture the relationships among the coupled rolling process parameters that influence the roll-bite contact mechanics is required to be coupled to accommodate the real time interactions between the structural dynamics and roll-bite mechanics. In addition to the aforementioned assumptions and simplifications in mill structural dynamic modeling, presence of both hard, and soft non- linearities as well as material non-linearities in the roll bite contact mechanics has led to exclusive use of linearized relationship in the current state-of-the-art chatter models, while 3D chatter models are non-existent in the literature. Accordingly, following the development of general-purpose 3D dynamic model, the dynamic simplified mixed-finite element method (D- SM-FEM) is coupled with a roll-bite process model. A critical part in replicating the dynamic interaction between the rolling process and the mill structural dynamics in this work relates to real-time variations in the “working point” or “operating point” relationship between specific rolling force and the plastic strain of the rolled strip which changes according to perturbations in the roll gap, position of entry/exit plane, entry/exit velocity, and tensions. These variations in the working point are incorporated in the presented chatter model via the concept of a “dynamic strip modulus” based on the secant (or tangent) relationship between the specific rolling force and plastic strain at the working point, but where the strip modulus is updated at every time-step. Case studies are presented using 4-high mill with aim to demonstrate the ability of the presented 3D chatter model to address the lack of available chatter models in literature employing 3D bulk body deformation effects, and to address some of the limitations identified above. The capabilities of the model to predict the stability, or dynamic instability is also illustrated with accompanying 3D plots showing the true mode shapes. Case studies are also undertaken to demonstrate the effect of asymmetric (with varying lower housing stiffness) mill stand assumptions. The results reveal interesting phase relationships not elucidated in previous research, as well as detailed effects from the 3D modeling on the strip profile and shape/flatness.Item Assessment of Flutter Prediction and Trends in the Design of Large-Scale Wind Turbine Rotor Blades(Published under licence by IOP Publishing Ltd.) Griffith, D. Todd; Chetan, Mayank; Griffith, D. Todd; Chetan, MayankWith the progression of novel design, material and manufacturing technologies, the wind energy industry has successfully produced larger and larger wind turbine rotor blades while driving down the levelized cost of energy (LCOE). Though the benefits of larger turbine blades are appealing, larger blades are prone to instabilities due to their long and slender nature, and one of the concerning aero-elastic instabilities of these blades is classical flutter. In this work we assess classical flutter prediction tools for predicting flutter speeds in the design of large blades. Flutter predictions are benchmarked against predictions of previous studies. Then, we turn to the main focus of the study, which is design to mitigate flutter. Trends in flutter speeds and flutter mode shapes are examined for a series of 100-meter blade designs. Then, a sensitivity study is performed to assess the impacts of blade design choices (e.g. materials choice and material placement) on flutter speed in a redesign study of a lightweight 100-meter blade with small flutter margin. A new design is developed to demonstrate the ability to increase the flutter speed while reducing blade mass through structural design.Item Characterization and Modeling of Mechanical Behavior of Sandwich Composites and 3D-printed Polymers at Meso and Nano Scales(August 2022) Cao, Dongyang; Lu, Hongbing; Haas, Zygmunt; Griffith, D. Todd; Qian, Dong; Li, WeiThis dissertation covers the topics of mechanical characterizations of sandwich composites and 3D printed polymers which can potentially be used in the wind turbine blades. Chapter 1 is studying the localized viscoelastic properties at the skin-core interphase of a sandwich composite using a viscoelastic nanoindentation technique. The skin-core interphase stiffness is one of the most important design factors in the sandwich composite. However, the stiffness distribution on the interphase remains unclear. Chapter 2 is studying the effects of resin uptake on the mechanical properties of sandwich composites under bending conditions. Improper resin uptake could lead to the resin starvation issue in the matrix or interphase and decrease the stiffness of the skin. So, defining an adequate range of the sandwich composite is essential in the design and optimization of the sandwich composite. And the effect of resin uptake on the sandwich composite has rarely been studied. Chapter 3 is studying the bonding quality of the displacement-controlled resistance welded; adhesive-bonded slender 3D-printed PLA beams through three-point bending experiments. The 3D printed polymer beams (e.g.; PLA, ABS, etc.) can act as the core material of the sandwich composite. However, due to the limitation of the building volume of the conventional 3D printer, a good bonding technique is needed to extend the volume of the structure with satisfactory structural integrity. How sufficient the 3D printed polymer is bonded together needs further investigation. Chapter 4 is studying the in-plane and out-of-plane shear strength and stiffness of the PVC foam core, balsa core, and 3D printed core sandwich composites by three- point bending and four-point bending tests. The core material of the sandwich composites plays a critical role in the structural integrity of the wind turbine blade. The 3D print process possesses a lot of advantages compared with the traditional manufacturing routine. However, whether