Solving custom transport equation in fluent

Jmaes · September 10, 2023, 09:05

Hello everyone!

I am trying to solve this transport equation attached. The velocity is constant so it comes out of the derivative. Anyway, I noticed that the units of the diffusivity in fluent are in kg/ms while the units in my source term is supposed to be m2/s. Furthermore, the rho has to go and instead of having mass flow rate as flux, I just want velocity. Is it possible to make these changes without a UDF?

LuckyTran · September 10, 2023, 15:03

Do a simple substitution of c2=c1/rho

Solve the transport equation for c2. Then create a field function c1=c2*rho.

Jmaes · September 10, 2023, 15:29

Thank you for the reply Lucky, I did figure out that fluent multiplies the terms by the density of fluid. And I suppose the I have to manually multiply in case of UDS diffusion and the source term since it asks for the value in kg/ms and for the units to be consistent. I did multiply the density in both diffusion and source terms but they do not return intended results meanwhile, they kinda do when the density is omitted. What do I do here?

Furthermore, when I divide the source term by porosity like in the original equation, the results are again not as intended. I want to add that the source term is being applied in a porous zone.

LuckyTran · September 10, 2023, 16:40

Honestly you shouldn't try to overcomplicate things.

Transport equations are based on conservation phenomena. That means you should find the expression of your "source term" in specific units. Consider for example internal energy. Heat capacity is what we measure in a calorimeter but we write the energy transport equation in specific heat capacity because, well, transport equations are transported properties and the mass flux is the carrier, the transporter.

Your source term clearly has the wrong units. Instead of trying to break the universe, just fix your source.

Jmaes · September 11, 2023, 08:10

Thank you Lucky,
I didn't understand what you meant by finding the source term in specific units. Do you mean just find the correct units?

And the transport equation is taken from a scientific article and I have made no chnages to it. C is in ug/L and q is in ug/g. So if I just have the density in g/L, wouldn't the units make sense?

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Help sought on axial compressor simulation	jyotir	OpenFOAM Running, Solving & CFD	0	November 17, 2021 10:49
Suppress twoPhaseEulerFoam energy	AlmostSurelyRob	OpenFOAM Running, Solving & CFD	33	September 25, 2018 17:45
HeatSource BC to the whole region in chtMultiRegionHeater	xsa	OpenFOAM Running, Solving & CFD	3	November 7, 2016 05:07
calculation stops after few time steps	sivakumar	OpenFOAM Running, Solving & CFD	7	March 17, 2013 06:37
Could anybody help me see this error and give help	liugx212	OpenFOAM Running, Solving & CFD	3	January 4, 2006 18:07

September 10, 2023, 15:03		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,676 Rep Power: 66	Do a simple substitution of c2=c1/rho Solve the transport equation for c2. Then create a field function c1=c2*rho.

September 10, 2023, 15:29		#3
Jmaes New Member James Ranawadia Join Date: Apr 2023 Posts: 14 Rep Power: 3	Thank you for the reply Lucky, I did figure out that fluent multiplies the terms by the density of fluid. And I suppose the I have to manually multiply in case of UDS diffusion and the source term since it asks for the value in kg/ms and for the units to be consistent. I did multiply the density in both diffusion and source terms but they do not return intended results meanwhile, they kinda do when the density is omitted. What do I do here? Furthermore, when I divide the source term by porosity like in the original equation, the results are again not as intended. I want to add that the source term is being applied in a porous zone.

September 10, 2023, 16:40		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,676 Rep Power: 66	Honestly you shouldn't try to overcomplicate things. Transport equations are based on conservation phenomena. That means you should find the expression of your "source term" in specific units. Consider for example internal energy. Heat capacity is what we measure in a calorimeter but we write the energy transport equation in specific heat capacity because, well, transport equations are transported properties and the mass flux is the carrier, the transporter. Your source term clearly has the wrong units. Instead of trying to break the universe, just fix your source.

September 11, 2023, 08:10		#5
Jmaes New Member James Ranawadia Join Date: Apr 2023 Posts: 14 Rep Power: 3	Thank you Lucky, I didn't understand what you meant by finding the source term in specific units. Do you mean just find the correct units? And the transport equation is taken from a scientific article and I have made no chnages to it. C is in ug/L and q is in ug/g. So if I just have the density in g/L, wouldn't the units make sense?