Laminar or Turbulent
 

Dear All Before I use a CFD software, I must determine the flow is laminar or Turbulent. But how can I decide, especially for those unsteady flow? Sometimes we can make sure based on the Reynolds number, but I don't think it accurate. Based on experiment? How if the flow is unsteady? Please advise.

W.W.

Re: Turbulent or Laminar
 

A necessary condition for a turbulent flow is that the spatial gradients of the fluctuating velocities are non-zero.

For unsteady flows you just need to use a more sofisticated decomposition than Reynolds averaging, it really depends on your application.

Re: Turbulent or Laminar
 

All flows are turbulent, it's just that laminar flows don't know it yet.

Re: Turbulent or Laminar
 

(1). I think, it is safer to assume that the flow is turbulent in the first place. (2). But if you don't know how to model the turbulent flows, then the only option is to assume that it is laminar. (3). By the time when you have some solutions and need to do validation, you will know whether the flow is laminar or turbulent.

Re: Turbulent or Laminar
 

Thank you very much. However I still wonder whether I could use experiment to testify, such as hot-wire or hot-film. Maybe this is away from CFD, but I think the combination should be better.

Regards.

Wood

Re: Turbulent or Laminar
 

(1). Yes, that is part of the validation.

Re: Laminar or Turbulent
 

Yes, I think before you run your CFD code, you have to know it is turbulent flow or not. I agree with you that Re number is not a accurate index, expecially for non-common flow.

Re: Laminar or Turbulent
 

I think, good turbulence modeling should be able to predict the laminar flow. I mean the Reynold strees item in Reynold time average quations should be calculated to zero by turbulece modelling in laminar flow. So, this kind of modeling can be applied to both laminar and turbulent flow.

Re: Laminar or Turbulent
 

Could you please give me some information on such models? RNG?

Re: Laminar or Turbulent
 

Sorry, I do not know much about this field. If this is no such a model, it will be worthwhile to develop one. What do you think?

Re: Laminar or Turbulent
 

Actually, I just want to apply a suitable method to solve my problem. I can not waste my time. Thanks anyway.