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unstable residual

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Old   November 26, 2017, 01:47
Default instable residual
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Abdel Rhman Al-Tawil
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I'm trying to simulate remotely controlled underwater vehicle "ROV" to get the coefficient of drag, lift force, drag force. however, I'm experiencing instability in the residual plot " especially in the continuity and k, omega" it starts stable until it reach 6e-3 then is start constant fluctuating around the value. I have tired to adjust under-relaxation factors no improvement.
here is model information
Analysis
1-steady fluid flow
2-turblent flow
Mesh info and Quality
meshing method tetrahedral patch independent
1- orthogonal quality rang from 0.30 to 0.99 "most of the elements above 0.70"
2-Aspect Ratio rang from 1.18 to 7.18 "most of the elements below 3"
3-skewness rang from 0 to 0.78 "most of the elements below 0.60"
4- Maximum Conner Angle rang from 71.56 to 152.60 "most of the elements below 112.5"
Fluent Model setup
1-k-omega SST
2-Fluid liquid water
3-Boundary Conditions inlet " velocity 1 m/s", outlet " pressure 0 pa" no pressure difference, Wall "ROV", Symmetry "rest of the planes to eliminate no slip conditions"
4-reference values specified
5- solution methods simple

my residual plot look something like this, it should be noted that my model encounter geometrical symmetry

Last edited by abdelrhmanaltawil; November 26, 2017 at 14:55.
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Old   March 1, 2020, 00:33
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Hi, I'm still learning CFD so I may not be able to help a lot but try to change your pressure-velocity coupling. For steady fluid flow, try using SIMPLE or SIMPLEC. These are iterative schemes used to adjust the pressure field to ensure continuity as the pressure field can't be obtained directly (with the exception of using the "Coupled" pressure-velocity coupling scheme, which is computationally expensive).

https://www.afs.enea.it/project/nept...ple-vs-simplec

Check out the differences between SIMPLE and SIMPLEC in this link. Depending on your situation, you can choose between SIMPLE and SIMPLEC.

Hope this helps
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ansys, drag coefficient, fluctuating residuals, fluent

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