Find Center of Pressure?

Doug68 · December 22, 2012, 08:45

Carrying on with the brute force and ignorance approach...

Using the method shown further up the thread I set up to create results on 11 CoR's 100mm apart on a straight line on the XZ plane, this covered the 1m where I guessed the CoP was somewhere within.

Plotting out the results as shown below where the line crosses the X axis torque is 0, therefore that's the CoP in the X direction.

GPan1 · December 22, 2012, 16:27

I'm 99% sure the moments are due to the vortexes shed by the wheel, which oscillate and why I don't get 100% convergence. Eliminating the wheel moments / atleast in x and z axes (which seems reasonable) and having y as non-zero, I can get two equations in the form of x and z, and use simple simultaneous equations to solve, however it's dependant on the CofR I pick.

That said your latest method makes the most sense of all methods I've tried, because centre of pressure will be where the moments are zero, so plotting My/Mx/Mz from several CofR's should give you the location. Genius!

Always looking for something more complex than it should be

Edit - had a think, but that's till dependant on the CofR because that will give you the position of zero torque at 0 height, which means CP would occur at more than one position, and you'd need to do a lot of iterative solutions before finding it?

Doug68 · December 26, 2012, 19:46

Yeah, you are right my last effort will give the zero torque point at the elevation stated.

more thought required

Doug68 · January 2, 2013, 03:11

So now I believe that finding the CoP using the data on forces.dat on its own is not possible.
Regardless of the number of files produced for different CoR's.

I think the most that can be proven is that there will be a line upon which the CoP lies.

I'm pretty certain the method that needs to be followed is as julien.decharentenay describes in post 2 of this thread.

openfoam_user · February 12, 2013, 09:45

Hi,

following the link below you will find a paper called "Forces and moments in CFD analysis" and written by Zdeněk Říha.

http://ebookbrowse.com/riha-forces-a...pdf-d253480950

Comments about this paper are welcome.

Regards,
Stephane.

December 22, 2012, 16:27		#22
GPan1 New Member Join Date: Nov 2012 Location: Scotland, UK Posts: 19 Rep Power: 13	I'm 99% sure the moments are due to the vortexes shed by the wheel, which oscillate and why I don't get 100% convergence. Eliminating the wheel moments / atleast in x and z axes (which seems reasonable) and having y as non-zero, I can get two equations in the form of x and z, and use simple simultaneous equations to solve, however it's dependant on the CofR I pick. That said your latest method makes the most sense of all methods I've tried, because centre of pressure will be where the moments are zero, so plotting My/Mx/Mz from several CofR's should give you the location. Genius! Always looking for something more complex than it should be Edit - had a think, but that's till dependant on the CofR because that will give you the position of zero torque at 0 height, which means CP would occur at more than one position, and you'd need to do a lot of iterative solutions before finding it? Last edited by GPan1; December 23, 2012 at 12:25.

February 12, 2013, 09:45		#25
openfoam_user Senior Member stephane sanchi Join Date: Mar 2009 Posts: 314 Rep Power: 18	Hi, following the link below you will find a paper called "Forces and moments in CFD analysis" and written by Zdeněk Říha. http://ebookbrowse.com/riha-forces-a...pdf-d253480950 Comments about this paper are welcome. Regards, Stephane. FJSJ likes this.

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December 26, 2012, 19:46		#23
Doug68 Member Join Date: Nov 2010 Posts: 62 Rep Power: 15	Yeah, you are right my last effort will give the zero torque point at the elevation stated. more thought required

January 2, 2013, 03:11		#24
Doug68 Member Join Date: Nov 2010 Posts: 62 Rep Power: 15	So now I believe that finding the CoP using the data on forces.dat on its own is not possible. Regardless of the number of files produced for different CoR's. I think the most that can be proven is that there will be a line upon which the CoP lies. I'm pretty certain the method that needs to be followed is as julien.decharentenay describes in post 2 of this thread.