the 3D- printed engineering core has the potential to replace the conventional core materials needs further attention. Chapter 5 is studying the annealing effect on the flexural strength and modulus, and the bonding quality of adhesive bonded, and thermoformed 3D-printed PLA beams and 3-inch chords by three-point bending experiments. The Fused Deposition Modeling (FDM) possess is one of the suitable manufacturing processes for the small-scale and thermoplastic wind turbine blades. However, what types of 3D printing parameters are suitable for thermoplastic wind turbine blade fabrication is still not clear. Also, the engineering structure fabricated by the FDM process usually needs to strengthen the deposited layer adhesion to increase its durability. Whether the annealing treatment is beneficial to the durability still needs further investigation. Debonding at the core–skin interphase region is one of the primary failure modes in sandwich composites under shear loads. As a result, the ability to characterize the mechanical properties at the interphase region between the composite skin and core is critical for design analysis. This work intends to use nanoindentation to characterize the viscoelastic properties at the interphase region, which can potentially have mechanical properties changing from the composite skin to the core. A sandwich composite using a polyvinyl chloride foam core covered with glass fiber/resin composite skins was prepared by vacuum-assisted resin transfer molding. Nanoindentation at an array of sites was made by a Berkovich nanoindenter tip. The recorded nanoindentation load and depth as a function of time were analyzed using viscoelastic analysis. Results are reported for the shear creep compliance and Young’s relaxation modulus at various locations of the interphase region. The change of viscoelastic properties from higher values close to the fiber composite skin region to the smaller values similar to the foam core was captured. The Young’s modulus at a given strain rate, which is also equal to the time-averaged Young’s modulus across the interphase region was obtained. The interphase Young’s modulus at a loading rate of 1 mN/s was determined to change from 1.4 GPa close to composite skin to 0.8 GPa close to the core. This work demonstrated the feasibility and effectiveness of nanoindentation-based interphase characterizations to be used as an input for the interphase stress distribution calculations, which can eventually enrich the design process of such sandwich composites. Resin uptake plays a critical role in the stiffness-to-weight ratio of wind turbine blades in which sandwich composites are used extensively. This work examines the flexural properties of nominally half-inch thick sandwich composites made with polyvinyl chloride (PVC) foam cores (H60 and H80; PSC and GPC) at several resin uptakes. We found that the specific flexural strength and modulus for the H80 GPC sandwich composites increase from 82.04 to 90.70 (𝑘𝑘𝑘𝑘 ∙ 𝑚𝑚) ⁄ 𝑘𝑘𝑘𝑘 and 6.03 to 7.13 (𝑀𝑀𝑘𝑘 ∙ 𝑚𝑚) ⁄ 𝑘𝑘𝑘𝑘, respectively, with 11.0 % resin uptake reduction, which stands out among the four core sandwich composites. Considering reaching a high stiffness-to-weight ratio while preventing resin starvation, 32 to 38 % and 40 to 45 % resin uptakes are adequate ranges for the H80 PSC and GPC sandwich composites, respectively. The H60 GPC sandwich composites have lower debonding toughness than H60 PSC due to stress concentration in the smooth side skin-core interphase region. The failure mode of the sandwich composites depends on the core stiffness and surface texture. The H60 GPC sandwich composites exhibit core shearing and bottom skin-core debonding failure, while the H80 GPC and PSC sandwich composites show top skin cracking and core crushing failure. The findings indicate that an appropriate range of resin uptake exists for each type of core sandwich composite, and that within the range, a low-resin uptake leads to lighter blades and thus lower cyclic gravitational loads, beneficial for long blades. Fused deposition modeling (FDM) is often used in additive manufacturing of materials such as thermoplastics and metals. Due to the size limitation in the building volume of an FDM system, fusion joining two or more FDM printed parts allows upscaling to manufacture larger structures by the assembly of printed subcomponent parts. In this work, a displacement-controlled joining process was used, rather than a force-controlled process previously reported. The bonding quality of the resistance welded, adhesive bonded slender polylactic acid (PLA) beams was investigated using three-point bending experiments. The slender beams were fabricated by FDM with a 0.3 mm layer thickness. Several infill patterns were explored, and it was determined that the 3D-triangular infill pattern gives the highest flexural properties. Three types of metal mesh, namely 30% (open area fraction)/ 0.11 mm (open size) Ni-Cu, 34%/0.07 mm Ni-Cu, and 36%/0.25 mm Co-Ni, were used as the heating elements. The micrographs of the PLA slender beams resistance welded by the three types of metal meshes show that there are no voids formed in the interphase region. Process parameters were varied, including the power output, mesh opening size, wire diameter, wire resistivity, initial joining pressure, displacement rate, and total displacement traveled. The mechanical properties of the resistance welded beams are compared with those of the corresponding beams printed continuously on FDM. Optimum process parameters were determined for the configuration investigated. In general, a smaller opening size, smaller wire diameter, and higher wire electrical resistivity are preferred in the resistance welding under a low current to reach a higher bonding quality. A higher wire diameter with a larger opening size yields a higher flexural modulus. The flexural strength does not depend on