# Problem with convergence residuals

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April 30, 2011, 04:36
Problem with convergence residuals
#1
New Member

Tom Janda
Join Date: Apr 2011
Location: Czech
Posts: 7
Rep Power: 8
Hi, I´m student, now I´m working on my thesis work on topic Car body. I have problem with value of residual.

I created model of sports car (half model exactly) and wind tunnel (size tunnel: 7 metres width, 6 metres high, 90 length). This all was imported to Star-ccm+. Mesh have 1 241 000 cells (polyhedral).

Flow conditions are: Three dimensional, Gas, Cell quality remediation, Coupled flow, Motion are stationary, Constant density, Time is steady, Viscous regime was selected Turbulent, Reynolds - Averaged Navier Stokes, K - Omega turbulence, SST (Menter) K-Omega, All y+ Wall Treatment.

On the start of tunnel is condition: velocity inlet (28 km/h, and in the end of tunnel is condition pressure outlet (0 Pa). Car and road have condition Wall, other side of tunnel have condition Symmetry.

Program calculted residuals as you can see on picture. I read somewhere about boundary of convergence is to be 10-4. During 800 iterations curves residual are decreased, and after were not change to the 2800 iteration. Then I stopped it . I´m tried change turbulent flow on laminar on 2850 iterations. I´m tried show of flow (pics enclosed), and I don´t know if the results with this mistake residuas can be right. My computer calculated this results 2 days, approximate numb is 50 iteration/hour (Pentium i3, 8gb ram)

Attached Images
 Residuals - Kopie.JPG (52.2 KB, 69 views) 11 - Kopie.JPG (91.6 KB, 47 views) Streamlines8 - Kopie.jpg (82.2 KB, 45 views)

 August 2, 2011, 02:37 #2 Member   David Stanbridge Join Date: Apr 2010 Location: Norwich, UK Posts: 59 Rep Power: 9 There is likely to be flow separation and large eddy generation in the model. Therefore the "steady" model cannot be used. Have you tried changing to transient. I would not trust the results at this time. Can you post a copy of the mesh file and I can try and run it here.

 August 2, 2011, 14:49 Problem with convergence residuals #3 New Member   Paul Hancock Join Date: Mar 2009 Location: Bellingham, WA Posts: 25 Rep Power: 10 There have been a few posts here questioning the validity of using CCM+'s residuals as the only measure of convergence. For external aerodynamic analyses, I would suggest that you set up Reports, Monitors and Plots for Force and/or Force Coefficient and Inlet Pressure.

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