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Convergence problems after mesh refinement
Posted 2 déc. 2020, 03:49 UTC−5 Fluid & Heat, Computational Fluid Dynamics (CFD), Heat Transfer Version 5.5 0 Replies
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Hello,
I'am trying to solve an unsteady conjugate heat flow problem for an uniformly heated cylinder in a laminar flow of water. The cylinder is located at the centerline and the inlet boundary is located at 
and outlet boundary at 
. The height of the domain is equal to 
.
All differential equations are dimensionless and are dependent on the Reynolds, Prandtl and Richardson numbers. In this case these numbers are equal to 20, 7 and 1 respectively.
Note that the equations are nondimensionlized by setting 
and 
equal to 1 and 
respectively for the laminar flow module. Furthermore, bouyancy is added by using the boussinesq approximation and is included in the y term of the volume force as 
for which, 
is the dimensionless temperature.
For the heat transfer module 
is set to 1 and 
to 
The boundary conditions are: (Comsol, 2019)
Inlet
with 
with 
and 
Upper and lower boundary
Cylinder
with 
Outlet conditions
with 
Mesh
The mesh is mapped with quads elements and are non uniformly distributed. The statistics are shown below:
Number of elements 133200
Minimum skewness 0.4995, average 0.9889
Minimum volume versus circumradius 0.2049, average 0.774
Minimum condition number 0.2692, average 0.7876
Minimum growth rate 0.9628, average 0.9849
Boundary minimum element size 0.0134
Solver
I'am using a seggregated solver
Convergence
Up to around 100000 elements the convergence is good but for finer meshes the error of the segregated solvers shows a fluctuating error after a time of around 40 seconds. The residuals also increase significantly in this period of time. These plots can be found in the attachment. Furthermore, after the error appears ''May have reached a singularity. Time: 40.593993119146752 s.Last time step is not converged.'' A plot containing undefined values of the variable comp1.T appears and is also included in the attachment. Just before the simulation quits the timestep also decreases significantly. The reciprocal of the time step plot is included in the attachment.
Can anybody help me with this problem. The effort is greatly appreciated.
Best regards,
Sven Beekers
Master graduate University of Technology Eindhoven
Reference Comsol, (2019). CFD's Module - User's Guide. Comsol
Attachments:
Hello Sven Beekers
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