Showing papers by "Andreas Bechmann published in 2014"

Evaluation of the wind direction uncertainty and its impact on wake modeling at the Horns Rev offshore wind farm

Abstract: Accurately quantifying wind turbine wakes is a key aspect of wind farm economics in large wind farms. This paper introduces a new simulation post-processing method to address the wind direction uncertainty present in the measurements of the Horns Rev offshore wind farm. This new technique replaces the traditional simulations performed with the 10 min average wind direction by a weighted average of several simulations covering a wide span of directions. The weights are based on a normal distribution to account for the uncertainty from the yaw misalignment of the reference turbine, the spatial variability of the wind direction inside the wind farm and the variability of the wind direction within the averaging period. The results show that the technique corrects the predictions of the models when the simulations and data are averaged over narrow wind direction sectors. In addition, the agreement of the shape of the power deficit in a single wake situation is improved. The robustness of the method is verified using the Jensen model, the Larsen model and Fuga, which are three different engineering wake models. The results indicate that the discrepancies between the traditional numerical simulations and power production data for narrow wind direction sectors are not caused by an inherent inaccuracy of the current wake models, but rather by the large wind direction uncertainty included in the dataset. The technique can potentially improve wind farm control algorithms and layout optimization because both applications require accurate wake predictions for narrow wind direction sectors. © 2013 The Authors. Wind Energy published by John Wiley & Sons, Ltd.

Near wake Reynolds-averaged Navier–Stokes predictions of the wake behind the MEXICO rotor in axial and yawed flow conditions

Abstract: In the present paper, Reynolds-averaged Navier–Stokes predictions of the flow field around the MEXICO rotor in yawed conditions are compared with measurements. The paper illustrates the high degree of qualitative and quantitative agreement that can be obtained for this highly unsteady flow situation, by comparing measured and computed velocity profiles for all three Cartesian velocity components along four axial transects and several radial transects. Copyright © 2012 John Wiley & Sons, Ltd.

The Science of Making Torque from Wind 2014 (TORQUE 2014)

Abstract: The 186 papers in this volume constitute the proceedings of the fifth Science of Making Torque from Wind conference, which is organized by the European Academy of Wind Energy (EAWE, www.eawe.eu). The conference, also called Torque 2014, is held at the Technical University of Denmark (DTU) 17–20 June 2014. The EAWE conference series started in 2004 in Delft, the Netherlands. In 2007 it was held in Copenhagen, in 2010 in Heraklion, Greece, and then in 2012 in Oldenburg, Germany. The global yearly production of electrical energy by wind turbines has grown approximately by 25% annually over the last couple of decades and covers now 2–3% of the global electrical power consumption. In order to make a significant impact on one of the large challenges of our time, namely global warming, the growth has to continue for a decade or two yet. This in turn requires research and education in wind turbine aerodynamics and wind resources, the two topics which are the main subjects of this conference. Similar to the growth in electrical power production by wind is the growth in scientific papers about wind energy. Over the last decade the number of papers has also grown by about 25% annually, and many research based companies all over the world are founded. Hence, the wind energy research community is rapidly expanding and the Torque conference series offers a good opportunity to meet and exchange ideas. We hope that the Torque 2014 will heighten the quality of the wind energy research, while the participants will enjoy each others company in Copenhagen. Many people have been involved in producing the Torque 2014 proceedings. The work by more than two hundred reviewers ensuring the quality of the papers is greatly appreciated. The timely evaluation and coordination of the reviews would not have been possible without the work of sixteen ''section editors'' all from DTU Wind Energy: Christian Bak, Andreas Bechmann, Ferhat Bingol, Ebba Dellwik, Nikolay Dimitrov, Gregor Giebel, Martin O L Hansen, Dorte Juul Jensen, Gunner Larsen, Helge Aagaard Madsen, Jakob Mann, Anand Natarajan, Ole Rathmann, Ameya Sathe, Jens Norkaer Sorensen and Niels Norkaer Sorensen, who are all co-editors of these proceedings. The resources provided by the Center for Computational Wind Turbine Aerodynamics and Atmospheric Turbulence funded by the Danish Council for Strategic Research grant no. 09-067216 and the Danish Ministry of Science, Innovation and Higher Education Technology and Production, grant no. 11- 117018 are gratefully acknowledged. We are also immensely indebted to the very responsive help and support from the editorial team at IoP, especially Sarah Toms and Anete Ashton, during the reviewing process of these proceedings. We are looking forward to meeting you in Copenhagen and also to Torque 2016, which will take place at the Technical University of Munich, Germany. Roskilde, Denmark, June 2014 Ebba Dellwik, Ameya Sathe and Jakob Mann Technical University of Denmark

Complex terrain wind resource estimation with the wind-atlas method: Prediction errors using linearized and nonlinear CFD micro-scale models

