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.