the types of materials used for the Joule heating meshes; in addition, it does not depend on the mesh opening area fraction and wire diameter. The beam samples joined by Ni/Cu mesh (34%/0.07 mm Ni-Cu metal mesh) possesses 96%, 94%, and 88% of the flexural strength, modulus, and maximum allowable strain, respectively, of the one continuous FDM printed sample; this sample gives the flexural properties closest to the continuously printed sample. The adhesive, cohesive, substrate and opening crack failure modes were captured for the resistance welded, adhesive bonded, and continuously printed slender beams. The substrate failure mode is the most desired failure mode which correlated to the highest bonding strength. Surface strain concentration was found in the bonding region. The normal strain dominates when the flexural load is applied to the continuously printed sample. Whether the 3D printed artifacts with the Fused Deposition Modeling (FDM) process remains further investigation in structural engineering applications, especially in wind turbine industries. In this work, the effect of resin uptake on shear strength and stiffness of compression-molded H60 PVC foam core, end-grain balsa core, and PLA lattice core sandwich composites were systematically studied through three-point and four-point bending tests. The surface strain and failure modes were investigated by DIC and fractured images. The skin/core bonding quality is shown to affect the in-plane and out-of-plane shear strength and stiffness of the 3D printed lattice core sandwich composites. The PLA filament is chosen for the 3D printed core material with sufficient strength to weight ratio and cost-effectiveness. The out-of-plane and in-plane shear strength and stiffness of two types of core sandwich composites were investigated by three-point bending and four-point bending tests. In the 3D printed core sandwich composites, based on the information on specific out-of-plane shear strength and stiffness, the optimized resin uptake regime of PLA core sandwich composites is from 20.43% to 22.86%. In the PLA core sandwich composites, the specific out-of-plane shear strength and stiffness increase from 19.22% to 20.43% of resin uptake, and then decrease from 22.86% to 25.79%. Both low and high amounts of resin in the interphase region of the 3D printed core sandwich composites lead to low bonding strength. The in-plane shear strength and stiffness of three types of core sandwich composites were computed based on the out-of-plane shear strength and stiffness. The specific in-plane shear stiffness of 3D printed and raw balsa core sandwich composites are similar. The UV light curing epoxy-based agent coated balsa core sandwich composites possess 8.19% and 15.16% higher specific in-plane shear stiffness than 3D printed core sandwich composites within all resin uptake regimes. The UV light curing epoxy-based agent coated balsa core sandwich composites possess 9.81% and 17.44% higher specific in-plane shear stiffness than 3D printed core sandwich composites within an optimized resin uptake regime. The 3D printed core stands out among conventional PVC core materials and is similar to conventional balsa core materials in shear and specific shear strength and stiffness. As a result, this work presents some new and important findings that support the greater use of additive manufacturing of core materials in applications such as wind turbine blades. A few subjects still need to be studied in the future regarding the PLA lattice core sandwich composites, including the performance under high cycle fatigue, debonding toughness in the interphase region, and strategies for the interphase stiffness enhancement. Small-scaled additively manufactured wind turbines give a solution for power generation in rural areas and large-scale wind turbines performance prediction by on-site construction, hence reducing the construction error and impact on the climate change. This work mainly investigates the effects of annealing on the structural performance of 3D printed short beams and 3-inch chords (smooth, adhesive bonded, thermoformed) by three-point bending experiments. The effect of vertical shells, horizontal shells, bed temperature, nozzle temperature, and printing resolution was systematically studied through three-point bending experiments. The 3D printed beams with 2 vertical shells and 4 horizontal shells possess the highest specific flexural modulus (4.81 × 10 3 ± 37.24 𝐺𝐺𝐺𝐺𝐺𝐺 (𝑘𝑘𝑘𝑘 𝑚𝑚 3⁄ )⁄ ). The 3D printed beams with 3 vertical shells and 3 horizontal shells possess the highest specific flexural strength (105.17 ± 1.17 𝐺𝐺𝐺𝐺𝐺𝐺 (𝑘𝑘𝑘𝑘 𝑚𝑚 3⁄ )⁄ ). The 60 °C bed temperature and 215 °C nozzle temperature achieve the best-deposited layers adhesion quality. The 0.6 mm nozzle diameter is an adequate resolution to 3D print a small-scale wind turbine blade with satisfactory efficiency and complexity. The effect of the annealing time on the 3D printed short beams and 3- inch chord were studied by three-point bending experiments. The 70 °C with 0.5 hours is an adequate annealing plan for a constant temperature annealing treatment due to a relatively small shrinkage under a unit heating time by a higher temperature for the smooth and adhesive bonded type 3D printed structure. For the thermoformed 3D printed structure, no annealing treatment is needed for maximizing the strength to weight ratio, annealing treatment is suggested for maximizing the modulus to weight ratio. The thermal buckling and edge wrapping were captured under the 70 °C/0.5 hour annealing treatment for 2 vertical shells and 4 horizontal shells 3D printed beams. So, a sufficient thickness of the vertical side should be achieved to encounter the thermal buckling and edge wrapping. Annealing treatment is not suitable for the 3-inch chord due to low flexural strength, flexural stiffness improvement, and a high level of volumetric shrinkage