Abstract: Using the Wind Atlas methodology to predict the average wind speed at one location from measured climatological wind frequency distributions at another nearby location we analyse the relative prediction errors using a linearized flow model (IBZ) and a more physically correct fully non-linear 3D flow model (CFD) for a number of sites in very complex terrain (large terrain slopes). We first briefly describe the Wind Atlas methodology as implemented in WAsP and the specifics of the “classical” model setup and the new setup allowing the use of the CFD computation engine. We discuss some known shortcomings of the linear orographic flow model (BZ) and possible modifications that could be considered, including the established RIX method. 1 Wind Atlas methodology The wind atlas methodology [2] was designed for horizontal and vertical extrapolation of mean wind conditions for use in wind power resource estimation. That is, if one has long term measured wind data (speed and direction) from some point (met mast location) at some height above ground, the method is used to estimate the wind conditions (wind speed frequency distributions per direction sector) at some other point of interest (hub height of wind turbine). The method assumes that winds in the points considered are governed by the same large(meso-) scale wind forcing. In practice this means that the horizontal distance over which the method can be meaningfully applied depend on the scales of the overall climate and of the scales of flow modifications introduced by surface inhomogeneity (roughness differences, thermal differences, hills and mountains). The methodology involves two distinct parts: The “Wind Atlas Analysis” in which the measured wind data are transformed using micro-scale models to estimate the local influences at the measuring point, subtracting these and using Rossby number similarity theory (“ Geostrophic drag law”) [5] to give a “Wind Atlas data set” , figure 1. Figure 1: Wind Atlas Analysis, adapted from [2]. The user input is indicated in green, the internal machinery as light blue blocks and orange arrows, results in red. The second part is the “Wind Atlas Application” in which the same micro-scale models are used to introduce the flow perturbations at the location and height of interest (e.g. the real or prospective location and hub height of a Wind Turbine) (figure 2). Figure 2: Wind Atlas Application, adapted from [2] The “Wind Atlas Analysis and Application Program” (WAsP) contain these two parts with the built-in orographic flow model (BZ) [2, 6] and the simple “internal boundary layer” (IBL) model for surface roughness inhomogeneities [2, 7]. Recently the WAsP has been modified to allow these internal (essentially) linear models to be replaced by external models, e.g. nonlinear models based “Reynolds Averaged NavierStokes” (RANS) equations. Such models require considerable more computing resources than standard WAsP and therefore these model simulations must currently be run on a remote cluster, figs 3. The terrain maps are transferred via the web (the clouds in the figure) together with specification of the target area to the remote cluster where grid generation and the running of the Ellipsys model takes place. The results (flow perturbations relative to specified far upstream inflow logarithmic profile) are returned via the web as a “result volume” giving the calculated flow corrections per upstream wind direction in a regular horizontal grid and at several levels above the ground. Within WAsP the internal models are thus replaced with interpolation within the result volume. The purpose of this paper is to compare the skill in making cross predictions in very complex terrain of the two WAsP configurations, which we will denote WAsP-IBZ for the “standard” version with the linear internal flow models, and WAsP-CFD for the configuration using the remote Ellipsys model. Figure 3: Wind Atlas Analysis using the Ellipsys RANS model, replacing part of the left blocks in figure 1. The corresponding Wind Atlas Application uses the same replacement of the two left column blocks (but in figure 2).

Atmospheric stability and complex terrain: comparing measurements and CFD

Abstract: For wind resource assessment, the wind industry is increasingly relying on Computational Fluid Dynamics models that focus on modeling the airflow in a neutrally stratified surface layer. So far, physical processes that are specific to the atmospheric boundary layer, for example the Coriolis force, buoyancy forces and heat transport, are mostly ignored in state-of-the-art flow solvers. In order to decrease the uncertainty of wind resource assessment, the effect of thermal stratification on the atmospheric boundary layer should be included in such models. The present work focuses on non-neutral atmospheric flow over complex terrain including physical processes like stability and Coriolis force. We examine the influence of these effects on the whole atmospheric boundary layer using the DTU Wind Energy flow solver EllipSys3D. To validate the flow solver, measurements from Benakanahalli hill, a field experiment that took place in India in early 2010, are used. The experiment was specifically designed to address the combined effects of stability and Coriolis force over complex terrain, and provides a dataset to validate flow solvers. Including those effects into EllipSys3D significantly improves the predicted flow field when compared against the measurements.

Canopy structure effects on the wind at a complex forested site

Abstract: We investigated the effect of the canopy description in a Reynolds-averaged Navier-Stokes method based on key flow results from a complex forested site. The canopy structure in RANS is represented trough the frontal area of canopy elements per unit volume, a variable required as input in canopy models. Previously difficult to estimate, this variable can now be easily recovered using aerial LiDAR scans. In this study, three approaches were tested which were all based on a novel method to extract the forest properties from the scans. A first approach used the fully spatial varying frontal area density. In a second approach, the vertical frontal area density variations were ignored, but the horizontally varying forest heights were kept represented. The third approach ignored any variations: the frontal area density was defined as a constant up to a fixed tree height over the whole domain. The results showed significant differences among the cases. The large-scale horizontal heterogeneities produced the largest effect on the variability of wind fields. Close to the surface, specifying more details about the canopy resulted in an increase of x – y area-averaged fields of velocity and turbulent kinetic energy.

Modifications to the current WAsP engine for Online WAsP

Abstract: max. 2000 char) This report documents the work performed in work package 3 of the Online WAsP EUDP project (Online WAsP for Small Wind Turbines). Specifically it is deliverable D3.1 “Report on modifications required to update current WAsP calculation engine”. DTU Wind Energy Report-I-0206(EN) April 2, 2014