“You can feel the air vortices behind aeroplanes if you stand at the end of the runway at Frankfurt Airport,” says Martin Oberlack. The professor likes to use everyday examples to illustrate his subject: “Hot water sloshes about more quickly in a saucepan than cold water,” he says. This is because it is less viscous and therefore more prone to turbulence. “If you stir honey, hardly any vortices form,” Oberlack explains.

This brings us straight to the heart of the problem. “Turbulence has an unpleasant property,” says Oberlack: the vortices do not occur in a single size, but across a whole spectrum – from large to minuscule. And the more fluid – or, in technical terms, the less viscous – a medium is, the smaller the vortices that occur. Because air is low-viscosity, this problem is particularly acute in aerodynamics, for example in aeroplanes. The limited computing power of computers makes it practically impossible to simulate even the smallest vortices, something developers would very much like to do in order to optimise their designs. Less turbulence means less drag and therefore fuel savings. Even the smallest savings would be welcome: “In aviation, every percentage point of fuel saved counts,” says Oberlack.

The alternative to simulations is wind tunnel testing using aircraft or vehicle models. However, these are smaller than the originals, meaning that the turbulence does not correspond to that of the real aircraft – this can only be partially compensated for, for example by using a colder, faster airflow. Yet even these experiments have their limits. Variables such as pressure or flow velocity cannot be measured with arbitrary precision. “You either measure with high resolution in a small part of the model, or everywhere, but with less detail,” explains Oberlack. Computer simulations, on the other hand, have a major advantage: “For every point in space and every point in time, we have access to all the data,” says the aerospace engineer.

Simulations do benefit from the fact that supercomputers, such as the one at the Jülich Research Centre, are becoming ever more powerful. But this is of little comfort to Oberlack: “If things continue at the same pace, it will be 40 years before we are able to simulate a commercial aircraft on a supercomputer,” he says. His team is therefore working on simplified mathematical methods. Together with colleagues from the Polytechnic University of Valencia, they achieved breakthroughs in 2022 that were published in renowned specialist journals.

The research team found a shortcut to useful results with a reasonable computational cost. “As engineers, we’re often not interested in every single data point; statistical values, such as the mean or variance of the pressure, are sufficient,” explains Oberlack. Yet even calculating comprehensive statistics was impossible before the breakthrough – this would have required solving an infinite number of equations. Thanks to Oberlack’s transformations, only a finite number of equations now need to be solved – a task feasible for supercomputers, but still computationally intensive.

Now the team of around 20 is taking the next step: in 2025, a latest-generation exascale supercomputer went into operation in Jülich. Computing time on ‘Jupiter’ is highly sought after by scientists. “We’ll be able to run a very large simulation there over the coming months,” says Oberlack. The researchers aim to use this to investigate the velocity distribution at individual points in the simulated model in greater detail. This deviates from the “normal distribution”, as applies, for example, to body height – people taller than two metres or shorter than one metre are very rare. “With turbulence, it’s different,” explains Oberlack. Extreme deviations upwards or downwards – that is, extremely high or low air speeds – are relatively common. This is of interest to the Darmstadt-based group, which hopes to gain deeper insights into the nature of turbulence. If they succeed, turbulence can be simulated with significantly less computational effort. Then, at the start of the runway, you might no longer feel a breath of air when an aeroplane flies overhead.

Christian J. Meier

“International Symposium on Turbulence and Shear Flow Phenomena” (TSFB)

From 28 to 31 July, one of the most important conferences in the field of turbulence research, the “International Symposium on Turbulence and Shear Flow Phenomena” (TSFB), will take place in Heidelberg. In this interview, the main organiser, Professor Martin Oberlack, Head of the Department of Fluid Dynamics at TU Darmstadt, talks about the importance of turbulence research in general and the role of the TSFB in particular.