Investigating the interplay between vortex dynamics and compressibility in transonic airfoil flutter

Airfoil flutter characteristics and limit cycle oscillation (LCO) behavior are profoundly influenced by compressibility effects. Non-linear mechanisms, such as shock-induced separation have been shown to significantly reduce the critical speed for flutter resulting in the so-called "transonic dip". Despite extensive research into this domain, many of the key mechanisms are still not fully explained. This is partly due to the large parameter space involved and the limited use of high-fidelity simulations. In this work, we focus on the aerodynamic instabilities arising from pitching oscillations of a NACA0012 airfoil at a Reynolds number of 10,000. Direct numerical simulations based on a high-order compressible flow immersed boundary method (ViCComp3D) are used for this purpose. The influence of Mach number and frequency on the flutter boundary is categorized using an energy map approach based on forced pitching oscillations. Through this approach, we identify LCOs which are highly dependent on the free-stream Mach number. New insights into the transonic dip mechanisms are provided through detailed analysis of the unsteady aerodynamic forces and flow visualization.