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March 3, 2012, 18:02 
Center of pressure (confirmation/question)

#1 
Senior Member

Hello,
I've been doing some vehicle aerodynamics in Fluent and have reached a conundrum. As many people probably do, I've set up an external flow around the Ahmed body which was modeled and oriented according to: So, the origin of the coordinate system is at the ground plane, centerline of the car and coincident with the rear surface, just like in the sketch for Ahmed body at http://www.cfdonline.com/Wiki/Ahmed_body (however Z axis is longitudinal for me, X is lateral and Y is vertical).. I think we can all agree that the center of pressure is the point on the body where the resulting forces of drag and lift act, and as such, the moment around the CoP should be zero.. That said, would anyone like to explain why the following happens: I'd go to Reports > Forces > and fix the X coordinate as 0 because it's smack dab in the middle of the goddamn body.. print out a CoP location and it says y=0.20841 and z=1.0992 which would put the CoP at the somewhat correct height but a whole meter behind the body. I'm aware that the CoP location could actually be outside of the body in some cases, so I double checked this by trying a Moment report where one thing remains unclear to me.. For the moment center I've put x=0, y=0.20841, z=1.0992 and then for the moment axis fields, logic dictates that we want a moment around the X axis and to form this axis I'm specifying another point with the same Y and Z coordinates but putting the X coordinate at x=1... Moment reported is 34.4 Nm. To double check and compare to what was read in the Help files I've tried putting the moment axis to (1 0 0) which yielded 51.6 Nm but still not a desired zero. In my quest to get a reported moment as zero I did the following: CoP report > fixed the X as 0 > this gave the same values as previously stated, then I took the reported Y value of 0.20841 m and fixed the Y at this value and reported another CoP which gave X~0 (2.3e9) and Z=0.22237 m. When these two values were put in moment center (0 0.20841 0.22237) and for moment axis (1 0 0) the moment was 0.019 Nm which is probably close enough. Same result when I put in (1 0.20841 0.22237) for moment axis  which I still think is the correct way to define a vector endpoint for the axis.. only the X should be different. So, why the hell doesn't it report a correct CoP location when X is fixed to zero? That is the only coordinate that is actually known when doing CFD on a symmetrical body, so it should have no problem calculating the Y and Z coordinates so that the reported moment is ~0.. Someone please explain. If I'm making some huge mistake here, please point it out and educate me! I am aware that the most useful thing to do is probably report a moment around the center of gravity which for this Ahmed body is at (0 0.19005 0.52584) and the reported moment for moment axis (1 0 0) is 10.93 Nm and for moment axis (1 0.19005 0.52584) is 9.54 Nm. (Still think the latter moment axis vector is the correct one). Following the "right hand rule" this would mean the moment is lifting the rear of the body which would correlate with information that's available on the body in question. However, in aerodynamics there seems to be a sign convention that a positive moment is one that increases the angle of attack (pitch up) and opposite for a negative moment. This convention means that a positive moment in aerodynamics would be clock wise while in mechanics a positive moment is counter clock wise.. I know for a fact that Fluent follows the "mechanics" right hand rule when determining a rotating wall condition direction of rotation, so, does the same apply for a moment? /edit Manual calculation of moment around the CoG with lift and drag forces reported by Fluent yields a moment of 10.951 Nm which correlates with the moment axis of (1 0 0) instead of the one that I thought correct.. but why? Surely the easiest way to specify a line/vector is through two points so why would they keep the two coordinates unimportant and fixed at zero? If a point of origin in 3D space is (0 0.19005 0.52584) then the coordinates of the second point to specify a vector in the X direction would need to be (1 0.19005 0.52584).. Did Fluent coders to the opposite for user's convenience or am I missing something? Last edited by scipy; March 3, 2012 at 18:35. 

March 3, 2012, 20:55 

#2 
Senior Member

OK.. it seems that the clever software uses the "moment axis" fields as a unit vector, meaning it only needs a point (which is the moment center) and a vector who's length is 1 in the direction of one of the 3 axes..
Still doesn't explain why the CoP report doesn't work right away with X fixed at 0. 

March 5, 2012, 07:54 

#3 
Senior Member

No one? Really? Someone must have noticed this before.. or did you all blindly trust that Fluent's CoP location was correct?
Why doesn't it work with X=0?! 

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