[M29]

Hi everyone.

I'm currently analyzing the dry cask which cooling the spent fuel assembly. The spent fuel assembly is 9000W/m3 heat source.

But I have a problem in my case.

The "Reynolds number" problem occurred when i analyzed my case through Laminar model in the atmosphere domain(you can see that area in figure 1). So, i changed the turbulence model from Laminar(none) to SST.

After analysis, the Reynolds number problem disappeared in the atmosphere domain but it occurred in the gap domain(you can see the gap domain in figure 2). The gap domain is very thin hollow cylinder and the thickness is 6mm.

I can't understand why the problem is occurred. The Reynolds number in the gap domain is about 52.128. I think it has small value and even i used the SST turbulence model.

summary
Laminar model -> Reynolds number error occurred in Atmosphere domain,
SST model -> Reynolds number error occurred in gap domain.

Please help me... I can't believe my results because of this error.

[ghorrocks]

The Reynolds Number "error" really should be a warning only. It is just saying that you should check whether this flow is laminar or not, and if it is turbulent you should use a turbulence model. So rather than just do what it says you should look at this flow and work out whether it is turbulent or not.

Note the Rayliegh number is the non-dimensional number of relevance to free convection flows to determine whether it is turbulent or not, not Reynolds Number. (For most free convection cases, anyway).

Also I note your residuals are not converging very well. Refer to this FAQ for that issue: https://www.cfd-online.com/Wiki/Ansy...gence_criteria

[Opaque]

From the convergence diagnostics, I can tell you are running multiple Mass and Momentum/Heat Transfer subsystems

Unless you setup the case using beta features with non-constant domain physics (and you know what you are doing after that), I am afraid your setup is missing a few domain interfaces definitions.

You may be solving a different problem than you expect.

Hope I am mistaken

[M29]

Originally Posted by ghorrocks

The Reynolds Number "error" really should be a warning only. It is just saying that you should check whether this flow is laminar or not, and if it is turbulent you should use a turbulence model. So rather than just do what it says you should look at this flow and work out whether it is turbulent or not.

Note the Rayliegh number is the non-dimensional number of relevance to free convection flows to determine whether it is turbulent or not, not Reynolds Number. (For most free convection cases, anyway).

Also I note your residuals are not converging very well. Refer to this FAQ for that issue: https://www.cfd-online.com/Wiki/Ansy...gence_criteria

Thank you for your answer!!
I understand about Rayleigh number that distinguish the Turbulent and Laminar. So, Can i neglect the "warning of Reynolds number?" (because the Turbulent and Laminar is decided by Rayleigh number instead of Reynolds number in the Free convection system.)

I have a question in terms of Reynolds number.
I actually used the turbulence model(SST) in my case. But the Reynolds number warning occurred in the gap domain. I think that this is due to the laminar flow in the gap domain. is it correct?(actually the Reynolds number in the gap domain is about 50.)

[M29]

Originally Posted by Opaque

From the convergence diagnostics, I can tell you are running multiple Mass and Momentum/Heat Transfer subsystems

Unless you setup the case using beta features with non-constant domain physics (and you know what you are doing after that), I am afraid your setup is missing a few domain interfaces definitions.

You may be solving a different problem than you expect.

Hope I am mistaken

Thank you for your answer!!
i actually checked the all interfaces but there is no problem.. I read your answer and i understood that the Reynolds number error occurred because of the wrong interfaces, right??
Then,, i will check interfaces again!!

[ghorrocks]

If you know Rayleigh number is the important parameter to determine whether the flow is laminar or turbulent then you can ignore the message. But you say the Reynolds number in the gap is 50 - you should not be using Reynolds numbers if Rayleigh number is the important parameter. You have to be consistent. You should work out the Rayleigh number in the gap and determine if that Rayleigh number is turbulent.

[M29]

Originally Posted by ghorrocks

If you know Rayleigh number is the important parameter to determine whether the flow is laminar or turbulent then you can ignore the message. But you say the Reynolds number in the gap is 50 - you should not be using Reynolds numbers if Rayleigh number is the important parameter. You have to be consistent. You should work out the Rayleigh number in the gap and determine if that Rayleigh number is turbulent.

Thank you very much!!
Let me ask you one more question. Are results i got correct regardless of this error(warning)?
For example, because the Reynolds number is exceeded in the special point in the gap domain, so the results i got are wrong. So, the warning occurred.

[ghorrocks]

The warning/error is nothing to do with whether your simulation is accurate. As discussed, Rayleigh number is the important parameter, not Reynolds number.

For the simulation to be accurate you need to consider many more issues than just the Reynolds or Rayleigh numbers. See FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F