Coherence resonance in an airfoil flow*

We present experimental evidence of coherence resonance in the flow around a prototypical low-Reynolds-number airfoil prior to stall. This flow undergoes a Hopf bifurcation at a critical Reynolds number, transitioning from a fixed point to a limit cycle characterized by self-excited low-frequency oscillations. We subject the flow to white noise of varying intensities, both before and after the Hopf point, and analyze the autocorrelation and spectral characteristics of the aerodynamic lift signal. Our findings reveal that the coherence factor reaches a maximum at an intermediate noise intensity, which is a signature of coherence resonance. To model the noise-induced dynamics, we use a stochastically forced Van der Pol oscillator, with its parameters determined through an output-only system identification procedure based on the Fokker–Planck equation. We find that the model predictions align well with our experimental results, underscoring the universality of coherence resonance in nonlinear dynamical systems near a Hopf bifurcation. This work establishes the potential for using the noise-induced dynamics arising from coherence resonance to develop early warning indicators of self-excited global instabilities in airfoil flows.