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 John August 2, 2002 06:46

Turbulent Schmidt Number

Hi. For the past few months I have been modelling the movement of neutrally buoyant dye through a pipe with the species model in Fluent 5.5. I have found that altering the turbulent Schmidt number from 0.5 to 1.5 makes a 300% difference to the results. I know Fluent claims that a value of 0.7 is generally OK, but does anybody know if there is more detailed information for pipe flow. Thanks John

 Michael Malin August 13, 2002 07:06

Re: Turbulent Schmidt Number

I don't know about your precise configuration or modelling practices, but for classical developed pipe flow with passive heat or species transport, the turbulent Prandtl/Schmidt number varies from around 0.9 at the wall to 0.6 at the flow axis. Experimental data exists which permits determination of the Prandtl number variation across the pipe. It can be deduced from measurements of the mean velocity profile, mean concentration profiles and the radial profiles of the Reynolds shearing stress and radial turbulent concentration/heat flux.

 John August 13, 2002 13:08

Re: Turbulent Schmidt Number

Thanks for your reply. Can you give me the reference for variation in the turbulent Schmidt number for a pipe? Also is the Prandtl and Schmidt number the same thing? Thanks once again. John

 Michael Malin August 14, 2002 06:50

Re: Turbulent Schmidt Number

I will dig out the reference. The Prandtl and Schmidt number will be the same when dealing with passive contaminents.

 Michael Malin August 15, 2002 02:31

Re: Turbulent Schmidt Number

The references are:

Launder, "Heat and Mass Transport" in Turbulence, Chapter 6, Ed. Bradshaw, Springer, Berlin, (1978).

Reynolds, "The prediction of turbulent Prandtl and Schmidt numbers", Int.J.Heat Mass Transfer, Vol.18, p1055, (1975).

Hinze, "Transport of a scalar quantity in wall turbulence" in Turbulence, 2nd Edition, Chapter 7, McGraw Hill, (1975).

 John August 27, 2002 09:23

Re: Turbulent Schmidt Number

Sorry for the delay, but I now have the references. I will have a look and let you know how I get on. Thanks once again for your time. John

 John August 29, 2002 11:04

Re: Turbulent Schmidt Number

I have now looked through the references - I had now idea the issue was so complicated! I have found a further reference you may be interested in:

The height dependence of the turbulent Schmidt number within the boundary layer, Koeltzsh 2000, from the journal of atmospheric environment (I think).

Thanks once again for your time. John

 IET May 31, 2015 22:36

In Fluent, the default schmidt number is 0.85 for conventional fluid (air and water et al.), but for heavy metal (lead bismuth eutectic et al.), its schmidt number is larger than 1.0. So I want to set it 4.22 instead of 0.85. But I don't know how to do it? Use UDF? Please help me!

 CeesH June 1, 2015 05:04

Hi,

If you go to viscous to set the turbulence model, there is a column on the right with a number of constants. if you have species enabled, this will include the turbulent schmidt number as well.

Best,
Cees

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