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Old   March 26, 2003, 02:12
Default laminar-turbulent
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MAtthias
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Hi together,

im working with a commercial cfd-software which offers some standard turbulence models (k-eps,k-eps +RNG). Now i wanted to run a simulation of a incompressible, adiabatic and steady state one phase flow through a nozzle to determine which geometric modification i must do to obtain a lamniar flow at the same pressure drop.

The problem i have:

- before launching the calculation i have to switch on laminar or turbulent flow in the cfd-solver.

- i expected of the calculation that when a critical reynolds number is reached the flow will be turbulent and a laminar calculation will be divergent

- but, i can calculate even a flow through the nozzel at a Reynolds-number of 3000 or higher with laminar configuration of the solver

-> what are the reasons (physical, mathematical)? -> what happens when im calculating a laminar flow with turbulent configuration of the solver ? -> how can i determine the flow change (lam->turb)

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Old   March 26, 2003, 04:18
Default Re: laminar-turbulent
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Manosh
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A Stability analysis may help here. By which you can find a critical Reynolds number (R_c)with the wavenumber/frequency. Then you can determine that when you are in R>R_c the flow will become turbulent and laminar when R<R_c.

Manosh
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Old   March 26, 2003, 04:19
Default Re: laminar-turbulent
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Manosh
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Missing last few words!

when R<R_c.

Manosh
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Old   March 26, 2003, 04:22
Default Re: laminar-turbulent
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Manosh
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oh! why less than sign doesn't work!!

I wanted to write

When R less than R_C.

Manosh
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Old   March 26, 2003, 04:22
Default Re: laminar-turbulent
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MAtthias
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Manosh:

how do you perform a stability analysis and whats up with my simulation when gets a flow turbulent
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Old   March 26, 2003, 05:38
Default Re: laminar-turbulent
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andy
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Commercial codes are typically designed to be robust and, as a consequence, clobber the onset of physical instabilities along with numerical instabilities. If you wish to study the onset of turbulence then you must use a code with appropriate modelling assumptions and definitely ones with no/negligible numerical dissipation. High Reynolds number turbulence models are irrelevant.

If you accelerate the flow hard enough it will stay laminar or relaminarize if turbulent. To be honest, you are probably better off spending some time reading about the physics rather than bashing away with what is most likely an inappropriate CFD code.

If you are still keen to pursue the CFD modelling then I would have a word with your CFD supplier who may well have an appropriate in-house code. Alternatively, there are a number of research groups in Germany with suitable software.

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Old   March 26, 2003, 05:54
Default Re: laminar-turbulent
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MAtthias
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great,

to be honest also, im rather new at cfd. So therefore i wanted to study the effects in a flow not discuss the capabilities of different codes.

questions: -> why does the flow changes (lam-turb) -> why does the flow relaminarize by accelerating hard enough. -> geometric influence: i thought by modifing the outflow geomtry of the nozzle and model a sharp edge (divergend nozzle)i would enforce a turbulent flow and vice versa -> what effects has the change from lam to turb at the outflow land of the nozzle if there is at a small distance in flow direction a plane with two small bores
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Old   March 26, 2003, 07:24
Default Re: laminar-turbulent
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Manosh
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M Atthias:

I have no idea how to tackel stability analysis with commercial packages. Sorry! Andy's answer (below) has good insight.

If you want to know initial to details of stability analysis, the following book may help you.

The theory of hydrodynamic stability / by C.C.Lin.

Manosh
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Old   March 26, 2003, 07:27
Default Re: laminar-turbulent
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MAtthias
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Manosh,

do you have some answers to the questions i wrote replying to andys text?
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Old   March 26, 2003, 08:17
Default Re: laminar-turbulent
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Manosh
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questions: -> why does the flow changes (lam-turb)

Answer:

The laminar flow is acted on by small disturbances in it. These disturbances may come from the entrance or from the wall roughness or mathematically the presence of non-linear terms in the Navier-Stokes equation is one who can automatically genarate disturbances/instabilities. Now question is how these disturbances behave. If these die in time then the flow still in laminar stage. If not, these disturabces grow and transition from laminar to Turbulence occur, and flow changes from laminar to turbulent.

Manosh
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Old   March 28, 2003, 14:02
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Ted
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Old   March 28, 2003, 14:09
Default Re: laminar-turbulent
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Ted
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- before launching the calculation i have to switch on laminar or turbulent flow in the cfd-solver.

- i expected of the calculation that when a critical reynolds number is reached the flow will be turbulent and a laminar calculation will be divergent

A: Laminar solution does not have to be divergent. On coarse grids, in fact, it will be just fine (but incorrect). Also, flow generally does not become turbulent upon reaching critical Re, which is the threshold below which flow is guaranteed to be laminar, nothing more.

- but, i can calculate even a flow through the nozzel at a Reynolds-number of 3000 or higher with laminar configuration of the solver

A; sure, see above

-> what are the reasons (physical, mathematical)? -> what happens when im calculating a laminar flow with turbulent configuration of the solver ? -> how can i determine the flow change (lam->turb)

A: depending on the model you use, you will get from somewhat incorrect results to very incorrect ones. Typically once you turn on turbulence, the model increases the flow's viscosity, but at least an order of magnitude.

As to how to determine whether a flow is transitioning from laminar to turbulent, can't tell you as this is a subject of my work, which will hopefully get a Nobel.
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