06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7

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Institute: search.filters.UBSInstitut.Institut für FlugzeugbauStart date: 2023

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    Experimental investigation of low-frequency sound and infrasound induced by onshore wind turbines
    (2024) Blumendeller, Esther; Cheng, Po Wen (Prof. Dr.)
    Climate change has a global impact and is increasingly affecting our environment. This is driving the continuous expansion of renewable energies, with wind energy playing a major role. As wind energy becomes more widespread, an increasing number of people will live near wind turbines in complex terrain. In such scenarios, wind turbines are often positioned at elevated locations, while residents live in valleys. In complex terrain, such as a steep escarpment, local turbulence, wind speed, and direction are strongly influenced by topography, contributing to the complexity of sound propagation or impacts the background noise situation in valleys, for example, due to shielding effects. The operation of wind turbines is associated with both visual and sound-related impact, with sound being generated at various frequencies. There is a growing interest in low-frequency sound and infrasound, characterized by long wavelengths that propagate over considerable distances without significant attenuation. This is in contrast to higher-frequency sound, and might increase the impact of wind turbine sound at residential areas located several hundred meters or a few kilometers away from the wind farm. In the context of complex terrain, this work investigates wind turbines in complex terrain as sources of low-frequency sound and infrasound. The investigations on characterization of sound generation and propagation are based on measurements in the vicinity of two wind farms. Measurements were conducted within four measurement campaigns at two wind farms located close to an escarpment at the Swabian Alb in Southern Germany over a period of about nine month. Acoustic data was obtained in the proximity of the wind turbines and at residential buildings in 1–1.7km distance to the wind farms in municipalities located within a valley. Besides acoustic measurements including the infrasonic frequency range, a comprehensive data set with ground motion data, wind turbine operating data, meteorological data and data from a noise reporting app supports the investigation. Two aspects require analysis: Firstly, the aspect of generation and propagation of wind turbine low-frequency sound and infrasound in complex terrain, and secondly, the relation with annoyance. Results show that sounds within the infrasonic range assigned to the blade passage at the tower are transmitted through the air over distances of 1 km. Low-frequency sounds were found to be amplitude-modulated and were investigated as amplitude modulation. Infrasound and amplitude modulation occurrences were more likely during morning, evening and night hours and during atmospheric conditions with positive lapse rate, vertical wind shear and low turbulence intensity. The occurrence of both infrasound and amplitude modulation was typically observed during rated rotational speed but below-rated power. To allow predictions, a standard prediction method was extended to include the lowfrequency sound and infrasound range and adapted to the measurement data in order to apply it to complex terrain. The sound level difference of the measured data aligns well with the predictions within the frequency range of 8 Hz and 250 Hz. Investigations regarding outdoor-to-indoor sound reductions showed influences from structural resonances and room modes, which depend on the characteristics of the building and the specific room under investigation. Combining acoustic measurements with annoyance reports showed that rated wind turbine operation appears to be a contributing factor in annoyance ratings obtained through a noise reporting app, ranging from “somewhat” to “very” levels. Furthermore, the analysis indicates that varying levels of annoyance at a distance of 1km from the wind farm, both outside and inside buildings, do not correspond to significant differences in the averaged and A-weighted sound pressure levels. Overall, this work contributes to a better understanding of the low-frequency sound and infrasound generated from wind turbines and provides insight into the sound characteristics of measured wind turbine sound at residential locations in complex terrains.
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    Electrical conductivity of monolithic and powdered carbon aerogels and their composites
    (2024) Kröner, Jessica; Platzer, Dominik; Milow, Barbara; Schwan, Marina
    The electrical conductivity of powdered carbon aerogels is one of the key factors required for electro-chemical applications. This study investigates the correlation between the structural, physical, mechanical and electrical properties of pure and activated carbon aerogels, as well as aerogel-composites. The thermal activation with carbon dioxide led to higher electrical conductivity and a decrease in density and particle size. Furthermore, the influence of applied force, compressibility of aerogels and aerogel composites on electrical conductivity was studied. A number of different carbonaceous powdered additives with various morphologies, from almost spherical to fiber- and flake-like shaped, were investigated. For two composites, theoretical values for conductivity were calculated showing the great contribution of particle shape to the conductivity. The results show that the conductive behavior of composites during compression is based on both the mechanical particle arrangement mechanism and increasing particle contact area.
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    Control co-design optimization of floating offshore wind turbines with tuned liquid multi-column dampers
    (2024) Yu, Wei; Zhou, Sheng Tao; Lemmer, Frank; Cheng, Po Wen
    The technical progress in the development and industrialization of floating offshore wind turbines (FOWTs) over the past decade has been significant. Yet, the higher levelized cost of energy (LCOE) of FOWTs compared to onshore wind turbines is still limiting the market share. One of the reasons for this is the larger motions and loads caused by the rough environmental excitations. Many prototype projects tend to employ more conservative substructure designs to meet the requirements for motion dynamics and structural safety. Another challenge lies in the multidisciplinary nature of a FOWT system, which consists of several strongly coupled subsystems. If these subsystems cannot work in synergy, the overall system performance may not be optimized. Previous research has shown that a well-designed blade pitch controller is able to reduce the motions and structural loads of FOWTs. Nevertheless, due to the negative aerodynamic damping effect, improvement in the performance by tuning the controller is limited. One of the solutions is adding tuned liquid multi-column dampers (TLMCDs), meaning that there is a structural solution to mitigate this limiting factor for the controller performance. It has been found that the additional damping, provided by TLMCDs, is able to improve the platform pitch stability, which allows a larger blade pitch controller bandwidth and thus a better dynamic response. However, if a TLMCD is not designed with the whole FOWT system dynamics taken into account, it may even deteriorate the overall performance. Essentially, an integrated optimization of these subsystems is needed. For this paper, we develop a control co-design optimization framework for FOWTs installed with TLMCDs. Using the multi-objective optimizer non-dominated sorting genetic algorithm II (NSGA-II), the objective is to optimize the platform, the blade pitch controller, and the TLMCD simultaneously. Five free variables characterizing these subsystems are selected, and the objective function includes the FOWT's volume of displaced water (displacement) and several motion and load indicators. Instead of searching for a unique optimal design, an optimal Pareto surface of the defined objectives is determined. It has been found that the optimization is able to improve the dynamic performance of the FOWT, which is quantified by motions and loads, when the displacement remains similar. On the other hand, if motions and loads are constant, the displacement of the FOWT can be reduced, which is an important indication of lower manufacturing, transportation, and installation costs. In conclusion, this work demonstrates the potential of advanced technologies such as TLMCDs to advance FOWTs for commercial competitiveness.