on the lengthwise.Item Data-driven Predictive Models for Manufacturing Glass Fiber Composites and 3D-printed Metals Using Neural Networks and X-ray Imaging(May 2023) Zhang, Runyu 1994-; Lu, Hongbing; Hansen, John H. L.; Li, Wei; Ryu, Ill; Qian, Dong; Griffith, D. ToddAdvanced manufacturing requires a close monitoring of process parameters, and real-time control for rapid response to fine-tune the process conditions to produce high-quality products. While multi-physics models provide high-fidelity simulation results, the computational time involved prohibits from those to be used directly for feed-back control. The physics-based trained data- driven models have the capabilities to replicate the multiphysics simulation results at the fast speed required for design optimization and process control. The data-driven models take inputs such as component geometry, process parameters, material properties, and output outcomes in the components including temperature profile, residual stress, microstructural evolution, and material property distribution. This investigation will focus on developing data-driven models for two specific manufacturing processes, namely vacuum-assisted resin infusion molding (VARIM) and wire arc additive manufacturing (WAAM). The data-driven predictive models are established using deep machine learning (ML). Several ML models are implemented, including deep convolutional neural network (CNN), for processing spatial information; recurrent neural network (RNN), and long short-term memory (LSTM) for processing temporal information. The manufacturing of large wind turbine blades requires well-controlled processing conditions to prevent defect formation such as thermal waves. The VARIM process is the most prevalent method implemented in the industry and is often studied and optimized using the physics-based finite element models that provide accurate computational capabilities but suffer from high computational costs in the meantime. Considering the limitations, an ML approach that employs a deep CNN and RNN/LSTM model is established to predict the spatial-temporal temperature distribution during the VARIM process. The ML model is trained with the “big data” that are generated from the physics-based high-fidelity simulations, validated by a lab-scale VARIM experiment conducted in the factory setting. Once fully trained, it can provide “real time” predictions of the blade manufacturing process. Powder-based additive manufacturing (AM) process, such as direct energy deposition (DED), is widely used in fabricating metallic functional gradient materials (FGM) parts, which have mechanical properties changing with locations in a part, since multiple metal powders are mixed and used in the DED process. Hybrid manufacturing, including the DED and machining processes, to fabricate stainless steel 316L/Inconel 718 FGM specimens are experimentally studied. The molten pool evolution during the printing is observed; influences of the machining process on the printed parts due to milling, including the surface roughness, and the hardness of the specimens are evaluated. Towards the goal of sustainability and eco-friendly process for manufacturing, the wire-feed-based AM process using WAAM provides porTable freeform fabrication capability, along with precision manufacturing at both small- and large-scales. However, internal defects such as porosities are often formed in additively manufactured metal components, the defects will nucleate, grow, and coalesce to form cracks under loads, leading to eventual catastrophic failure. To understand this failure process under loading, full-field porosity evolution in a WAAM aluminum alloy cylinder under tension is observed with in-situ X-ray micro-computed tomography (μCT). The analysis is performed with the assistance of a CNN algorithm that provides rapid analysis of over thousands of slice images at various strains. The results provide quantitative evaluations of the evolution of macropores inside the WAAM specimen under tension.Item Design of Large Wind Turbine Rotors Through Passive and Active Load Mitigation Strategies(2022-05-01T05:00:00.000Z) Chetan, Mayank; Griffith, D. Todd; Guo, Xiaohu; Rotea, Mario A.; Malik, Arif; Jin, YaqingWind energy over the years has positioned itself to become a primary source of renewable energy and this is attributed to the reduction in the Levelized Cost of Energy (LCOE). Historically, this is accomplished by an increase in tower heights which allow access to higher wind speeds, and also by increasing rotor diameters which allow for more power capture. However, there are significant challenges that come with these large turbines like aeroelastic instabilities of the blades due to their long and slender nature, and the need for more robust turbine components that can withstand the larger loads associated with large turbines. This has motivated the development of design strategies that incorporate different methods of load alleviation to achieve optimized wind turbine designs that can result in lower LCOE. This dissertation presents various methods of designing wind turbine rotors that take advantage of passive and active load reduction strategies. First, classical flutter is addressed for the design of large blades. Flutter is an aeroelastic instability that contributes to fatigue damage, or in the worst case can least to sudden catastrophic turbine failure. A comprehensive evaluation of flutter behavior including classical flutter, edgewise vibration, and flutter mode characteristics for two- and three-bladed wind turbine blade designs is carried out. Further, a study is performed to evaluate mitigation of flutter in the design process via structural redesign by evaluating the effect of leadingvi edge and trailing edge reinforcement on flutter speed and hence demonstrates the ability to increase the flutter speed and satisfy structural design