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Yung February 4, 2006 11:36

Free slip and no slip.
How rough ness can be found out?what is the formula?Second thing I have not understood about free slip and no slip ? I want to know that when free slip and no slip should be provided?

Mick February 10, 2006 14:32

Re: Free slip and no slip.
Look in POLis under things like Roughness, Wall etc The formula is in POLIS somewhere.

I think people tend to use the language Slip and no slip when they are talking about two-phase flows.

For example in two phase the second phase may be a solid and this may be completely convected by the carrier fluid (no slip), or it may be pushed along at a different speed to the carrier fluid (slip).

Jim March 2, 2006 10:56

Re: Free slip and no slip.
Yeah maybe, but more likely he is interested in rarefication/ganulation, where the non-slip boundary condition becomes invalid, and wall slip must be allowed. This occurs when the Knudson number is very high (Kn > 0.01). Free slip is essentially the same as a symmetry in that there is no stress at the wall.

With regards to wall roughness, you generally use an equivilant sand roughness scale. The best reference I can think of is Frank White's Viscous Flow, but google is your friend. Try searching for Moody diagram as a nice academic starting point.

Mike March 2, 2006 12:04

Re: Free slip and no slip.
PHOENICS is equipped with roughness wall functions that employ the sand-grain roughness formulae of Jayatilleke. These formulae fit the data of Moody, Nikaradse, etc very well. In these wall functions, the roughness parameter and sub-layer resistance function, that appear respectively in the log laws for momentum and energy, are made a function of the roughness Reynolds number. The formulae are given here:

There is also an option in PHOENICS to use the fully-rough log law of the wall, which is often used to describe wind profiles in external aerodynamic problems. This law employs a user-specified effective roughness length.

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