From low-frequency oscillations to Markovian bistable stall dynamics

An experimental study is conducted on a thin symmetric airfoil at stall. Below a critical Reynolds number, the flow exhibits low-frequency oscillations (LFOs) characterized by a broadband peak in the aerodynamic force spectrum. Beyond this threshold, the LFOs are replaced by intermittent random switches between two states of either high or low lift (attached or detached flow). The states are explored randomly in time for a fixed angle of attack, contrary to the classical hysteresis often observed in airfoil flows at stall, where both states are absorbing. We model this using a continuous Markov chain and extreme value theory, a framework that can determine the system bifurcation points.