ENGINEERING AND CFD
Enhanced Aerodynamics of Wind Turbines
Principal Investigator:
Thorsten Lutz
Affiliation:
Institute of Aerodynamics and Gas Dynamics, University of Stuttgart (Germany)
Local Project ID:
pr94va
HPC Platform used:
SuperMUC of LRZ
Date published:
Introduction
Within the present project, the aerodynamic behavior of modern wind turbines has been investigated by the use of CFD. Three main sub-topics have been studied:
• The effect of inflow turbulence on the transient turbine loads.
• The possibility to control the rotor loads by applying trailing edge flaps to the outer part of the rotor blade.
• The analysis of the flow around the turbine nacelle.
Additionally, investigations have been conducted on the impact of complex terrain, aero-elastic rotor blade deformation and complex inflow conditions.
Results and Methods
All simulations have been performed with the finite-volume CFD code FLOWer, developed by the German Aerospace Center (DLR) within the MEGAFLOW project. It solves the Navier Stokes equations in an integral form using different turbulence RANS and hybrid RANS/LES models.
The simulations requiring the highest resources were the ones regarding inflow turbulence, where 236,000 CPU hours have been used for each case (6 cases in total). The used space in the $HOME is around 50%, while in the $WORK is around 92% of the available budget. The used code is not producing an exceeding amount of files that is why, up to now, no problem was faced in the file storage.
The influence of inflow turbulence has been studied within the European project AVATAR [1], using the reference rotor of the project with radius R=102.88 m and simulating three different levels of turbulence intensities (TI) at the turbine location, see table 1. As can be seen, the inflow turbulence has an impact on the mean power and thrust (denoted CP and CT respectively) showing a continuous increase with increasing TI.
Cases | 10L | 10M | 10H |
| TI [%] | 4 | 7.3 | 12.5 |
| CP | 0.411 | 0.431 | 0.457 |
| CT | 0.614 | 0.627 | 0.643 |
Figure 1: Interaction of the rotor with a high level of atmospheric turbulence.
Copyright: IAG, University of StuttgartTurbine loads and fluctuations are increasing with the rotor diameter. Active trailing edge flaps [2] at the outer rotor blade part are a modern aerodynamic concept for load and fatigue control, since flaps allow modifying the airfoil lift at given angle of attack. The result of the research was that the flap needs to be centered at 80-85% on the blade radius with an extension of 10-15% along chord in order to avoid separation, depending on the turbine.
Figure 2: Harmonic flap oscillation (6p) – Vortices in the blade wake
Copyright: IAG, University of StuttgartWithin the national AssiST research project, CFD analysis of the flow around a wind turbine nacelle have been performed in order to improve the understanding of the complex flow physics and to derive means to reduce flow separation in this area and in this way to increase the turbine’s efficiency. Simulations have been compared to a simulation of an isolated rotor, see Figure 3.
In the connection area between the blade and nacelle, the interference of the two boundary layers create a thicker one that cannot overcome the adverse pressure gradient of the blade and separation occurs. To avoid this, the corner of the nacelle has been rounded in order to increase the radius where the boundary layer is thicker. As can be seen in Figure 4, the separation is in this way eliminated resulting in a slight increase of axial and driving forces.
Figure 3: Vortex around the rotor. Isolated rotor (left) rotor with nacelle (right).
Copyright: IAG, University of Stuttgart
Figure 4: Surface streamlines of the flow around the baseline nacelle (left) and an optimized nacelle (right)
Copyright: IAG, University of StuttgartOn-going Research / Outlook
The project has been prolongated in order to study the following topics:
• aero-elastic effects on wind turbines in complex terrain.
• vibration and acoustic analysis by onshore wind turbines.
Within the project WINSENT (WINd Science and ENgineering in complex Terrain) [3] new numerical models are developed in order to take into account turbulent inflow conditions, complex terrain [4] and aeroelasticity. Complex terrain is a terrain whose topology and roughness is influencing the atmospheric boundary layer (ABL), see Figure 5. Results will be compared within different institutions involved in the project and with field results from the two research wind turbines that are going to be erected in Stöttener Berg, in South Germany. Fluid Structure Interaction (FSI) CFD-CSD coupled simulations [5] will be run in cooperation with TU Munich using both beam and shell elements to model the turbine structure properties.
Figure 5: Velocity distribution in complex terrain.
Copyright: IAG, University of StuttgartWithin the national project TremAc, emission and immission of vibrations and low frequency noise from wind turbines are simulated. There can be different causes for noise, like non-uniform inflow conditions, atmospheric turbulence, flow separation, tower impact, rotor tilt and especially aero-elasticity. In particular for this last point, the coupling between the CFD flow solver FLOWer and the MBS solver SIMPACK has been extended.
Simulations including turbulent inflow conditions, aeroeleasticity and/or complex terrain are really expensive because they require fine meshes, small timesteps and high order numerical schemes. Most of the simulations have been run on SuperMUC phase 2 and with the upcoming machine SuperMUC-NG it is hoped to increase the simulation velocity, saving in this way also computational time.
References and Links
[1] http://www.eera-avatar.eu/
[2] E. Jost, T.Lutz and E. Krämer. 2016. A parametric CFD Study of Morphin Trailing Edge Flaps on a 10 MW OFFshore Wind Turbine. Energy Procedia, vol. 94, pp. 53-60
[3] www.windfors.de/english/test-site.html
[4] T. Lutz, C. Schulz, P. Letzgus and A. Rettenmaier. 2017. Impact of Complex Orography on Wake Develpment: Simulation Results for the Planned WindForS Test Site. Wake Conference, 2017.
[5] M. Sayed, T. Lutz, E. Krämer, S. Shayegan, A. Ghantasala, R. Wüchner and K.-U Bletzinger. 2016. High fidelity CFD-CSD aeroelastic analysis of slender bladed horizontal-axis wind turbine. Torque conference (2016), J. Phys.: Conf Ser. 753 042009
Research Team:
Giorgia Guma, Eva Jost, Levin Klein, Thorsten Lutz (PI), Pascal Weihing
Scientific Contact:
Dr.-Ing. Thorsten Lutz
Institut für Aerodynamik und Gasdynamik
Arbeitsgruppe Luftfahrzeugaerodynamik und Windenergie
Universität Stuttgart
Pfaffenwaldring 21, D-70569 Stuttgart (Germany)
e-mail: thorsten.lutz [@] iag.uni-stuttgart.de
NOTE: This report was first published in the book "High Performance Computing in Science and Engineering – Garching/Munich 2018":
LRZ Project ID: pr94va
November 2018