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The separated flow over a wall-mounted bump geometry under harmonicoscillations of the inflow stream is investigated by direct numericalsimulations. The bump geometry gives rise to streamwise pressure gradientssimilar to those encountered on the suction side of low-pressure turbine (LPT)blades. Under steady inflow conditions, the separated-flow laminar-to-turbulenttransition is initiated by self-sustained vortex shedding due toKelvin-Helmholtz (KH) instability. In LPTs, the dynamics are furthercomplicated 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 ofthe inflow conditions. Three inflow oscillation frequencies and threeamplitudes are considered. The frequencies are comparable to the wake-passingfrequencies in practical LPTs. The amplitudes range from 1% to 10% of theinflow total pressure. The dynamics of the separated flow are studied byisolating the flow components that are respectively coherent with anduncorrelated to the inflow oscillation. Three scenarios are identified. Thefirst one is analogous to the steady inflow case. In the second one, the KHvortex shedding is replaced during a part of the inflow period by the formationand release of a large vortex cluster. The third scenario consists solely ofthe periodic formation and release of the vortex cluster; it leads to aconsistent reduction of the separated flow length over all the period comparedto the steady inflow case and would be the most desirable flow condition in apractical application.

The impact of harmonic inflow variations on the size and dynamics of the separated flow over a bump