A deep understanding of the fundamental mechanisms that drive turbulence is essential if we are to advance our knowledge of turbulent flows, which are present in many different fields, including nature, technology, and industry. These flows are unstable, irregular and seemingly random or chaotic. Their velocity varies irregularly over time and space, promoting the transport and mixing of matter, heat and momentum. The Navier–Stokes equations, which describe turbulence, have been around for 200 years. This means that we know a lot about turbulence. For instance, we know that the energy dissipation rate is constant at intermediate scales and that several scaling laws are valid (as proposed by A. Kolmogorov in his 1941 phenomenological theory). However, less is known about the smallest scales, where extreme events occur and where there is little experimental evidence.

In this group we investigate the smallest scales of both idealized and atmospheric turbulence. Below is a brief synopsis of the current projects:

Using the Göttingen Variable Density Turbulence Tunnel (VDTT), we can produce unique environments that allow us to explore fundamental questions of turbulent flows. We are presently involved in two main lines of research:

Stochasticity of turbulent flows

Turbulence has strong and seemingly random fluctuations. Conversely, the Navier-Stokes equations, which describe turbulence, are deterministic. The question of whether turbulence is random or deterministic is key to predicting flows in technology and in nature. Edward Lorenz posed this question in his pioneering work on the chaotic butterfly effect. Using the VDTT we can generate turbulence using an active grid, which allows us to repeat the similar initial conditions up to 30,000 times. This way, we can discriminate the deterministic and the random parts of the velocity field, giving us insights into the predictability of turbulent flows.

The dissipation range in turbulence.

We study the smallest scales of motion in turbulent flows by using helium and the active grid. This gives us experimental insights on the extreme phenomena occurring at these scales, including fluctuations of the velocity gradients and the exponential decay of the turbulent kinetic energy.

In collaboration with Dr. Bagheri and Prof. Dr. Bodenschatz

Understanding the individual and relative motion of particles is fundamental to understanding transport processes in complex flows. Large-scale transport in atmospheric flows is well understood, with monitoring stations and satellite observations providing valuable data. However, our knowledge of smaller-scale processes, such as those ranging from 100 metres to 50 kilometres, and the associated concentration fluctuations, is limited. Consider, for example, the transport of particles or tracers originating from the ground, such as gases or aerosols being released, smoke from a fire, or particles being blown into the atmosphere. To study local atmospheric transport, we conduct field experiments in which we release balloons simultaneously and equip them with small, lightweight radiosondes. We then study their Lagrangian dispersion and the local atmospheric conditions.