1. Summary
The project concerns the mutual interaction between wind turbine aerodynamics, turbine wakes, terrain affected flow and atmospheric turbulence, which is not accounted for in state of the art modelling. This will be achieved by combining and exploring the knowledge and methodologies of leading national and international research groups within the fields of aerodynamics,
computational fluid mechanics, atmospheric physics and wind energy. The project is partitioned into five clearly defined tasks: Rotor aerodynamics, wakes and clusters, wind farms, siting in complex terrain and atmospheric boundary layers. In all of the tasks the focus is on creating the methodologies to handle the mutual interaction between the ambient turbulence and the wind turbine.
The physical outcome of the project is a set of reliable and verified simulation tools, capable of bridging the multi-scale flow phenomena connected with operating wind turbines in the atmospheric boundary layer, and the application of these in the further development of wind energy. The overall aim of the activity is to manifest Denmark as the world leading player within wind turbine aerodynamics and atmospheric turbulence, both with respect to research, technology and education.
2. Objective of the project
The objective of the project is to strengthen the coherence of Danish research in wind energy within the fields of wind turbine aerodynamics and atmospheric boundary layer turbulence. The aim of this collaborative research effort is to develop novel computational techniques that are capable of covering a large range of length scales from some few micrometers to several
kilometres. The newly developed tools will provide fundamental insight into the interaction between atmospheric turbulence and wind turbine aerodynamics. This will lead the way to more
efficient wind turbines and wind farms, optimized with respect to location and wind resources. Ultimately, the new knowledge generated within the center will provide the foundation for
increasing the production of wind energy world wide.
The objective of the project is to develop computational methodologies and physical models capable of coping with multiple scales and apply them to combined wind turbine aerodynamics
and atmospheric physics problems.
3. The main results of the project
The main outcome of the project is a set of computational methodologies and software packages that enables designers of wind turbines and wind turbine systems to predict power and
aerodynamic loadings of wind turbines and clusters of wind turbines with a much higher accuracy than today. Today, most wind power predictive tools are based on simplified modelling
assumptions and ad-hoc empirical corrections. The developed models are in this project based on first principle assumptions (Navier-Stokes equations) with intelligent approximations to deal with the inherent large range of length scales that characterizes the interaction between wind turbines and the atmospheric boundary layer. The project partners have access to a number of very efficient PC-clusters, which places them in a unique position to carry out large parametric studies which are necessary for the planned activities. As an additional outcome of the project, a set of engineering predictive tools based on input from results of the advanced computing algorithms will be developed. Among these are linearized atmospheric flow models that eventually are incorporated in the commercial pc-based flow models WAsP and WAsP Engineering which are available from Risø.
4. Background and hypothesis of the project
Background
The complexity of wind turbine aerodynamics:
The aerodynamics of wind turbines is highly complex in that the length scales that govern the performance of a wind turbine range from the thickness of the very thin boundary layer
developing on the rotor blades to the height of the atmospheric boundary layer, which can be several kilometers. While many physical processes in wind turbine aerodynamics and
atmospheric turbulence are well understood and may be accounted for independently, the increasing size of modern wind turbines imposes a growing demand to treat the design process as
a synthesis problem that takes into account the interaction between several scales in both space and time. This, however, is currently out of reach, due to the fact that state-of-the-art design tools are fundamentally based on a segmented approach.
The typical segmented approach:
It is well-known that many modelling approaches often make crude approximations for some of the scales involved. For example, when performing a detailed rotor calculation the turbulence scales and stratification of the atmosphere are largely ignored. Similarly, when simulating the atmospheric boundary layer of large wind farms the turbines are only crudely represented. In cases involving atmospheric boundary layers it is also required to employ different physical models, including heat transfer, radiation and multiple fluid phases, which is presently not included in flow models for wind energy applications.
On the experimental side, a fragmented approach is the standard, treating each subset of scales independently. At the smallest scale, airfoil performance data are measured in low-turbulent wind tunnels under non-rotating conditions. A few wind tunnel experiments of rotors in well defined non-turbulent inflow conditions exist; however, the cost of such experiments is
prohibitive and cannot be performed for each new rotor being designed. Often measurements on full scale wind turbines only include integral quantities with a lack of information about blade aerodynamics and detailed inflow conditions. Measurements in the atmospheric boundary layer inconnection with siting of wind turbine farms in complex terrain or near forests are based on only a few measuring positions. Typically one reference mast is located in a position with undisturbed flow conditions and one or two masts strategically placed at positions where turbines are planned to be placed. In this case, the limited resolution of the inflow and wake characteristics prohibits the investigation of turbine to turbine or park to park interaction.
