Flow in the descending aorta. A turn of the screw or a sideways glance?

F luid dynamics is an enormously complex field of study, both experimentally and theoretically, but is well outlined in a recent overview of the topic.' The nonlinear partial differential equations that govern fluid flow yield such a bewildering array of possible solutions that study of fluid dynamics in physics curricula is frequently deferred until such esoteric subjects as general relativity and quantum electrodynamics have already been considered. Experimental study of fluid flow is equally complex, because one must characterize a fluid system by relating four dependent variables (pressure and the x, y, and z components of velocity) to four independent variables (the x, y, and z spatial coordinates and time).