  The separated flow over a wall-mounted bump geometry under harmonic
oscillations of the inflow stream is investigated by direct numerical
simulations. The bump geometry gives rise to streamwise pressure gradients
similar to those encountered on the suction side of low-pressure turbine (LPT)
blades. Under steady inflow conditions, the separated-flow laminar-to-turbulent
transition is initiated by self-sustained vortex shedding due to
Kelvin-Helmholtz (KH) instability. In LPTs, the dynamics are further
complicated by the passage of the wakes shed by the previous stage of blades.
The wake-passing effect is modeled here by introducing a harmonic variation of
the inflow conditions. Three inflow oscillation frequencies and three
amplitudes are considered. The frequencies are comparable to the wake-passing
frequencies in practical LPTs. The amplitudes range from 1% to 10% of the
inflow total pressure. The dynamics of the separated flow are studied by
isolating the flow components that are respectively coherent with and
uncorrelated to the inflow oscillation. Three scenarios are identified. The
first one is analogous to the steady inflow case. In the second one, the KH
vortex shedding is replaced during a part of the inflow period by the formation
and release of a large vortex cluster. The third scenario consists solely of
the periodic formation and release of the vortex cluster; it leads to a
consistent reduction of the separated flow length over all the period compared
to the steady inflow case and would be the most desirable flow condition in a
practical application.
