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Matthew A Lackner - One of the best experts on this subject based on the ideXlab platform.
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Improved Free Vortex Wake Models Of Floating Offshore Wind Turbines
34th Wind Energy Symposium, 2016Co-Authors: Evan M. Gaertner, Shu Jian Liu, Matthew A LacknerAbstract:Floating offshore wind turbines offer significant advantages compared to onshore and near-shore wind turbines in terms of their ability to be located in deep waters. However, due to the motion of floating platforms in response to wind and wave loading, the aerodynamics are substantially more complex. Traditional aerodynamic models and design codes do not adequately account for the floating platform dynamics. Turbines must therefore be over designed due to loading uncertainty and are not fully optimized for their operating conditions. Previous research at the University of Massachusetts Amherst developed the Wake Induced Dynamics Simulator, or WInDS, a Free Vortex wake model of wind turbines that explicitly includes the velocity components from platform motion. This work presents the ongoing development of the WInDS model: the inclusions of a LeishmanBeddoes dynamic stall model, solution acceleration through parallel computing, and aeroelastic coupling with FAST, NREL’s computer-aided engineering tool. These developments enable the analysis of the unsteady aerodynamics of floating wind turbines in a fully-coupled simulation. Both the effect of aerodynamics and hydrodynamics on the structural motion, and the resulting change in the aerodynamics due to structural motion are captured. A sample case is presented to demonstrate these capabilities. The presented improvements to WInDS will enable future research and analysis of the complex, coupled aero-hydro-elastic behavior of floating offshore wind turbines.
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Modeling Dynamic Stall for a Free Vortex Wake Model
Wind Engineering, 2015Co-Authors: Evan M. Gaertner, Matthew A LacknerAbstract:Floating offshore wind turbines in deep waters offer significant advantages to onshore and near-shore wind turbines. However, due to the motion of floating platforms in response to wind and wave loading, the aerodynamics are substantially more complex. Traditional aerodynamic models and design codes do not adequately account for the floating platform dynamics. Previous research at the University of Massachusetts, Amherst developed the Wake Induced Dynamics Simulator, or WInDS, a Free Vortex wake model of wind turbines that explicitly includes the velocity components from platform motion. WInDS rigorously accounts for the unsteady interactions between the wind turbine rotor and its wake, however, as a potential flow model, the unsteady viscous response in the blade boundary layer is neglected. This work addressed this concern through the integration of a Leishman-Beddoes dynamic stall model into WInDS. Several improvements to the Leishman-Beddoes dynamic stall model are proposed to improve the synthesis of...
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development of a Free Vortex wake method code for offshore floating wind turbines
Renewable Energy, 2012Co-Authors: Thomas Sebastian, Matthew A LacknerAbstract:Offshore floating wind turbines (OFWTs) present unique aerodynamic analysis challenges. Motion–derived velocity perturbations in the wake necessitate higher–fidelity aerodynamic analysis methods than the ubiquitous momentum balance techniques currently in use. A more physically–sound approach is to model the wake generated by a wind turbine rotor as a Freely convecting lattice, using the resultant inflow to estimate rotor loads, as it done with a Free Vortex wake method (FVM). The FVM code Wake Induced Dynamics Simulator (WInDS) was developed at the University of Massachusetts at Amherst to predict the aerodynamic loading and wake evolution of an OFWT to a higher degree of accuracy than is possible via momentum balance methods. A series of validation cases were conducted to provide some basis for applying WInDS to floating wind turbine cases, for which no aerodynamic experimental data is currently available. The results from these tests show that WInDS is able to accurately predict the aerodynamically–derived loads and wake structures generated by various fixed and rotary–wing cases, and may therefore be applied to more complex cases, like OFWTs, with a degree of confidence.
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Curved Vortex Filaments in Free Vortex Wake Analysis of Floating Wind Turbines
2011Co-Authors: Friedemann Beyer, Thomas Sebastian, Denis Matha, Matthew A LacknerAbstract:Floating offshore wind turbines experience complex aerodynamic phenomena due to additional Degrees-of-Freedom (DOF) and transient flow field conditions, which common aerodynamic modeling techniques based on momentum balance methods are not capable of taking into account. Free Vortex Methods (FVM) represent an alternative approach based on potential flow theory and capture the physics of the flow field more precisely with reasonable computational costs, but only recently have gained importance even though they have been state-of-the-art in helicopter research for decades. The focus of this study by Beyer et al. [1] is the development, validation and application of a curved Vortex Filament Model (VFM). In contrast to commonly used straight-line segments, this higher-order approach simulates the local curvature of the wake more accurately resulting in an increase in computational efficiency because fewer and larger curved elements can be used to represent a Vortex filament .
