Theory and computations are presented for the receptivity of a three-d
imensional boundary-layer flow on a flat surface to a large-scale low-
frequency periodic external forcing. The basic-state boundary layer an
d the disturbances are governed by the classical (non-interactive) Pra
ndtl equations with a prescribed pressure gradient. The disturbance so
urces in the case of (i) injection/suction through the wall or (ii) fr
ee-stream oscillations interacting with irregularities in the wall geo
metry are assumed to be concentrated close to a sharp leading edge whe
re the main boundary layer is approximated by a similarity solution of
Loos (1955). It is shown that time- and spanwise-periodic perturbatio
ns introduced by (i) or (ii) develop downstream as a spatially growing
instability wave of the type examined in Timoshin (1995), modified ho
wever by the effects of the flow nonparallelism. An algebraic form of
instability in the boundary layer on a rounded leading edge is also di
scussed.