The Foppl model for the incompressible now about the circle is conside
red. The major feature of this model is the understanding of the conve
ctive mechanism that gives rise to the onset of the instability in the
now past the circle. Through this model the real flow is approximated
by means of a potential held with two singular points, counter-rotati
ng vortices, placed symmetrically behind the circle. This field config
uration is topologically analogous to the actual steady field for Reyn
olds numbers below the critical value corresponding to the onset of th
e first instability. The intrinsic instability of the Foppl field to s
mall perturbations may be described by a second-order Linear dynamical
system. In this paper the instability behaviour of the Foppl held dur
ing transient motions is studied. It is shown that a reduction of the
instability growth rate may be induced by a positive acceleration of t
he asymptotic stream or by a pulsating asymptotic stream. In this last
case the effect is directly displayed by a reduction of the temporal
growth rate of the perturbation with respect to that belonging to the
steady state. In the first case the reductive effect is appreciated th
rough a modified form of Shen's criterion. The relevance of these resu
lts to experimental observations of the real flow is discussed.