 mjmiddle
Ansys Employee

Using cylindrical coordinate systems can be confusing, or any CS that has an angular axis, for that matter. The main reason is that all elements and nodes in APDL have Cartesian CS. So while you can specify preprocessing values (CSYS) using a cylindrical CS, you can never really retrieve results (RSYS) in a cylindrical coordinate system. In either case it is using pseudo cylindrical CS. It rotates element or nodal coordinate systems (Cartesian CSs) to be aligned with local cylindrical directions at every element centroid or node. In the case or preprocessing (loads/constraints), those directions may only agree at the start of the analysis if any angular loading or deformation occurs throughout the analysis. Refer to the Ansys help section (https://ansyshelp.ansys.com/): //Mechanical APDL// Basic Analysis Guide // 7. The General Postprocessor (POST1) // 7.3. Additional POST1 Postprocessing

Radial or Z displacements alone will not cause a problem with results in cylindrical CS and are easy to interpret. When angular displacements occur (by loading or deformation), then both cylindrical X and Y will be hard to interpret and my not be very useful to report in cylindrical CS.

For reporting results in a true cylindrical CS, I find it best to use a a Cartesian RSYS where x,y coordinates are in the plane of r,theta. Then convert for each node. Assuming UPCOORD was not done:

UR = sqrt((NX(Id)+UX(Id))**2 + (NY(Id)+UY(Id))**2) – sqrt(NX(Id)**2 + NY(Id)**2)

Use atan2(y,x) to get cylindrical angle (same as tan(y/x), except it accounts for angle quadrants due to signs of x and y).

UTheta = atan2(NY(Id)+UY(Id), NX(Id)+UX(Id)) - atan2(NY(Id), NX(Id)) 