the calculation of the velocity potential associated to constant sink/source panels and vortex rings (constant doublets) at a given point in space is quite complex. open vogel reduces the complexity level by projecting the quadrilateral panels in its normal direction, so that they can be represented by a flat panel (triangular panels do not need to be projected since they are always flat). the calculation of the velocity potential under such situation can be found in this reference. of course, this method has some associated leakage, which depends on how well panels are approach by their flat counterpart.