You are correct - the moments about the center of pressure should be zero. What geometry are you working with? How did you find the center of pressure? If the moments about the point you are measuring are non-zero, then that location is not the center of pressure.

Are you working with an airfoil? If so, the aerodynamic center is NOT the same as the center of pressure. The aerodynamic center is the location where the moment does not change with angle of attack. However, for a cambered airfoil, the moment about the aerodynamic center is finite.

my geometry attached to this post ,my geometry is symmetry and you can see in this picture the velocity contour around the body and also pressure on surfaces of body

i find center of pressure in fluent by using reports->forces->center of pressure

and assuming z value of center of pressure to be zero because of the shape of my body.

then fluent gives me y and x values of center of pressure , then when i check these values for moment , the moment is not zero .

so have you any idea to find the center of pressure ?

I would plot the moment as a function of x-location and/or y-location and see where it crosses zero. This should help you narrow it down. It could be that you're really close, but that this geometry produces very strong moments.

so how  do you plot moment as a function of x-location in ansys fluent ? I can't find any way to do this in ansys fluent

Shayan,

I'm not terribly familiar with the current version of Fluent. However, if you were able to check the moment about a single point in the domain, you should be able to check the moment about any point. I would start a spreadsheet and start to check points along, say, the x-axis, or an axis that crosses near the alleged center of pressure. You should be able to sample a bunch of points by hand, and plot the results in a spreadsheet.