requirements (such as fatigue) while maintaining or even reducing blade mass. This flutter structural mitigation study is conducted for two wind turbine designs, one a two-bladed rotor and the other a three-bladed rotor. Second, a new rotor design methodology is developed to integrate active load control in the form of controllable gurney flaps. A comprehensive sequential iterative design procedure is developed that integrates aerodynamics, structural, and baseline turbine control system design with advanced active load control into a design process. This procedure also takes into account the contribution of loads on all major components of the turbine. To realize the best LCOE reduction solution, new methods to evaluate blade structural properties are developed wherein, the reductions in damage equivalent loads (i.e.; fatigue loads) due to a generic active load control system are mapped to structural design improvements in terms of blade mass reduction, cost reduction in other major turbine components, and LCOE reductions that result from integration and redesign of the turbine with the active load control system. Third, using the design methodologies established, newer rotor designs with a larger rotor radius are explored to examine the impacts of the active load control system. These rotors take advantage of the fatigue load reductions due to the controllable gurney flaps integrated into the design. The effect of the controllable gurney flaps is evaluated for various blade and non-blade component loads on the turbine. This methodology results in larger rotors that have increased energy capture and reduced LCOE’s. For this study, two different turbine operating strategies are followed, one limits the turbine power to that of the baseline while the other allows the turbine to extract more power at higher wind speeds. Finally, a method is introduced to support the realization of new passive and active load mitigation strategies by improving prototype wind turbine development. A novel method of developing a multi-fidelity digital twin structural model of a wind turbine blade is presented. The digital twin model development methodology, presented herein, involves a novel calibration process to integrate a wide range of information including design specifications, manufacturing information, and structural testing data (modal and static) to produce a multi-fidelity digital twin structural model: a detailed high-fidelity model (i.e., 3D FEA) and consistent beam-type models for aeroelastic simulation. Digital twin models are useful to cost-effectively evaluate the performance of new technologies in the field like novel downwind rotors and controllable gurney flaps. Finally, the new methodology is demonstrated for an as-built two-bladed downwind prototype rotor resulting in a multi-fidelity digital twin model which has a 1% match in mass properties, 3.2% in blade frequencies, and 6% in deflection to the as-built blade. The rotor examined is the SUMR – Demonstrator (SUMR-D), which was installed on the Controls Advanced Research Testbed (CART-2) wind turbine at the National Wind Technology Center. The digital twin model developed here was utilized to design controllers to safely operate SUMR-D in field tests, which are providing additional data for further evaluation and development of the multi-fidelity digital twin structural model.Item Integrated System Design for a Large Wind Turbine Supported on a Moored Semi-Submersible Platform(MDPI AG, 2018-10-22) Liu, J.; Thomas, E.; Manuel, L.; Griffith, D. Todd; Ruehl, K. M.; Barone, M.; Griffith, D. ToddOver the past few decades, wind energy has emerged as an alternative to conventional power generation that is economical, environmentally friendly and, importantly, renewable. Specifically, offshore wind energy is being considered by a number of countries to harness the stronger and more consistent wind resource compared to that over land. To meet the projected “20% energy from wind by 2030” scenario that was announced in 2006, 54 GW of added wind energy capacity need to come from offshore according to a National Renewable Energy Laboratory (NREL) study. In this study, we discuss the development of a semi-submersible floating offshore platform with a catenary mooring system to support a very large 13.2-MW wind turbine with 100-m blades. An iterative design process is applied to baseline models with Froude scaling in order to achieve preliminary static stability. Structural dynamic analyses are performed to investigate the performance of the new model using a finite element method approach for the tower and a boundary integral equation (panel) method for the platform. The steady-state response of the system under uniform wind and regular waves is first studied to evaluate the performance of the integrated system. Response amplitude operators (RAOs) are computed in the time domain using white-noise wave excitation; this serves to highlight nonlinear, as well as dynamic characteristics of the system. Finally, selected design load cases (DLCs) and the stochastic dynamic response of the system are studied to assess the global performance for sea states defined by wind fields with turbulence and long-crested irregular waves.Item New Methods for Digital Twin Modelling of Wave and Wind Energy Systems(2020-08) Haus, Liliana C.; 0000-0001-5145-5628 (Haus, LC); Griffith, D. ToddRenewable energy is a rapidly growing sector within the realm of electricity generation. While the field itself is incredibly broad, wind and wave energy are the primary focus of this thesis. Wind turbines have existed for hundreds of years, with their first application to electricity production occurring over 100 years ago. Wave energy, on the other hand, is a comparatively new field, having been seriously explored within only the past 20 years. However, both sectors are intimately related to the field of structural