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Problem in convergence regarding constant viscosity and UDF related Viscosity.

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Old   May 9, 2016, 08:43
Default Problem in convergence regarding constant viscosity and UDF related Viscosity.
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Alex Skerhut
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Hey!
I am defining a shear and temperature dependent viscosity as a user defined function in Fluent but the code diverges since the value of strain rate is used in denominator and at initial iterations the strain rate will be low which will lead to large fluctuations and hence divergence of the solution according to me.

I wish to run the simulation at constant viscosity for few iterations and then shift to the viscosity defined by UDF taking the final iterations value of constant viscosity simulations as initial conditions( mainly strain rate) for viscosity defined by UDF.

When I run the simulations at constant viscosity the solution converges with the message "Divergence detected - temporarily relaxing and trying again...".

The initial message that comes is
# Divergence detected in AMG solver: pressure coupled -> Decreasing coarsening group size!
# Divergence detected in AMG solver: pressure coupled -> Increasing relaxation sweeps!
# You may try the enhanced divergence recovery with (rpsetvar 'amg/protective-enhanced? #t)

I am not able to understand the origin of the divergence. In initial simulations there was no such errors but now this error is occurring and I am unable to understand the origin of this error.

I have kept the under-relaxation factor for energy as 0.9 and Explicit Relaxation factor for pressure as 0.2. Solution methods are coupled scheme, Least squared cell based gradient, standard for pressure and second order upwind for momentum and energy.

Thanks for the help in advance.
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convergence fluent, convergence problems, udf code

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