Unable to get unsteady result for natural convection problem at critical Rayleigh no.
 

Hello everyone
I am using buoyantBoussinesqPimpleFoam solver to simulate the natural convection problem in a 3D square cavity of aspect ratio 1:1:1. My fluid inside the cavity is water.

For this case the critical Rayleigh number given, for which the flow will become unsteady, is 5e8. I tried to simulate this case at same Rayleigh number and even at 6e8 Rayleigh number but the results were showing steady state behavior. I also tried to simulate my results with different combinations of schemes like Euler and Crank Nicolson temporal schemes and also tried to change the Relaxation factors but this did not help either.

I am using openfoam 5.0 for the simulations. And I am also learning openfoam by running different cases.

Please help me in this regard, I have also checked similar kind of query on the CFD forum but that did not help my case.

I have also attached my case file with google drive link.
https://drive.google.com/file/d/1WL9...ew?usp=sharing

Someone please look at my case and help me.

Regards
Rahul

Not getting same results.
 

Hi everyone,

So I again ran the simulation on the above mentioned post of natural convection at 5e8 Rayleigh number. And the result looked like a steady state result to me (Figure 1). But someone told me to zoomed in to the probe signal and after zooming in I could see some fluctuations(Figure 2). But I am not satisfied, is that data really unsteady?

Also there is one more thing, that I want to ask. I stopped the simulation after 1440 seconds then I restart the simulation from the same time step. But after restarting the simulation it showed a complete different behavior and after some time oscillations were starting to damp (Figure3). I don't know why this was happening with this simulation. So someone please tell me if there is something wrong with case set up or not. I am using openfoam version 5.0 for the simulations. I have attached a link of my case file in my previous post.

So please help me if anyone has the idea why this was happening in this case.

Regards
Rahul


Figure 1
Attachment 73090


Figure 2
Attachment 73091


Figure3
Attachment 73092