Partial cut plane

Hello,

1) I have a 3D geometry. It is a pipe and I would like to do 2D cut plane. In my case I want to do partially. It means that I do not want to cut for all diameter (between boundaries) of pipe but for example 1/3 of diameter. Actually I need to take maximum velocity from desired area of pipe. Is it possible somehow? If not is it possible to add a function that will take maximum velocity from desired area?

2) I want also to change angle of cut plane by +/- 5 degrees. Can anybody explain me the plane entry method: point and normal? Is it a point and a perpendicular normal vector to the cutting plain? Am I right?

Thanks in advance.