Cavitation is a phenomenon that causes shipping a lot of problems – damage to propellers, noise in the marine environment and vibrations in the ship to name a few. Determined to study this process in depth, Professor Tom van Terwisga of the Department of Maritime and Transport Technology (MTT) and Professor Jerry Westerweel of the Department of Process and Energy (P&E) worked together to design a new type of Cavitation Tunnel – a one-year Cohesion project that led to the construction of an experimental facility that is unique in the world.
“We were already co-operating a lot,” says van Terwisga, referring to the MTT and P&E departments, “and we did have a very old Cavitation Tunnel from the 1960’s. But it was leaking – not just water, it was also taking in air, which meant the air content of the water in the tunnel changed continuously during operation.” As it’s recently become clear that air content in water is an important factor in causing vibration problems in ships with cavitating propellers “…we wanted to have a new facility in order to study exactly how cavitation develops and what we can do about it.”
Tiny bubbles making holes in steel
Cavitation happens when bubbles of vapour form in water under low-pressure conditions – and then suddenly collapse when the pressure increases again, creating little shockwaves. When this is repeated over and over again, as happens with pumps and propellers for example, it can cause metal fatigue. “You can actually see the same process in your kettle at home. As the water heats up, you see bubbles being created at the bottom, which grow as the temperature approaches boiling point – at which point, it starts to make quite a bit of noise as well. Well you can also get the same effect, not by increasing the temperature, but by decreasing the pressure.” This is the same phenomenon that requires water in our kitchen kettle to reach 100 degrees Centigrade before it boils yet on top of Mount Everest where the ambient pressure is much lower it’s already boiling at around 60 degrees Centigrade. “And if you lower the pressure even more, water starts bubbling at 15 degrees Centigrade, which is sea-water temperature - and that’s what happens in the vicinity of the propeller.”
You can actually see the same process in your kettle at home. As the water heats up, you see bubbles being created at the bottom, which grow as the temperature approaches boiling point – at which point, it starts to make quite a bit of noise as well.
Fish and sea mammals
Not only do cavitation bubbles cause damage, they also cause radiated noise around the propeller: “Aside from engine noise, cavitation is the main source of noise in a ship,” says van Terwisga. “At lower speeds, machinery noise is dominant but at 10 knots or more, which is typical of most sea-going ships and ferry-boats, cavitation on the propeller is unavoidable and it becomes the dominant source for radiated noise, which is very disturbing to fish and sea mammals.”
Nowadays it’s possible to design propellers that avoid cavitation but they come at the cost of energy efficiency, which is not ideal in a world that wants to reduce carbon emissions: “So we need to find a balance between acceptable cavitation, acceptable radiated noise from the propeller and yet the highest possible efficiency.”
From Cohesion project to Multi-phase Flow Tunnel
In 2016, van Terwisga and Westerweel proposed a Cohesion project with the goal of specifying and designing a new type of cavitation tunnel – one that operates on sea water: “Salinity has a significant effect on cavitation and all cavitation tunnels built so far are operating in fresh water yet most ships sail on salt water. So the director of the maritime research institute said to me if you want to build a new cavitation facility, make sure that it’s unique and fill it with salt water, and we said Yes that fits in seamlessly with our plans.”
After a year, working with a young researcher, Swaraj Nanda, van Terwisga and Westerweel had a draft design: “And that starting point was sufficiently convincing to get funding from the Dutch Science Foundation, NWO, to elaborate a more practical design for the new cavitation tunnel and build it. This became part of the “AQUA” programme on the effects of water quality on multi-phase flows in maritime applications. So really the Cohesion project seeded the plans for the further development of what was to become our Multi-phase Flow Tunnel.”
As is often the case however, the project became much more expensive than the original budget but happily the Dean of ME, Theun Baller, stepped in to provide the rest of the funding: “Theun was willing to take the risks, both financial and technical, so he played a major role for which we’re very grateful.”
So really the Cohesion project seeded the plans for the further development of what was to become our Multi-phase Flow Tunnel.