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-   -   Laminar & Eddies ?? (https://www.cfd-online.com/Forums/fluent/30678-laminar-eddies.html)

 Bharath December 14, 2002 14:46

Laminar & Eddies ??

Hello

I would like to knw if an eddy, vortices,,,can be created in laminar flows through ..ducts ,channels etc ?? Could u site me an example of the occurence same.. Also,...if such interactions are present..is it wise to assume Steady State assumptions for CFD problems...??

Thanks

Bharath

 Stanislav Pagur December 16, 2002 07:37

Re: Laminar & Eddies ??

Hi Bharath,

Vortices creation is unusual for laminar flows (because of defenition of laminar flow "rot(v)=0") Also circulation velocity is constant - if your flow just have a vortex - it can't disappears by flow-flow interaction (vortex can be swallowed only by onother vortex or by wall). I know an examle of such vortex - pipe with elbows in two dimensions - flow is laminar, but central vortex is stable.

BR, Stanislav

 Bharath December 16, 2002 13:51

Re: Laminar & Eddies ??

hello Stanislav

consider a case of BAckward Facing Step...in a rectangular duct due to sudden increase in the diameter of the duct ..is it possible that the vortex generated ...is due to a Laminar flow ?

Thanks

Bharath

 kim December 19, 2002 07:10

Re: Laminar & Eddies ??

From very basic fluid mechanics: rot(v)=0 is definition of POTENTIAL flow, not laminar. In laminar flow the condition rot(v)=0 does NOT hold.Which means that vortices can also appear in laminar flow. Using CFD means that one has to know the fluid mechanics well. Results are useless if you do not know what you compute and how to compute it (assumptions, BCs, etc.)

 sue December 19, 2002 12:40

Re: Laminar & Eddies ??

What's the differnce between vorticity (rotation) and eddy?

 Stanislav Pagur December 24, 2002 05:57

Re: Laminar & Eddies ??

Diar kim, please give an example of laminar but not potential flow

Stanislav

 Marc December 31, 2002 12:24

Re: Laminar & Eddies ??

I completely agree with Kim.
rot(v) means potential flow. Potential flow is always laminar, but laminar flow is not necessarily a potential flow. You will not find potential flow in reality. With some simplifications you can despribe special cases analytically. The potential theory ends when you have walls, which are not free of friction. A simple example for laminar flow, which violates the potential conditions, is a simple pipe flow below critical RE number. The potential condition will only be satisfied in the centerline. In the rest of the flow field you will find rotation.
Greetings Marc

 kim January 3, 2003 06:39

Re: Laminar & Eddies ??

Marc is absolutely right. VORTICITY is present in every laminar flow as result of no-slip condition and viscosity.Just try to rewrite Navier-Stokes eq. in the form of angular velocities and you will immediately find that rot(v)=0 does not hold for laminar flow. As an example of laminar flow take the simpliest one:Couette flow (flow between 2 paralel plates).As a result of shear stress there is VORTICITY. It is nice we talk about CFD, but we should always know the basic principles. Sincerely Kim

 lgp January 21, 2003 13:13

Re: Laminar & Eddies ??

Hi Kim, I have one question in my mind. Will you tell me more about it.

In my problem i have got a tank of size 1m X 1m X 1m. the inlet is only 6mm jet flow(fluid is water) with Re=13,000, outlet is 0.003m X 1m. Now my doubt is regarding the use of K-E turbulence model for simulating the entire tank. Major part of the tank is having cell Reynolds Number less than 200. hence if i use K-E realisable model for simulating the flow in the tank, will it be correct? Is there any method for such problem( both laminar and turbulent flow in a same domain without any demarcation)?

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