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 gravis May 4, 2010 11:03

Wall function formulation in CFX and Fluent

Dear all,

I am trying to compare the wall function implementation in CFX and Fluent, and there are a few things I don't understand from reading the documentation. The problem is as follows:

As I understand it, at least in CFX the reason for using the form of nondimensionalized distance and velocity having superscript "*" rather than "+" is due to the law of the wall becoming singular at separation points. The definition of the dimensionless wall distance y* is the same in both codes: , Also, the dissipation is calculated by the same expression in both codes.

First, in CFX the friction velocity is replaced by (eq. 2.222) to get rid of the singularity issue. My interpretation is that the following is then done:
1. yields
1. and yields
1. and yields

In Fluent on the other hand, a dimensionless velocity is defined by:

(eq. 4.12-1)
where
(eq. 4.12-2)

My guess is that the latter expression is used for the same reason as (eq. 2.222) is used in CFX, i.e. to cope with the singularity issue. My interpretation is that the following is done in Fluent:
1. is calculated by the expression for above, which then yields from (eq. 4.12-1)
1. is used in (eq. 4.12-2) to yield

I assume that is equivalent to in the law of the wall, in the same way as (eq. 2.222) is equivalent to the friction velocity .

If the dimensionless velocity in CFX (equivalent to Fluent's ) is . Then, it seems to me that the wall function implementation differs between the codes. Are they different, or have I misunderstood something?