Farewell to Darmstadt

view from balcony

Another part of my life came to an end in October. Writing about it puts the question in which attitude I do it. I want to write thankfully. In 2009, I came to Darmstadt. First I worked at the Technical University, then at the accelerator GSI. I wrote a monography. For 10 years I took part in the workshop band of Jürgen Wuchner who died in 2020. Eight times I participated in the Jazz Conceptions, a one week summer workshop. I enjoyed the vibrant music scene, especially the concerts at Jazz Institute and at Knabenschule. The Frankfurt Radio Big Band played at Centralstation. Darmstadt also has a Staatstheater (public theatre) with opera. Each summer the Jürgen Wuchner Workshop Band gave a promenade concert in Orangerie Garden if it did not rain. I feel sad that I had to leave friends. Certainly will I return as a visitor.

Wavy Falling Films

As an example from the broad field of wavy falling films we take a simulation created with OpenFOAM. The picture displays a water falling film of a Reynolds number of 60 being harmonically excited with a frequency of 13 Hz. Gravitation points from the left to the right. The control volume is 600 units long (in the direction of the stream). One unit is the thickness of the corresponding smooth film. The height (perpendicular to the stream) is equal to 4 units being magnified by a factor of 50.

The red colour marks the water, the blue the air. Downstream a certain pattern can be recognized: A large solitary wave and in front of it small capillary waves.

Falling Films

In process engineering a falling film is a thin liquid film which flows down an inclined plate by the action of gravity. Falling films are employed in various fields, like the food industry, the pharmaceutical industry or in power plants. They have the advantage of a good heat transfer and a small hold up. Furthermore, they have a large surface for the exchange between gas and liquid (like in gas scrubbing).

A falling film flowing down a vertical wall is always hydrodynamically unstable. However, if the Reynolds number is very small, the waves may not be seen.

In the EU project EasyMED I simulated evaporating seawater falling films. Turbulence wires were employed to improve the heat transfer.

Current Leads

Current leads are a cryo-electrical component of the circuit of a superconducting magnet, a pair for each electrical circuit. One end, the warm terminal, has ambient temperature, about 300 K (Kelvin). The other end, the cold terminal, has the temperature of liquid helium, approximately 4 K.

If the cross-section of the conductor is too large, a lot of heat comes from the warm to the cold terminal. On the other hand, if the cross-section is too small, the electrical current will cause a significant heating and the current leads may burn through. The designer has to find a compromise.

There are current leads which are only cooled at the cold terminal. This is the conduction cooled type. The vapour cooled current leads guide the helium vapour all along the whole length from the cold to the warm terminal. Sometimes the temperature is fixed in between with liquid nitrogen (77 K) by the employment of a thermal anchor.

Computational Fluid Dynamics

In CFD a computer solves the discretized equations on a numerical grid which should be fine enough in order not to neglect important features of the flow in between. Beside of the real experiment and pure theory it has established itself as the third column of fluid dynamics. In contrast to the real experiment, the numerical experiment offers much more data. Pure theory has the disadvantage that it can only provide solutions of few simple problems. It shall not be concealed that by far not all situations of interest can be simulated accurately. For example, in most cases turbulent flows have to be modelled which involves simplifications.

A mocking interpretation of CFD is Colors For Dollars, since the visualisation is very colourful and the software in most cases expensive. Alas, this is only the opinion of the mockers! 🙂