06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

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

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Open Access
Acoustic and seismic emissions from wind turbines
(2017) Calarco, Francesca; Cheng, Po Wen; Zieger, Toni; Ritter, Joachim
With regards to the interdisciplinary “TremAc” Project funded by the German Federal Ministry for Economic Affairs and Energy, this paper examines acoustic and seismic emissions generated by wind turbines with the aim of identifying a better understanding of their interaction. Measurement campaigns will be carried out in the field around a single wind turbine plant and results in terms of acoustic and seismic signals will be correlated and then evaluated in relation to environmental factors such as wind speed, wind direction and temperature as well as to data related to the wind turbines-specifications (e.g. rotation speed).
Open Access
Advances on reduced-order modeling of floating offshore wind turbines
(2021) Lemmer, Frank; Yu, Wei; Steinacker, Heiner; Skandali, Danai; Raach, Steffen
Aero-hydro-servo-elastic modeling of Floating Offshore Wind Turbines (FOWTs) is a key component in the design process of various components of the system. Different approaches to order reduction have been investigated with the aim of improving structural design, manufacturing, transport and installation, but also the dynamic behavior, which is largely affected by the blade pitch controller. The present work builds on previous works on the SLOW (Simplified Low-Order Wind Turbine) code, which has already been used for the above purposes, including controller design. While the previous rigid rotor model gives good controllers in most cases, we investigate in the present work the question if aero-elastic effects in the design model can improve advanced controllers. The SLOW model is extended for the flapwise bending and coupled to NREL's AeroDyn, linearized and verified with the OlavOlsen OO-Star Wind Floater Semi 10MW public FOWT model. The results show that the nonlinear and linear reduced-order SLOW models agree well against OpenFAST. The state-feedback Linear Quadratic Regulator (LQR) applied with the same weight functions to both models, the old actuator disk, and the new aero-elastic model shows that the LQR becomes more sensitive to nonlinear excitation and that the state feedback matrix is significantly different, which has an effect on the performance and potentially also on the robustness. Thus modeling uncertainties might even be more critical for the LQR of the higher-fidelity model.
Open Access
Albian to Campanian explosive island arc volcanism in Costa Rica : a new basis for plate reconstructions of western Caribbean
(1994) Calvo, Claudio; Bolz, Angela
In dieser Zusammenfassung werden konkrete Hinweise auf Subduktionsprozesse während der Unterkreide in Südzentralamerika präsentiert. Somit bildete der entsprechende mittelamerikanische Inselbogen seitdem die westliche Grenze der Karibischen Platte.
Open Access
Analysis of floating offshore wind turbine hydrodynamics using coupled CFD and multibody methods
(2013) Beyer, Friedemann; Arnold, Matthias; Cheng, Po Wen
The focus of this study is the application of a higher order hydrodynamic modeling technique for the analysis of Floating Offshore Wind Turbine dynamics. This approach is based on a coupling between Multibody and Computational Fluid Dynamics methods. Results of the translational and rotational platform displacement are presented for a basic free-decay simulation in surge direction in still water. A comparison to linear hydrodynamics is presented. Additional, pressure mapping is demonstrated.
Open Access
Automation in laser scanning for cultural heritage applications
(2005) Böhm, Jan; Haala, Norbert; Alshawabkeh, Yahya
Within the paper we present the current activities of the Institute for Photogrammetry in cultural heritage documentation in Jordan. In particular two sites, Petra and Jerash, were recorded using terrestrial laser scanning (TLS). We present the results and the current status of the recording. Experiences drawn from these projects have led us to investigate more automated approaches to TLS data processing. We detail two approaches within this work. The automation of georeferencing for TLS data is presented along with our approach for automated feature extraction.
Open Access
Collective pitch feedforward control of floating wind turbines using lidar
(2015) Schlipf, David; Simley, Eric; Lemmer, Frank; Pao, Lucy; Cheng, Po Wen
In this work a collective pitch feedforward controller for floating wind turbines is presented. The feedforward controller provides a pitch rate update to a conventional feedback controller based on a wind speed preview. The controller is designed similar to the one for onshore turbines, which has proven its capability to improve wind turbine control performance in field tests. In a first design step, perfect wind preview and a calm sea is assumed. Under these assumptions the feedforward controller is able to compensate almost perfectly the effect of changing wind speed to the rotor speed of a full nonlinear model over the entire full load region. In a second step, a nacelle-based lidar is simulated scanning the same wind field which is used also for the aero-hydro-servo-elastic simulation. With model-based wind field reconstruction methods, the rotor effective wind speed is estimated from the raw lidar data and is used in the feedforward controller after filtering out the uncorrelated frequencies. Simulation results show that even with a more realistic wind preview, the feedforward controller is able to significantly reduce rotor speed and power variations. Furthermore, structural loads on the tower, rotor shaft, and blades are decreased. A comparison to a theoretical investigation shows that the reduction in rotor speed regulation is close to the optimum.
