The role of nonlinear wave - Wave interactions in laminar-turbulent transition

The transition to turbulence in the wake flow behind a flat plate is investigated experimentally at freestream velocity 8 m/s, Reynolds number 550, and background turbulence level 0.05 percent in the subsonic wind-tunnel facility described by Miksad et al. (1982). Results for both natural transitions (induced by the random acoustic background) and controlled transitions (induced by coherent acoustic excitation) are presented in extensive graphs and characterized in detail, with a focus on the role of difference-mode generation in nonlinear interactions between low-frequency waves and two or more higher-frequency waves. It is shown that the nonlinear stages of the transition can be analyzed in terms of a duality between nonlinear wave interactions and modulations; this approach leads to helpful distinctions between natural and controlled transition.