Aerodynamics for wind turbines

The aerodynamics of wind turbines is greatly affected by turbulent conditions in the wind field. As such, the aerodynamics must be optimized to the wind field, and this is done by describing the latter in highly realistic conditions. Researchers have a stated aim to make this description available for uses including site assessment. Computational fluid dynamics (CFD) is being used more and more, for which Fraunhofer IWES mainly uses the open code OpenFOAM.

Wind turbines are machines driven by aerodynamics. Loads, performance, and also most of the noise they produce depend directly on the aerodynamics of the turbines. Developments that improve the aerodynamics of wind turbines are therefore an ongoing requirement.

Vorticity of a 2 MW wind turbine

At Fraunhofer IWES one main focus is on the enhancement of numerical methods for a better prediction of the aerodynamics using CFD within OpenFOAM. Most of the important vortex structures are already visible by using time-averaged simulations based on the Reynolds-Averaged Navier-Stokes (RANS) equations (see graph).

Analogy of simulation and experiment

In contrast, computations using the DDES approach („Delayed-Detached-Eddy-Simulation“) capture more turbulent structures and thus can deliver many detailed results as expensive experiments. The video shows the analogy of simulation and experiment: velocity field as with PIV (“Particle Image Velocimetry”), separation behaviour as with UV oil films and flexible pressure distributions as with many pressure tubes.

Displaying core factors: wind speed, revolutions per minute, power of a single blade, pitch angle (clockwise)

Beside the computation of full rotor blades within aero-servo-elastic conditions, the focus of our work lays on the computation of aerodynamic applications, so-called add-ons, for the improvement of the wind turbine aerodynamics. Also the automatized optimization of airfoils or blades is a central topic.

Control behavior can be analyzed in detail via aero-servo simulations in order to improve the accuracy of certified load simulation tools. The picture shows the behaviour of a wind turbine under an extreme operating gust (EOG) at rated wind speed.

Add-ons such as vortex generators (VG) are installed on blades more and more. In OpenFOAM VGs can be simulated using the BAY model (from Bender, Anderson and Yagle) or they can be fully-resolved in the mesh. The picture shows the results of using the BAY model at the airfoil NACA 63-415.

[1]  According to Bender, Anderson, and Yagle

Airfoil optimization can be done via the adjoint approach, since then the optimization is independent from the number of design parameters, which basically means that each airfoil point can be an individual design parameter. For example, the graphs show a lift optimization in which a predefined lift coefficient shall be reached.

Validated CFD tools are used for design and optimization of aerodynamic geometries, where complete 3D flow effects are also considered. For the individual design of blades, airfoils can be analyzed in detail or improved by validated simulation methods.  Validated simulation methods can be used for analyzing airfoils or blades in detail or for further improvements of the individual blade design. These methods consider the relevant turbulence of the flows which is especially important for blade design. Also blades for uncommon use such as vertical axis wind turbines (VAWT) or downwind turbines can be designed or improved by these tools.

Efficient power monitoring with dynamic power curve

Stochastic methods provide a broad range of analysis for an environment characterized by an incoming turbulence. In collaboration with ForWind at the university of Oldenburg, Fraunhofer IWES promotes these methods, e.g. CTRW wind field model, continuous time random walk model as well as the dynamic power curve for power monitoring. A main method is the analyses of noise profiles. Different sources of noise and deterministic dynamics can be separated in a signal. This process can be applied on all kinds of data, which are influenced by deterministic and random parts.

The dynamic power curve is a quick and cost effective method to monitor the power output of wind turbines and whole wind farms. The software is based on stochastic examination method, which enables the user to determine a turbine´s power curve in only a few days by using hub anemometer and power output data. The generated data provide an overview of the wind turbine functionality for manufacturers, operators, service companies and public utilities. Deviating behavior of same type turbines is reliably detected, so that improvements of turbines and wind farms result in a reduction of yield losses.

The continuous time random walk (CTRW) model

The base of a realistic load calculation for wind turbines is a good model of the incoming wind field. However, wind field generators based on Gaussian distributions have problems to reproduce the correct distribution of changes in wind speed compared to real wind fields. In contradiction to the model, a realistic wind field comprises a large amount of incidences and its peak wind speeds outstrip the calculated Gaussian prediction by far. The “continuous time random walk” (CTRW) model is able to generate correlated wind fields with realistic conditions. In collaboration with ForWind at the university of Oldenburg, Fraunhofer IWES offers a wind field generator based on the CTRW method.

Fraunhofer IWES uses primarily the open source code OpenFOAM or the derivative FOAMextended for CFD simulations. Even though the code is open, it is quite challenging to understand the program and apply it efficiently. Fraunhofer IWES offers OpenFOAM courses for users with different levels of knowledge:

  • Introductions to OpenFOAM
  • Courses for the use of OpenFOAM in site assessment or aerodynamics
  • Courses on programing in OpenFOAM