Open Access
A comparison between transient heat transfer measurements using TLC and IR thermography
(2017) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens von
Narrowband thermochromic liquid crystals (TLCs) and infrared thermography (IRT) are compared in the context of spatially resolved and transient heat transfer measurements. For accurate measurements the TLC coating was calibrated with a stationary method before the experiment. The IRT camera was in-situ calibrated with a surface thermocouple. A good agreement on temperature was achieved for both methods. The TLC data as a single-point-in-time measurement was evaluated for a time-independent heat transfer coefficient hTLC. The surface temperature history measured with the IRT camera enables an additional evaluation for a time-dependent hIR(t). In the case of one-dimensional heat conduction situations and late TLC indications, hTLC and hIR(t) agree well after the first 10 s of the experiment. An investigation of the heat transfer in the wake region of a vortex generator illustrated the influence of lateral conduction. This effect is taken into account by an analytical-empirical correction method for the TLC data. For the IRT data an evaluation method based on an analytical solution of the three-dimensional heat conduction equation is presented. Equally to the one-dimensional case the evaluation methods considering the lateral conduction effects agree well after 10 s while the other methods deliver erroneous results.
Open Access
A comparison between transient heat transfer measurements using TLC and IR thermography
(2016) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens von
Narrowband thermochromic liquid crystals (TLCs) and infrared thermography (IRT) are compared in the context of spatially resolved and transient heat transfer measurements. For accurate measurements the TLC coating was calibrated with a stationary method before the experiment. The IRT-camera was in-situ calibrated with a surface thermocouple. A good agreement on temperature was achieved for both methods. The TLC data as a single point measurement was evaluated for a time-independent heat transfer coefficient ℎTLC. The surface temperature history measured with the IRT-camera enables an additional evaluation for a time-dependent ℎCF(t). In the case of one-dimensional heat conduction situations and late TLC indications ℎTLC and ℎCF(t) agree well after the first 10 s of the experiment. An investigation of the heat transfer in the wake region of a vortex generator illustrated the influence of lateral conduction. This effect is not taken into account by any method and leads to greater differ-ences between ℎTLC and ℎCF(t).
Open Access
Comparison of feedforward and model predictive control of wind turbines using LIDAR
(2012) Schlipf, David; Pao, Lucy Y.; Cheng, Po Wen
LIDAR systems are able to provide preview information of wind disturbances at various distances in front of wind turbines. This technology paves the way for new control concepts such as feedforward control and model predictive control. This paper compares a nonlinear model predictive controller and a feedforward controller to a baseline controller. Realistic wind "measurements" are obtained using a detailed simulation of a LIDAR system. A full lifetime comparison shows the advantages of using the wind predictions to reduce wind turbine fatigue loads on the tower and blades as well as to limit the blade pitch rates. The results illustrate that the feedforward controller can be combined with a tower feedback controller to yield similar load reductions as the model predictive controller.
Open Access
Conceptual design of floating wind turbines
(2013) Sandner, Frank; Cheng, Po Wen
The need for different numerical models with varying degrees of simplification for the conceptual design of a floating offshore wind turbine is the focus of this paper. While parts on the component level can be designed apart from the others the overall dynamics on the system level have to be assessed from the beginning. Starting with very simple models and identifying the significant contributions to the system behavior while going step by step to more detailed ones makes a successful dimensioning possible. The significant effect of the blade pitch controller on the system dynamics is analysed and preliminarily designed with a simple 1-degree of freedom (dof) model. Further on the section forces at tower base and the distributed platform loads are calculated with a 9-dof multibody system with simplified aerodynamics and Morison equation allowing a pre-dimensioning of the structure.
Open Access
Correlation-model of rotor-effective wind shears and wind speed for lidar-based individual pitch control
(2015) Haizmann, Florian; Schlipf, David; Cheng, Po Wen
In this work the spectra based model of the correlation between lidar systems and wind turbines is extended from rotor-effective wind speed only, to rotor-effective wind speed and linear horizontal and vertical shear components. This is achieved by the incorporation of a model based wind field reconstruction method solving a set of linear equations with the least-squares method. The model allows to optimize a lidar system’s measurement configuration for a specific wind turbine a-priori by means of direct and fast spectra calculations. Furthermore, it allows to assess the filter parameters to be expected and needed for the application of lidar-assisted control. By extending the model to rotor-effective linear shears, the results can be used for lidar-assisted individual pitch control.
