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-   -   Velocity profiles (http://www.cfd-online.com/Forums/fluent/30040-velocity-profiles.html)

Rex July 17, 2002 03:35

Velocity profiles
 
Hi all, I've been simulating turbulent flow through an annulus with Fluent and have been looking at the velocity profiles, some of which look dubious (some look asymmetric, some show a steep sides with a small flat profile in the middle). I want to compare these with 'hand calculated' velocity profiles, so does anyone know of the equation to calculate annular velocity profiles?

Laika July 17, 2002 09:20

Re: Velocity profiles
 
Watch out with flow in an annulus. Flow instabilities can occur!!

Laika, still orbiting

Lanre July 18, 2002 15:16

Re: Velocity profiles
 
Since the flow is turbulent, the velocity profile would be a correlation. I don't know of any in the literature...haven't looked yet. What is the gap to diameter ratio? If large, i.e., 1/100, use the turbulent velocity profile between flat plates as a close approximation.

Peter July 20, 2002 19:15

Re: Velocity profiles
 
There is a equation for a flat plate relating velocity, BL thickness, free stream velocity and perpendicular distance to the wall: (u/u_fs)=(y/delta)^(1/n) Usually for turbulent flows you take n=7 although some other values (8,9) are also valid. Cheers

Matt July 23, 2002 19:35

Re: Velocity profiles
 
I have had a number of issues with correct velocity profiles for simple geometries. With turbulent flows, you need to be very careful with your y+ value and placement of the 1st grid point from the wall. Use the right wall function.

Good luck with things. FLUENT 6.02 has developed new wall functions to ease the restrictions on the strict adhearance to the y+, but you still need to be careful or you will get bad results. I have actually found the most luck with the k-omega turbulence model for a RANS solver of internal flow.

Matt


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