NuTTS 2016: Propellers and ships in waves

PROPELLER SIMULATION

Multiphysics simulation, applied to propellers in particular, was at the heart of the researchers' discussions. To simulate the behaviour of a propeller as realistically as possible, researchers agree that turbulence, transition and cavitation models must be taken into account.

Turbulence is the state of a fluid, liquid or gas, in which the microstructure of the flow consists of eddies - eddies which vary constantly in size, location and orientation. Turbulent flows are therefore characterised by a very disorderly appearance, and unpredictable and complex behaviour.

As no flow is turbulent from the onset, applying a turbulence model throughout the flow is unphysical. Thus, increasingly, a transition model is also applied for propeller simulation. The transition from laminar flow to turbulence, in hydrodynamics, can be illustrated by the point at which the stream of water from a tap transforms into drops. Each flow undergoes this transition - modelling it provides better simulation of the flow on a propeller.

A cavitation phenomenon can also be observed (see video) on the tips of propeller blades, i.e. where the pressure is the lowest.

The pressure is sometimes so low that gas or vapour bubbles are created, which leads to problems. Indeed, this phenomenon not only reduces the performance of the propeller, it is also noisy, which is catastrophic in the defence industry for example. Moreover, these vapour bubbles implode when the pressure rises, creating violent shock waves in the water and damaging the propellers.

To study this phenomenon in order to improve propeller shapes, researchers have been trying for some time to incorporate cavitation models into the simulation of propeller behaviour.

NuTTS 2016 highlighted two points: cavitation models are improving and the combination of the three models (turbulence, transition, cavitation) is becoming more and more common.  Today, we are able to simulate propellers with turbulence, transition and cavitation at the same time. The question is: does it really make sense to combine these models as they stand, given that they were developed separately? A lot of thought and experimentation is now underway as to how these three models - and maybe others - can be combined.

SHIPS IN WAVES
The behaviour of ships in waves was also a significant theme in the discussions. The objective of this research is to improve our understanding of ship movement in waves, the extra resistance created by the waves and the deformation of ships under their effect in order to determine the best hull shapes. In this respect, researchers are turning increasingly towards automatic shape optimisation, creating software to determine the best shapes for ships. In the long run, the best geometry of a ship could be defined automatically. Auke van der Ploeg, of the Maritime Research Institute Netherlands (MARIN), was awarded the 2016 Landrini Prize for his work on shape optimisation.

NuTTS, A ONE-OF-A-KIND CONFERENCE
In line with the NuTTS tradition, participants share their meals and accommodation. This is an opportunity for researchers to discuss their latest results, their ongoing experiments, the problems they face, etc. in an informal context. Everything is set up to foster exchange during this two-day interlude, the setting for which was Oléron island for this 19th edition.

NuTTS 2017
The 20th edition will be held in Wageningen, Netherlands, from 1st to 3rd October 2017. It will be organised by Auke van der Ploeg and Thomas Lloyd of MARIN.