First measurements in the Oldenburg wind tunnel

How do rotor blades deform in wind gusts?

In the research project TurbuMetric, scientists from the Jade University of Applied Sciences, the University of Oldenburg and the University of Applied Sciences Emden/Leer are investigating how wind turbines deform during turbulent inflow conditions. "The aim of our project is to develop measurement methods that show the deformation of the rotor blades while simultaneously measuring the wind field," says project leader Prof. Dr. Thomas Luhmann from the Jade University. "This knowledge will help to improve strategies for reducing loads and extending the service life of the rotor blades or to design new blades that operate well even in turbulent flows". There are already findings on the behavior of rotor blades in normal wind, but no suitable methods are available yet for measuring the behavior in extreme gusty wind situations.

First measurements in the wind tunnel
The scientists have now carried out first experimental tests in the large wind tunnel of the Center for Wind Energy Research (ForWind) at the Institute of Physics at the University of Oldenburg. This large research device, which is unique in its dimensions and characteristics, allows the researchers to generate turbulence in a controlled way and thus study realistic wind fields and their interaction with wind turbines. For the measurements the scientists use a model wind turbine with rotor blades of 90 centimeters in length – about 70 times smaller than in reality. The blades run at 600 rotations per minute, so that the tip of the blade reaches a speed of 57 meters per second. "We therefore have to develop a highly dynamic measuring method that covers the entire surface of the rotor blade and works in a non-contact way so that the measurements are not affected," explains Luhmann.

Make tiny particles visible and detect them
In order to be able to measure turbulent flow and deformation of the rotor blades simultaneously, two methods are combined: The Particle Image Velocimetry (PIV) method and photogrammetric measurements.
With the PIV method, a laser makes tiny particles visible, which are added to the air. High-speed cameras record the movements of the passing particles and the change of their positions from one image to the next is used to calculate flow direction and velocity. Each camera takes up to 12,000 images per second, which results in a very high temporal and spatial resolution. "Of particular interest is, when and how the flow detaches from the rotating blade and which wind situations lead to the largest impacts and thus to the greatest dynamic loads on the components of the wind turbine," explains Dr. Gerd Gülker, who is leading the sub-project at ForWind.

In the video the rotor blade of the model wind turbine runs in slow motion from top to bottom through the picture. The tiny particles make the turbulences of the air flow around the rotor blade visible. The video consists of single images taken at a rate of 5,000 frames per second.

The PIV method is combined with photogrammetric measurements. Four high-speed cameras observe the entire wind rotor and capture 200 images per second. "From the recorded image data, we can derive 3D point clouds and determine their deformations," explains Simon Nietiedt, researcher at the Jade University. "The result provides data for the entire surface of the rotor blades and is highly accurate – despite the high dynamics of the measured object". The scientists face the challenge to combine both systems in such a way that all data is represented in one system in a temporally and geometrically correct way.

About the project
The research project "TurbuMetric – Optical 3D measurement techniques for the acquisition of dynamic fluid-structure interactions in turbulent wind environments" is a joint project of the Jade University of Applied Sciences, the University of Oldenburg and the University of Applied Sciences Emden/Leer. Starting in 2018 it is granted for three years by the Lower Saxony Ministry of Science and funded by the European Regional Development Fund (EFRE) with around 1.3 million euros.

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