Pressure distribution and flow development in unsteady incompressible laminar boundary layers

Abstract

It is shown that in an unsteady flow the friction drag is always accompanied by the form drag whose magnitude is comparable to that of the former and that the pressure around the unsteady boundary layer can be far from that of the inviscid irrotational flow. The unsteady boundarylayer equations and boundary conditions for the external potential flow are modified accordingly and the flow around a circular cylinder which is set impulsively to move in a constant velocity is analysed using these modified boundary-layer equations. The solutions are in power series of\(\sqrt \tau \) rather than τ where τ is the dimensionless time elapsed since the onset of motion, and the form drag, like the friction drag, decreases from infinity in inverse proportion to\(\sqrt \tau \), when τ is small.