Open Access
Coupled MBS-CFD simulation of the IDEOL floating offshore wind turbine foundation compared to wave tank model test data
(2015) Beyer, Friedemann; Choisnet, Thomas; Kretschmer, Matthias; Cheng, Po Wen
A two MW floating offshore wind turbine is currently developed within the EU-FP7 project FLOATGEN. A wave tank test of the floater model at 1/32th scale has been performed in extreme wave conditions. In the present study numerical calculations of the floating foundation with regular waves using coupled MBS-CFD methods are compared to experimental data enabling a validation. Results of the wave elevation, floater motion and mooring line tension show a very good correlation. Flow phenomena like vortex shedding at the hull of the floater are shown. The presented methodology provides detailed knowledge allowing analysis of wave impact and resulting load assessment of floating offshore structures.
Open Access
Development of a wind turbine LiDAR simulator
(2009) Schlipf, David; Trujillo, Juan José; Basterra, Valeria; Kühn, Martin
Remote sensing techniques like LiDAR offer many novel applications to the wind energy community, e.g. fast and accurate measurements of inflow and wake wind fields from the turbine nacelle. The prospects of such a new technique are evaluated with a software tool simulating a nacelle-based LiDAR system. The paper presents the implementation and application of a simulator that has been conceived to support the design of wind field scanning procedures. The tool helps to optimize the hardware setup, scanning trajectories and frequency. Furthermore it can be coupled with an aeroelastic code with the aim of developing a predictive control based on remote sensing.
Open Access
Direct speed control using LIDAR and turbine data
(2013) Schlipf, David; Fleming, Paul; Kapp, Stefan; Scholbrock, Andrew; Haizmann, Florian; Belen, Fred; Wright, Alan; Cheng, Po Wen
LIDAR systems are able to provide preview information of the wind speed in front of wind turbines. One proposed use of this information is to increase the energy capture of the turbine by adjusting the rotor speed directly to maintain operation at the optimal tip-speed ratio, a technique referred to as Direct Speed Control (DSC). Previous work has indicated that for large turbines the marginal benefit of the direct speed controller in terms of increased power does not compensate for the increase of the shaft loads. However, the technique has not yet been adequately tested to make this determination conclusively. Further, it is possible that applying DSC to smaller turbines could be worthwhile because of the higher rotor speed fluctuations and the small rotor inertia. This paper extends the previous work on direct speed controllers. A DSC is developed for a 600 kW experimental turbine and is evaluated theoretically and in simulation. Because the actual turbine has a mounted LIDAR, data collected from the turbine and LIDAR during operation are used to perform a hybrid simulation. This technique allows a realistic simulation to be performed, which provides good agreement with theoretical predictions.
Open Access
Down-regulation and individual blade control as lifetime extension enablers
(2018) Pettas, Vasilis; Cheng, Po Wen
As more and more wind turbines are coming close to the end of their design lifetime, evaluation of end of life strategies is becoming highly relevant. Moreover, as turbine technology matures and wind farms grow larger, lifetime extension becomes a financially attractive option compared to re-powering and decommissioning. Present work suggests control strategies, namely down-regulation and individual blade control, as lifetime extension enablers. The concept of using them as retrofit control implementations is explained. Their individual and combined potential in fatigue load reduction is evaluated, along with their effect on other performance and pitch system metrics. Finally, the possible period of extension, beyond the nominal 20 years, is evaluated in an example case where the retrofit control strategy is applied after 15 years of baseline operation. The aeroelastic simulations are performed with a 10 MW reference wind turbine, according to load certification standards. Results show that the two methods complement each other in load alleviation. The pitch actuator demands are also significantly decreased when the two methods are combined.
Open Access
Efficient integration of aerial and terrestrial laser data for virtual city modeling uUsing LASERMAPs
(2005) Böhm, Jan; Haala, Norbert
One of the first steps during evaluation of terrestrial LIDAR measurement is the co-registration of the data sets, which are collected from different stations and their transformation to a suitable reference coordinate system. As it is demonstrated within the paper, this can be achieved automatically, if low-cost components are used for direct georeferencing parallel to LIDAR measurement. By these means the position and orientation of the laser scanner can be determined. Further refinement is feasible, if the terrestrial scans are aligned to a DEM from airborne LIDAR or existing 3D building models. Frequently, terrestrial lasers scanning (TLS) is used to collect building facades at a large amount of geometric detail. For this type of application, the integration of existing 3D building models from airborne data collection as complimentary data source is especially suitable for the modeling step. Due to viewpoint restrictions of airborne data collection, the amount of detail, which is available for the building facades, is very limited. Thus, after alignment, the terrestrial scans are ideally suited to refine these models. However, as it is demonstrated in the second part of the paper, the large effort required for explicit modeling can be avoided if so-called LASERMAPs are extracted from TLS and mapped against the coarse 3D building model.
