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Old   August 29, 2012, 04:50
Default Drag Coefficient Calculation
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This is a very basic question, but it's puzzling me at the moment and I'm hoping someone might be able to shed light on it for me, it's probably very obvious I just can't see it!

I'm running some simulations in Phoenics on ground vehicles and retrieving drag and CD values for comparison purposes.

The values I retrieve for CDx are very close to what I would expect of the simulation, however the Phoenics calculation uses the dimensional length to calculate each CD axis:

Cd = Force /(Dynamic head * Normalisation area)

Whereas typically it would be frontal area rather than length in the x-direction that would be used to calculate the CD.

What confuses me is that the CDx values output by Phoenics are always extremely close to what I'd expect from the simulation, but if I calculate the CD using force values and frontal area rather than length along the axis, the CD value is much larger than it should be and consequently the Force values calculated must be larger than they should be?

To give an example:
Integrated forces for all included objects
Fx = 2.778401E+03
Fy = -4.078467E+02
Fz = 8.727589E+01
Ftot= 2.809532E+03

Normalisation areas:
AREAx = 1.650000E+01
AREAy = 4.000000E+00
AREAz = 3.165000E+00

Reference density = 1.189000E+00, Reference velocity = 2.500000E+01
Free-stream dynamic head = 3.715625E+02

Drag coefficients based on total forces:
(Cd = Force /(Dynamic head * Normalisation area)
Cdx = 4.531888E-01
Cdy = 2.744133E-01
Cdz = 7.421450E-02
__________________________________________________ _

Phoenics gives a CDx of .45 which is the CD result I'd expect from this case, however if I calculate it myself from the frontal area the CD is around .67.

Any insight and re-aligning in the right direction is much appreciated.
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Old   August 29, 2012, 05:07
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Ah I believe I've noticed my mistake:

In the reference areaX, Y, Z in the CD calulation settings I've placed the object size on that axis not it's area.

So to fix this I'd put the frontal area in the 'Reference Area in X' section.

However this means that the force in N calculated by my setup is far too large and giving incorrect values.

Are my assumptions correct?
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