ABSTRACT
The flow over a cylindrical object is much more complicated than flow over a flat plate. Most of that complexity arises because at some point, the solid surface begins to curve away from the mean flow direction of the fluid. Fluid flowing around that curve finds it increasingly difficult to stay in communication with the solid surface. Centrifugal force thrusts the fluid outward, away from the object. If the fluid is flowing very slowly, frictional forces enable it to negotiate the curve. As the fluid flows faster the centrifugal force increases. Eventually, this force overcomes the viscous and the intermolecular forces keeping the fluid following the shape of the object. At this point we say the boundary layer separates. Part of the fluid flies off of the cylinder, and the remaining fluid goes into a spin behind the surface. The situation is akin to driving a car around a curve. If one drives slowly there is no problem negotiating the curve. The faster one drives the more force one must exert on the road to keep the car in the direction of the curve. If one increases one’s speed still further one can either be thrown off the curve or one can overcome the friction of tires on the road, lose traction, and begin to spin out behind the curve.
