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  • 1.
    Gantasala, Sudhakar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Detection of blade icing and its influence on wind turbine vibrations2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Wind turbine installations in extreme conditions like cold climate have increased over thelast few years and expected to grow in future in North America, Europe, and Asia regionsdue to good wind resources and land availability. Their installed capacity could reach186 GW by the end of 2020. The cold climate sites impose the risk of ice accumulationon turbines during the winter due to the humidity at low temperatures. Since theatmospheric and operating conditions of the wind turbine leading to blade icing varystochastically in space and time, the resulting ice accumulation is completely random, itis even different for turbines within the same site. Ice accumulation alters aerofoil shapesof the blade, affecting their aeroelastic behavior. The icing severity at different locationsof the blade and their non-uniform distribution on blades have a distinct influence onturbine power output and vibrations. The current thesis proposes a methodology toinvestigate such behavior of wind turbines by considering the structural and aerodynamicproperty changes in the blade due to icing. An automated procedure is used to scalesimulated/measured ice shape on aerofoil sections of the blade according to a specifiedice mass distribution. The aeroelastic behavior of the blades is simulated considering thestatic aerodynamic coefficients of the iced aerofoil sections. The proposed methodologyis demonstrated on the National Renewable Energy Laboratory (NREL) 5 MW baselinewind turbine model. The method can be leveraged to analyze the influence of icing onany wind turbine model.De/Anti-icing systems are installed on the turbines to mitigate the risks associatedwith icing. It is essential to detect icing at the early stage and initiate these systems toavoid production losses and limit the risks associated with ice throw. Ice accumulationincreases blade mass and its spatial distribution changes natural frequencies of the blade.A detection technique is proposed in this thesis to characterize ice mass distributionon the blades based on its natural frequencies. The detection technique is validatedusing experiments on a small-scale cantilever beam and 1-kW wind turbine bladeset-ups and its effectiveness is also verified on large-scale wind turbine blades usingnumerical models. The proposed technique has the potential for detecting ice masseson large wind turbines operating in cold climate as it requires only first few naturalfrequencies of the blade. These natural frequencies are usually excited by the turbulentwind in operation/standstill conditions and they can be estimated from the vibrationmeasurements of the blade.

    The full text will be freely available from 2019-11-15 08:00
  • 2.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    A Preliminary Experiment to Excite and Identify Modal Frequencies of a Rotor in the Rotating Frame of Reference2019In: Proceedings of the 10th International Conference on Rotor Dynamics – IFToMM / [ed] Katia Lucchesi Cavalca, Hans Ingo Weber, Cham: Springer, 2019, p. 265-277Conference paper (Refereed)
    Abstract [en]

    The current work uses two types of excitation on a rotating shaft to identify its modal frequencies. The first one is a non-contact excitation where an oscillating magnet is placed near the shaft, eddy currents generated by the oscillating magnetic field excites vibrations in the shaft. In the second type of excitation, a miniature electrodynamical exciter powered by a decoder amplifier board is placed on the shaft to excite vibrations with predefined frequencies in a signal (mp3 format) stored on the USB flash drive connected to the board. The shaft is rotated at different speeds and vibration accelerations are measured using a small data logger placed on the shaft while excited using these two excitation systems. These two types of asynchronous excitation on the shaft excites both forward and backward whirl vibration modes of the rotor system. The modal frequencies are identified at the peak amplitudes in the waterfall plots of the measured vibration accelerations to a chirp excitation of the shaft. A Campbell diagram is plotted with the identified modal frequencies of the shaft in the rotating frame of reference.

  • 3.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    A Preliminary Experiment to Excite and Identify Modal Frequencies of a Rotor in the Rotating Frame of Reference2019In: Proceedings of the 10th International Conference on Rotor Dynamics: IFToMM / [ed] Cavalca, Katia Lucchesi; Weber, Hans Ingo, Springer, 2019, Vol. 3, p. 265-277Conference paper (Refereed)
    Abstract [en]

    The current work uses two types of excitation on a rotating shaft to identify its modal frequencies. The first one is a non-contact excitation where an oscillating magnet is placed near the shaft, eddy currents generated by the oscillating magnetic field excites vibrations in the shaft. In the second type of excitation, a miniature electrodynamical exciter powered by a decoder amplifier board is placed on the shaft to excite vibrations with predefined frequencies in a signal (mp3 format) stored on the USB flash drive connected to the board. The shaft is rotated at different speeds and vibration accelerations are measured using a small data logger placed on the shaft while excited using these two excitation systems. These two types of asynchronous excitation on the shaft excites both forward and backward whirl vibration modes of the rotor system. The modal frequencies are identified at the peak amplitudes in the waterfall plots of the measured vibration accelerations to a chirp excitation of the shaft. A Campbell diagram is plotted with the identified modal frequencies of the shaft in the rotating frame of reference.

