Center of pressure-local and global coordinate

Soolmaz · September 20, 2012, 03:29

Hi there,
I need to find the center of pressure on a surface. I have read several previous posts relate to this subject, but I finally could not get a reasonable result.

What I did for finding CoP was going through M = r x F. I read M and F values in CFX-post function calculator for X,Y and Z dirctions and found CoP = (CoPX, CoPY,CoPZ). However, the point CoP is not located on the desired surface!! Any suggestions?

Thanks for help
Soolmaz

ghorrocks · September 20, 2012, 07:28

For a 3 dimensional surface the centre of pressure is not on the surface, in general. But the line of net force on the surface, acting through the CoP, will pass through the surface most of the time (but again, I am sure you can dream up cases when it does not).

Soolmaz · September 20, 2012, 08:35

My surface is a surface of a cylinder so it is 3D. Till now, I tought CoP should be on the surface! Can you explain about it is not in general on the surface?
Can you help me to understad how I can find CoP?
I got two ideas:
1. solving the equation M = r x F which leads to a point rather far from the body!
2. I read in previous posts: 0=F_x*x+F_y*y and M_z = -F_x*y+F_y*x so it gives x and y values. But if I solve it for M in other directions, I will get other values. I do not know how to deal with them. (I do not understand the idea).

What is your advice for finding CoP for a 3D surface?

Thanks,
Soolmaz

singer1812 · September 20, 2012, 13:00

The center of pressure is a point that the force passes through that provides no moment. The problem you will run into is that this is not a unique point. Any point along the force vector will provide you with this condition.

One way around this is to fix one of the point locations, preferably that locates the COP within the body. For example, if you were looking at a missile, you might choose the location to be on the centerline of the geometry and solve for the other 2 locations.

Soolmaz · September 21, 2012, 02:05

Dear Edmund,
Thanks for nice explanation of CoP, it really helped me to understand the concept. Can you please tell me how I can find this force vector so then I can choose the desired location?

Soolmaz

singer1812 · September 21, 2012, 09:03

You can the force on the body out in Post for each component. force_z,force_y and force_x.

That is the vector.

Soolmaz · September 21, 2012, 09:08

I did the same, both F and M in x, y and z-direction. Then how I can find CoP location in my coordinate from it?

Thanks
Soolmaz

singer1812 · September 21, 2012, 09:11

You can use the equations from your first post. M=rxF. Post will give you M and F. Fix one of the x,y,z to an appropriate value, solve for the other two.

September 20, 2012, 03:29	Center of pressure on a surface	#1
Soolmaz New Member Soolmaz Join Date: Aug 2012 Location: Sweden Posts: 19 Rep Power: 13	Hi there, I need to find the center of pressure on a surface. I have read several previous posts relate to this subject, but I finally could not get a reasonable result. What I did for finding CoP was going through M = r x F. I read M and F values in CFX-post function calculator for X,Y and Z dirctions and found CoP = (CoPX, CoPY,CoPZ). However, the point CoP is not located on the desired surface!! Any suggestions? Thanks for help Soolmaz Last edited by Soolmaz; September 20, 2012 at 05:09.

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September 20, 2012, 07:28		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	For a 3 dimensional surface the centre of pressure is not on the surface, in general. But the line of net force on the surface, acting through the CoP, will pass through the surface most of the time (but again, I am sure you can dream up cases when it does not).

September 20, 2012, 08:35		#3
Soolmaz New Member Soolmaz Join Date: Aug 2012 Location: Sweden Posts: 19 Rep Power: 13	My surface is a surface of a cylinder so it is 3D. Till now, I tought CoP should be on the surface! Can you explain about it is not in general on the surface? Can you help me to understad how I can find CoP? I got two ideas: 1. solving the equation M = r x F which leads to a point rather far from the body! 2. I read in previous posts: 0=F_xx+F_yy and M_z = -F_xy+F_yx so it gives x and y values. But if I solve it for M in other directions, I will get other values. I do not know how to deal with them. (I do not understand the idea). What is your advice for finding CoP for a 3D surface? Thanks, Soolmaz

September 20, 2012, 13:00		#4
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	The center of pressure is a point that the force passes through that provides no moment. The problem you will run into is that this is not a unique point. Any point along the force vector will provide you with this condition. One way around this is to fix one of the point locations, preferably that locates the COP within the body. For example, if you were looking at a missile, you might choose the location to be on the centerline of the geometry and solve for the other 2 locations.

September 21, 2012, 02:05		#5
Soolmaz New Member Soolmaz Join Date: Aug 2012 Location: Sweden Posts: 19 Rep Power: 13	Dear Edmund, Thanks for nice explanation of CoP, it really helped me to understand the concept. Can you please tell me how I can find this force vector so then I can choose the desired location? Soolmaz

September 21, 2012, 09:03		#6
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	You can the force on the body out in Post for each component. force_z,force_y and force_x. That is the vector.

September 21, 2012, 09:08		#7
Soolmaz New Member Soolmaz Join Date: Aug 2012 Location: Sweden Posts: 19 Rep Power: 13	I did the same, both F and M in x, y and z-direction. Then how I can find CoP location in my coordinate from it? Thanks Soolmaz

September 21, 2012, 09:11		#8
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	You can use the equations from your first post. M=rxF. Post will give you M and F. Fix one of the x,y,z to an appropriate value, solve for the other two.