SimScale CAE Forum

Passive scalar sources


Hello all,
in case of defining a volumetric passive scalar source, is the value represent the flux per the volume of the source or the volume of the domain?
for example, the average passive scalar concentration in space with volume of 100x100x20 m3 increased from 100 mg/m3 to 300 mg/m3 within an hour,
is it correct to calculate the flux for volumetric passive scalar by dividing 200mg/m3 (the difference in the concentration) over time (60x60 s)
flux in this case equal to 0.055 mg/m3.s
or i should multiplies the value (0.055) by the volume of the domain (so flux equal 11111.11 mg/s) and then divide it over the passive source volume

best regards,


Hi @Mo_Elsebaei!

Let me get back to you soon! Will go through your questions step-by-step (@CFD-SQUAD, feel free to jump in :slight_smile: ). Please also make sure to attach your project link in this thread so that we can easily find your project (assuming that you have created one already) :smiley:

Cheers and enjoy your weekend!



Hi @Mo_Elsebaei
Just go through the following documentation to know more about your case.



Thanks for jumping in here so quickly Hamza!

@Mo_Elsebaei, please feel free to mark Hamza’s answer as the solution if he could answer your questions.




first, thank you for answering, @HamzaBaig @jousefm
the example in the documentation is in fact deal with the amount of passive scalar in gram or kilogram and not with the concentration; that is what cause the confusion. and that is why i put a numerical example in the question

so if i had the concentration in mg/m3, should i calculated the amount of passive scalar (in gram or kilo gram) that were added to the domain within an a hour (as i did in the example) in order to use it as the flux value in the case of passive scalar source or i can use it as it is without any calculation as flux value for the case of volumetric passive scalar source; but if the volume in fact is the source volume and not the volume of the domain then i can not use it in that way! in fact i should do the same as the first case, to calculate the amount of passive scalar and then divide it over the volume of the source.
if you can clarify it based on the example i will highly appreciated
and can you please tell me what the result (T) value in both cases present?


Hi @Mo_Elsebaei
I completely got your point, i will be answering your question with a numerical example shortly.


Hi @Mo_Elsebaei
In passive scalar source, values interpretation is all upon the user. Thus one can define a passive scalar flux as 500 1/s and interpret it as a flux of 500 g/s or 0.5 kg/s as per one’s convenience.
Similarly in VPSS as 500 g/m3.s or 0.5 Kg/m3.s.

Below you can see comparison between cases for two different passive scalar source values, one defined as 500 1/s (interpreted in grams) while other as 0.5 1/s (interpreted in kilograms). We see that the final results (in this case is shown as T1) are only scaled by the equivalent factor (1000 here, see legends), whereas the actual dynamics remain unaltered.

Same is true for VPSS.

Hope you get the answer of your question.


What is the unit of T