dynamics, and the goal of producing electricity at a rate competitive with traditional sources of energy plays a large role in the development of new technologies in these fields. In this respect, validated design tools are one of the key needs for achieving (in the case of wave energy) and maintaining (in the case of wind energy) a market-competitive levelized cost of energy (LCOE). In this thesis, new methods for developing digital twin models based on dynamic modelling and multiple uses of the resulting models are derived, detailed, and discussed for both wave and wind energy systems. Wave energy is the focus of the first half, in which a coupled dynamics model of a point absorber-style wave energy converter (WEC) and the bridge to which it is mounted is developed and validated against experimental data. The primary benefits of the resulting model are its simplicity and decreased simulation time relative to other available WEC models while still providing an acceptable degree of accuracy. Such a model would be particularly useful in the realm of controls system development, which will become increasingly important in the field for larger-scale devices. The model is then utilized to explore the relationship between power production and fatigue damage in the realm of wave energy. This issue is of great importance in the young field of wave energy as the feasibility of WECs lies in their ability to produce power at a sufficiently low LCOE. The second half of this thesis focuses on wind energy and a novel technique for developing an aero-structural digital twin of an existing utility-scale 1.5MW wind turbine. In this technique, experimental data from an operating wind turbine is used to calibrate the properties of a baseline turbine model to represent the dynamic behavior of a target wind turbine. The blade aerodynamics and structural dynamics are the primary focus of this technique, in which the power curve and thrust coefficient data of an experimental turbine is used to calibrate the blade aerodynamic properties of the model, and experimental blade total mass, center of gravity, and natural frequencies are used to calibrate the model blade mass and stiffness property distributions, respectively. A primary benefit of this methodology is its relative ease of implementation in the creation of models of multiple similar turbines (such as those in a fleet) once a baseline model is in place. In short, the overarching goal of this thesis is to provide and explore two unique dynamic model development methodologies for existing renewable energy systems (i.e., digital twins) and explore their potential benefits in the realm of LCOE reduction.Item New Models for Flutter and Edgewise Instability Analysis of Vertical and Horizontal Axis Wind Turbines for Land-based and Floating Offshore Conditions(August 2023) Ahsan, Faraz; Griffith, D. Todd; Ng, Yu Chung Vincent; Jin, Yaqing; Koeln, Justin; Li, YaoyuWind energy is a vital part of renewable energy sector that is increasingly becoming popular to reduce the adverse effect of traditional power production methods in increasing the global temperature. As the demand for wind energy increases, the sizes of the blades of wind turbines are also increasing with the availability of novel materials and manufacturing techniques. On the other hand, these very large wind turbines might be susceptible to design challenges and instability problems because of their sheer size which typically are not concerns for relatively smaller turbines. This has motivated the development of models to predict the unstable behavior of very large vertical axis wind turbines (VAWTs) and horizontal axis wind turbines (HAWTs). This work presents modeling method of rotor-platform system for offshore floating vertical axis wind turbines. Effect of structural design parameters on flutter instability of 2-bladed and 3-bladed VAWTs are studied. An analysis is presented on the effect of floating platform on flutter behavior of rigid body and flexible modes of vibration of the coupled system. A fundamental understanding of how the floating system impacts the resonance and flutter properties of VAWT is sought and presented. Further study has been performed on the impact of aerodynamic modeling assumptions that are conventionally implemented to predict flutter of wind turbines. The shortcomings of simplifying assumptions of standard aerodynamic theory have been demonstrated, and new aerodynamic model is developed to address those shortcomings. Then, this new model is applied to both horizontal axis wind turbines as well as vertical axis wind turbines. Comparative analysis is done of the effect of standard and new aerodynamic model in terms their predictive capability of flutter for both land-based and floating vertical axis wind turbines. Large number of horizontal axis wind turbines with varying sizes and geometry are studied for flutter and edgewise instability with the newly developed aerodynamic model. Similarly, vertical axis wind turbines are examined with the newly developed aerodynamic model. This study also aims at validating numerical models with experimental results. To achieve that goal, a subscale floating VAWT system is manufactured, and experimental test is performed on it to extract modal dynamic properties. The measured structural properties are used to calibrate the rotor model, and free decay test results are used to generate a floating platform model. Finally, the rotor and platform model are coupled and modal analysis (frequency analysis) is performed and the model is further refined by comparing the test results and model predictions. Key findings of this dissertation confirm that moving a VAWT from land-based to floating configuration has the potential to alleviate both resonance and flutter concerns. Developed new aerodynamic model shows higher flutter prediction of tower, propeller and edgewise modes