  • 4.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Luneno, Jean-Claude
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Identification of ice mass accumulated on wind turbine blades using its natural frequencies2018In: Wind Engineering: The International Journal of Wind Power, ISSN 0309-524X, E-ISSN 2048-402X, Vol. 42, no 1, p. 66-84Article in journal (Refereed)
    Abstract [en]

    This work demonstrates a technique to identify information about the ice mass accumulation on wind turbine blades using its natural frequencies, and these frequencies reduce differently depending on the spatial distribution of ice mass along the blade length. An explicit relation to the natural frequencies of a 1-kW wind turbine blade is defined in terms of the location and quantity of ice mass using experimental modal analyses. An artificial neural network model is trained with a data set (natural frequencies and ice masses) generated using that explicit relation. After training, this artificial neural network model is given an input of natural frequencies of the iced blade (identified from experimental modal analysis) corresponding to 18 test cases, and it identified ice masses’ location and quantity with a weighted average percentage error value of 17.53%. The proposed technique is also demonstrated on the NREL 5-MW wind turbine blade data.

  • 5.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Luneno, Jean-Claude
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Influence of Icing on the Modal Behavior of Wind Turbine Blades2016In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 9, no 11, article id 862Article in journal (Refereed)
    Abstract [en]

    Wind turbines installed in cold climate sites accumulate ice on their structures. Icing of the rotor blades reduces turbine power output and increases loads, vibrations, noise, and safety risks due to the potential ice throw. Ice accumulation increases the mass distribution of the blade, while changes in the aerofoil shapes affect its aerodynamic behavior. Thus, the structural and aerodynamic changes due to icing affect the modal behavior of wind turbine blades. In this study, aeroelastic equations of the wind turbine blade vibrations are derived to analyze modal behavior of the Tjaereborg 2 MW wind turbine blade with ice. Structural vibrations of the blade are coupled with a Beddoes-Leishman unsteady attached flow aerodynamics model and the resulting aeroelastic equations are analyzed using the finite element method (FEM). A linearly increasing ice mass distribution is considered from the blade root to half-length and thereafter constant ice mass distribution to the blade tip, as defined by Germanischer Lloyd (GL) for the certification of wind turbines. Both structural and aerodynamic properties of the iced blades are evaluated and used to determine their influence on aeroelastic natural frequencies and damping factors. Blade natural frequencies reduce with ice mass and the amount of reduction in frequencies depends on how the ice mass is distributed along the blade length; but the reduction in damping factors depends on the ice shape. The variations in the natural frequencies of the iced blades with wind velocities are negligible; however, the damping factors change with wind velocity and become negative at some wind velocities. This study shows that the aerodynamic changes in the iced blade can cause violent vibrations within the operating wind velocity range of this turbine.

  • 6.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Luneno, Jean-Claude
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Investigating How an Artificial Neural Network Model Can Be Used to Detect Added Mass on a Non-Rotating Beam Using Its Natural Frequencies: A Possible Application for Wind Turbine Blade Ice Detection2017In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 2, article id 184Article in journal (Refereed)
    Abstract [en]

    Structures vibrate with their natural frequencies when disturbed from their equilibrium position. These frequencies reduce when an additional mass accumulates on their structures, like ice accumulation on wind turbines installed in cold climate sites. The added mass has two features: the location and quantity of mass. Natural frequencies of the structure reduce differently depending on these two features of the added mass. In this work, a technique based on an artificial neural network (ANN) model is proposed to identify added mass by training the neural network with a dataset of natural frequencies of the structure calculated using different quantities of the added mass at different locations on the structure. The proposed method is demonstrated on a non-rotating beam model fixed at one end. The length of the beam is divided into three zones in which different added masses are considered, and its natural frequencies are calculated using a finite element model of the beam. ANN is trained with this dataset of natural frequencies of the beam as an input and corresponding added masses used in the calculations as an output. ANN approximates the non-linear relationship between these inputs and outputs. An experimental setup of the cantilever beam is fabricated, and experimental modal analysis is carried out considering a few added masses on the beam. The frequencies estimated in the experiments are given as an input to the trained ANN model, and the identified masses are compared against the actual masses used in the experiments. These masses are identified with an error that varies with the location and the quantity of added mass. The reason for these errors can be attributed to the unaccounted stiffness variation in the beam model due to the added mass while generating the dataset for training the neural network. Therefore, the added masses are roughly estimated. At the end of the paper, an application of the current technique for detecting ice mass on a wind turbine blade is studied. A neural network model is designed and trained with a dataset of natural frequencies calculated using the finite element model of the blade considering different ice masses. The trained network model is tested to identify ice masses in four test cases that considers random mass distributions along the blade. The neural network model is able to roughly estimate ice masses, and the error reduces with increasing ice mass on the blade.

