Learning Forum

[Benjamin Walz]

Hello.

 

I am trying to simulate the indoor air flow inside a room model.

More specifically, I want to calculate thermal comfort indices according to ISO 7730 (PMV, PPD and Draught rate).

How do I determine the local turbulent intensity or the variation of the air velocity, which I need to calculate the draught rate?

Is there an already existing UDF that I can use for that? Unfortunaely, I am not used to write C-Code.

 

Any help regarding this topic would be greatly appreciated.

Regards

Benjamin

[Rob]

It's the local velocity, so velocity magnitude that's used. You may also want to account for solar gain (heating) as that's a significant part of comfort. 

Yes, there is a UDF, but as you'll know from reading the rules when you signed up I (as Ansys staff) can't share it on here. You may want to have a look on the Customer Portal; students may not have access but staff may if your site has a maintained licence. 

[Benjamin Walz]

Hi Rob, thank you for your answer.

My current model does not take solar radiation (eg through a window) into account. I will model solar radiation in a later step.

The local velocity of air is used to calculate the PMV, but as far as I know, for the Draught rate I need a measure for local turbulence. I am slightly confused with the terminology here, since in german there is something called "Turbulenzgrad", which translates approximately to "turbulence intensity". Its defined locally as follows:

TU = 100 * standart deviation of velocity / mean velocity    [%]    This is how i measure turbulence in the laboratory. The measurements are used to "calibrate" my CFD-Model.

I found following equation in Fluent theory guide:

Tu = 100/u *(k*2/3)^0.5    ,unit is also % i guess.

Do these two expressions describe the same?

I do have access to the customer portal, I couldnt find something on this in Solutions. Can I ask for a hint where exactely to look for such a udf?

 

[Rob]

I searched for PMV but I also have a staff level view so it may be slightly different.  In the UDF I have I can't see any turbulence model values. 

In the equation, you've got "k" rather than standard deviation of velocity, so you'll need to check the model definitions: the Theory Guide is fairly comprehensive.