of land-based and floating VAWT compared to the prediction by standard aerodynamic model. For large HAWT blades, the new aerodynamic model has more impact on 3-bladed case than on 2- bladed case in terms of flutter and edgewise instability RPM prediction. Validation study on modal dynamics of floating VAWT confirm reasonably accurate modeling of coupled rotor-platform floating model.Item Performance Analysis of Large-scale NGSO Satellites-based Radio Astronomy and Sky Radio Quiet Zones(December 2023) Fan, Zhixuan 1993-; Minn, Hlaing; Griffith, D. Todd; Fumagalli, Andrea; Ali, Mohammed Zamshed; Saquib, MohammadLarge-scale non-geostationary orbit (NGSO) satellite communication (SatCom) systems (SCSs) are emerging to play an important role for future global wireless communication. However, satellite communication systems with more than thousands of satellites raise a serious concern of radio frequency interference (RFI) to the ground-based radio astronomy system (RAS). This situation becomes more serious as the SatCom industry is rapidly expanding the number of communicating satellites which increases RFI, while the RAS is also advancing with enhanced radio astronomical observation (RAO) capability requiring better protection against RFI. To address this impending RFI issue, this dissertation focuses on two approaches, namely, space-based RAS and sky radio quiet zone. First, the satellite-based RASs including low earth orbit (LEO)-based or medium earth orbit (MEO)-based RASs are considered to lower the impact from SCSs on the higher orbit onto the ground RASs and the lower orbit RASs. We find out the required SCS emission mask for each RAS so that both systems can avoid RFI. Then, we investigate three typical radio astronomy metrics such as maximum baseline distance (MBD), the number of simultaneously observing telescopes, and the signal to interference plus noise power ratio (SINR) performance for NGSO satellite-based RAS. Additionally, we also explore the advantages of NGSO satellite-based RAS from a communication side. Our analysis shows that the large-scale NGSO satellite-based RAS can offer more spectrum access to both SCS and RAS. Secondly, we analyze RFI from the large-scale NGSO SatCom system to a ground-based RAS to identify dominant RFI contributors and then propose two types of sky radio quiet zone (SRQZ), namely, telescope-centered (TC)-SRQZ and RAO direction-centered (DC)-SRQZ. We investigate peak RFI and average RFI suppression characteristics of the two SRQZ types when applied individually alone and jointly. We evaluate the RFI characteristics with/without SRQZs for a few representative ground RAS receiver locations under a low earth orbit SatCom system as well as a medium earth orbit SatCom system. We present and discuss extensive RFI performance results and their dependency on the specifics of the RFI scenarios. These results show that appropriately designed SRQZs provide significant RFI suppression. We also offer guidance on the choice of SRQZ type/deployment, related parameter settings, and practical implementation aspects.Item Structural Design and Optimization of Sub-scale and Extreme-scale Wind Turbine Rotors(2021-12-01T06:00:00.000Z) Yao, Shulong; Griffith, D. Todd; Cho, Kyeongjae; Qian, Dong; Zhang, Jie; Malik, ArifThe main objectives of this dissertation are to develop some new design and optimization solutions for large/extreme-scale (up to 50 MW) wind turbine rotors, as well as a structural design method for a sub-scale wind turbine blade for manufacturing and field testing. To accomplish these objectives, a series of 13.2 MW downwind rotors is designed and optimized. A key question to enable large rotor designs is how to configure and optimize structural designs to constrain blade mass and cost while satisfying a growing set of challenging structural design requirements. In this dissertation, the performance of a series of three two-bladed downwind rotors with different blade lengths (104.3 meters, 122.9 meters, and 143.4 meters) all rated at 13.2 MW is investigated. The primary goals are to achieve 25% rotor mass and 25% LCOE (levelized cost of energy) reduction. A comparative analysis of the structural performance and economics of this family rotors is presented. To further explore optimization opportunities for large rotors, the new results in a root optimization and a spar cap design study are presented. The structural design solutions that achieve 25% rotor mass reduction in a SUMR13i design (104.3 meters) and 25% LCOE reduction in a SUMR13C design (143.4 meters) are provided. A new sub-scale field-prototype design solution is also developed to realize the dynamics, structural response, and distributed loads (gravitational, aerodynamic, centrifugal) that are characteristics of a full-scale large, modern wind turbine rotor. The challenge lies in producing a structural design meeting two competing objectives: novel scaling objectives that prescribe the sub-scale blade to have low mass and stiffness; and traditional structural safety objectives that drive the design to have high stiffness and mass. A 20% gravo-aeroelastically scaled wind turbine blade is developed successfully that satisfies these competing objectives. First, it achieves close agreements for non-dimensional tip deflection and flap-wise blade frequency (both within 2.1% error) with a blade mass distribution constrained to produce target gravitational and centrifugal loads. Second, the entire blade structure is optimized to ensure a safe, manufacturable solution meeting strict strength requirements for a testing site that can experience up to 45 m/s wind gusts. Next, 50 MW wind turbine rotors with blades’ length over 250 meters are designed and optimized. Key questions in this work include: what is the structural limit for the size of a wind turbine rotor to be feasible or cost-effective, and what are the technologies and