Turbulence is an ubiquitous phenomenon – and one of the great mysteries of physics. A research team from Oldenburg has now managed to generate real storm turbulence in the wind tunnel of the Wind Energy Research Center (ForWind).
Strong storms often seem to leave behind accidental destruction: While the roof tiles of a house are blown away, neighboring property may not be damaged at all. What causes these changes are wind gusts – or, as physicists say, local turbulence. It results from large-scale atmospheric flow, but so far, it is impossible to predict in great detail.
Experts from the University of Oldenburg and the Université de Lyon have now paved the way to study small-scale turbulence: The team led by Oldenburg physicist Prof. Dr. Joachim Peinke managed to generate turbulent currents in a wind tunnel. The streams resembled those that occur in large galleries. The team has found a way to literally cut a slice out 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 devices in which, for example, the effects of turbulence on wind turbines can be realistically investigated,” says Peinke.
The most important parameter that characterizes the turbulence of a flow is the so-called Reynolds number: This physical quantity describes the ratio of kinetic energy to frictional forces in a medium. In simple terms, you can say: The larger the Reynolds number, the more turbulent the flow. One of the biggest mysteries of turbulence is its statistics: Extreme events such as strong and sudden gusts of wind occur more often if you look at smaller scales.
“Eddy turbulence of a stream becomes sharper on a smaller scale,” explains Peinke, who heads the research group Turbulence, Energy Wind and Stochastics. In a severe storm – that is, when the Reynolds number is high – a fly is affected by much more interesting conditions than, for example, by an airplane. The specific reasons for this are not well known: the physical equations describing fluids have not yet been solved when it comes to turbulence. This task is one of the famous problems of the millennium of mathematics, in the solution of which the Balta Institute of Mathematics in the USA has decided from one million dollars each.
In the large wind tunnel of the Wind Energy Research Center (ForWind), the Oldenburg team has now managed to generate more turbulent wind conditions than ever before. Compared to previous experiments, the researchers increased the Reynolds number by a hundred times and thus simulated conditions similar to those encountered in a real storm. “We still do not see an upper limit,” says Peinke. “The turbulence created is already very close to reality.”
Experiments in the wind tunnel
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 network, which was developed for specific requirements in the old Oldenburg wind tunnel. The structure, measuring three by three meters, is located at the beginning of the wind tunnel and consists of almost a thousand small, diamond-shaped aluminum wings. The metal plates are removable. They can be rotated in two directions through 80 horizontal and vertical axes. This allows wind researchers to selectively block and reopen small areas of the wind tunnel mouth for a short time, causing the air to rotate. “With the active network – the largest of its kind in the world – we can generate many different dark areas in the wind tunnel,” explains Lars Neuhaus, who is also a member of the team and played a key role in the study. .
For the experiments, the team changed the motion of the grid in a chaotic manner similar to the conditions occurring in turbulent airflow. They also changed the power of the fans irregularly. Thus, in addition to small-scale turbulence, the air flow generated a greater 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, real storm turbulence,” explains co-author Dr. Michael Hölling. The physicist also chairs the International Wind Tunnel Testing Committee at the European Wind Energy Academy (EAWE). This storm turbulence came out 10 to 20 meters behind the active network.
Swirls on a small scale
“By arranging the network and the wind tunnel fans, we have generated a large-scale disturbance measuring about ten to one hundred meters. At the same time, a small-scale blur measuring several meters and less appeared spontaneously. However, we still do not know exactly why, ”explains Hölling. As he and his colleagues report, this new approach makes it possible to escalate atmospheric turbulence important to wind turbines, aircraft, or houses to a size of one meter in the wind tunnel. This will allow researchers to conduct realistic experiments with miniature models in the future – in which extreme explosions occur just as often as in real storms.
Reference: “Generating Atmospheric Turbulence with an Unprecedented Large Number of Reynolds in a Wind Tunnel” by Lars Neuhaus, Michael Hölling, Wouter JT Bos and Joachim Peinke, October 9, 2020, Physical review letters.
DOI: 10.1103 / PhysRevLett.125.154503
Funding: Federal Ministry of Economic Affairs and Energy of Germany, Ministry of Science and Culture of Lower Saxony, German Research Foundation.