research & development
Figure 3: Computed wake. Left: Optimized hull. Right: initial hull
Figure 4: INSEAN E779A open-water computation by FreSCo. (top) Pressure field, limiting-streamlines and iso-surface of -Cpn>1.5 for the advance coefficient of (J)=0.88. (bottom) Open-water diagram
Figure 5: Delft-Wing computation by FreSCo. (top) Iso-surfaces (0.25, 0.5, 0.90) of vapour illustrating the cavity-surface for one instant. (bottom) Vapour volume fraction distribution and velocity vector field for mid-section of the wing for the same instant
CFD in ship hydrodynamics
vertical axis and the increase of the Wake Object Function (WOF) on the horizontal axis. Each point gives the computed values for one hull form variation. There is a clear envelope, a ‘Pareto front’, that indicates the best that can be achieved. A compromise between decrease of resistance and WOF is clearly required. This front is hardly influenced by the grid density, which lends much con- fidence to the results. From the submitted designs, MARIN’s optimised hull form was finally selected as the most promising. A 3.1% reduction in viscous resistance was predicted, with an 8% sacrifice in WOF. A model has been built and measurements were performed at SSPA. A 3.4% decrease in viscous resistance and 11.5% increase in WOF were measured. The predictions were thus confirmed and also the qualitative change of the wake field agreed with these predictions (Figures 2 and 3).
The techniques that enable these systemat- ic variations will now be further automa- tised. After trial applications, optimisation for the lowest resistance and best wake can be introduced to practical ship design.
Cavitation nuisanc In the Propulsion and Cavitation work package, emphasis was placed on the development of tools for the prediction of cavitation nuisance: pro- peller induced pressure fluctuations and cavitation erosion. For a proper assessment of cavitation nuisance, an accurate predic- tion of the time-dependent cavitation volume is required. It has already been demonstrated that the lower blade-rate harmonics of the pressure fluctuations are effectively predicted by the potential flow panel code PROCAL. But for higher frequen- cies and for cavitation erosion risk assess- ments, more detail in the cavity behaviour is needed. Therefore, MARIN joined forces with its counterpart in Hamburg, HSVA, to develop a multi-phase, viscous flow CFD code called FRESCO. This code has been extensively tested and validated for more than four years, leading to a code that sim- ulates the dynamic cavitation behaviour. A review of the capabilities of multiphase RANS codes to address a cavitating propel- ler in a wakefield is given in [2]. In addition, exploratory propeller design exercises were carried out. Results clearly
showed the importance of including pressure pulse and cavitation erosion constraints when aiming for the highest efficiencies. If these constraints are not included this will lead to unrealistically high efficiencies at unacceptable cavitation nuisance.
To meet the ever more stringent require- ments on fuel reduction and exhaust emis- sions, MARIN plans to develop an integrated optimisation of an aftbody-propeller-rudder configuration. A detailed cavitation nuisance assessment is therefore, a necessary prerequisite. These FRESCO developments have come just at the right time.
1. Raven, H.C., van der Ploeg, A., Starke, A.R., and Eça, L., “Towards a CFD-based prediction of ship performance --- Progress in predicting resistance and scale effects’’, Int. Jnl of Maritime Engineering, Trans. RINA Vol.150 – A4, 2008.
2. Salvatore, F., Streckwall, H. and Van Terwisga, T., “Propeller cavitation modelling by CFD – Results from the VIRTUE 2008 Rome Workshop”, First International Symposium on Marine Propulsors SMP’09, Trondheim, June 2009
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