# unstable residual

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 November 26, 2017, 01:47 instable residual #1 New Member   Abdel Rhman Al-Tawil Join Date: Nov 2017 Posts: 1 Rep Power: 0 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.

 March 1, 2020, 00:33 #2 Member   Join Date: Jun 2017 Posts: 54 Rep Power: 8 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

 Tags ansys, drag coefficient, fluctuating residuals, fluent