UDF Macro for Mass Diffusivity using Kinetic Theory

souza.emer · December 6, 2018, 09:08

Hello everyone

I want to use the Kinetic Theory model for Mass Diffusivity and use its value, for each species, in a UDF to calculate the reaction rate. My question is, which macro should I use? Is it one of those two?

C_DIFF_L(c,t); Laminar species diffusivity

C_DIFF_EFF(c,t,i); Effective species diffusivity

If not, how can I call it?

Thanks for the attention.

obscureed · December 6, 2018, 11:42

Hi there,

The macros that you mention are for accessing values, not setting them. For defining, look at DEFINE_DIFFUSIVITY. If you want to include the effects of turbulence on diffusion (which, in a turbulent situation, you definitely do), you need to add this to the diffusivity of a User-Defined Scalar. (The manual's example makes this clear.) For a species, I *think* turbulence effects are added separately -- this is what the Theory Guide appears to say, but I don't remember checking in detail.

But then, there is a built-in version of kinetic theory for diffusivity, and this uses the molecular masses, species by species. (In fact, it seems to calculate it for each species-species pair, which does not seem possible in the UDF. Hmm.)

Good luck!
Ed

souza.emer · December 6, 2018, 12:49

Hello

Yeah, I know that Macros are used for accessing the values, that's why I'm in doubt because I'm not sure of how I could use the value of diffusivity calculated by the Kinect Theory when I set this previously at the materials menu in Fluent.
I do not want to use the macro to modify the current diffusivity coefficient calculated by Kinect Theory theory, I just want to use the value which Fluent calculates.

I think you thought that I wanted to modify it, which I do not want to, yet.

Do you know how I can do this? Or make any suggestions?

Best

rewol · December 6, 2018, 14:12

You can use DEFINE_NET_REACTION_RATE macro to calculate reaction rates and inside the code you can utilize C_DIFF_L(c,t) or C_DIFF_EFF(c,t,i) to access mass diffusivities of the species.

obscureed · December 6, 2018, 20:11

Hi Souza.Emer,

Yes, you are correct that I assumed you wanted to define the values. Sorry, this is a more frequent request than accessing values in UDF (because you can access the values in normal postprocessing if you want).

For accessing the built-in values, without any turbulent influence, C_DIFF_L is what you want. But check the manual for syntax -- C_DIFF_L(c,t) is not correct.

You can do this accessing inside several types of UDF -- rewol mentions one, though that is specific to the kinds of model that will accept it. DEFINE_ON_DEMAND is a general option: first look up a domain pointer, then find some cell or face zones, and loop through some faces or zones.

Good luck!
Ed

Weiqiang Liu · May 28, 2019, 15:28

hello, can I ask a question under this thread?
I checked the syntax in manual and found the correct syntax is C_DIFF_L(c,t,i,j). I am wondering is the diffusion coefficient I am accessing with this macro is binary diffusion coefficient? what if I want to access mixture averaged diffusion coefficient or multi-component diffusion coefficient？

December 6, 2018, 09:08	UDF Macro for Mass Diffusivity using Kinetic Theory	#1
souza.emer Member Emerson Join Date: May 2018 Posts: 35 Rep Power: 8	Hello everyone I want to use the Kinetic Theory model for Mass Diffusivity and use its value, for each species, in a UDF to calculate the reaction rate. My question is, which macro should I use? Is it one of those two? C_DIFF_L(c,t); Laminar species diffusivity C_DIFF_EFF(c,t,i); Effective species diffusivity If not, how can I call it? Thanks for the attention.

December 6, 2018, 14:12		#4
rewol Member Refik Join Date: Dec 2014 Location: Turkey Posts: 58 Rep Power: 11	You can use DEFINE_NET_REACTION_RATE macro to calculate reaction rates and inside the code you can utilize C_DIFF_L(c,t) or C_DIFF_EFF(c,t,i) to access mass diffusivities of the species. souza.emer likes this.

December 6, 2018, 20:11		#5
obscureed Senior Member Join Date: Sep 2017 Posts: 246 Rep Power: 12	Hi Souza.Emer, Yes, you are correct that I assumed you wanted to define the values. Sorry, this is a more frequent request than accessing values in UDF (because you can access the values in normal postprocessing if you want). For accessing the built-in values, without any turbulent influence, C_DIFF_L is what you want. But check the manual for syntax -- C_DIFF_L(c,t) is not correct. You can do this accessing inside several types of UDF -- rewol mentions one, though that is specific to the kinds of model that will accept it. DEFINE_ON_DEMAND is a general option: first look up a domain pointer, then find some cell or face zones, and loop through some faces or zones. Good luck! Ed souza.emer likes this.

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December 6, 2018, 11:42		#2
obscureed Senior Member Join Date: Sep 2017 Posts: 246 Rep Power: 12	Hi there, The macros that you mention are for accessing values, not setting them. For defining, look at DEFINE_DIFFUSIVITY. If you want to include the effects of turbulence on diffusion (which, in a turbulent situation, you definitely do), you need to add this to the diffusivity of a User-Defined Scalar. (The manual's example makes this clear.) For a species, I think turbulence effects are added separately -- this is what the Theory Guide appears to say, but I don't remember checking in detail. But then, there is a built-in version of kinetic theory for diffusivity, and this uses the molecular masses, species by species. (In fact, it seems to calculate it for each species-species pair, which does not seem possible in the UDF. Hmm.) Good luck! Ed

December 6, 2018, 12:49		#3
souza.emer Member Emerson Join Date: May 2018 Posts: 35 Rep Power: 8	Hello Yeah, I know that Macros are used for accessing the values, that's why I'm in doubt because I'm not sure of how I could use the value of diffusivity calculated by the Kinect Theory when I set this previously at the materials menu in Fluent. I do not want to use the macro to modify the current diffusivity coefficient calculated by Kinect Theory theory, I just want to use the value which Fluent calculates. I think you thought that I wanted to modify it, which I do not want to, yet. Do you know how I can do this? Or make any suggestions? Best

May 28, 2019, 15:28		#6
Weiqiang Liu Senior Member Weiqiang Liu Join Date: Feb 2018 Posts: 278 Rep Power: 9	hello, can I ask a question under this thread? I checked the syntax in manual and found the correct syntax is C_DIFF_L(c,t,i,j). I am wondering is the diffusion coefficient I am accessing with this macro is binary diffusion coefficient? what if I want to access mixture averaged diffusion coefficient or multi-component diffusion coefficient？