Transonic flutter

Flutter is an aeroelastic instability characterized by self-sustained oscillations of an elastic solid due to an incidental small disturbance. The energy for sustained oscillations of the elastic structure is provided by the airstream flowing around it. 

The mechanism of energy transfer from the fluid to the solid, or vice versa, during aeroelastic flutter differs as the lifting surface operates in different flow regimes. For instance, in what is termed as classical bending-torsion flutter, the phase difference between two degrees of freedom of oscillation of the elastic solid is often a prerequisite for this instability to occur; whereas flow separation is the main cause of stall flutter. Given the rather diverse causes of aeroelastic flutter, the choice of fluid model used varies with the conditions in which the aeroelastic system operates. 

An important feature of transonic flow is the presence of regions of subsonic and supersonic flow partitioned by a shock over the solid lifting surface, and the interaction of shock and boundary layer. The supersonic region is usually terminated by a strong shock and the appearance of the shock increases the drag force considerably. Viscous effects can change the strength and position of the shock, and if the shock induces a very high pressure gradient across it, then this could lead to shock-induced separation. Consequently, these flow characteristics may drastically lower the dynamic pressure, at a given Mach number, where aeroelastic instability sets in. 

The  work done in our laboratory focuses on the role of shock displacement and shock–boundary layer interactions on transonic flutter, in particular, the energetics of power flow from the airflow to the elastic solid in those regions where shock–boundary layer interactions are significant, with the intent of understanding the mechanisms that cause a drop in the flutter boundary leading to an early aeroelastic flutter instability: the transonic f lutter dip. 

Pradeepa T. Karnick and Kartik Venkatraman (2017). “Shock-boundary layer interaction and energetics in transonic flutter”. In: Journal of Fluid Mechanics 832, pp. 212–240. https://doi.org/10.1017/jfm.2017.629.