Particle Paths, Fields of Flow Relative to Axes Fixed in Fluid

Abstract

In a field involving the relative motion of a solid and a fluid, we may refer the motion either to a set of axes fixed in the body or to a set assumed as fixed relative to the fluid at a great distance from the body. Thus standing on a bridge and watching the water flow by about one of the bridge piers, we associate ourselves with the solid obstacle—the pier—and refer the motion to axes considered as fixed in that body. Again if we are standing on a bridge over an arm of a lake in which the water is at rest relative to the earth and note the motion set up in the water as a small boat approaches, passes the point of observation and recedes in the distance, we associate ourselves with the earth or with the outlying body of still water and refer the motion to axes considered as fixed relative to the outlying and undisturbed fluid.