What CFD can or cannot do?

[nico]

Hi,


I am begining to "play" with CFD. I d like to know what CFD can and cannot do in term of resistance and lift calculation (does CFD calculate everything: viscous resistance, induced, pressure etc). What can do common CFD (Fluent,CFX, etc..) in terms of appendages resistance and hull resistance, for us yacht designers?

Thank you

[burakreis]

hi,

in short cfd can calculate everything you asked. on the other hand it is not so easy to get (and interpret) realistic results with cfd. very good experience on cfd and some good knowledge with fluid dynamics are essential. these two take time to achieve otherwise you can get pretty colourfull post-processings which can only be misleading.

fluent, cfx and starcd are the most widely used general purpose cfd codes. for appandage resistance, any of them (there are also many more good software) can be excellent analysis tool for a yacht designer. do not forget that for the sharp results on a yacht appandage, a fsi (fluid structure interaction) analysis must be used extensively. this type of analysis is the combination of cfd and fea. for example, a deep bulb keel produce some lift when sailing and this lift force distribution on the keel bents the structure and the flow conditions on this new (real) shape will be different then the undeformed keel indeed. of course this type of analysis only applicable on the state of the art vessels (because it needs long time for modeling the problem and very long computational time to solve it) there are also many software for fsi such as ansys, fidap etc. i can specially advice to check "cfdrc" for this type of analysis.

for the hull resistance, adapting a general purpose cfd code is not easy due to the hard nature of free surface flows (for the wave resistance). a specially developed code will be better for this purpose but i think they are very expensive for the most yacht designers. for example the most widely used hull resistance analysis code, shipflow is around 80.000$ per seat by the way this type of software could not catch the accuracy of tank testing yet but they are good for comparing and optimazing parametrically transformed hull models.
common cfd software can also simulate freesurface flows but their solution methods is not effective for a parametric hull analysis yet...

best regards,
burakreis