THE PRESSURE DISTRIBUTION AROUND UNSTEADY BOUNDARY-LAYERS

The effect of an unsteady boundary layer on the pressure field around a bluff body has been investigated. It is found that in an unsteady flow the friction drag is always accompanied by a form drag whose magnitude is comparable with that of the former, and thus the pressure field around the unsteady boundary layer can be very different from that of an inviscid irrotational flow. The definition of the displacement thickness is modified accordingly and interpreted as a measure of the momentum of fluid trapped in the boundary layer rather than as the distance displaced laterally by the retardation of the flow in it. The result is consistent with previous specific numerical and analytical descriptions of these boundary-layer flows.