Experimental investigation of transonic buffeting, frequency lock-in and their dependence on structural characteristics

Abstract

Transonic buffet is a phenomenon that appears in compressible flow around an airfoil and plays a substantial role in the limitation of the flight envelope of commercial aircraft. If the structural natural frequencies of the wing are similar to the buffet frequency, the oscillation of the compression shock, fluid, and structure may interact (transonic buffeting) and lead to strong loads and potential structural failure. Numerical research has shown that structural parameters can have a significant effect on the onset point of shock oscillations as well as on the typology of the fluid–structure interaction, which presented classical structural excitation, modal veering, or frequency lock-in.The aim of this research is a systematic experimental investigation to examine the structural effects and the lock-in phenomenon. It provides a partial validation of the numerical results available in the literature. A lightweight, elastically-suspended wing model (OAT15A profile) was tested in the Trisonic Wind Tunnel Munich. Optical measurement techniques were deployed to non-intrusively observe the flow-induced density gradient field (background-oriented Schlieren) and the structural deformation and displacement of the wing (digital image correlation). Mass ratios varied from 282 to 322 and the half-chord-based, reduced natural pitch frequency ranged from 0.169 to 0.280. The experimental results confirm the existence of frequency lock-in, an interaction dominated by the structural mode that presents high but limited pitch amplitudes for natural pitch frequencies above the natural buffet frequency. The effects of mass ratio and natural pitch frequency on the interaction are discussed. A substantial effect of the mass ratio on the onset of buffeting and the resulting pitch amplitude for frequency lock-in was discovered.