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Convergence issue- residuals of continuity

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Old   December 7, 2010, 01:20
Default Convergence issue- residuals of continuity
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Hello ,
I'm modeling a laminar coolant(water) flow through a circular tube of

Inner Diameter = 0.1 m
Inlet velocity of coolant = 0.02 m/s
Inlet temperature = 293 Kelvin
There is a uniform heat flux = 1000 W/m^2 applied to the outer surface(wall).

I modeled the inner tube as a fluid and applied a uniform heat flux to the wall. However , when I try to plot my residuals with the following solver settings

convergence criteria for continuity, x,y,z velocity = 1e-6
convergence criteria for energy equation = 1e-9

the residuals of the continuity equation do not converge. they seem to decrease initially and then keep bouncing around at 1e-3 .

I tried changing the under relaxation factors , but this ddint not help all that much.

I'm using a SIMPLE scheme , with 2nd order upwind discretization for momentum and energy

Any help or views on this would be greatly appreciated.
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Old   January 18, 2011, 11:26
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Are you solving it in steady state or time-dependent? Try time-dependent and see if it works. Try solving for a long period until values of velocity or temperature between two continuous time step does not change significantly.

Just a suggestion.
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Old   January 18, 2011, 19:10
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obvious things but check
1) grid is scaled (Mesh>Scale)
2) bc's are all set correctly (inc materials and heat flux definition)
3) grid quality is good enough (with prisms for heat transfer ideally)
4) grid is refined enough to capture gradients (you could adapt), though if too refined you can pick up transience

display contours of residuals to see where it is struggling
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Old   February 7, 2016, 13:01
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I have a similar problem, where in an open channel flow all the variable converge (residuals < 10^(-3)), whereas the continuity equation has residual constant around 10^(-1). The flow seems to not change any longer during iterations.

Thank you
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continuity, convergence, residuals fluent, underrelaxation

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