[bkm]

I am solving heat transfer on rectangular channel on water. I tried to solve problem with laminar flow model. But I got convergence issue .. it is fluctuate Between range of 10-3 to 10-4.. imbalance is in under 0.2 % ...and when I tried to it with SST Model it converge. So is it correct to use this Model on laminar flow case

[evcelica]

First off, the laminar model should converge. I'd suggest using a better quality mesh.

As far as if SST can predict laminar flow, well, that depends. Is it giving the same answer as the laminar model? If not, then no. In some cases the SST will resolve to low/zero turbulence, but that is not always the case.

Have you included enough of an inlet length for your flow to fully develop before the heat transfer? The SST model will have turbulent fluid coming in the inlet, as set by your boundary condition. This would take time/distance to convert to laminar using the SST model. Check your Viscosity ratio in your results, see how turbulent the SST model is showing your flow to be.

Bottom line though, fix your laminar model if you know the flow is laminar. use an output monitor on some result of interest to see when your result of interest has converged.

[ghorrocks]

I have a bit of a different take on this than Erik.

This FAQ describes the procedure for non-convergence: https://www.cfd-online.com/Wiki/Ansy...gence_criteria

Laminar flows can have problems converging where they are actually turbulent flows, or where transient laminar flow structures exist. (This is assuming you have a good quality mesh and the simulation is correctly set up.) If you say the flow should be laminar then it is still quite possible that a laminar steady state simulation will not converge because of transient flow structures. This is very common in heat transfer simulations with natural convection - the natural convection tends to have transient laminar structures. If this is the case the only way to proceed is to do a transient laminar simulation.

If you solve a laminar simulation with a turbulence model you are adding extra dissipation to the model. This dissipation is not real, it is a product of the turbulence model you are using, but it can have the effect of damping out these transient flow structures and apparently converging. The SST turbulence model is better than most turbulence models as when the turbulent kinetic energy goes to zero (it is exactly zero in a laminar flow, by definition) the turbulent viscosity also goes to zero, so SST does not add much dissipation to the model and you might get away with it. But k-epsilon based models are well known to have far too much dissipation in the low Reynolds number regimes because as k goes to zero the turbulent viscosity goes to a finite value, and this is false additional dissipation. This is why the k-e model is a bad choice for low Reynolds number flows, and you either need to modify it or use a k-omega based model like SST which does give zero turbulent viscosity at zero turbulent kinetic energy.

Hopefully that explains things a bit.

[bkm]

Originally Posted by ghorrocks

I have a bit of a different take on this than Erik.

This FAQ describes the procedure for non-convergence: https://www.cfd-online.com/Wiki/Ansy...gence_criteria

Laminar flows can have problems converging where they are actually turbulent flows, or where transient laminar flow structures exist. (This is assuming you have a good quality mesh and the simulation is correctly set up.) If you say the flow should be laminar then it is still quite possible that a laminar steady state simulation will not converge because of transient flow structures. This is very common in heat transfer simulations with natural convection - the natural convection tends to have transient laminar structures. If this is the case the only way to proceed is to do a transient laminar simulation.

If you solve a laminar simulation with a turbulence model you are adding extra dissipation to the model. This dissipation is not real, it is a product of the turbulence model you are using, but it can have the effect of damping out these transient flow structures and apparently converging. The SST turbulence model is better than most turbulence models as when the turbulent kinetic energy goes to zero (it is exactly zero in a laminar flow, by definition) the turbulent viscosity also goes to zero, so SST does not add much dissipation to the model and you might get away with it. But k-epsilon based models are well known to have far too much dissipation in the low Reynolds number regimes because as k goes to zero the turbulent viscosity goes to a finite value, and this is false additional dissipation. This is why the k-e model is a bad choice for low Reynolds number flows, and you either need to modify it or use a k-omega based model like SST which does give zero turbulent viscosity at zero turbulent kinetic energy.

Hopefully that explains things a bit.

Thank You Glenn and Erik!!
it means i can use SST model for laminar flow.Correct!!

I have quite simple structure rectangular plate with one side inlet another side outlet diagonally. My solution was converged for 1 lpm flow rate of water with Laminar model. But it is not converging for 3 lpm flow rate reaming physics and everything is same for steady state. I have refined mesh but there was no improvement in results. Thats why i used SST model ( i thought that may be some of element may create turbulence ). Even i running it for transient case with laminar model may be it will work. I am waiting for results.

Thank you once again

[ghorrocks]

it means i can use SST model for laminar flow.Correct!!

No. It is not as simple as that. If your flow is steady state laminar and you use the SST turbulence model the turbulence model is likely to give effectively zero turbulent viscosity, meaning that your answer probably will be reasonably accurate (only with a small amount of extra dissipation).

If your flow is transient laminar and you use the SST turbulence model there is a good chance the turbulence model will generate too much dissipation and damp out the transient flow, which is wrong. You are likely to get a big error in this case.

I have quite simple structure rectangular plate with one side inlet another side outlet diagonally. My solution was converged for 1 lpm flow rate of water with Laminar model. But it is not converging for 3 lpm flow rate reaming physics and everything is same for steady state. I have refined mesh but there was no improvement in results. Thats why i used SST model ( i thought that may be some of element may create turbulence ). Even i running it for transient case with laminar model may be it will work. I am waiting for results.

In this case using the SST turbulence model is wrong. A transient laminar model is correct. Note you will need to do time step, mesh and convergence criteria sensitivity studies to work out what you need for time step and mesh size, and convergence criteria.