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Technology & Innovation

FLOW TUNNEL

Text: Tomas van Dijk

A new flow tunnel in the building of 3mE (Mechanical, Maritime and Materials Engineering) is replacing its 1950s predecessor. In it, scientists are studying the impact of air lubrication on a ship’s fuel consumption and how vapour bubbles can affect a ship’s propeller.

The huge stainless-steel colossus weighing 14 tonnes, with more than 17 m³ of water flowing through it, covers three floors of the building. “We’ve turned a ship inside-out”, says fluid dynamics researcher Dr Cornel Thill. Inside it, is the water and (a model of) the ship’s propeller, and protruding on the outside are the ribs and supports that prevent the tank becoming deformed by more than 0.4% at an operating pressure of two bar. It will be used for two types of research: air lubrication and cavitation. Air bubbles can reduce the energy for ship propulsion by up to 15%. “We aim to improve our understanding of that friction reduction”, says Thill. Since bubbles behave differently in salt- and freshwater, the tunnel has been equipped for both marine and inland waters.

Vapour bubbles

The other research area, cavitation, concerns the formation of vapour bubbles in water next to a ship’s propeller, which can become damaged. Provoking cavitation often requires the use of negative pressure or vapour pressure, whereas a positive pressure of two bars is required for air lubrication research up to a depth of 20 m. These conflicting requirements resulted in a complex design. Taking five years to develop, the tunnel cost €1.5 million, which was shared by TU Delft and the Dutch Research Council (NWO).

There are additional steel girders in various places to support the weight.

 In the cellar, a 110-kW pump propels the water upwards.

 The water then flows slowly (up to 1.56 m/s) through a wide flat box.

 The box is used to capture and remove any air bubbles.

Inside, there are three sets of seven parallel roof-shaped channels to capture the bubbles.

 After two sharp curves and a honeycomb for equalisation, the flow increases up to 13 m/s.

 In front of the window, air can be added for air-lubrication experiments.

 The polycarbonate windows (4 cm thick) on the front, underneath and back ensure a perfect view of the experiment.

 Force transducers in the ceiling measure the friction of the flowing water.

 There are storage tanks in the cellar for salt- and freshwater not in use.

© Stephan Timmers