[aestas]

Hello,my friend.
Recently i have been working on a plasma simulation problem.I have a problem in defining the energy equation.

In the Fluent 6.3 manual,fluent define the energy equation like this:
1111.jpg

and i made my equation similar to that form:
123.jpg

How ever in the UDS theory section, fluent solve the equation like this
123123123.jpg

if i make my energy equation into this form, it will have different source terms.

so i have a question, what scalar does the energy equation solve?T or h or else? what source terms should i define?

[chittipo]

Hi,
sorry, I deleted my earlier post as I am not totally sure...

[pakk]

In the UDS section, the Fluent manual describes how Fluent solves the equations for a UDS. It does not mention the energy equation there. Energy is not a UDS.

[chittipo]

Hi, now i feel bit comfortable to reply.
Let us say your equation t1 = t2 + t3
t1 = convection
t2 = diffusion
t3 = source
you do not have transient term.

Is it possible to transform "h + 0.5 u^2" to T. Then it is good. there are two ways.


1. keep only "rho and v" multiplication factor in convection term (t1) and take remaining coefficient to other side. This is only possible you have constant coefficient.
In this case your diffusion term (lambda/this_coefficient) and source term would have division with this coefficient.

2. keep resulted new coeficient with convection term ("t1") itself. But, there should be some where in UDS panel, there should be an option to select user defined convection term. By default it is "mass flow". Select this user defined convection, provide your "rho*v*this_coefficient" as a flow term. The corresponding macro "DEFINE_UDS_FLUX"

If there are additions/substractions, they should go to source term (as you rightly said before).

through DEFINE_DIFFUSIVITY() evaluated as it is now units lamda (W/m.s). If there are any coefficients they should be part of this Macro. e.g., lamda*x*y/z

through DEFINE_SOURCE() you get t3

Note: all the units should match. It appears t1, t2 are in W/m3.s. Therefore t3 should also be in same units


NOTE: I have last used FLUENT in 2012. If there is something changed i am sorry.

[aestas]

Originally Posted by pakk

In the UDS section, the Fluent manual describes how Fluent solves the equations for a UDS. It does not mention the energy equation there. Energy is not a UDS.

Thanks my friend. You mean that i should follow exactly the form of energy equation defined in fluent? I just need to define the source term in my equation(equation 1 and source one in the attachment file)?
123123.jpg

[aestas]

Originally Posted by chittipo

Hi, now i feel bit comfortable to reply.
Let us say your equation t1 = t2 + t3
t1 = convection
t2 = diffusion
t3 = source
you do not have transient term.

Is it possible to transform "h + 0.5 u^2" to T. Then it is good. there are two ways.


1. keep only "rho and v" multiplication factor in convection term (t1) and take remaining coefficient to other side. This is only possible you have constant coefficient.
In this case your diffusion term (lambda/this_coefficient) and source term would have division with this coefficient.

2. keep resulted new coeficient with convection term ("t1") itself. But, there should be some where in UDS panel, there should be an option to select user defined convection term. By default it is "mass flow". Select this user defined convection, provide your "rho*v*this_coefficient" as a flow term. The corresponding macro "DEFINE_UDS_FLUX"

If there are additions/substractions, they should go to source term (as you rightly said before).

through DEFINE_DIFFUSIVITY() evaluated as it is now units lamda (W/m.s). If there are any coefficients they should be part of this Macro. e.g., lamda*x*y/z

through DEFINE_SOURCE() you get t3

Note: all the units should match. It appears t1, t2 are in W/m3.s. Therefore t3 should also be in same units


NOTE: I have last used FLUENT in 2012. If there is something changed i am sorry.

I am so appreciate your reply, thank you so much my friend.

i think the method you meant is the equation 2 and source term 2 in the below picture.

It's sure that this form meet the standard UDS equation in fluent, but i am not sure whether the build-in equation should be defined like this...

[chittipo]

Hi,
what do you mean by 'built in equation...defined like this?'
where do you want to use this equation?

[aestas]

Originally Posted by chittipo

Hi,
what do you mean by 'built in equation...defined like this?'
where do you want to use this equation?

This is the energy equation in fluent...

[pakk]

Dear aestas,

It is not clear what you want to know. You want to know something about the energy equation, and something about sources, but what exactly?

[aestas]

Originally Posted by pakk

Dear aestas,



It is not clear what you want to know. You want to know something about the energy equation, and something about sources, but what exactly?

well, i wonder why you do not understand that..



it's easy to say, i activated the energy equation in fluent, and in my model the energy equation has the form in the picture.As you can see, i made two forms of energy euqation, euqation 1 has the form like the fluent defined in In the Fluent 6.3 manual, whereas equation 2 has the form like the UDS equation solved by the equation defined in the in the UDS theory section.

I don't know which form should be used, because it has totally different diffusion term and source term(which should be defined in the energy source term in fluent).


123.jpg

[pakk]

Originally Posted by aestas

I don't know which form should be used, because it has totally different diffusion term and source term(which should be defined in the energy source term in fluent).

Used for what?
Do you want to write the energy equation in a report?
Do you want to make your own CFD code that does the same as what Fluent does?

As long as you don't say what you want to do, I can not help you.

[aestas]

Originally Posted by pakk

Used for what?
Do you want to write the energy equation in a report?
Do you want to make your own CFD code that does the same as what Fluent does?

As long as you don't say what you want to do, I can not help you.

I want used it as the energy equation in fluent, i am working on a plasma problem, my energy equation is deduced from plasma theory, and i donot konw which form meet the energy equation requirements in fluent, because i have to define source terms of energy, you understand?

[pakk]

Originally Posted by aestas

I want used it as the energy equation in fluent, i am working on a plasma problem, my energy equation is deduced from plasma theory, and i donot konw which form meet the energy equation requirements in fluent, because i have to define source terms of energy, you understand?

No, I don't know what you mean with "use it as the energy equation in Fluent". Fluent does not allow you to choose an energy equation.

But maybe you mean to say: I want to add a source term for the energy equation, and I don't know exactly how. Is that what you want? Then please ask that, and don't ask something else.

[aestas]

Originally Posted by pakk

No, I don't know what you mean with "use it as the energy equation in Fluent". Fluent does not allow you to choose an energy equation.

But maybe you mean to say: I want to add a source term for the energy equation, and I don't know exactly how. Is that what you want? Then please ask that, and don't ask something else.

yes, I want to add a source term for the energy equation

and you can see i have two forms of source term...which one should i pick?

[pakk]

If you want to add an energy source, you should use the energy equation.
If you want to add a UDS source, you should use the UDS equation.

So in your case, just use the energy equation.

[aestas]

Originally Posted by pakk

If you want to add an energy source, you should use the energy equation.
If you want to add a UDS source, you should use the UDS equation.

So in your case, just use the energy equation.

ok, i see. Thank you so much.