Many large-scale flows in nature are thermally driven and subsequently shaped due to rotation of the planet or star on which these flows exist. Examples abound: Earth’s atmosphere and oceans, its liquid-metal interior (the so-called outer core), the interior of the giant gas planets in our solar system; even the outer layer of our Sun is flowing largely according to these basic forces. The effects of rotation are not intuitive: it is, for example, the reason why low-pressure areas on the northern hemisphere always rotate counterclockwise and high-pressure areas clockwise, a result of the dominant so-called geostrophic force balance between pressure gradient and Coriolis force. In deeper layers than the rather shallow atmosphere rotation generally leads to an organisation of the flow into columnar whirls or vortices aligned with the rotation axis. Researchers have found that this classical picture, taken mostly from relatively small-scale laboratory experiments, changes when looking at the limit of very large rotating systems (think of planets and stars). Instead, a new state called “geostrophic turbulence” is found, the properties of which are not quite known yet.
We want to explore this new state with a unique custom-designed experiment and with cutting-edge computer simulations. One of the most important aspects to know is the amount of heat that can be transported through a layer of gas or liquid. This is a major contribution to any energy budget model for these natural flows. We can directly measure the heat transport in our large experiment, a rotating cylinder with a height of up to 4 m, filled with water, heated from below and cooled from above. By the sheer size of this experiment we can access the new geostrophic turbulence state better than any other previous effort. The numerical simulations provide additional information: we can look at every detail of the flow. This step is also carried out in the experiment, though we can only see the flow in a small cross-section of the entire cylinder. Still, this will be enough to compare results from experiment and simulation..
The results will bring a better understanding of the natural flows mentioned above. We will give valuable input to get a grip on the origin and generation of Earth’s magnetic field, that is formed in the outer core and shields us from harmful radiation from space. We get to know more about the interior of the giant gas planets and the Sun. Additionally, these results may also contribute to climate modelling.
The main conclusions at the end of the project are several. We have gained an understanding of the various flow features observed in rapidly rotating convection by identifying the principal forces (e.g. inertia, Coriolis force, viscous effects). We have reproduced these flow features in great detail in the experiment. One surprising result is the generation of a so-called wall mode, a strong and persistent circulatory flow near the sidewall, that is quite decisive for the overall flow behaviour. The wall mode has been studied in detail, though its effect on the bulk flow is not yet completely clear. Another major finding is the formation of large-scale domain-filling vortex structures, both in simulations and in experiments, a sign of upscale transfer of energy contrary to the usual energy transfer down the spatial scales. A significant step towards true extrapolation to large-scale flows has been set, although new questions (like the wall mode) have popped up along the way.
