Contra-rotating propellers aim at improved performance, in comparison with the conventional design of isolated propellers. Part of the tangential momentum gained by the flow through the front propeller is indeed recovered to produce additional thrust by the rear propeller. In this sense, contra-rotating propellers can be considered an energy-saving device, having the purpose of improving the efficiency and reducing the environmental impact of shipping.
Unfortunately, very little is known about the complex flow physics of this interesting technology, as demonstrated by the limited literature currently available on the subject. Performing physical experiments is challenging and expensive, even more than for conventional propellers.
Computational fluid dynamics is a powerful tool to complement their results. However, high-fidelity simulations are required to reproduce accurately the flow physics of this class of propellers, resulting in the need of huge computational resources on Tier-0 supercomputers, at the forefront of the current capabilities.
For instance, the interaction between the tip vortices shed by the two propellers plays a crucial role in the process of instability of the whole wake system and the resulting wake signature, in terms of turbulence statistics and hydro-acoustics.
An improved insight is required, for the best exploitation of this technology by engineers through optimal design solutions. In this project high-fidelity, state-of-the-art fluid dynamic simulations on a computational grid consisting of 7.2 billion points will be conducted on a system of two contra-rotating propellers, for which detailed physical experiments are already available for validation purposes. For comparison, also simulations of the isolated front and rear propellers working alone will be performed. They will help us isolating the effect of the mutual interaction between the two propellers on performance, turbulence, dynamics of the major vortices, wake instability and hydro-acoustics.