New experimental data is available:
Recently, a series of experiments have been planned and several of these are already in full progress to bridge the gap between the different flow regimes ranging from airfoil and rotor
blade aerodynamics, rotor aerodynamics, wake aerodynamics to the characteristics of the atmospheric boundary layer. The DAN-Aero experiment (Aug. 2009) aims at bridging the gap between airfoil performance, boundary layer transition, rotor wake properties and the Atmospheric Boundary Layer (ABL) inflow. A parallel experiment The Siemens Boulder Full Scale Experiment performed by Siemens Wind Power (2009-2011), will provide similar data on a modern commercial Mega Watt turbine. Within the TOPFARM project (2007-2010) a series of
high resolution LIDAR measurement campaigns of turbine inflow and wake characteristics are undertaken. The TOPFARM experimental campaign will, in part of the time, be run in parallel
with the DAN-Aero experiment thus creating a unique full-scale dataset enlightening the interaction of flow regimes ranging from scales characteristic for the blade boundary layer to the
large scale turbulent eddies in the ABL. The Bolund experiment (Dec. 2007- Feb. 2008) provides high-resolution information on the flow field (-10 mast positions with ~30 sensors) in a terrain with almost vertical slopes, and two other experiments provides data over and near forests (Falster and Sorø). Finally, a large scale project within the framework of the National
Programme for Research Infrastructure has been initiated to deliver a LIDAR based 3D Wind Scanner for measuring the 3D wind and turbulence field around a full scale modern wind turbine, with the aim of providing new information about inflow and wake properties [1] -[3].
A Unique Opportunity
A unique opportunity thus exists for manifesting Denmark as a leading player in wind energy by creating a flow center for wind turbine aerodynamics and atmospheric turbulence, which can use the new knowledge generated from the above-mentioned experiments to develop and validate new multi-scale flow models that are necessary to design the multi-megawatt wind turbines of the future.
Hypothesis
The basic hypothesis of the present center is that the mutual interaction between the small scales related to the turbine and the large scales of the atmospheric boundary layer is essential, when estimating both the loads that a turbine structure experiences as well as the power production of the turbine.
It is the aim of the flow center to quantify these effects, and to form the theoretical foundation for developing computational multi-scale methods and to implement them to become
engineering predictive tools, by developing computational tools and exploring the newest experimental data. Additionally, by combining research groups traditionally working with different models and subsets of the multi-scale problem, a unique possibility exists to generate a new unified approach. Today, there already exists a variety of methods to simulate large scale flow structures using Reynolds-Averaged Navier-Stokes models (RANS) or Large Eddy Simulation (LES) techniques. The partners behind this application have been among the main drivers in introducing these techniques into the field of wind power [4]-[15]. Although some of the methods are now used as industrial tools to analyse specific parts of a wind turbine, there is an acute need to develop and validate methods capable of covering several different length and time scales in a unified computation. Such techniques are presently under development using different approaches, but the techniques have not yet matured to a level at which they can be used for practical applications, e.g. [9]. A main part of the project concerns validation of the developed computing codes using new experimental data.
5. Innovative value, impact and relevance of the project
The project concerns development of computational tools for describing in detail velocity fields, turbulence properties, pressure distributions and energy contents in all types of wind energy
applications, ranging from wind turbine airfoils to large scale wind farms. The innovative value of the project is to develop a unified approach for dealing with all relevant length and time scales and parameterize this into engineering models. Scientifically, the relevance of the project lies in deriving the different elements needed to go from one length scale to the next, and to formulate these into operational computational tools. These tools will be employed to derive better wind input for the aerodynamic models as well as providing a better representation of wind turbines into meteorological models. As an example, turbulence properties in wind farms, that today largely are unknown, will be derived from the model and result in a much better description of the wind conditions needed for designing turbines in wind farms. At the other end of the spectrum, the influence of a complete wind farm on the atmospheric boundary layer will be determined using body and turbulence sources into the flow equations and the outcome of the computations will be parameterized into low-dimensional turbulence models of use when planning and designing new wind farms. The relevance of the project lies in the possibility of obtaining a much more precise description of all relevant physical properties in wind turbine aerodynamics and atmosphere turbulence, and exploiting this to derive simple engineering models of practical use for designing wind turbines and wind farms.
6. Project's methodology and anticipated results
The project will focus on combining Reynolds averaged based methods (RANS) and LES, theoretical development of Multi Point Closures and Variational Multi Scale turbulence models,
for the application to the unified turbine, terrain and atmospheric boundary layer problem. As basic computing algorithm, the incompressible Navier-Stokes solver, EllipSys3D, a solver
developed at Risø and DTU, specifically designed for incompressible flows, will be applied. This code, which has been developed by two of the partners behind the application (Risø-DTU and
MEK-DTU), is based on a very efficient in-house developed multi-block/multi-grid approach aimed specifically for running efficiently on PC-clusters. During the project, the EllipSys3D
code will be supplemented with new algorithms for treating multiple scales. Some of these are already implemented, such as the actuator disc/line technique, whereas others are still
hypothetical or under development.
The models will be applied to a series of tasks, where each task bridges the traditional partitioned application of present day numerical models. The applications will be compared to the detailed experimental field campaigns by Risø-DTU, other projects partners, and to work by the external partners, several of these covering the multi scale problem. Additionally, the different tasks will be used for mutual inter-comparison. Finally, the results will be utilized to derive mean velocity fields as well as Reynolds stresses and turbulence intensities, and will form the basis for the development of low-dimensional inhomogeneous turbulence models for use in standard engineering codes. The project will provide valuable results since it to a large extent relies on the existing and well functioning computing code, EllipSys3D, and a large amount of new experimental data from ongoing and recent measurements campaigns. As some of the ideas for treating the inherent multiscale physics are somewhat hypothetical, their reliability will first be explored and verified during the course of the project. However, we do not consider this to be a problem that will jeopardise the overall success of the project.