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PhD Defence Ruben Verschoof

Affecting drag in turbulent Taylor-Couette flow

Ruben Verschoof is a PhD student in the research group Physics of Fluids. His supervisor is prof.dr. D. Lohse from the faculty of Science and Technology. 

Approximately 90% of the world trade is transported by ships. This means that a small percentage of fuel savings massively impacts the overall fuel consumption, costs and greenhouse gas emissions. Air lubrication — using bubbles or an air layer as lubricating layer between the hull of the ship and the surrounding water — is seen as one of the most promising techniques to efficiently reduce the friction. On the other hand, vessels suffer from biofouling, and thus hulls becomes rough by attached barnacles, algae, slime and corrosion. The goal of this work is two-fold: (1) we want to understand how wall roughness influences turbulent flows, and (2) we want to understand how the friction can be efficiently reduced with air lubrication.

These types of flows are difficult to study: they consist of multiple phases — both air and water — and are highly turbulent, which means that the fluid movement is extremely chaotic. Furthermore, full scale ship experiments are extremely costly, and computer simulations are not yet advanced enough to fully resolve these types of flows. Therefore, a simplified test environment is necessary.

In the work leading to this thesis, Ruben and his colleagues studied wall roughness and air lubrication in a so-called “Taylor-Couette” setup. Taylor-Couette flow, the flow between 2 concentric, independently rotating cylinders, is one of the paradigmatic systems in which the physics of fluids are studied. Its advantages are multiple: it is a closed system with a well-defined balance between energy input and dissipation, it is well-accessible by experimental and numerical tools, and, thanks to its simple geometry, it can be build with high precision .

We found that large, deformable bubbles are crucial to efficiently reduce the drag. Furthermore, we explained that the bubble characteristics are largely influenced by the contaminations in the water. Therefore, laboratory results, often measured using distilled water, cannot be applied directly to the conditions encountered in seawater. A rough wall can make bubble drag reduction useless, as the roughness forces bubbles away from the wall.

We also studied one particular type of air lubrication: the use of air cavities. An air cavity is an air layer developed downstream of a “cavitator”, a sharp edged obstructing rib. We found that air cavities efficiently reduce the friction, but the friction is enhanced by the cavitator. Therefore, the balance between added drag by the cavitator and drag reduction is subtle.

One difficult question in the field of fluid mechanics is: how can lab results be upscaled to larger, or faster flow systems (systems with a higher Reynolds number)? It is generally not known a priori whether, and how, this is possible. We found that wall roughness triggers the so-called “asymptotic ultimate turbulence regime”. In this regime, results can be upscaled to arbitrary large dimensions or velocities without any further difficulties.