problem in convergence with Rayleigh above 1e7; Please Help me!!!!!
 

hi everybody

I'm analyzing a stack that is axisymmetric with boussinesq. my Rayleigh number is 1e9 and convergence didn't obtain. the results are wrong. when I changed the gravity acceleration to 0.098 that corresponding Rayleigh is 1e7 the solution converged. how can I reach to convergence with Rayleigh above 1e7??
please help me

thank you

Can you post more details about the problem? Please define the Raleigh number.

thank you meangreen,
assume you have a stack that it's geometry is axisymmetric. the air with T=405K and V=0.08m/s enters and the stack is in the surroundings with ambient T and P. my outlet boundary condition is pressure-outlet. a picture of model is attached.

Rayleigh number is a measure of laminar or turbulence. if it was greater than 10^7 the viscous model is turbulent and if it was below 10^7, it is laminar:
Ra=(g*beta*(T2-T1)*L^3*rho)/(mu*alpha)

I overcome this problem by changing the Specification Method to Intensity and Hydraulic Diameter and Turb. Intensity=7% and Hydraulic Diameter=150mm at inlet and outlets.

the solution converged but the results seems to be wrong. I can't understand where I make mistake that this results obtained.

Isn't here anybody can help me!!!!!! Please.