UDS for positive scalar
I would like to use UDS to calculate the concentration field. But the results are negative (small) in some parts of the domain. Would it affect a lot on the solution? and if that's true, how can I solve a positive scalar by uds? Can I just set the negative values to zero using udf?
Thanks a lot for your advising. Jun Li 
Re: UDS for positive scalar
There are couple of things to do.
Generally I check for small negative values and set them to zero inside a DEFINE_ADJUST routine. Scalars should never be negative (see Patankar). The other option, which you should also consider is to add or modify your source term  Patankar also describes this  so that it is linearised with respect to the scalar being solved and forces the solution to zero as zero is approached. In practice even with some sort of source term linearisation I find that I still need to reset small negatives to zero. gnerally we are talking about numbers of order 1.0e4 to 1.0e6 in magnitude so they don't effect the solution if reset to zero. However it you leave them they may grow exponentially and interact with your other equations in an adverse way, so its best to reset them to zero. Hope it helps Greg ps Patankar  Numerical heat Transfer 1980  the first book to read if you're new to CFD and finite volume methods. 
Re: UDS for positive scalar
I appreciate it, Greg. I am doing it now and will let people here know what will happen when setting the negatives to zero.
Jun 
Re: UDS for positive scalar
No much change happens in my case if resetting the negatives to zero. Jun

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