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-   -   DRAG coefficient prediction (http://www.cfd-online.com/Forums/openfoam-solving/94733-drag-coefficient-prediction.html)

thinkagain November 23, 2011 20:42

DRAG coefficient prediction
 
3 Attachment(s)
Hello

I'm trying to determinate the polars for an airfoil 2D and I created a test case with a NACA 0012 on openFOAM 2.0.1

I obtained very good value for lift and moment coefficient, while the drag coefficient is very strange related with the ABBOTT results

I attached the compare between my results and the ABBOTT

- the fvScheme and fvSolution are the same of the airfoil 2D tutorials

- the boundary conditions are:
{
version 2.0;
format ascii;
class volVectorField;
object U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 1 -1 0 0 0 0];

internalField uniform (69.62 0 7.32);

boundaryField
{
inlet
{
type freestream;
freestreamValue uniform (69.62 0 7.32);
}

outlet
{
type freestream;
freestreamValue uniform (69.62 0 7.32);
}

body
{
type fixedValue;
value uniform (0 0 0);
}

leftAndright
{
type empty;
}
}

{
version 2.0;
format ascii;
class volScalarField;
object p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 2 -2 0 0 0 0];

internalField uniform 0;

boundaryField
{
inlet
{
type freestreamPressure;
}

outlet
{
type freestreamPressure;
}

body
{
type zeroGradient;
}

leftAndright
{
type empty;
}
}

{
version 2.0;
format ascii;
class volScalarField;
object nut;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 2 -1 0 0 0 0];

internalField uniform 1.8766e-05;

boundaryField
{
inlet
{
type freestream;
freestreamValue uniform 1.8766e-05;
}

outlet
{
type freestream;
freestreamValue uniform 1.8766e-05;
}

body
{
type nutUSpaldingWallFunction;
value uniform 0;
}

leftAndright
{
type empty;
}
}

{
version 2.0;
format ascii;
class volScalarField;
object nuTilda;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 2 -1 0 0 0 0];

internalField uniform 7.25e-05;

boundaryField
{
inlet
{
type freestream;
freestreamValue uniform 7.25e-05;
}

outlet
{
type freestream;
freestreamValue uniform 7.25e-05;
}

body
{
type fixedValue;
value uniform 0;
}

leftAndright
{
type empty;
}
}

rho = 1.225 kg/m^3
nu = 1.45e-05 m^2/s^2

- I used the Spalart-Allmaras turbulence model

the yplus is near 90, that would be good, becuase is in the logaritmic BL

I attached my mesh

There is anyone that could help and explain why I obtained those results

Last questions:
in the forceCoeff.C file if these


vector totForce = fm.first().first() + fm.first().second();
vector totMoment = fm.second().first() + fm.second().second();

scalar liftForce = totForce & liftDir_;
scalar dragForce = totForce & dragDir_;
scalar sideForce = totForce & sfDir_;
scalar pitchMoment = totMoment & pitchAxis_;

scalar Cl = liftForce/(Aref_*pDyn);
scalar Cd = dragForce/(Aref_*pDyn);
scalar Cy = sideForce/(Aref_*pDyn);
scalar Cm = pitchMoment/(Aref_*lRef_*pDyn);

what is fm.first() and the other term to calculate the totForce?

Thank you for every useful answers! I need help!

Andrea


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