[foam_noob]

Hello everyone,

I am currently simulating the airflow over a car. I know what the results should look like, but after changing the boundary condition numerous times I am still unable to get realistic results (tried with both kepsilon model and komega model). I posted a picture of my simulation below (blended out some walls) as well as added the boundary conditions.

fvSchemes and fvSolution were taken from either the motorbike tutorial (for k omega model) or pitzDaily tutorial (k epsilon model). I am calculating with simpleFoam. R, omega and k were also taken from these tutorials.

Appreciate the help .

Patrick


P.S Which model (k epsilon / k omega model) is better for this calculation?

epsilon:

 
 
dimensions      [0 2 -3 0 0 0 0];
 
internalField   uniform 14.855;
 
boundaryField
{
 
“car_.*”
{
    type            epsilonWallFunction;
    value           uniform 14.855;
}
 
road_noslip
{
    type            epsilonWallFunction;
    value           uniform 14.855;
}
symmetry
{
    type            symmetryPlane;
}
topwall
{
    type            slip;
}
sidewall
{
    type            slip;
}
inlet
{
    type            fixedValue;
    value           uniform 14.855;
}
outlet
{
    type            zeroGradient;
}
roadslip
{
    type            slip;
}
}

nut:

dimensions      [0 2 -1 0 0 0 0];
 
internalField   uniform 0;
 
boundaryField
{
 
 “car_.*”
 {
     type            nutWallFunction;
     value           uniform 0;
 }
 road_noslip
 {
     type            nutWallFunction;
     value           uniform 0;
 }
 symmetry
 {
     type            symmetryPlane;
 }
 topwall
 {
     type            calculated;
     value           uniform 0;
 }
 sidewall
 {
     type            calculated;
     value           uniform 0;
 }
 inlet
 {
     type            calculated;
     value           uniform 0;
 }
 outlet
 {
     type            calculated;
     value           uniform 0;
 }
 roadslip
 {
     type            calculated;
     value           uniform 0;
 }
}

p:

dimensions      [0 2 -2 0 0 0 0];
 
internalField   uniform 0;
 
boundaryField
{
 
 “car_.*”
 {
     type            zeroGradient;
 }
 
 road_noslip
 {
     type            zeroGradient;
 }
 symmetry
 {
     type            symmetryPlane;
 }
 topwall
 {
     type            slip;
 }
 sidewall
 {
     type            slip;
 }
 inlet
 {
     type            zeroGradient;
 }
 outlet
 {
     type            fixedValue;
     value           uniform 0;
 }
 roadslip
 {
     type            slip;
 }
}

U:

dimensions      [0 1 -1 0 0 0 0];
 
internalField   uniform (27.77 0 0);
 
boundaryField
{
 “car_.*”
 {
     type            fixedValue;
     value           uniform (0 0 0);
 }
 
 road_noslip
 {
     type            fixedValue;
     value           uniform (0 0 0);
 }
 symmetry
 {
     type            symmetryPlane;
 }
 topwall
 {
     type            slip;
 }
 sidewall
 {
     type            slip;
 }
 inlet
 {
     type            fixedValue;
     value           uniform (27.77 0 0);
 }
 outlet
 {
     type            zeroGradient;
 }
 roadslip
 {
     type            slip;
 }
}

k:

dimensions      [0 2 -2 0 0 0 0];
 
internalField   uniform 0.24;
 
boundaryField
{
 “car_.*”
 {
     type            kqRWallFunction;
     value           uniform 0.24;
 }
 road_noslip
 {
     type            kqRWallFunction;
     value           uniform 0.24;
 }
 symmetry
 {
     type            symmetryPlane;
 }
 topwall
 {
     type            slip;
 }
 sidewall
 {
     type            slip;
 }
 inlet
 {
     type            fixedValue;
     value           uniform 0.24;
 }
 outlet
 {
     type            inletOutlet;
     inletValue      uniform 0.24;
     value           uniform 0.24;
 }
 roadslip
 {
     type            slip;
 }
}

omega:

dimensions      [0 0 -1 0 0 0 0];
 
internalField   uniform 1.78;
 
boundaryField
{
 “car_.*”
 {
     type            omegaWallFunction;
     value           uniform 1.78;
 }
 road_noslip
 {
     type            omegaWallFunction;
     value           uniform 1.78;
 }
 symmetry
 {
     type            symmetryPlane;
 }
 topwall
 {
     type            slip;
 }
 sidewall
 {
     type            slip;
 }
 inlet
 {
     type            fixedValue;
     value           uniform 1.78;
 }
 outlet
 {
     type            fixedValue;
     value           uniform 1.78;
 }
 roadslip
 {
     type            slip;
 }
}

R:

dimensions      [0 2 -2 0 0 0 0];
 
internalField   uniform (0 0 0 0 0 0);
 
boundaryField
{
 “car_.*”
 {
     type            zeroGradient;
 }
 road_noslip
 {
     type            zeroGradient;
 }
 symmetry
 {
     type            symmetryPlane;
 }
 topwall
 {
     type            slip;
 }
 sidewall
 {
     type            slip;
 }
 inlet
 {
     type            fixedValue;
     value           uniform (0 0 0 0 0 0);
 }
 outlet
 {
     type            zeroGradient;
 }
 roadslip
 {
     type            slip;
 }
}

[foam_noob]

problem solved

[bjr]

Do you like your drag results?

[foam_noob]

I haven't had the chance to implement liftDrag from OF 1.2. I have to do that when I have time. If you know a different way to calculate the drag coefficient with OF please share .

[bjr]

functions
{
forces
{
type forces;
functionObjectLibs ("libforces.so");
outputControl timeStep;
outputInterval 1;

patches (OBJECT_OBJECT);
pName p;
Uname U;
log true;
rhoName rhoInf;
rhoInf 1.17;
magUInf 0.0045;
CofR (1.25 0 4);
}

forceCoeffs
{
type forceCoeffs;
functionObjectLibs ("libforces.so");
outputControl timeStep;
outputInterval 1;

patches (OBJECT_OBJECT);
pName p;
Uname U;
log true;
rhoName rhoInf;
rhoInf 1.17;
magUInf 0.0045;
CofR (1.25 0 4);

liftDir (0.0 1.0 0.0);
dragDir (1.0 0.0 0.0);
pitchAxis (0 0 0);

lRef 0.5;
Aref 12.56;

}


I use something like this in my /system/controlDict file. You must alter for your case, but this is a start. I'm not sure in which version that a forces function object (vocabulary?) was introduced off-hand.

Regards,
Ben Racine

[bjr]

The following thread is the reference for the above statements.
http://www.cfd-online.com/Forums/ope...es-v1-6-a.html

[foam_noob]

Hey Ben,

Thanks for the code. I tried it and it worked without any problems.

I have a couple of question regarding this code.

1. Which patches should I use for my simulation? Do I use every patch that defines the vehicle (I split the car into multiple parts: wheels, windshields and so on)?

2. And what is lRef? How do I calculate it?

3. Do you know a method for calculating Aref?

Appreciate the help

Patrick Wang

[bjr]

1. Which patches should I use for my simulation? Do I use every patch that defines the vehicle (I split the car into multiple parts: wheels, windshields and so on)? Yes

2. And what is lRef? How do I calculate it?
I believe it's the the length-scale used for finding Re number.

3. Do you know a method for calculating Aref?
Aref is probably best done by doing a 2D frontal area projection of your surface model in some CAD package... as I believe it's typical to use frontal area in vehicle aerodynamics when calculating drag coefficients.

Hope this helps and someone can correct if I'm wrong anywhere.
Ben

[baklanton]

Hi Patrick,

I am also experiencing an issue which is similar to the one you explained in your first post. Could you please explain how you solved it?

Thank you!