# Diffusion Coef. of a Scalar

 Register Blogs Members List Search Today's Posts Mark Forums Read

 August 16, 1999, 20:30 Diffusion Coef. of a Scalar #1 C.H. Huang Guest   Posts: n/a Sponsored Links 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. Thanks C H Hung

 August 17, 1999, 10:01 Re: Diffusion Coef. of a Scalar #2 Tom Plikas Guest   Posts: n/a 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.

 August 18, 1999, 03:48 Re: Diffusion Coef. of a Scalar #3 Jin Wook LEE Guest   Posts: n/a 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

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post vivek070176 OpenFOAM Programming & Development 10 December 24, 2014 00:48 Niklas Wikstrom (Wikstrom) OpenFOAM Running, Solving & CFD 122 June 15, 2014 06:20 ternik OpenFOAM 0 September 22, 2010 05:10 cmv Siemens 0 April 18, 2005 06:21 cmv Siemens 1 April 15, 2005 05:42