Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - TROCONVEX (Turbulent Rotating Convection to the Extreme)
Teaser
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...
Summary
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.
Work performed
The main progress on the experimental side of the research is the completion of the experimental setup (a picture is enclosed). It is now the tallest rotating convection experiment in the world, with an unprecedented range of experiment conditions to consider. So much so, in fact, that we are continuously gathering data. Every single experiment may last anywhere from one to four days. The most interesting experiments take the longest; they are at conditions for which rotation is truly dominant in the flow and buoyant energy input is rather low. We have also made sure to benchmark the setup by comparing to earlier heat transfer measurements for convection without rotation and a very good agreement is reached. Currently we are measuring at a height of 2 m, after which the full 4 m setup will follow.
Concurrently with the heat transfer measurements we have been preparing the experiment for optical flow measurement inside the cylinder. A new transparent section has been designed that can be put in place of one of the other cylinder sections. It has been ordered; once it is finished we can start working on illumination and placement of cameras that can follow the motion of tiny tracer particles suspended in the flow to get a picture of the flow inside, to look for the new geostrophic turbulence state.
The results from the experiments are further elucidated by accompanying numerical simulations. We have been able to match the experimental and numerical results in the range of parameters where that is possible, so we are quite certain of their validity. In the simulations we have additionally considered different working fluids. The most interesting observation is the formation of a large-scale vortex (a picture showing vorticity, a measure for the amount of swirl in the flow, positive for anticlockwise and negative for clockwise swirl) in convection of a fluid with a high thermal conductivity, like a liquid metal. The large-scale vortex has thus far only been observed in computations with artificial stress-free boundary conditions; here it is observed for realistic no-slip boundary conditions as in the experiment. Such vortices could be the flow structures causing swirling, helical currents in the Earth’s liquid-metal outer core that generate Earth’s magnetic field.
Final results
In the remainder of the project we will use our experiment to dive deeper into the regime of rapidly rotating convection. The best part is yet to come: the 4 m setup. With both heat transfer and flow measurements we can learn a lot about the flow and form predictions for the even larger natural flows alluded to before.
At the same time we continue the numerical simulations to get detailed information on many aspects of this flow that are not directly available from the experiment. An important example of such information is the flow in the boundary layers. These are the layers which form very close to the bottom and top plate which take care of the matching of the flow in the central part to the solid plates. It is well-known that these layers, with thickness down to a fifth of a millimetre in the most extreme experiments, are still highly decisive in the overall flow behaviour. We can study these in detail in the simulations. Additionally, the effects of changing the fluid as described before will be considered. The flow behaves quite differently when considering a liquid metal (as in the Earth’s core), a gas (as in the giant gas planets), water (as in the ocean), or maybe even a more viscous liquid.
The final goal of the project is to compose all this information into a model to predict heat transfer in the rapidly rotating regime of turbulent convection. This is a very welcome tool for people modelling the natural flows mentioned before, that they can expand with the decisive extra effects that their system will inevitably have. Extra physics such as magnetic fields, large density and/or temperature changes due to pressure, internal heating, radiative heat transfer, etc. will certainly play its role and can be added to the basic flow model.