# Initial residuals not converging and stay high values

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April 30, 2019, 14:01
Initial residuals not converging and stay high values
#1
Member

Joe lee
Join Date: Nov 2018
Posts: 59
Rep Power: 7
I am trying to do the mesh convergence test by increasing the mesh refinement to see at what level will the drag force converge. I used simpleFoam as I want to solve for steady-state solution. However, I found that when the mesh refinement level increases, the residual values especially Ux, Uy and p keep increasing. Here is the residuals of the mesh (I will call this meshA) with refinement level = (edge = surface = region = 2; no layer):

And for mesh (I will call this meshB) with refinement level = (edge = surface = region = 4; boundary layer = relatvie 0.3; no. of layer = 3 (81.1% generated)):

Also, the forceCoeffs become unstable for high refinement level(meshB), while it is stable at low refinement level(A).

forceCoeffs of meshA:

forceCoeffs of meshB:

What may be the cause? Does this imply the solution maybe transient?
Thanks a lot!

FYI, here is my controlDict for both settings:
Quote:
 application simpleFoam; startFrom latestTime; startTime 0; stopAt endTime; endTime 600; deltaT 1; writeControl timeStep; writeInterval 20; purgeWrite 4; writeFormat binary; writePrecision 6; writeCompression off; timeFormat general; timePrecision 6; runTimeModifiable true; functions { #include "streamLines" #include "cuttingPlane" #include "forceCoeffs" }
fvSolution:
Code:
```solvers
{
p
{
solver          GAMG;
smoother        GaussSeidel;
tolerance       1e-7;
relTol          0.01;
}

Phi
{
\$p;
}

U
{
solver          smoothSolver;
smoother        GaussSeidel;
tolerance       1e-8;
relTol          0.1;
nSweeps         1;
}

k
{
solver          smoothSolver;
smoother        GaussSeidel;
tolerance       1e-8;
relTol          0.1;
nSweeps         1;
}

epsilon
{
solver          smoothSolver;
smoother        GaussSeidel;
tolerance       1e-8;
relTol          0.1;
nSweeps         1;
}
}

SIMPLE
{
nNonOrthogonalCorrectors 0;
consistent yes;
residualControl
{
U		1e-4;
p		1e-4;
"(k|epsilon)"	1e-4;
}
}

potentialFlow
{
nNonOrthogonalCorrectors 10;
}

relaxationFactors
{
equations
{
U               0.9;
k               0.7;
epsilon       0.7;
}
}

cache
{
}```

fvSchemes:
Code:
```ddtSchemes
{
}

{
default         Gauss linear;
}

divSchemes
{
default         none;
div(phi,k)      bounded Gauss upwind;
div(phi,epsilon)  bounded Gauss upwind;
}

laplacianSchemes
{
default         Gauss linear corrected;
}

interpolationSchemes
{
default         linear;
}

{
default         corrected;
}

wallDist
{
method meshWave;
}```
Thank you very much!!

 May 1, 2019, 14:18 #2 New Member   jawahar sivabharathy Join Date: Jan 2011 Posts: 12 Rep Power: 15 Hi Jinjolee, What is the Reynolds number and what geometry are you computing the flow for?

May 1, 2019, 14:44
#3
Member

Joe lee
Join Date: Nov 2018
Posts: 59
Rep Power: 7
Quote:
 Originally Posted by sjsivabharathy Hi Jinjolee, What is the Reynolds number and what geometry are you computing the flow for?
Hi,
My geometry is a cyclist riding on a bike(as shown in the figure below).

And I am measuring for the air drag from the external flow with inlet velocity of 12.5m/s, so if I am correct(sorry for my poor fluid background), the Reynolds number should be:
(12.5*1.638)/(1.5*e-5) = 1365000
where 1.638 is the wheelbase length of the bike(I take it as characteristic length)

Thanks.

 October 25, 2019, 21:09 #4 New Member     Igor Leonardo Join Date: Jul 2016 Location: Brazil - SP - SJC Posts: 20 Rep Power: 9 Hello Jinjolee, Im facing similar issue. Did you manage to solve your problem?

 Tags convergence, residual, simplefoam, steady-state, transient