Determining flow type of natural convection in cylindrical cavity

Rozpruwacz · July 2, 2021, 09:13

Hello,

I'm just out of college and staring my career in CFD. I have a question regarding natural convection in closed cavities.

How do I determine flow type for my CFD simulation?

I know that we can judge flow type by Reynolds (Re) number in forced convection and Rayleigh (Ra) number, which is a product of Grashof (Gr) and Prandtld (Pr) numbers, in natural convection. However, only critical values of Ra that i've found is 10^9 for vertical plate cooling (found in Y.Cengel "Heat Transfer. A Practical Approach." as well as T.L Bergman et all "Fundamentals of Heat and Mass Transfer") and 10^6 for squared cavity (found here: https://www.researchgate.net/publica...nclosed_cavity).

How do I proceed when my geometry is not one of those two. For example consider "cylinder in cylinder" cavity presented on the picture below:

.

I tried to find some resources with similar geometry and H/R ratio but I was not able to find anything. How then can judge character of my flow?

Best Regards,
Wojtek

piu58 · July 2, 2021, 11:20

I don't understand you case fully.

We have the natural convection, which mostly happens in a boundary layer (it is driven there).

Do we have a rotation cylinder in a fixed surroundings or do we have a rotation cell?

Rozpruwacz · July 8, 2021, 02:28

It is simply a cylindrical heat source with gas layer of thickness R outside. No rotation, arrows are just to show where fluid flow is happening. I just provided an example - my question is more general: How to judge weather flow is turbulent or laminar in natural convection in cavities.

piu58 · July 8, 2021, 08:02

I assume that you simulate temperature with the flow. Starting turbulence somewhere in the region means that you have a steep gradient of temperature: Laminar means you have mostly head conduction, a diffusion like process. In turbulence all is mixed and the process is convection dominated.

ships26 · July 8, 2021, 08:29

Nevermind, I just re-re-read your question and you have found the article on convection-driven cavities...
This might be a starting point, since they simulate non-square cavities. However, you might have to dig deeper into bibliography to find out about transition itself.

Also, keep in mind that this is a 2D study and not an axisymmetric one.

Rozpruwacz · July 10, 2021, 02:55

Thanks for the replies and sources, i'll look into them

July 2, 2021, 11:20		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	I don't understand you case fully. We have the natural convection, which mostly happens in a boundary layer (it is driven there). Do we have a rotation cylinder in a fixed surroundings or do we have a rotation cell? __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

July 8, 2021, 08:02		#4
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	I assume that you simulate temperature with the flow. Starting turbulence somewhere in the region means that you have a steep gradient of temperature: Laminar means you have mostly head conduction, a diffusion like process. In turbulence all is mixed and the process is convection dominated. aerosayan likes this. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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July 8, 2021, 02:28		#3
Rozpruwacz New Member Join Date: Jul 2021 Posts: 4 Rep Power: 4	It is simply a cylindrical heat source with gas layer of thickness R outside. No rotation, arrows are just to show where fluid flow is happening. I just provided an example - my question is more general: How to judge weather flow is turbulent or laminar in natural convection in cavities.

July 8, 2021, 08:29		#5
ships26 New Member Francisco Join Date: Sep 2018 Location: Portugal Posts: 27 Rep Power: 7	Nevermind, I just re-re-read your question and you have found the article on convection-driven cavities... This might be a starting point, since they simulate non-square cavities. However, you might have to dig deeper into bibliography to find out about transition itself. Also, keep in mind that this is a 2D study and not an axisymmetric one.

July 10, 2021, 02:55		#6
Rozpruwacz New Member Join Date: Jul 2021 Posts: 4 Rep Power: 4	Thanks for the replies and sources, i'll look into them