I know quite well what a dipole is and in general what multipole moments are (in the context of, for instance, electrodynamics). What I find myself confused by is something called a "force dipole" in fluid dynamics. I believe that it is the same thing as a "stresslet". There are many papers about micro-organisms at low Reynolds numbers which talk about the velocity fields around "force dipoles", and these fields are always dissimilar to what I understand as the field around a "dipole". In fact, the field looks more like that of a pure "quadrupole" to me. As an example, I am attaching a figure cut out from the paper "Hydrodynamics of self-propulsion near a boundary: predictions and accuracy of far-field approximations" (S. E. Spagnolie and E. Lauga, Journal of Fluid Mechanics 700, 105-147 (2012)). To me it is only the field in part (b), here called as that of a "source dipole", which looks like a dipole field, not the one in part (a). Is this merely a matter of convention, that fluid dynamicists refer to as a "force dipole" what would generally be a called a quadrupole? Any light shed on the matter would be appreciated.