SimScale CAE Forum

What is the unit of T


Hi all,
What is the unit of T (in passive scalar transport)?


Hi @Mo_Elsebaei,
Have a look at the following explanation from SimScale documentation regarding your query.
It is important to understand the correct interpretation of units used for passive scalar sources. Since passive scalars do not affect the dynamics of fluid flow, units for passive scalars are independent to the unit system of simulation. Thus one can define a passive scalar flux as 100 1/s and interpret it as a flux of 100 g/s or 100 kg/s as per one’s convenience. However, these interpreted units must be kept consistent throughout the simulation setup (for values specified in initial conditions, boundary conditions etc). Scale of the results will directly correspond to absolute value of input variables.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 are only scaled by the equivalent factor (1000 here, see legends), whereas the actual dynamics remain unaltered. Thus the relative distribution of passive scalar remains the same. This is evident from following figure

Passive scalar distribution for flux = 500/s

Passive scalar distribution for flux = 0.5/s



thank you @HamzaBaig for your reply
i saw this and have read it but still did not get an answer, simply what is T unit ? in case of volumetric source or the normal source what is T stand for? i understood that units of passive scalars are independent to the unit system but that does not mean that there is no unit for T


Hi @Mo_Elsebaei,
I just realized that we have discussed the topic in the recent past.



Thank you again @HamzaBaig ,
it is a different question than the previous topic
we have discussed the input (flux), now im asking about the unit of T which is the output
simply what is T unit?


@tcakir @jousefm


Hi @Mo_Elsebaei and thanks for tagging!

T is not a temperature but a concentration, it works as a tracker but has no density or anything like that. Does that help?




thank you so much for the reply, i do understand the concept of it as tracker, but tell me please if the following is correct or not: T indicates how much of the flux is in an area but it is not concentration that can be calculated from the flux that we have define as input
can we assume that the value of T is the area share amount of uniform concentration
so if we calculated a uniform distribution of the passive scalar in the boundary box , t will equal how much of the amount of the uniform concentration is at any point in the map
for example, if the uniform distribution = 2 g/m3 then concentration in point 1 (T in this point equal 4) equal 4x 2= 8g/m3
and if T in point 2 equal to 0.1 then concentration is equal to 0.2; make sense?!?! :laughing::laughing::sweat_smile: