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-   -   Modified the motorBike tutorial, getting very high drag (http://www.cfd-online.com/Forums/openfoam-meshing-snappyhexmesh/108418-modified-motorbike-tutorial-getting-very-high-drag.html)

ben1793 October 23, 2012 05:37

Modified the motorBike tutorial, getting very high drag
 
I wanted to test the drag on a car design that I had made so from the help of the users of this forum I managed to replace the motorbike with the car in the wind tunnel.

I opened the forceCoefs.dat file to view my drag coefficients, there was a huge list of very big numbers up to about 2000 and the minimum being about 1000, this was the same for lift and down force also.I

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

patches ( "motorBike.*" );
pName p;
UName U;
rhoName rhoInf; // Indicates incompressible
log true;
rhoInf 1; // Redundant for incompressible
liftDir (0 0 1);
dragDir (1 0 0);
CofR (0.72 0 0); // Axle midpoint on ground
pitchAxis (0 1 0);
magUInf 20;
lRef 1.42; // Wheelbase length
Aref 0.75; // Estimated
<- EDIT: changed this to 100.75 and am suddenly getting lower drag coefficients, they are still very high but getting numbers like 19.0
}

Please could you tell me how to fix this problem, I know the drag coefficient should only be around 0.2 but I am getting numbers up to about 5000 now.

EDIT: So after changing the area I notice I am getting better results, so I am guessing I need to find the estimated area of my car to get any good results, does anyone know how I can do this through OpenFOAM or paraView, and also when it says area does it mean frontal area?

Thank you

colinB October 23, 2012 08:03

Dear Ben,

indeed the reference area is important and has to be correct!

However take care that there are "several" reference areas possible:

the cross-section of the car or the "wetted surface" which is for
single flow basically the whole surface.

From my experience in two phase flows the wetted surface is used
(e.g. ship resistance) and for single phase flows the cross-section is used
(e.g. flow around a NACA profile)

I'm not sure whether there are exceptions of that rule and which one is
valid for your case, but I guess google will help you ;-)

regards

plucas October 23, 2012 09:09

You can get the cross-sectional area through paraview. Google it and you will find it. I recommend just using your CAD software however because it is easier.

s.m August 13, 2013 14:36

Quote:

Originally Posted by plucas (Post 388120)
You can get the cross-sectional area through paraview. Google it and you will find it. I recommend just using your CAD software however because it is easier.


Dear Plucas,
would you please explain more that how can we get the cross-sectional area through paraview?

Thank you very much.:)

nimasam August 16, 2013 02:25

one rough way, i guess, you can create an slice, then you can saw in information tab box boundness


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