Transonic shock buffet plays a substantial role in the limitation of the flight envelope of commercial aircraft. This flow unsteadiness appears in compressible flows around a wing at sufficiently high angles of attack or Mach number. The phenomenon is associated with a mostly periodic shock-wave/boundary-layer interaction. If the structural natural frequencies of the wing are similar to this "natural" buffet frequency, a fluid-structure interaction may be induced. The resulting coupled oscillation of compression shock, fluid, and structure, called transonic buffeting, can lead to high-amplitude wing oscillations and structural failure. Numerical research has shown that structural participation can alter the onset point of shock oscillations. Furthermore, the typology of the fluid-structure interaction was found to present different natures: the classical structural excitation by the aerodynamic phenomenon of buffet, the frequency lock-in where the coupled oscillation appears at the natural pitch frequency, and the intermediate transition, so-called "veering" region. This doctoral thesis presents a systematic experimental investigation of transonic buffeting in a cumulative form based on three journal publications with the aim of providing an experimental validation of the numerical observations. For this purpose, an experimental setup is designed, manufactured, and integrated in the Trisonic Wind Tunnel Munich. The design consists of a lightweight, two-dimensional, supercritical wing (OAT15A) with an optional pitching degree of freedom and variable torsional spring stiffness and mass distribution. Two optical measurement techniques are deployed for the non-intrusive observation of the flow-induced density gradient field (background-oriented schlieren) and the structural displacements of the wing (digital image correlation). The first configuration of the fixed, rigid wing with inhibited pitching flexibility sets the basis of this work by providing an overview of the natural buffet characteristics in the given facility. The flow development is characterized for various angles of attack and Mach numbers with particular attention to the steady shock motion (regular or inverse), the onset points and the dominant shock oscillation frequencies. Two subsequent detailed measurement campaigns with released pitching flexibility were conducted, whereby the distinct fluid-structural interaction of transonic buffeting with limit-cycle oscillations of the wing was obtained. The first campaign focused on the effect of structural flexibility with the aim of providing experimental validation for an alteration of the buffet(ing) onset characteristics. By comparison with the natural buffet case, a shift and a change of slope of the onset boundary were detected, which provided proof for the numerical works available in the literature. In the second campaign, the natural pitch frequency (close to the natural buffet frequency) and the mass ratio, as well as the angle of attack were varied at a constant Mach number. The aim was the investigation of the different patterns of transonic buffeting and the respective dominant modes. The experimental results confirm the existence of the regions of fluid-dominated, veering, and structurally-dominated interaction. The latter, transonic frequency lock-in, is detected for natural pitch frequencies above the natural buffet frequency and presents high but limited pitch amplitudes. Furthermore, the effects of mass ratio and natural pitch frequency on the corresponding shock and pitch amplitudes and respective region boundaries are presented and discussed. A substantial effect of the mass ratio on the onset boundary of buffeting and, unexpectedly, the resulting pitch amplitude for frequency lock-in were discovered. The experimental observations highlight the importance of the application of experiments or simulations of fluid-structure coupling during the aircraft development to consider the limiting effects of transonic buffet(ing).    «

Transonic shock buffet plays a substantial role in the limitation of the flight envelope of commercial aircraft. This flow unsteadiness appears in compressible flows around a wing at sufficiently high angles of attack or Mach number. The phenomenon is associated with a mostly periodic shock-wave/boundary-layer interaction. If the structural natural frequencies of the wing are similar to this "natural" buffet frequency, a fluid-structure interaction may be induced. The resulting coupled oscillati...    »