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 Giovanni Ieria December 21, 1999 06:27

Ship Wall Roughness

I am a student of Calabria University (Italy), I am developing a thesis about CFD for ship. If I use Wall Functions in order to model the boundary layer, I must specify two roughness parameters which I don't know: Ks and Cks. May you help me indicating some typical values for ship hulls? Thanks in advance

 Sung-Eun Kim December 21, 1999 10:12

Re: Ship Wall Roughness

When you model "hydrodynamically smooth" ship hulls, you don't need to specify the two wall roughness parameters, i.e., Ks (roughness height) and Cks (roughness constant). The defaults are 0.0 and 0.5, respectively, which is directly applicable to "hydrdynamically smmoth" walls. Scaled model ships tested in towing tanks have all "hydrodynamically smooth" surfaces, unless roughness is deliberately put on the surfaces to study the effects of roughness, which is very rare.

Roughness effects on ships' resustance can become an issue when you deal with full scales ships whose surfaces are actually "fouled" in the sea. I understand ship designers use the so-called "roughness allowance" for the roughness when estimating total resistance of full-scale ships. In the CFD modeling, what you can do in this situation is to specify your best estimate of the roughness height (e.g. of the average height of biological creatures on ship hull) for Ks and use the default value of Cks.

Please keep in mind that Cks is calibrated using roughened pipes with sand roughness and Fluent Inc. doesn't have much expertise how Cks should be varied for different roughness types.

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