Hi,

In a laminar flow there are velocity gradients, but no turbulence present. Therefore not in every flow where velocity gradients are present necessarily turbulence must be present too.

For a CFD solver - unless you are not doing DNS - it is input knowledge of the simulating person, whether the studied flow is laminar, turbulent or in transitional stage. In order to derive this input knowledge for the CFD solver, CFD text book information about flow regime transition based on Reynolds number estimates are used. The CFD solver by itself is not able to find out about the flow regime. Because you as the scientist have to prescribe in the CFD solver setup the appropriate modeling for the flow, i.e. no modeling if the flow is laminar, a transition model if the flow is transitional and an appropriate turbulence model if the flow is really turbulent.

Best regards,

Dr. Th. Frank.

Dear Dr. Thomas Frank

Thanks for your valuable answer.

So it means that Reynolds number is the only estimation we could use to describe the flow is Laminar or turbulent?

In a case, where flow is in Laminar regime acc to Reynolds number but has irregular structure in certain regions, how can we predict the nature of flow?

In my study of low Reynolds number, I compared the turbulent model with the Laminar to predict the flow numerically, and surprisingly flow with Laminar seems to be more turbulent than the turbulent model results.

Dear Hafiz A. Ahmed :

Have gone into this discussion : https://www.researchgate.net/post/Is_a_complete_mathematical_description_of_turbulence_possible ?

If not, I'm sure you will find it very interesting, the subject you stated in your question is discussed here.

Regards !

Q-criterion may give you some insights. You can calculate it in fluent.

I agree with Tomas Frank, think about the coute or poiseulle flow, you have certainly velocity gradients but not turbulence. As @mahdi mentioned the W criterion could be used, but it does not provide you with quantitative information about turbulence.