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Shock-buffet and aerodynamic resonance

2-d RANS-Simulation

At transonic flow conditions, beyond critical angles of attack the flow around airfoils exhibits shock-induced flow separation. The latter is usually accompanied with the occurrence of an aerodynamic resonance, i.e. the occurrence of initially damped aerodynamic natural oscillations of low frequency (see Fig.1).  Ultimately, following a Hopf bifurcation, these flow oscillations can occasionally turn into self-excited limit cycle oscillations of large amplitude – the so calles transonic buffet or shock buffet (see Fig. 2).

The flow physics of the feedback mechanism underlying these flow oscillations, as they can be observed in wind tunnel experiments and CFD simulations, is widely unknown. Until today no adequate theory exists, that predicts frequency and damping of the natural oscillations based on the steady background flow field.

Aerodynamic resonance peak 1 Aerodynamic resonance peak 2

Literature:

• Thormann, Reik and Nitzsche, Jens and Widhalm, Markus (2012)Time-linearized simulation of unsteady transonic flows with shock-induced separation. In: Proceedings of the 6th European Congress on Computational Methods in Applied Sciences and Engineering (ECCOMAS 2012). TU Wien. ECCOMAS 2012 - 6th European Congress on Computational Methods in Applied Siences and Engineering, 10.-14. Sept. 2012, Wien, Österreich. ISBN 978-3-9502481-9-7.
• Nitzsche, Jens and Giepman, Rogier (2013)Numerical experiments on aerodynamic resonance in transonic airfoil flow. In: New results in numerical and experimental fluid mechanics VIII Notes on numerical fluid mechanics and multidisciplinary design, 121. Springer-Verlag. Seiten 349-357. ISBN 978-3-642-35679-7. ISSN 1612-2909. 
• Nitzsche, Jens (2009)A numerical study on aerodynamic resonance in transonic separated flow. IFASD 2009 - International Forum on Aeroelasticity and Structural Dynamics, 21-25 June 2009, Seattle, WA (USA).