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Convergence problems after mesh refinement

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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



0 Replies Last Post 2 déc. 2020, 03:49 UTC−5
COMSOL Moderator

Hello Sven Beekers

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