Open Access
Flatness-based feedforward control of wind turbines using Lidar
(2014) Schlipf, David; Cheng, Po Wen
Current lidar technology is offering a promising opportunity to take a fresh look at wind turbine control. This work evaluates a flatness-based feedforward approach, that allows to calculate the control action based on trajectories of the rotor speed and tower motion using wind measurements. The trajectories are planned online considering actuator constrains to regulate the rotor speed and minimize tower movements. The feedforward signals of the collective pitch and generator torque update can be combined with conventional feedback controllers. This facilitates the application on commercial wind turbines. Simulations using a realistic lidar simulator and a full aero-elastic model show considerable reduction of tower and shaft loads.
Open Access
FlexDyn - a new OpenFAST structural dynamics module for a general, user defined wind turbine topology
(2021) Lemmer, Frank; Pérez Brovia, Santiago; Skandali, Danai; Raach, Steffen
In the present work, FlexDyn, a new structural dynamics module for the OpenFAST framework is developed. FlexDyn can generate structural equations of motion through a formalism, given user-defined rigid and elastic bodies and associated Degrees of Freedom (DOFs). The Newton-Euler formalism uses beam models with shape functions for a reduced-order representation in the same way as ElastoDyn of OpenFAST. The equations of motion are formulated in minimal coordinates, equally to ElastoDyn. FlexDyn is fully integrated into the OpenFAST framework with a coupling to AeroDyn and the new SubDyn module for FE representations of floating substructures (Jonkman, et al., 2020), among others. The formalism was previously implemented and verified in the low-order aero-hydro-servo-elastic code SLOW (Lemmer, et al., 2020). The objective of the presentation is to show the methodology of the formalized generation of equations of motion and first results of the new FlexDyn module for OpenFAST. The use case is an improved aero-elastic model, which includes the torsional DOF of the blades. The torsional DOF is not included in the ElastoDyn module but can potentially contribute to the motion and load response of the blades. The fidelity level of this use case of FlexDyn is higher than that of ElastoDyn but still below that of BeamDyn, which is a full FE representation of the blades. For this reason, the computational performance is still in the range of ElastoDyn, taking advantage of the order reduction.
Open Access
From point samples to surfaces - on meshing and alternatives
(2005) Böhm, Jan; Pateraki, Maria
Terrestrial laser scanners deliver a dense point-wise sampling of an object’s surface. For many applications a surface-like reconstruction is required. The most typical example is the visualization of the scanned data. Traditional approaches use meshing algorithms to reconstruct and triangulate the surface represented by the points. Especially in cultural heritage, where complex objects with delicate structures are recorded in highly detailed scans, this process is not without problems. Often long and tedious manual clean-up procedures are required to achieve satisfactory results. After summarizing our experience with current meshing technology we therefore explore alternative approaches for surface reconstruction. An alternative approach presented within this paper is point splatting. We have developed an algorithm to compute a suitable surfel representation directly from the raw laser scanner data. This results in a speedy and fully automated procedure for surface reconstruction. The properties of the different approaches for surface reconstruction are discussed considering a practical example from the field of cultural heritage. The Panagia Kera in Kritsa near Agios Nikolaos on the island of Crete was chosen as a suitable example.
Open Access
Georeferencing of terrestrial laserscanner data for applications in architectural modeling
(2005) Schuhmacher, Sara; Böhm, Jan
Commonly several measurements originating from different sensor systems and stations are combined in order to completely model an architectural site. An important step preceding data fusion is the geometric alignment of the measurements with a geodetic coordi-nate system, or in other words georeferencing the measurements. This paper investigates methods for the georeferencing of terrestrial laserscanning data. Both sensor- and data-driven methods are considered. We propose the integration of a GPS receiver and a digital compass for automated georeferencing. We detail on how virtual city models and aerial LIDAR data can be utilized for georeferenc-ing by point cloud matching. The proposed methods are evaluated in practical experiments and compared to conventional methods. The results obtained are presented and illustrated.