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Severe storms often seem to leave random destruction behind – as the shingles of a house are blown away, neighboring property may not be damaged at all. What causes these differences are gusts of wind or, as physicists say, local turbulence. It is the result of large-scale atmospheric flows, but until now it is impossible to predict in detail.
Experts from the University of Oldenburg and the Université de Lyon have now pioneered the study of small-scale turbulence: the team led by Oldenburg physicist Prof. Dr. Joachim Peinke managed to generate turbulent flows in a wind tunnel. The flows resembled those that occur during great winds. The team found a way to literally cut a slice of a storm, the researchers report in the journal Physical Review Letters. “Our experimental discovery makes our wind tunnel a model for a new generation of such structures in which, for example, the effects of turbulence on wind turbines can be studied realistically,” says Peinke.
The most important parameter that characterizes the turbulence of a flow is the so-called Reynolds number: this physical quantity describes the relationship between the kinetic energy and the forces of friction in a medium. In simple terms, you can say: the higher the Reynolds number, the more turbulent the flow. One of the biggest mysteries of turbulence is its stats: extreme events like strong and sudden gusts of wind occur more frequently when looking at smaller scales.
Unsolved equations
“Turbulent eddies in a flow become more severe on smaller scales,” explains Peinke, head of the Turbulence, Wind Energy and Stochastics research group. In a strong storm, i.e. when the Reynolds number is high, a fly is therefore affected by much more pleasant flow conditions than, for example, an airplane. The specific reasons for this are not well known: the physical equations that describe fluids are not yet solved when it comes to turbulence. This task is one of the famous mathematical problems of the millennium, on which the Clay Mathematics Institute in the United States has invested a million dollars each.
In the large wind tunnel of the Center for Wind Energy Research (ForWind), the Oldenburg-based team has now managed to generate more turbulent wind conditions than ever. Compared to previous experiments, the researchers increased the Reynolds number one hundred times and then simulated conditions similar to those encountered in a real storm. “We don’t see an upper limit yet,” says Peinke. “The turbulence generated is already very close to reality”.
Wind tunnel experiments
The Oldenburg wind tunnel has a 30 meter long test section. Four fans can generate wind speeds of up to 150 kilometers per hour, which corresponds to a category 1 hurricane. To create turbulent airflow, the researchers use a so-called active grid, which was developed for special requirements in the Great Oldenburg Wind Tunnel. The structure, three meters by three, is located at the beginning of the wind tunnel and is composed of almost a thousand small diamond-shaped aluminum wings. The metal plates are movable. They can be rotated in two directions via 80 horizontal and vertical shafts. This allows wind researchers to selectively block and reopen small areas of the wind tunnel nozzle for a short time, causing an air swirl. “With the active network – the largest of its kind in the world – we can generate many different turbulent wind fields in the wind tunnel,” explains Lars Neuhaus, who is also a member of the team and played a key role in this study.
For the experiments, the team varied the movement of the grid in a chaotic manner similar to the conditions that occur in turbulent airflow. They also changed the power of fans erratically. Thus, in addition to the small-scale turbulence, the airflow generated a larger movement in the longitudinal direction of the wind tunnel. “Our main finding is that the wind tunnel flow combines these two components into a perfect and realistic storm turbulence,” explains co-author Dr. Michael Hölling. The physicist also chairs the International Committee for Wind Tunnel Tests of the European Wind Energy Academy (EAWE). This storm turbulence emerged 10 to 20 meters behind the active grid.
Small-scale swirls
“By adjusting the wind tunnel grille and fans, we generated large-scale turbulence ranging in size from ten to one hundred meters. At the same time, small-scale turbulence with dimensions of a few meters and less appeared spontaneously. However, we don’t know. yet exactly why “, explains Hölling. As he and his colleagues report, this new approach reduces atmospheric turbulence related to wind turbines, aircraft or homes down to a size of one meter in the wind tunnel. This will allow researchers to conduct realistic experiments with miniaturized models in the future, where extreme gusts occur with the same frequency as real storms.
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Materials provided by Oldenburg University. Note: The content can be changed by style and length.
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