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Richard Stevens (promotion date: 30 June 2011)

Rayleigh-Bénard Turbulence

Promotion date: 30. June 2011

Promoters: Prof. Detlef Lohse (Twente) and Prof. Herman Clercx (Eindhoven)

Turbulent convection in a layer of fluid contained between two parallel plates heated from below and cooled from above - known as Rayleigh-Bénard (RB) convection - continues to be a topic of intense research.

This thesis covers three main topics: high Rayleigh number thermal convection, rotating RB convection, and 2D RB convection.

In rotating RB convection we find that when the rotation rate is increased, the heat transport first increases before it rapidly decreases. Furthermore, it is found that the onset of heat transport enhancement occurs with a sharp transition, which coincides with a transition between two different turbulent flow states: one dominated by a large convection roll in the whole cell for weak rotation, and one dominated by heat transport in local vertically-aligned vortices for strong rotation.

We analyzed the spontaneous flow reversals in 2D RB convection in computer simulations and found for intermediate Pr numbers, there is a diagonal large scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner flow rolls play a crucial role in the large-scale wind reversals we observe in this system. The results of these simulations were confirmed by experiments performed in the group of Prof. Ke-Qing Xia in Hong Kong.

Was your thesis project fundamental in nature or more application driven?

The research topic is fundamental in essence but applications are clearly visible at the background, in very different areas of interest even. These include heat transfer in industrial processes as well as water or air currents in weather forecasts, climate modeling and planetary processes.

The thesis theme is strongly connected to my master project in which I investigated the influence of rotation on heat transfer. The topic is relevant for the optimization of industrial applications. An example is the efficient separation of carbon dioxide (CO2) from nitrogen or methane gas in so-called rotational phase separators (RPS). For this project I received the Shell master prize.

What was the special research angle in your thesis project?

One could say that theory, experiments and numerical simulations were combined in a way never done before in this research area. Most of our simulations, which revealed the flow structures in detail, were carried out on the SARA supercomputer Huygens.

Experiments were carried out at the Turbulence and Vortex Dynamics group in Eindhoven, led by professor Herman Clercx and professor GertJan van Heijst. A specially built turntable is available there, which can be controlled very precisely and possesses a diameter of almost two meters. In addition, we collaborated with many groups around the world, for example with the group Guenter Ahlersin Santa Barbara and with Roberto Verzicco from the university of Rome.

The combination of experiments and simulations is appealing to me. Using simulations one can vary freely parameters and media used in a realistic environment, however using assumptions and simplifications of course. Coming to exciting outcomes from here, a next step is to build clever experiments performing real measurements in a tangible environment.

Coming further in the thesis project, I developed an in-depth feeling of the matter under study, being able to overview the whole system and being aware of the details playing a decisive role.

Did your research lead to nice publications?

Thirteen publications appeared, like in: Physical Review Letters, Journal of Fluid Mechanics, New Journal of Physics and Physics of Fluids, for example. Also I gave about twenty presentations at (inter)national conferences.

What are your future plans?

After my thesis defense, I am going to work on a post-doc position with professor Detlef Lohse in the Physics of Fluids group at the Mesa+ institute.

What, in your opinion, is important for Mesa+ to stay successful in the future?

The right academic atmosphere is decisive, I believe. Performing a rich variety of experiments and studying different topics is important for scientific progress. Some ideas will proof successful; others might not. But one of course does not know in advance.