I am postprocessing turbulent channel flow data from a 3D1H simulation, in which I used a coordinate mapping to impose a waviness on the bottom wall and specified  NUMMODES="9" TYPE="GLL_LAGRANGE_SEM" in inhomogeneous directions. To analyze the data, I calculate the gradient (FieldConvert -m mapping -m gradient) of instantaneous fields and write the output in vtu format.  

After the conversion to physical coordinates in Paraview, I extract a horizontal plane from the resulting vtk fields where I see some spurious peaks on element borders in the streamwise derivative fields (u_x, v_x, w_x). I attached a figure showing an example of this, where on the left the problematic "u_x" is shown and on the right "u_x" derived from the continuity equation is shown, i.e., "-v_y-w_z". It can be seen that the fields are the same except some artifacts on element borders in direct "u_x". 

Is this maybe a bug?

Thanks for any feedback.

Regards,