approaches needed to achieve large rotors. The largest wind turbine design in prior work is a 25 MW rotor, and here a 50 MW rotor design is considered, the largest ever design with blades’ length over 250 meters, which is 2.5 times the length of a football field. This dissertation shows that a 50 MW design is indeed possible from a detailed engineering perspective and presents a series of aero-structural blade designs for 50 MW wind turbine rotors, and a critical assessment of technology pathways and challenges for such extreme-scale rotors. The rotor design for the 50 MW rotor begins with Monte Carlo simulations focused on optimizing the carbon spar cap design, which is found to be a major cost driver in the blade design. Further, a study of blade root fatigue performance is performed, which is found to be the key limitation for the extreme-scale machine at 50 MW scale. A baseline, initial design results in a 250-meter blade with a mass of 500 metric tons. This initial study indicates a significant opportunity for improvement through the aero-structural design; thus, an aero-structural design and optimization study is performed to reduce the blade mass/cost and achieves more mass/cost-effective 50 MW rotors that result in more than 25% mass reduction and over 30% cost reduction by determining the optimal blade chord and the optimal airfoil thickness for the best aerodynamic and structural performance. Wind turbine blade reliability is critical once blades go into service in operation to avoid costly repairs and lost revenue due to turbine downtime resulting from blade damage. With increased size of the blade, especially for the extreme-scale blades, the blade can experience a more complicated loading. A new method utilizing the panel behavior for structural health monitoring and nondestructive damage detection is examined in this dissertation. The localized panel resonance (panel mode) is identified by the experimental modal test of the BSDS blades at Sandia National Laboratories and numerical analysis of an open-source BSDS design model. Then the study is extended to a larger, novel concept SUMR-D blade design, which was presented in Chapter 3. Some classical wind turbine damage modes are simulated based on the SUMR-D ANSYS model, including shear webs disbonding simulations and trailing edge disbonding simulations. A relation is established to correlate the panel mode with the damage size, which can be applied to structural health monitoring applications. For the shear webs damage cases, a relation is established to correlate the panel mode with panel buckling performance as a function of damage size based on numerical results and analytical formulae, which has potential applications in the nondestructive buckling capacity evaluation.Item Tissue Characterization Using H-scan Ultrasound Imaging(2022-12-01T06:00:00.000Z) Tai, Haowei 1991-; Hoyt, Kenneth; Griffith, D. Todd; Brown, Katherine; Hansen, John H.L.; Tamil, LakshmanBreast cancer is the second leading cause of mortality among women and affects more women than any other type of cancer. Around 43,600 women in the U.S. died in 2021 from breast cancer. Clinical studies have demonstrated that an early neoadjuvant response is a better predictor of the patient’s recurrence-free survival than pathological complete response. Therefore, mammography, ultrasound (US), and magnetic resonance imaging (MRI) have been widely used to determine tumor response by tracking changes in tumor size using guidelines provided by the Response Evaluation Criteria in Solid Tumors (RECIST). However, measurable changes in tumor size may not be detectable until after multiple cycles of chemotherapy. In the interim, high cost and unnecessary patient toxicity may be incurred for therapy regimens. Further, intratumor heterogeneity poses a fundamental treatment challenge because different tumor subregions might have different drug sensitivities. This implies that some therapeutic strategies might not be effective against the whole tumor. Therefore, the use of noninvasive US for quantitative tissue characterization has become an exciting research prospect. Herein the challenge is to find hidden patterns in the US data to reveal more information about tissue function and pathology that cannot be seen in the conventional US images. Circumventing some of the limitations associated with traditional tissue characterization approaches, a new modality has been proposed for the US classification of acoustic scatterers, such as cancer cells. Termed H-scan US imaging, this technique relies on matching a model that describes US image formation to the mathematics of a class of Gaussian-weighted Hermite polynomials. In short, it reveals the local frequency dependence of different sized scatterers in soft tissue. In this dissertation work we demonstrate: (1) application of a novel frequency-dependent attenuation correction technique improves H-scan US imaging sensitivity to subtle changes at tissue depth. (2) propose 3-D H-scan imaging technique to capture data from the entire tumor burden, visualization of any heterogenous tissue patterns, and fundamentally improve any tissue characterization strategy and treatment response determination and (3) propose volumetric H-scan US imaging to visualize breast cancer changes during response to drug treatment including apoptotic activity, which is a hallmark feature of effective anticancer therapy. Our overarching hypothesis is that volumetric H-scan US imaging can detect early response to chemotherapy in breast cancer tumors and provide vital prognostic data on treatment response and tumor progression. Consequently, this would provide a new and safe approach to exploring the tumor response to chemotherapy as early as possible and maximize effective therapy for an individual patient, reduce morbidity, and constrain escalating health care costs associated with overtreatment.