Laminar-turbulent transition mechanisms over widely-spaced suction holes

An experimental study was performed on a large array of discrete suction perforations mounted on a flat plate with no sweep. Suction is a useful technology for stabilizing laminar boundary layers, thus delaying the onset of transition to turbulence, and the associated drag increase. This study did not focus on the stabilizing effects of suction, but rather the destabilizing effects associated with localized flow fields around individual perforations. Historically two distinct transition mechanisms have been identified associated with destabilizing suction or 'over-suction': which mechanism dominates is decided by the perforation spacing. In this study new insights are provided into the physical nature of the widely spaced 'over-suction' mechanism. In addition a second/new mechanism associated with widely spaced perforations has been discovered and the physical process of which is explained. In summary, the pre-existing mechanism is dominated by secondary instability of counter-rotating vortices generated by the suction perforations. In the new mechanism, transition through 'over-suction' is governed by non-linear interactions between low frequency modes analogous to travelling cross-flow waves, generated by the suction and low frequency oblique modes generated by a resonant triad in an N-type transition process.