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 Peter July 10, 2001 18:17

Surface roughness

Hi to all,

Some engineers insist that surface roughness is a manufacturing parameter and that should be taken into account only when the piece is build up. My question is: how this parameter is considered when using CFD, for instance when doing airfoil design?

Thanks

 John C. Chien July 10, 2001 19:40

Re: Surface roughness

(1).In airfoil design, surface is considered smooth. so, the roughness is not considered. (2). Roughness is a problem for pipes (casting or machined), so there are charts and tables to account for the loss, see Schlichting's book of The boundary layer theory for details.

 Michael Malin July 11, 2001 06:07

Re: Surface roughness

When using high-Reynolds-number turbulence models in CFD, wall-roughness effects are simulated by the specification of an equivalent "sand-grain" roughness height (as mentioned by John Chien), or alternatively by means of an effective roughness height as in, for example, atmospheric boundary layers where the effective height is related to the size of the roughness elements on the surface terrain.

With 'sand-grain' roughness, the 'roughness' parameter for a smooth wall E is a constant in the log law, and for a rough wall E is expressed as a function of the roughness Reynolds number by means of empirical laws based on the work of Nikuradse(Schlichting's book) and others.

 Peter July 11, 2001 13:28

Re: Surface roughness

It is clear that in pipes, surface roughness is important and there are correlations for that. My question was focused in airfoil profiles, since I know people working on that does not take it into account any surface roughness considering that the wall is smooth. To which extent that assumption is valid?? Can interpolating splines for the geometry introduce any surface roughness in CFD??

Thanks

 John C. Chien July 11, 2001 15:53

Re: Surface roughness

(1). In airfoil design, or CAD, the cubic is commonly used standard. (2). Other has used quartics or slightly higher order, but they normally do not introduce surface roughness. (3). Spline curve with higher order can introduce strange shapes, so care must be taken there. SO, if you use cubic or quartic, you should be all right. (that is, limit the control points to four or five points per curve segment)

 Peter July 11, 2001 17:26

Re: Surface roughness

People I contacted do use cubic splines. That is probably the reason why they don't consider surface roughness at all.

OK

 Bart Prast July 12, 2001 03:16

Re: Surface roughness

Has anybody any experience in modelling two-phase induced pressure losses with surface roughness? I mean modelling a very thin liquid film in a gas stream by surface roughness. The exchange of impuls between gas and liquid could be captured with a surface roughness. Anybody?

 ken elms July 13, 2001 11:32

Re: Surface roughness

Surface finish by a manufacturing process is important for numerous reasons from fitting together components to handling processes flows. Hydraulic pump impellers from practical everyday uses to the basis [assumptions- in flow modelling and navier stokes equations etc. SCHLICHTING-1979],PIPE FLOW EQUATIONS[ROUGHNESS FACTORS-CONCRETE,PLASTICS,STEEL,ETC]. Its not just for engineers either.

 Khurram July 18, 2001 11:56

Re: Surface roughness

I am intersted in validation of skin friction drag obtained from CFD. can any body help me?

 Herve July 19, 2001 11:29

Re: Surface roughness

I am too! My experience is that for low ks/ks+ it is in general quite possible to relate physical and num. roughness values. When roughness is high or complex, it is difficult to relate both.The grain roughness approache doesn't work anymore...

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