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C.H. Huang August 16, 1999 20:30

Diffusion Coef. of a Scalar
Dear Sir: I am trying to model the pollution concentration distribution in clean rooms using the User-Defined Scalar Transport Mode in Fluent 5. The convective flux was computedwith mass flow rate and the K-E turbulent model was turn-on.. Can anyone let me know how to specify/calucate the diffusion coef of a scalar (pollution concentration, C)). Threre is no input panel for diffusion coef of a scala of a scala in theinput panel of the model of Fluent 5. Initially, I think the code will do the caluculation internally, however I found the value is 1.0 as I am examine the results, i.e.very very strong diffusion.

In K.E model, diffusion coef of a scala=(sum of molecular viscosity + trubulent viscosity)/segama C. segama C= 1.0. Since I am uisng K-E turbulent model, why diffusion coef. =1.0 is resulted.


C H Hung

Tom Plikas August 17, 1999 10:01

Re: Diffusion Coef. of a Scalar
If your diffusion coefficient is constant, then you can specify it's value in the materials panel under UDS diffusivity. If it is a function of your flow, then you have to write a user defined function. See the help section on user defined functions. In this case, you will use DEFINE_DIFFUSIVITY.

Jin Wook LEE August 18, 1999 03:48

Re: Diffusion Coef. of a Scalar
Dear C.H. Huang

You said segama C =1.0. However, in your equation, it seems to be turbulent Schmidt number(usually represented as sigma C), not mass diffusivity. Its value is, in general, order of unity and as far as I know default vaule in your code(Fluent) is 0.7. For turbulent flow with k-e model, the effect of molecular diffusivity is fairly small because turbulent effet is predominant. And you don't need to change this default value unless you want to obtain very accurate solution. Actually, exact value of turbulent Schmidt number is very hard to obtain.

Hoping it helps.

Sincerely, Jin Wook LEE

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