Passive scalar

alialemana · March 6, 2018, 16:13

Hello everyone,

I can't find anything about my problem in the internet so I guess it is quite obvious but I can't find the solution:

I want to simulate the transportation of a passive scalar. I have two inlets and in both flows water --> water-water-simulation.
The problem is by defining the mixture with just one fluid (water) the species mass fractions option in the inlet boundary condition is disabled. So how should I define the concentration (one inlet 1, other 0)?

I would really appreciate your help!
Thanks a lot

CeesH · March 6, 2018, 16:55

I am not sure if I get what you mean - how do you define a mixture with just one species..? that's not a mixture, it's a pure liquid.

If you want to make a mixture with 2 species that both have the properties of water, just make 2 fluids with any name, say water_1 and water_2, that have the same physical properties, and combine them into one mixture. That will allow you to set the mass fraction of water_1 at the boundary as desired.

alialemana · March 6, 2018, 17:10

Dear CeesH,

thanks a lot for your fast reply. I does not have to be a mixture but as soon as I switch the species transport on I have to define a mixture or do you have a different idea how to simulate a passive scalar transport in a water-water simulation?

Greetings

CeesH · March 6, 2018, 17:13

yes, you will have to define a mixture in species transport. Just give the mixture and all components the properties of water (don't forget to switch density from ideal gas to volume weighted mixing!) and you should be good to go.

alialemana · March 6, 2018, 17:15

Ok perfect this is the way I already tried it but I thought there might be a easier or a more "official" way.
Thanks anyway!!

LuckyTran · March 7, 2018, 00:57

Passive scalar might be the right approach...

Having one passive scalar or only one mass fraction doesn't mean you only have one substance, it obviously implies there is at least another.

When you do species transport it is solved the same way. The difference is whether you have an active scalar, which needs a species transport or a passive scalar. If you only need to tag the origin of the fluid, then the passive scalar might be the way to go. In Fluent, passive scalar is performed via the "user-defined scalar"

alialemana · March 17, 2018, 12:24

Quote:

Originally Posted by LuckyTran

Passive scalar might be the right approach...

Having one passive scalar or only one mass fraction doesn't mean you only have one substance, it obviously implies there is at least another.

When you do species transport it is solved the same way. The difference is whether you have an active scalar, which needs a species transport or a passive scalar. If you only need to tag the origin of the fluid, then the passive scalar might be the way to go. In Fluent, passive scalar is performed via the "user-defined scalar"

Dear LuckyTran,

so you are saying species Transport is not the right approach for my passive scalar, it would be better using UDF? I tried it and the results were horrible.

March 6, 2018, 16:13	Passive scalar	#1
alialemana New Member Join Date: Jul 2017 Posts: 12 Rep Power: 9	Hello everyone, I can't find anything about my problem in the internet so I guess it is quite obvious but I can't find the solution: I want to simulate the transportation of a passive scalar. I have two inlets and in both flows water --> water-water-simulation. The problem is by defining the mixture with just one fluid (water) the species mass fractions option in the inlet boundary condition is disabled. So how should I define the concentration (one inlet 1, other 0)? I would really appreciate your help! Thanks a lot

March 7, 2018, 00:57		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	Passive scalar might be the right approach... Having one passive scalar or only one mass fraction doesn't mean you only have one substance, it obviously implies there is at least another. When you do species transport it is solved the same way. The difference is whether you have an active scalar, which needs a species transport or a passive scalar. If you only need to tag the origin of the fluid, then the passive scalar might be the way to go. In Fluent, passive scalar is performed via the "user-defined scalar" granzer likes this.

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March 6, 2018, 16:55		#2
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	I am not sure if I get what you mean - how do you define a mixture with just one species..? that's not a mixture, it's a pure liquid. If you want to make a mixture with 2 species that both have the properties of water, just make 2 fluids with any name, say water_1 and water_2, that have the same physical properties, and combine them into one mixture. That will allow you to set the mass fraction of water_1 at the boundary as desired.

March 6, 2018, 17:10		#3
alialemana New Member Join Date: Jul 2017 Posts: 12 Rep Power: 9	Dear CeesH, thanks a lot for your fast reply. I does not have to be a mixture but as soon as I switch the species transport on I have to define a mixture or do you have a different idea how to simulate a passive scalar transport in a water-water simulation? Greetings

March 6, 2018, 17:13		#4
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	yes, you will have to define a mixture in species transport. Just give the mixture and all components the properties of water (don't forget to switch density from ideal gas to volume weighted mixing!) and you should be good to go.

March 6, 2018, 17:15		#5
alialemana New Member Join Date: Jul 2017 Posts: 12 Rep Power: 9	Ok perfect this is the way I already tried it but I thought there might be a easier or a more "official" way. Thanks anyway!!