J. Gordon Leishman - One of the best experts on this subject based on the ideXlab platform.
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VALIDATION OF A Free-Vortex WAKE MODEL FOR WIND TURBINES IN YAWED FLOW
44th AIAA Aerospace Sciences Meeting and Exhibit, 2006Co-Authors: Sandeep Gupta, J. Gordon LeishmanAbstract:A time-accurate Free-Vortex wake model has been used to predict the skewed wake geometry behind an upwind wind turbine in yawed flow over a range of yaw angles and tip speed ratios. The predicted tip Vortex positions compared well with flow visualization measurements of the tip Vortex positions found in the experiments. Increasing wake skewness was found to cause the wake to roll up along its edges. Good agreement was achieved between the predicted and measured turbine thrust for the entire range of conditions used in the experiment.
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STABILITY OF METHODS IN THE Free-Vortex WAKE ANALYSIS OF WIND TURBINES
42nd AIAA Aerospace Sciences Meeting and Exhibit, 2004Co-Authors: Sandeep Gupta, J. Gordon LeishmanAbstract:An analysis of the stability and accuracy of some timemarching methods used in the Free-Vortex wake analysis of wind turbines has been performed. Both linear and nonlinear stability analyses have been conducted. Integration schemes examined included the Euler explicit, Adams-Bashforth, and two predictor-corrector schemes. The linear stability analysis is shown to give an upper bound in determining the stability of each numerical method. However, it is further shown that the induced velocity term in the governing equations of the wake makes the overall stability problem nonlinear. A nonlinear stability analysis of the various schemes was performed using the method of modified equations. The numerical convergence of the wake was further examined following the methodology of conventional computational fluid dynamics. A predictor-corrector scheme with two time-step backward difference approximation was shown to be optimum when considering the overall stability, dispersion, and computational cost of each integration method. NOMENCLATURE Cl
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Free-Vortex Filament Methods for the Analysis of Helicopter Rotor Wakes
Journal of Aircraft, 2002Co-Authors: J. Gordon Leishman, Mahendra J. Bhagwat, Ashish BagaiAbstract:The theoretical basis and the numerical implementation of Free-Vortex filament methods are reviewed for application to the prediction and analysis of helicopter rotor wakes. The governing equations for the problem are described, with a discussion of finite difference approximations to these equations and various numerical solution techniques. Both relaxation and time-marching wake solution techniques are reviewed. It is emphasized how the careful consideration of stability and convergence (grid-independent behavior) are important to ensure a physically correct wake solution. The implementation of viscous diffusion and filament straining effects are also discussed. The need for boundary condition corrections to compensate for the inevitable wake truncation are described. Algorithms to accelerate the wake solution using velocity field interpolation are shown to reduce computational costs without a loss of accuracy. Several challenging examples of the application of Free-Vortex filament methods to helicopter rotor problems are shown, including multirotor configurations, flight near the ground, maneuvering flight conditions, and descending flight through the Vortex ring state
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Transient Rotor Inflow Using a Time-Accurate Free-Vortex Wake Model
39th Aerospace Sciences Meeting and Exhibit, 2001Co-Authors: Mahendra J. Bhagwat, J. Gordon Leishman, L GlennAbstract:A time-marching Free-Vortex wake analysis was developed for application to the prediction of the aerodynamics of a helicopter rotor under transient or maneuvering flight conditions. The stability, accuracy and convergence of the time-marching algorithms was rigorously examined. A linearized analysis was used to determine the basic stability characteristics of the algorithms. A new time-marching algorithm is proposed to ensure numerical stability and convergence of the wake solution. The second-order accuracy and grid independent nature of the wake geometry solution is demonstrated. This algorithm is applied to the problem of transient rotor response resulting from time-varying changes in the rotor collective pitch inputs. Good agreement is shown between the predictions and experimental measurements.
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Accelerated, High Resolution Free-Vortex Wakes For Rotor Aeroacoustic Analysis
15th Applied Aerodynamics Conference, 1997Co-Authors: Ashish Bagai, J. Gordon Leishman, L GlennAbstract:Numerical acceleration algorithms have been developed to improve the computational efficiency of rotor Free-Vortex wake analyses. Two general methodologies were formulated: an adaptive grid sequencing algorithm, and a velocity field interpolation algorithm. The methods were found to produce up to an order of magnitude decrease in execution time, but without significant loss in predictive accuracy. Examples are shown for very high fidelity wake geometries required for rotor aeroacoustic analyses. t/jj jth rotor discretized azimuth location Ot kth collocation point along Vortex filament fl Rotor rotational frequency, rad/s
Thomas Sebastian - One of the best experts on this subject based on the ideXlab platform.
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development of a Free Vortex wake method code for offshore floating wind turbines
Renewable Energy, 2012Co-Authors: Thomas Sebastian, Matthew A LacknerAbstract:Offshore floating wind turbines (OFWTs) present unique aerodynamic analysis challenges. Motion–derived velocity perturbations in the wake necessitate higher–fidelity aerodynamic analysis methods than the ubiquitous momentum balance techniques currently in use. A more physically–sound approach is to model the wake generated by a wind turbine rotor as a Freely convecting lattice, using the resultant inflow to estimate rotor loads, as it done with a Free Vortex wake method (FVM). The FVM code Wake Induced Dynamics Simulator (WInDS) was developed at the University of Massachusetts at Amherst to predict the aerodynamic loading and wake evolution of an OFWT to a higher degree of accuracy than is possible via momentum balance methods. A series of validation cases were conducted to provide some basis for applying WInDS to floating wind turbine cases, for which no aerodynamic experimental data is currently available. The results from these tests show that WInDS is able to accurately predict the aerodynamically–derived loads and wake structures generated by various fixed and rotary–wing cases, and may therefore be applied to more complex cases, like OFWTs, with a degree of confidence.
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Curved Vortex Filaments in Free Vortex Wake Analysis of Floating Wind Turbines
2011Co-Authors: Friedemann Beyer, Thomas Sebastian, Denis Matha, Matthew A LacknerAbstract:Floating offshore wind turbines experience complex aerodynamic phenomena due to additional Degrees-of-Freedom (DOF) and transient flow field conditions, which common aerodynamic modeling techniques based on momentum balance methods are not capable of taking into account. Free Vortex Methods (FVM) represent an alternative approach based on potential flow theory and capture the physics of the flow field more precisely with reasonable computational costs, but only recently have gained importance even though they have been state-of-the-art in helicopter research for decades. The focus of this study by Beyer et al. [1] is the development, validation and application of a curved Vortex Filament Model (VFM). In contrast to commonly used straight-line segments, this higher-order approach simulates the local curvature of the wake more accurately resulting in an increase in computational efficiency because fewer and larger curved elements can be used to represent a Vortex filament .
Liu Tian - One of the best experts on this subject based on the ideXlab platform.
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Study on the Laser Wavefront Deformation Induced by the Free-Vortex Aerodynamic Window for High Energy Lasers
Chinese Journal of Lasers, 2003Co-Authors: Liu TianAbstract:This paper analyzes the principle of wavefront disturbed by the Free Vortex aerodynamic window(FADW) designed for the high power laser measurement using the Hartmann Shack (H S) sensing, and measures the wavefront when the FADW is running steadily using the H S sensor with 37 sub apertures experimentally. The wavefront attained by CCD detectors is sent to the high speed DSP wavefront processing units. And then it reconstructs the disturbed wavefront using the method of Zernike mode, and calculates the relative optical parameters such as the peak value (PV) of wavefront error, RMS and STREHL raton (R s) and so on.The results shows that the effects on the output laser wavefront of the FADW are mainly the beam deflexion and the beam divergence, otherwise the else high level wavefront errors are very low.
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Preliminary study on the compensation of the wavefront deformation induced by Free-Vortex aerodynamic window using AO system
High Power Laser and Particle Beams, 2002Co-Authors: Liu TianAbstract:The Free Vortex aerodynamic window(FADW) can take the place of the conventional crystal output window sealing the lower pressure laser cavity of high energy lasers effectively. But the supersonic flow of the FADW will introduce some deformation to the output high power laser beam so it is necessary to compensate the disturbed laser beam using the adaptive optical system. This paper studies the compensation of the wavefront deformation of the FADW under the ideal designing situation using the 37 subapertures indoor adaptive optical system.It discusses the principle of the adaptive optical system and gives the experimental method and compensaton results. The results show that this 37 subapertures indoor adaptive optical system can compensate the wavefront deformation of the FADW under the designing situation preferably.
Yi Shi - One of the best experts on this subject based on the ideXlab platform.
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The design and experimental investigations of supersonic Free-Vortex ADW
Experiments and Measurements in Fluid Mechanics, 2003Co-Authors: Yi ShiAbstract:Supersonic FreeVortex aerodynamic window (ADW) is employed to seal lowpressure lasercavity. In this article the procedures are described for determining FreeVortex aerodynamic window. FreeVortex nozzle is designed and the performance is measured. The experiments show that the FreeVortex aerodynamic window can operate on design parameter and has a good optical quality.
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Preliminary Study of Asymmetric Nozzle of Free-Vortex Aerodynamic Window
Journal of National University of Defense Technology, 2000Co-Authors: Yi ShiAbstract:The crystal window of high power laser can not work in gear because of the heat caused by absorption of high power beam . The supersonic Free Vortex segment is usually employed to seal low pressure laser cavity. In this article the procedures are described for determining Free Vortex segment properties. Characteristics method is used to design nozzle which produces Free Vortex segment.
