Experimental evidence of transient growth of energy before airfoil flutter

This paper presents experimental evidence of the transient growth of energy for the coupled-mode flutter of an airfoil. The phenomenon occurs even in linearly stable dynamical systems. Its application is new in the context of fluid–structure interactions where only theoretical and numerical studies of transient growth exist. The experimental set-up allows an NACA 0015 airfoil to oscillate in rotational and vertical degrees of freedom when it is subjected to air-flow. Measurements consist of time series of the two motions obtained by laser displacement sensors. Structural parameters are first estimated without air-flow. The transient evolution of energy is measured, and amplification is observed for a given set of initial conditions. Our experiments agree well with numerical simulations based on unsteady airfoil theory.