Question about CFX turbulence intensity
 

Hi

Here is my approach to detect the flow regime for flow in complex geometry:

1) Run the simulation by SST model (or other turbulence model)

2) Look at the turbulence intensity value to know the flow regime


Question:

1) Does this approach make sense?

2) What is the range of the turbulence intensity value corresponding to laminar flow? And transitional flow?

3) Could CFX output turbulence intensity in CFX post?

You should not need to do this. Reynolds number can be calculated for most geometries - no need to be exact, just close enough - and compared to the known transition points for pipe flow, flat plates or spheres/airfoils and you will know if you are laminar, turbulent or transitional. In the setup stages, that is all you need to know.

Other non dimensional numbers may also be useful, depending on the simulation. FOr instance Rayliegh number for natural convection flows.

Dear ghorrocks,

I am wondering if you know the turbulence intensity in a natural convection cases(in an enclosed domain) for a turbulent flow and laminar flow? Do you know how the TI should vary in percentage?
for instance more than 20%, should be called fully turbulent flow?

Thank you,
Hooman

Turbulence intensity is 0% for a laminar flow by definition - a laminar flow is not turbulent. So if the turbulence intensity is larger than 0 then the flow is turbulent.

For buoyancy driven flows the Rayleigh number is often a better measure. There is often a Rayleigh number which transitions from laminar to turbulent flow, just the same as in Reynolds Number.

http://en.wikipedia.org/wiki/Rayleigh_number

Dear ghorrocks,

Thank you for the reply. Basically, I knew and agree that Ra is important. Suppose you do not the Ra corresponds to transitional or turbulent flow. so maybe a good way is to calculate TI and judge based on that value, that flow is laminar, transitional or turbulent(low-medium or high turbulent)...
So, my concern is about that and I am wondering if you know a reference about the Ti variation in a natural convection case.

Regards,
Hooman

The simulation approach used for laminar flows is quite different to turbulent flows - you don't use a turbulence model for one. You need to determine in advance whether the flow is laminar or turbulent and select appropriate models. CFX has a turbulence transition model which is the only model in CFX which can handle laminar and turbulent flows and their transition - and it is only applicable under specific circumstances (flow over airfoils and airfoil like objects).