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Continuity Residual high but total mass convergence achieved?

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Old   February 8, 2016, 06:21
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, 10:24
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.

I hope this helps.
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continuity, mass, residual, residuals

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