Hello ,
I am surprised by the results of my 3d model. Its results remain unaffected for all values of parameters used in the model.
[@prj@frederichecht ]
can anyone suggest/comment to resolve it?

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Also, an error is popping up for the plot:

"plot: Sorry no ps version for such type of plot 6 "

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My code for cubic enclosure with
bottom surface hot,
upper surface cold, and
insulated for all other surfaces in Cubic enclosures are as follows:

I played with this for a while but could not get it t converge as written
or with some modifications. I did question your boundary conditions however.
If this is supposed to be energy and momentum, did you want to set U to
zero at top and bottom? Also you have a cNewton factor on eqn labelled x momentum
which does not look right. I can’t really tell much however since I can’t get it
to converge…

Hi @marchywka,
this coupled equation ( momentum and energy) takes care of the continuity equation through the penalty FEM approach.
Also, I have put velocity components zero at each surface due to the no-slip boundary conditions. In my case, it is converging but not affected by the value of the Rayleigh number.

Did you try plotting u_z? Fluids isn’t really my thing but apparently the
reynolds number involves the flow velocity and a transition to turbulent
flow. If the veolocity is zero what will happen? You won’t really see much
in any case until you approach a transition AFAICT.
I guess you could try removing the boundariy conditions on u on the top
or bottom or both and see what happens .

Still puzzled why it won’t converge on my fresh clean install…

It looks like I was just running out ot memory so I changed the elements
to P1 instead of P2. If you plot e3, your boundary conditions on surfaces 5 and 6
create zero flow right? Reomve those and plot u3. With the reynolds number at
1e8 i looks a lot different lol. Not sure any of this is right but if you are modelling
flow along z I’m not sure you cant to set flow to zero at both ends.

I hacked up your code a lot there may be other changes …