  • 7.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Luneno, Jean-Claude
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aeroelastic simulations of wind turbine using 13 DOF rigid beam model2016Conference paper (Refereed)
    Abstract [en]

    The vibration behavior of wind turbine substructures is mainly dominated by their first few vibration modes because wind turbines operate at low rotational speeds. In this study, 13 degrees of freedom (DOF) model of a wind turbine is derived considering fundamental vibration modes of the tower and blades which are modelled as rigid beams with torsional springs attached at their root. Linear equations of motion (EOM) governing the structural behavior of wind turbines are derived by assuming small amplitude vibrations. This model is used to study the coupling between the structural and aerodynamic behavior of NREL 5 MWmodel wind turbine. Aeroelastic natural frequencies of the current model are compared with the results obtained from the finite element model of this wind turbine. Quasi-steady aerodynamic loads are calculated considering wind velocity changes due to height and tower shadow effects. In this study, vibration responses are simulated at various wind velocities. The derived 13 DOF simplified model of the wind turbine enables to simulate the influence ofchange in parameters and operating conditions on vibration behavior with less computational effort. Besides that, the results of the simplified models can be interpreted with much ease.

  • 8.
    Gantasala, Sudhakar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Tabatabaei, Narges
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Numerical Investigation of the Aeroelastic Behavior of a Wind Turbine with Iced Blades2019In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 12, article id 2422Article in journal (Refereed)
    Abstract [en]

    Wind turbines installed in cold-climate regions are prone to the risks of ice accumulation which affects their aeroelastic behavior. The studies carried out on this topic so far considered icing in a few sections of the blade, mostly located in the outer part of the blade, and their influence on the loads and power production of the turbine are only analyzed. The knowledge about the influence of icing in different locations of the blade and asymmetrical icing of the blades on loads, power, and vibration behavior of the turbine is still not matured. To improve this knowledge, multiple simulation cases are needed to run with different ice accumulations on the blade considering structural and aerodynamic property changes due to ice. Such simulations can be easily run by automating the ice shape creation on aerofoil sections and two-dimensional (2-D) Computational Fluid Dynamics (CFD) analysis of those sections. The current work proposes such methodology and it is illustrated on the National Renewable Energy Laboratory (NREL) 5 MW baseline wind turbine model. The influence of symmetrical icing in different locations of the blade and asymmetrical icing of the blade assembly is analyzed on the turbine’s dynamic behavior using the aeroelastic computer-aided engineering tool FAST. The outer third of the blade produces about 50% of the turbine’s total power and severe icing in this part of the blade reduces power output and aeroelastic damping of the blade’s flapwise vibration modes. The increase in blade mass due to ice reduces its natural frequencies which can be extracted from the vibration responses of the turbine operating under turbulent wind conditions. Symmetrical icing of the blades reduces loads acting on the turbine components, whereas asymmetrical icing of the blades induces loads and vibrations in the tower, hub, and nacelle assembly at a frequency synchronous to rotational speed of the turbine.

  • 9.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Cervantes, Michel J.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gantasala, Sudhakar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Wind turbine aerodynamic modelling in icing condition: 3D RANS-CFD vs BEM method2018In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994Article in journal (Refereed)
  • 10.
    Tabatabaei, Narges
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gantasala, Sudhakar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Cervantes, Michel
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wind Turbine Aerodynamic Modeling in Icing Condition: Three-Dimensional RANS-CFD Versus Blade Element Momentum Method2019In: Journal of energy resources technology, ISSN 0195-0738, E-ISSN 1528-8994, Vol. 141, no 7, article id 071201Article in journal (Refereed)
    Abstract [en]

    Icing limits the performance of wind turbines in cold climates. The prediction of the aerodynamic performance losses and their distribution due to ice accretion is essential. Blade element momentum (BEM) is the basis of blade structural studies. The accuracy and limitations of this method in icing condition are assessed in the present study. To this purpose, a computational study on the aerodynamic performance of the full-scale NREL 5 MW rotor is performed. Three-dimensional (3D) steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for both clean and iced blade, as well as BEM calculations using two-dimensional (2D) computational fluid dynamics (CFD) sectional airfoil data. The total power calculated by the BEM method is in close agreement with the 3D CFD results for the clean blade. There is a 4% deviation, while it is underestimated by 28% for the iced one. The load distribution along the clean blade span differs between both methods. Load loss due to the ice, predicted by 3D CFD, is 32% in extracted power and the main loss occurs at the regions where the ice horn height exceeds 8% of the chord length.

  • 11.
    Thiery, Florian
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Gantasala, Sudhakar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Numerical evaluation of multilobe bearings using the Spectral Method2017In: Advances in Mechanical Engineering, ISSN 1687-8132, E-ISSN 1687-8140, Vol. 9, no 7Article in journal (Refereed)
    Abstract [en]

    Hydropower rotors and pumps have the specificity to be oriented vertically, meaning that the bearing forces have to be evaluated at each time-step depending on the position of the rotor for dynamical analyses. If the bearing forces cannot be evaluated analytically, a suitable numerical method should be used to calculate the pressure distribution over the bearing domain. This process can be computationally expensive as it should be performed for each discrete time-step. As a result, a comparison between the spectral method, the finite difference method, and the finite element method is performed to investigate which method is more adapted to dynamical analysis of the bearing. It is observed that the spectral method has the advantage of having a reasonable simulation time for any eccentricity magnitude with a moderate number of interpolation points. However, this method should be restricted to simple bearing models such as plain bearings or multilobe bearings due to the advantage of finding a global numerical solution directly on the entire bearing/pad domain

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