[wangy1767]

Hi all,
I have a small question about air ideal gas in CFX Pre.
Whether or not air ideal gas considers the properties of air e.g. convection, diffusion/dispersion and radiation etc.? If not, how should we set those properties in CFX Pre step by step in detail?

Many thanks in advance!
Br.

wang

[ghorrocks]

Air ideal gas can support all those physics. But you have to set the models up (eg buoyancy, something to diffuse and a radiation model) to use it.

[wangy1767]

Originally Posted by ghorrocks

Air ideal gas can support all those physics. But you have to set the models up (eg buoyancy, something to diffuse and a radiation model) to use it.

Many thanks! I can find and define buoyancy model and radiation model. But I can't find diffuse and convection (only exist buoyancy model?) model. Please show me where are they or how to define them: diffuse and convection model for ideal air.

Many thanks in advance!

[ghorrocks]

There are several types of buoyancy model. The first is to use a Boussinesq buoyancy model in an incompressible fluid - this is simplest and least accurate. A more accurate approach is to use an ideal gas with heat transer and let the ideal gas equation take care of the density difference. You will need to define a gravity vector for both of these approaches.

The convection model is built in - if the fluid is flowing it is convecting.

[wangy1767]

Originally Posted by ghorrocks

There are several types of buoyancy model. The first is to use a Boussinesq buoyancy model in an incompressible fluid - this is simplest and least accurate. A more accurate approach is to use an ideal gas with heat transer and let the ideal gas equation take care of the density difference. You will need to define a gravity vector for both of these approaches.

The convection model is built in - if the fluid is flowing it is convecting.

many thanks!
There is no Boussinesq buoyancy model in my CFX/Pre, or I cant find it. I dont know why?

In my CFX pre Domain-Basic settings, Buoyancy Model-Option only exists No Buoyant and Buoyant. I used Buoyant, I set already Gravity Vector and Buoy. Ref. Density **kg/m3. Is this Type 2nd approach as you said?

And where is Diffusion model? How to define it in Cfx-Pre? Is the diffusion model is also built in - if the fluid is flowing it is diffusing?

Many thanks in advance!

[ghorrocks]

There is no Boussinesq buoyancy model in my CFX/Pre, or I cant find it.

You have to enable the thermal model, then specify a gravity vector. Then it uses a boussinesq buoyancy model.

In my CFX pre Domain-Basic settings, Buoyancy Model-Option only exists No Buoyant and Buoyant. I used Buoyant, I set already Gravity Vector and Buoy. Ref. Density **kg/m3. Is this Type 2nd approach as you said?

You have to do this for ANY buoyancy model. This is where the gravity vector is defined and that is needed for all models.

And where is Diffusion model?

Diffusion of what?

If you mean of momentum then the viscosity does this. If you mean of heat then thermal conductivity does this. If you mean of another scalar the diffusivity of the scalar does this.

[wangy1767]

Originally Posted by ghorrocks

You have to enable the thermal model, then specify a gravity vector. Then it uses a boussinesq buoyancy model.



You have to do this for ANY buoyancy model. This is where the gravity vector is defined and that is needed for all models.



Diffusion of what?

If you mean of momentum then the viscosity does this. If you mean of heat then thermal conductivity does this. If you mean of another scalar the diffusivity of the scalar does this.

1) Do you mean to enable the Thermal Model in Domain/Fluid Models in CFX-Pre? But I only find Heat Transfer option in Domain/Fluid Models, there is no thermal model. Maybe since I selected Continuous Fluid not incompressible fluid, so I can't find a boussinesq buoyancy model?

2)I used CO2 as Ideal Air's additional variable, for Kinematic Diffusivity option, whether I should set Diffusivity Coefficient as 1.64*10^-5 m2/s here for CO2-Air?

Many thanks in advance!

[ghorrocks]

Sounds like you need more specific comments rather than general comments.

Can you describe what you are trying to do, and the results you hope to obtain and we can comment on the best way to achieve it?

[wangy1767]

Originally Posted by ghorrocks

You have to enable the thermal model, then specify a gravity vector. Then it uses a boussinesq buoyancy model.



You have to do this for ANY buoyancy model. This is where the gravity vector is defined and that is needed for all models.



Diffusion of what?

If you mean of momentum then the viscosity does this. If you mean of heat then thermal conductivity does this. If you mean of another scalar the diffusivity of the scalar does this.

Originally Posted by ghorrocks

Sounds like you need more specific comments rather than general comments.

Can you describe what you are trying to do, and the results you hope to obtain and we can comment on the best way to achieve it?

I simulated indoor CO2 concentration produced by some people inside the room, my method is to use CO2 as Ideal Air's additional variable, I'd like to ask whether this kind of way considers real CO2's properties in the air e.g. diffusion, convection (buoyancy) etc.?

Besides, for Kinematic Diffusivity option of CO2 as additional variable, whether I should set Diffusivity Coefficient as 1.64*10^-5 m2/s here for CO2-Air? Or other value I should define here?

For buoyancy model, Do you mean to enable the Thermal Model in Domain/Fluid Models in CFX-Pre? But I only find Heat Transfer option in Domain/Fluid Models, there is no thermal model. Maybe since I selected Continuous Fluid for Ideal Air Gas not incompressible fluid, so I can't find a boussinesq buoyancy model?



Many thanks in advance!

[ghorrocks]

An additional variable has convection and diffusion (if you choose the appropriate option). You could link it to a source term for bouyancy but if you want CO2 to affect buoyancy I would instead use a multi component mixture which will then allow for the different material properties between air and CO2.