Turbulence is one of the most important and at the same time one of the least understood phenomena in nature. Weather, flow conditions in bodies of water and in industrial chemical or biological reactors, the bloodstream: wherever liquids and gases are in motion, hierarchies of vortices determine how energy propagates and what effects it has locally. The limitations of current predictive models are due in large part to the limited understanding of turbulence and its interactions with other factors.
Researchers at ETH Zurich, together with partners from other research institutions, have now developed a novel experimental method to measure the energy of vortices in fluids much more accurately and, above all, more simply – on scales ranging from a few millimetres to hundreds of metres. The method paves the way for much better predictions and has the potential to help take our understanding of chaotic motion to the next level.
Small change, big effect
Markus Holzner and Stefano Brizzolara of the Environmental Fluid Mechanics group – a joint interdisciplinary research unit of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and the ETH Domain’s EAWAG Aquatic Research Institute – have tackled the problem of turbulence measurement with a completely new approach. While previous methods tracked the movements of spherical marker particles, the scientists’ new procedure detects the movements of the ends of rigid fibres floating in the liquid. What appears to be a small change in the experimental setup has enormous implications for measurement effort and accuracy.
The ends of a fibre provide the necessary data
The major difference with the new method is that the rotations of the ends of a single fibre provide all the statistical data necessary to characterise a vortex’s motion and energy. Until now, the researchers had to use a high number of marker particles, and this amount was constantly rising. This is because floating spheres automatically distribute themselves in a liquid and the mean distance between them increases rapidly as a result of turbulent diffusion.
