By-pass transition to airfoil flutter by transient growth due to gust impulse

We present an experimental study which shows that the mechanism known as transient growth of energy, can cause flutter instability of a nonlinearly flexible airfoil at a wind velocity below the linear critical flutter velocity. A flap mounted upstream a flexible airfoil in a wind tunnel generates a single gust which triggers the plunge and pitch oscillations. This gust is characterized using two-component hot-wire anemometry. For the first time experimental evidence is provided to confirm the theoretical scenario of a by-pass transition to flutter by transient growth. From an engineering point of view, transient growth might explain also the premature structural fatigue encountered in structures subject to wind.