Ivo Peters (promotion date: 29 June 2012)

Free surface flow focusing

Promotion date: 29. June 2012

Promotor: Prof. dr. Detlef Lohse

Assistant promotor: Dr. Devaraj van der Meer

Reducing the area through which a fluid is allowed to flow often leads to an increase of flow velocity.

When an axisymmetric object (like a circular disc or a sphere) impacts on a water surface, a surface cavity is formed, which collapses and after pinch-off, two jets are formed: one shooting upwards and one downwards. The velocities of the jets and also of the water surface just before the pinch-off, are usually much higher than the velocity of the impacting object. This is because the free surface evolves in such a way, that it focuses the liquid towards the axis of symmetry of the system.

We show that not only the liquid phase, but also the entrained air attains high velocities. A subtle interplay between the air flowing out and the water rushing in, eventually leads to an air flow that attains supersonic speeds.

By impacting discs with a non-axisymmetric disturbance, experimentally a dramatic effect is shown on the evolution of the collapsing cavity. Perturbations that initially existed - no matter how small – persist throughout the collapse in an oscillating manner, with growing relative amplitude. As a consequence, non-linear effects start to influence the shape of the cavity.

These findings, and others, can be used in, for example, research into inertial fusion energy and the formation of stellar jets.

The approach chosen in this thesis project, did it lead to special results?

The idea of pulling a disc through a water surface using a linear motor, to mimic the free fall of an object in a more controlled fashion, was done before in our group. Investigating the air flow and using irregular shaped discs however, led to some surprising results.

The entrained air is moving along with the disc, leaving behind an ever smaller opening for the air to escape to the atmosphere. Water is vigorously flowing towards the centre, pushing the air out at supersonic speeds. The work was an interaction of experiments, using laser sheet illumination of smoke particles inside the cavity to visualize the air flow, together with numerical simulations and theoretical analysis.

The use of different shapes of discs, and recording the unexpected cavity shapes using high speed cameras, led to amazing pictures which appeared recently on the cover of the Journal of Fluid Mechanics. Master student Oscar Enríquez - currently doing a PhD in the group - contributed to this substantially, experimenting with different kinds of disc shapes.

By experimenting with discs impacting on an oil layer, we could convincingly proof that the jet that is formed right after closure, a cavity is formed from liquid on the surface of the cavity.

Are applications in future possible with respect to these findings?

The project was curiosity driven in the first place. However, observing something in a lab does not mean that the same phenomenon does not exist outside the laboratory. For example, focusing a plasma to one single point using inertial forces, is a helpful tool in nuclear fusion energy processes. To control these processes it is very important to understand what happens to irregularities when a singularity is approached.

The research was subsidized by the Spinoza grant professor Detlef Lohse gained, some years ago. Therefore I was privileged to investigate scientific matters with a lot of freedom. This suited me very well, since I like to design and perform experiments on a short notice. The field of fluid mechanics can benefit from this approach since a lot of phenomena are still left awaiting for exploration. As a PhD I learned to write down the findings and ideas at an earlier stage than I used to. Doing so research obtains more direction and focus.

What are your future plans?

The next two years I am going to work at the University of Chicago as a post-doc. Here granular materials and suspensions are the main topic of research. For example, jets can occur in these systems too. Granular materials and suspensions are found everywhere, varying from grain and corn to avalanches.

Did you feel part of the Mesa+ community?

When I started, my project was part of the Impact institute within the University of Twente. The project was later transferred towards Mesa+, about half a year ago. I never performed any experiments in the famous cleanroom.

The Mesa+ Day was important though, as I could make a great contribution in studying immersion lithography processes. Here, a water droplet is sliding along between the lens and the silicon wafer surface to increase optical resolution. Increasing the sliding speed, the shape of these droplets show significant changes. Understanding how these shapes are formed is important in optimizing the working speeds in these processes.

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

I am a strong advocate of free and fundamental research. Universities and institutes should not be forced to perform application oriented research too much. In that case, one is already biased by the goal of the research. The charm and extra value should be to explore new fields and be surprised by the results there. Coincidence and chance should be give free space in these processes.