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Continuity Residual high but total mass convergence achieved?

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Old   February 8, 2016, 07:21
Default Continuity Residual high but total mass convergence achieved?
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Hi everyone,

I'm working on designing an aerodynamics package for a Formula Student team and am relatively new to CFD. One thing that has been bothering me is a high continuity residual I am getting for my simulations. My x,y,z-velocity residuals are all around 1e-5, and my k and omega residuals steady around 1e-3. However, my scaled continuity residuals stays around 1e-2.

Despite this, my mass flow rate between my inlet and outlet only differs by 0.0005% (all other BCs are moving walls). I know that residuals are ideally meant to drop 3 orders of magnitude at least for convergence but Fluent's scale factor for the cont. equation depends on the first 5 iterations.

Would this high continuity residual be justified given the total mass of the wind tunnel has converged?


My BCs are:
Inlet (15m/s)
Pressure Outlet
Symmetry Plane
Roof (moving wall -> 15m/s)
Wall (moving wall -> 15m/s)
Ground (moving wall -> 15m/s)
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Old   February 8, 2016, 11:24
Default Under-Relaxation factor
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InduSekhar Roy
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You can try decreasing the Under-Relaxation factor.

Setup-> Solution Controls ->

Try decreasing the "Under-Relaxation factor" for "momentum" from 0.7 to 0.6. You can try decreasing till 0.1, even 0.01. But, it is advised not decrease it below 0.1 to get accurate result.

Convergence

Convergence

I hope this helps.
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