# Passive Scalar Dispersion

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 April 20, 2013, 16:58 PLZ HELP!!! (Urgent) Passive Scalar Dispersion #1 New Member   Join Date: Apr 2013 Posts: 1 Rep Power: 0 Hi, I am new to FLUENT, and am trying to simulate an unsteady passive scalar dispersion on FLUENT 14.0. It is for a 2D axisymmetric sudden expansion geometry. I am using K-epsilon for turbulence modelling, with a 15m/s velocity inlet and 0.4% turbulence intensity. I am unsure how to simulate a passive scalar, and I would really appreciate any help in this, thank you! manast Last edited by manast; April 20, 2013 at 18:16.

 April 22, 2013, 10:56 Dispersion #2 New Member   Francois Join Date: Apr 2013 Posts: 2 Rep Power: 0 Hello, I'm new user of Ansys Fluent. I want simulate a passive scalar but i dont know which boundaries conditions, i can take. I study substances dispersion in the lentic water (pond, tank,...). Please, some persons can help me? Thanks

 April 23, 2013, 07:31 #3 Senior Member     A-A Azarafza Join Date: Jan 2013 Posts: 203 Rep Power: 6 __________________ Regard yours

April 23, 2013, 07:41
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A-A Azarafza
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User Defined Scalars (UDS) are used to solve any arbitrary generic transport equations, something like species transport equations. You can use species transport equations to solve only mass fractions of species, but you can use UDS to solve any transport equations you want, for instance, you can use them to solve electric potential or magnetic field or injecting a tracer into media to obtain RTD. Fluent allows you to define up to 50 UDS. An arbitrary generic Scalar( UDS) has a form like this:

As you see the left hand side of the generic equation has three terms. The first one is used in a transient (if you need to consider time), the second term is convective flux and the third one is diffusive flux. The right hand term is source which can be defined depending on your problem, for instance, if your scalars are consumed or produced.

is diffusivity which should be defined by user. it can be considered as isotropic or anisotropic. An anisotropic diffusivity is defined as a tensor. There are several methods for diffusivity in Fluent, you can access them via Fluent material panel.
F, is flux function which in case of mass flow, it is defined as multiplication of density by velocity. For other cases should be defined separately.
How to run a UDS in Fluent:
To run UDS in Fluent, first define the numbers of UDS in Define/ User defined/ Scalars. UDS index is used to recognize the UDSs and it starts from zero. It means that for instance, the second UDS has index 1, the fourth UDS has index 3 and so on. You need to select the solution zones for your scalars as well. If you interested in considering flux function, then define it too, otherwise leave it as none (In this case you only solve the diffusion term).
By doing this, you'll be directed to material panel where you need to define the diffusivity for your scalars as I mentioned above. One more thing, you have to adjust the scalars on wall boundaries of your solution zones too either as a specified value or flux.
That's a brief procedure of defining a UDS.
Hope it helps
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Last edited by A CFD free user; May 13, 2013 at 13:01.

 April 24, 2013, 11:59 Thank you #5 New Member   Francois Join Date: Apr 2013 Posts: 2 Rep Power: 0 Hello Azarafza, Thank you for your answers (helps). Best regards

 March 3, 2016, 07:27 #6 New Member   hassan hayder ismail Join Date: Feb 2016 Posts: 2 Rep Power: 0 hi ... is it possible to consider the UDS for studying the pollutant dispersion in a river?

March 5, 2016, 13:27
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A-A Azarafza
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Quote:
 Originally Posted by hassanhayder hi ... is it possible to consider the UDS for studying the pollutant dispersion in a river?
Yes. It is possible to use either species transport or generic scalar equations to define your model, but what is more important is pollutant's diffusivities and source terms for transport (if pollution is due to chemical reaction) which should be correctly considered in the model.
I hope it helps
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 March 7, 2016, 08:48 #8 New Member   hassan hayder ismail Join Date: Feb 2016 Posts: 2 Rep Power: 0 Thank you so much.

 Tags axisymmetric 2d fluent, fluent, passive scalar, sudden expansion, unsteady

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