# dispersion of vehicle pollutants

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 February 26, 2008, 10:09 dispersion of vehicle pollutants #1 Andy He Guest   Posts: n/a hi, i want to simulate the dispersion of the vehicle pollutants under idle condition using CFX. i assum the addition variable as the pollutants,which don't react with the air. the addition variable seems has no mass and shape. So that means it also has no gravity and buoyancy. Is this all right? Then, How to describe the addition variable(pollutants) diffuse? the kinematic diffusivity=10^-5? any others? the simulate result seems not very well! Can someone give insights to problem? i will be grateful!

 February 26, 2008, 13:37 Re: dispersion of vehicle pollutants #2 Fusion Guest   Posts: n/a The additional variable acts as a tracer into your flow field, it has no mass and it is passively transported by the flow (its second name is "passive scalar"). If your objectives involve the interaction between polluttants and gravitational field or flow field you must adopt a different approach by switching to some particle tracking model or a multi-phase flow.

 February 26, 2008, 17:38 Re: dispersion of vehicle pollutants #3 Glenn Horrocks Guest   Posts: n/a Hi, The additional variable approach is the simplest approach but as Fusion says it cannot take into account buoyancy or relative slip with air. If you do use the additional variable approach you will need to give it a sensible diffusivity - the default value is very low and effectively means no diffusivity for most cases. Glenn Horrocks

 February 27, 2008, 08:50 Re: dispersion of vehicle pollutants #4 Andy He Guest   Posts: n/a Many thanks for Fusion and Glenn Horrocks! In the domain of the CFX, the default value of the kinematic diffusivity is 10^-5,which means there is little effect for the dispersion. If i want to increase the diffusivity,which value is appropriate for the kinematic diffusivity.

 February 27, 2008, 17:51 Re: dispersion of vehicle pollutants #5 Glenn Horrocks Guest   Posts: n/a Hi, Good question! I have no idea. Do a literature search and post the result on the forum and we all will know. Glenn Horrocks

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