ABSTRACT
All real fluids have non-zero viscosity, whose effect is mainly confined to wall boundary layers or to shear layers in complex shear flows away from the wall, for example, in the recirculation regions of a sudden-expansion pipe flow. Outside the shear layers, the fluid may be considered ideal with zero viscosity and the flow field modeled as a potential or irrotational flow rendering considerable simplifications. The term “potential flow” originates from the fact that the three-dimensional velocity vector field of such a flow is obtainable from the gradient of a three-dimensional scalar potential function. Irrotational flow, on the other hand, connotes that the curl of the three-dimensional velocity vector field associated with the flow is zero. Since the curl of the gradient of a scalar function is identically zero from vector calculus, a potential flow must always be irrotational or vice versa. Potential flows are also known as ideal flows. They are isentropic in nature with constant total pressure everywhere. Most fluid flows, external flows in particular, are dominated by potential flows and, as discussed in Chapter 8, the static pressure distribution in a wall boundary layer is essentially imposed by the potential flow in the free stream away from the wall.
