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

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.

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

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

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).

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

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]

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]

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