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Feature 3 | CFD & HYDRODYNAMICS


proven robust and sufficiently accurate for most industrial applications. RANSE simulations with cavitation modelling have become part of modern design procedures for advanced propulsors. Rudders behind highly loaded propellers are susceptible to cavitation and associated erosion which endangers the ship. CFD is by now regularly employed to predict location and extent of cavitation on rudders in these cases. Te concerned regions are then often built


in more


enduring steel, unless local redesign avoids the formation of cavitation erosion.


Ship with rudder and propeller illustrating the trend towards modeling complete systems.


standard k-ε or k-ω turbulence models are adequate. In order to predict secondary flows better, more sophisticated models are needed. Te Reynolds-stress model (RSM) is frequently a popular and appropriate option. A special turbulence model is needed to predict transition from laminar to turbulent flow, e.g. when predicting resistance of a competitive sailing yacht. Such models are also available. For predicting noise sources, wall vibration etc., large-eddy-simulation (LES) or detached eddy simulation (DES) type of analyses with special subgrid-scale turbulence models are used. These are subject to research.


Modelling free- surface effects Free-surface flows (wave resistance, seakeeping, slamming, sloshing) are of prime interest


for naval architects.


Interface-capturing methods allow the simulation of highly nonlinear free-surface flows. Resulting quantities of engineering interest, e.g.


induced


loads in tanks with sloshing, are so well predicted that such simulations are widely accepted by classification societies for load determination in strength analyses. Despite the significant progress in free-surface modelling, research continues in this field, as the modelling of breaking waves can still be improved in terms of air mixing and turbulence interaction with the free surface.


Cavitation modelling In most propellers and several rudders, cavitation is unavoidable. If cavitation cannot be avoided, its effect on performance needs to be assessed. Despite theoretical shortcomings, cavitation models based on bubble dynamics have


Motion of floating bodies For a variety of seakeeping problems, simulations of flows and flow-induced motions of floating bodies (ships or offshore structures) are desired. Highly nonlinear motions (e.g. launching of free-fall lifeboats with subsequent water entry and resurfacing) are possible due to implicit simulations. Rigid-body motions of freely moving ships have been presented for a variety of applications including many industry projects. The simulations can handle in principle all complexity required in maritime applications, including multi-body configurations moving relative to each other, possible coupling between bodies (via elastic moorings, rigid connections, or


flexible links with constraints),


inclusion of external forces (e.g. thrusters, mooring, towing), or relative motion of system components (e.g. propellers).


Fluid-Structure- Interaction (FSI) Coupled simulation of flow and flow-induced deformation of


solid structures have evolved more recently for Analyses of ship without (left) and with (right) nozzle to assess quantitatively the fuel saving effect.


48


The Naval Architect July/August 2011


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