# Significance of residuals

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 August 16, 2014, 05:54 Significance of residuals #1 Member   Michael Kruger Join Date: Jul 2014 Location: South Africa Posts: 38 Rep Power: 11 I am busy with steady-state simulation of flow over a backward facing step and am interested in the turbulence of the flow. Although my velocity residuals are coming down nicely, the pressure residual is jumping around erratically and not coming down as I would have hoped. This made me wonder, if I am only really interested in the velocity field (in order to calculate wall shear stress and skin friction factor), how important is the pressure solution? i.e. if my pressure residual is high, but my velocity residuals are very low, is my velocity solution still accurate? Thanks in advance. __________________ ------------------------------------------------------- Michael

 August 16, 2014, 07:01 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,777 Rep Power: 71 you should specify the equation for the pressure you are talking about...Are you solving compressible o incompressible flow model?

 August 16, 2014, 07:04 #3 Member   Michael Kruger Join Date: Jul 2014 Location: South Africa Posts: 38 Rep Power: 11 I am using simpleFoam, an incompressible steady state solver, but I meant the question more in a general sense than to my specific problem. __________________ ------------------------------------------------------- Michael

 August 16, 2014, 07:16 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,777 Rep Power: 71 for steady state equations, the residual can be seen as the time derivative of the variable you want to solve. A non-vanishing residual is a unbalanced term you can see as a relevant time effect. But, for the pressure equation in incompressible flow you have to drive the residual to a very small value. It is a Lagrangian-type constraint as ensures that your velocity field is divergence-free. Therefore, a meaningful residual in pressure equation strongly affects the velocity!

 August 16, 2014, 07:23 #5 Member   Michael Kruger Join Date: Jul 2014 Location: South Africa Posts: 38 Rep Power: 11 Great thank you very much FMDenaro. __________________ ------------------------------------------------------- Michael

 Tags open foam, residuals, simplefoam