Looking for the ultimate effect
It is difficult to understand how propellers, hulls and rudders to work optimally together in calm water. Add the impact of waves and it seems next to impossible. But that challenge has been accepted by Arash Eslamdoost and his research colleagues.
The hull is designed separately, then a propeller is pressed into it. A little simplified, but it is more or less how it still works when it comes to designing ships. Each component is optimized individually instead of together.
”Today we have a pretty good track of what happens to the flow around a self-propelled hull in calm water, but when there are waves it becomes more problematic. The propeller performance and the interaction effects between propeller, hull and ship appendages are different in presence of waves”, Arash Eslamdoost, researcher at Chalmers, says.
There are two ways to study the flow, either by towing a small scale model in a towing tank or by numerical calculations. And it’s the latter approach that is used in the two-year research project Propeller-Hull Interaction Effects in Waves, which is conducted within the the Swedish Transport Administration's industry programme Sustainable Shipping, operated by Lighthouse.
”Taking the advantage of the state of the art CFD tools, we will develop a numerical method which is capable of capturing the sophisticated flow around a self-propelled hull in waves. It takes a lot of computer capacity and hasn’t been possible before.”
It takes time though. Just calculating the flows around a single specific wavelength takes one to two weeks for the computer to solve.
”Therefore, we have no opportunity to study all wavelengths but have to stick to the ones that are most interesting, such as the one that is as long as a ship. In that wavelength there are the largest movements”, says Arash Eslamdoost.
So far, the researchers have been able to measure how the hull works in waves without a propeller. At the the same time they have also been succesful in extracting the "wake image" or the speed distribution of the propeller.
- Now is the time to connect the wake image and the movements from the hull. We have all the results to do so and we’ve also succeeded in validating the method.
So, when a tool has been developed - how much can fuel consumption be reduced?
- We hope to be able to identify and map the critical speeds for a certain wavelength, ie those that create a resistance that requires a lot of power and energy. In this way, you can adapt the speed to achieve optimal energy consumption, says Arash Eslamdoost and continues:
- But to say exactly how much you can save is difficult, but I would think it is about five to ten percent in fuel consumption.
The project is carried out as a PhD project at the department of Mechanics and Maritime Science of Chalmers in collaboration with SSPA. A doctoral student Mohsen Irannezhad has been recruited and Rickard Bensow (Professor at Chalmers) and Martin Kjellberg (Technology Doctor, SSPA) is the supervisor and co-supervisor respectively. By the time Mohsen Irannezhad had obtained his licentiate degree further grant will be sought for him to carry out a second part of the project (scale effects) which will lead him towards a PhD degree.