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    Quantification of amplitude modulation of wind turbine emissions from acoustic and ground motion recordings
    (2023) Blumendeller, Esther; Gaßner, Laura; Müller, Florian J. Y.; Pohl, Johannes; Hübner, Gundula; Ritter, Joachim; Cheng, Po Wen
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    Graph transformation in engineering design : an overview of the last decade
    (2023) Voss, Christopher; Petzold, Frank; Rudolph, Stephan
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    Thermomechanical analysis of thermoplastic mono-material sandwich structures with honeycomb core
    (2024) Latsuzbaya, Temuri; Middendorf, Peter; Voelkle, Dietmar; Weber, Christoph
    The application of fiber-reinforced thermoplastic mono-material sandwich panels has many advantages, such as recyclability, reduction in processing cycle times, integration of additional elements by means of welding, and a great potential for in-line production. The most efficient way to produce a curved thermoplastic sandwich panel is thermoforming, which has several challenges. One of them is to achieve a higher thermal gradient in the panel. On the one hand, the temperature at the skin-core interface must exceed the softening point of the polymer to reach a sufficient bonding degree. On the other hand, the core should not be overheated and overloaded to avoid its collapse. Furthermore, several fiber distortions, such as wrinkles or buckles, can be developed during thermoforming. All these flaws have a negative impact on the mechanical performance of the sandwich structure. The objective of this study is the development of a simulation tool for the thermoforming process, which can replace the time-consuming trial-and-error-based method. Therefore, a coupled thermomechanical model was developed for a novel thermoplastic sandwich structure, which is able to predict the temperature distribution and its influence on the mechanical properties of the panel. Experimental trials were conducted to validate the thermomechanical forming model, which demonstrated a good agreement with numerical results.
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    Modelling and analysis of electro-mechanical interactions in wind turbines
    (2024) Lüdecke, Fiona Dominique; Cheng, Po Wen (Prof. Dr.)
    The contribution of wind energy to the energy transition is steadily increasing. This growing contribution is driven by two aspects: the construction of new turbines and the increase in turbine size. In particular, for offshore sites, the nominal power of new turbines is now up to 16 MW and rising. Two main drive-train concepts are used, the geared and the direct-drive. Direct-drives, the focus of this work, have a characteristically low speed in order to limit the blades' tip speed. This results in large generator diameters, which have reached around 10 m. Scaling laws show that structural support mass grows faster than active mass, contributing to power generation. Therefore, new design methods for mass reduction are desired. The generator design is optimised based on given input loads and must maintain the air gap between the rotor and stator at all times. Typically, this is achieved by very high main bearing and generator support structure stiffness requirements, limiting mass reduction potential. In this work, it is assumed that designing wind turbines based on component optimisation does not ensure the best system design. However, moving to a more system-oriented approach requires new, holistic modelling techniques to simulate the wind turbine system, including electromagnetic forces from the generator. The required system model is derived in this thesis by adding a radial degree of freedom to the state-of-the-art wind turbine model. Two generator models of different fidelity are coupled to the wind turbine model, an analytical and a finite element model. Influences of the model adaptations on the system behaviour are identified. Structural component interactions are analysed and the effects of modelling on interactions with the aerodynamic solver and controller are investigated. The results show that lower system modes can be affected in their natural frequency by the modelling. Furthermore, a new system mode is introduced which is related to the new degree of freedom. The controller shows a high excitation of the new system mode for specific parameter combinations. Frequency-dependent feedbacks into the aerodynamics are also identified. Based on the comparison of both generator models, the analytical generator model promises a good trade-off between accuracy and computation time. In a second step, the effects of the identified interactions on turbine loads inside and outside the drive-train are analysed at the main bearing, the tower top, the tower base and the blade root. For this purpose, the equivalent loads of both models are compared. The load comparison shows that components inside and outside the drive-train are affected by the modelling. In particular, the main bearings and the tower show significant changes in load. However, loads can be increased as well as decreased compared to the state-of-the-art model. This supports the hypothesis of this work that a system design optimisation will differ from the component optimisation result. Furthermore, it can be shown that the added radial degrees of freedom and the electromagnetic forces add up in some cases and cancel each other out in other cases for load level changes. Based on the results of this work, follow-up questions arise, including lifetime estimation. Overall, this work contributes to a better understanding of the electro-mechanical interactions in direct-drive wind turbines and provides insight into the modelling approaches required for their analysis. It thus promotes the way towards system-oriented design of wind turbines.
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    Round-Trip-Engineering im modellbasierten Ingenieurentwurf
    (2023) Schopper, Dominik; Rudolph, Stephan (PD Dr.-Ing.)