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Residuals not converging

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Old   August 28, 2020, 08:00
Default Residuals not converging
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Aniket Dalvi
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I am simulating Rayleigh Benard Convection between two horizontal plates for steady and laminar case. I am attaching a screenshot of my residuals along with convergence criteria. Any suggestions for reaching convergence will be highly appreciated.

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Old   September 2, 2020, 02:01
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Hello,

There are two possible reasons.

1. If your initial solution is very close to final converged solution, then residual monitors will not go down.

2. If flow is in transient nature, then it may not converge much.

In both scenario, it is better to put some monitor points at critical regions and monitor pressure or velocity. If it is flat, then solution can be considered as converged. If oscillation is too high at monitor points, then change to transient solver.
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Old   September 4, 2020, 06:03
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Quote:
Originally Posted by jsm View Post
Hello,

There are two possible reasons.

1. If your initial solution is very close to final converged solution, then residual monitors will not go down.

2. If flow is in transient nature, then it may not converge much.

In both scenario, it is better to put some monitor points at critical regions and monitor pressure or velocity. If it is flat, then solution can be considered as converged. If oscillation is too high at monitor points, then change to transient solver.
Thank you for replying,

I tried using unsteady and turbulent k-epsilon model. In this case, I ran the simulation for a flow time of 20 mins but failed to see any flow occur. I am attaching the temp and velocity contours.

velocity.png

contour-1.png
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Old   September 6, 2020, 20:51
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Hello,

For simulations like Rayleigh–Benard convection, at least transient simulation should converge. Then it looks, meshing & solver set up might be the reason for this convergence issue.

There are many questions you have to answer and think...
  1. Is mesh fine enough to capture the flow accurately? Hex or Tetramesh?
  2. What is the Y+ Value and mesh growth rate?
  3. Have you added boundary layers in the mesh?
  4. How you decided the flow is turbulent flow?
  5. What is the time step size you have chosen?
  6. Have you monitored the flow in any particular points?

I recommend to read some journal papers about Rayleigh-Benard convection to understand the phenomenon and underlying physics of this simulation. Also read Ansys documentation about natural convection simulations.

This might help you to get the converged solution quickly.
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