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Confused by COP in Fluent

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Old   February 13, 2020, 14:26
Question Confused by COP in Fluent
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Hello everyone,

firstly the obligatory not a native speaker, so I'm sorry for any spelling or grammar mistakes.

I'm not completely new to CFD but do still consider myself a newbie, who still has to learn a lot.

I already tried to find a solution for my problem using google, but didn't find anything useful.

I am running a series of simulations of a racecar in straightline and in cornering conditions in Fluent 2019R3. The simulations run fine and the results are close to real world measured data for downforce and drag. (for the straightline at least, I'm missing real world data for cornering conditions) The thing I can't wrap my head around is calculating the COP in Fluent. I know that there is the option to calculate the COP using the "Force" option under the results tab. But I have to input one of the coordinates for Fluent to be able to compute the other two. But I don't know any of my three coordinates. I tried an iterative process, where I would guess an initial value and try to work my way on from there, but the Values just keep on jumping around.

This is my case:
Case.jpg
I know, that the COP is the point where all the Moments equal zero. So I tried to calculate the COP by hand. I got these equations:

Mx = Fy*COPz - Fz*COPy

My = Fz*COPx - Fx*COPz

Mz = Fx*COPy - Fy*COPx

My coordinate system has it's origin behind the car. The Z=0 Point is exactly on the centerplane of the tires. My X-Axis is going forward in the direction of travel. My Y-Axis is going to the left and my Z-Axis is going up. I'll include a Picture of the Situation

Here are some values from one of the straightline simulations just in case you want to calculate it using some values:

Mx = -32,85 Nm

My = 141,79 Nm

Mz = 38,88 Nm

Fx = -38,02 N

Fy = -15,49 N

Fz = -95,38 N

I tried to solve this equation system manually, using Matlab and several different online calculators for equation systems. All of them Result in there being no specific solution. When doing it manually I allays get to a point where I lose everything in an equation and get to Mx=Mx, My=My or Mz=Mz.

It seems to me like the equations aren't unique and that I need more equations to be able to calculate it. But what would an equation like that look like?

It would be awesome if you could help me understand this problem and what I'm doing wrong, so I could apply that knowledge in different cases, such as the cornering state. I'm confident I can solve this for any future cases if I really understand it once.

Thank you in advance!
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Old   February 14, 2020, 05:34
Default Surface for COP
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COP is calculated either along a line or along a plane. That's how one of the coordinates is assumed to be always known. And usually, it is a known value. Which surface do you wish to calculate your CoP on? If it is top or bottom, e.g., then CoP would line on the plane of symmetry, except during turning. During turning, it would change with time.
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Old   February 25, 2020, 12:36
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Hey vinerm,
I'd like to calculate the COP of all surfaces of the car, so I can calculate the Forces acting on each wheel. So I can compare different packages in terms of downforce and balance. My simulations are all in steady state condition, so there is no changing of the plane of the COP with time as there is no difference in vehicle direction.
Does this help you answer my question?
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Old   February 25, 2020, 17:03
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Then, you can use the center point as the known coordinate. E.g., for any surface, such as, top or underbody of the car, center plane is the plane of symmetry. If the car is moving in straight direction, then CoP has to lie on this plane. Assuming width of the car is along y-coordinate, provide y value corresponding to center of the car width as known coordinate.

For the surfaces on the side, it would be little difficult but if you combine two surfaces from left and right and their CoP also has to lie on midplane.
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