Experimentelle Analyse von Stoßschwingungen bei der Stoß-Grenzschicht-Wechselwirkung in schallnaher Anströmung unter Einbringung künstlich erzeugter Schallwellen
Zusammenfassung der Projektergebnisse
To date, the mechanisms sustaining the shock-wave oscillations during buffet are not fully understood. A detailed understanding of the buffet mechanism, however, is crucial to shift the buffet boundary to higher Mach numbers or angles of attack and to find strategies to damp or even suppress buffet, which will enhance the operational performance of an aircraft. In this context, the current research project obtained fundamental and particularly valuable conclusions with respect to the mechanism underlying transonic buffet. Various models of transonic buffet flows over supercritical airfoils identify self-sustained large-scale shock oscillations on the suction side of the airfoil in combination with a phase-dependent thickening and thinning of the boundary layer aft the shock wave. As a result, this oscillatory shock movement modifies the pressure field on the wing inducing lift and drag oscillations. Usually, these observations are accompanied by vortex shedding at the trailing edge and the airfoil wake which potentially causes upstream traveling pressure waves. Typically, the UTW can be observed in experimental measurements using Schlieren based techniques like standard Schlieren approaches or Background-oriented Schlieren methods. From a numerical point of view, the UTW are present in highly resolved simulations. To capture all these flow features experimentally, BOS and PIV measurements for various airfoil and flow field configurations are conducted simultaneously, which are in good agreement to previously reported findings of the DRA-2303 airfoil. In the present investigation, buffet is observed at Ma = 0.73 and α = 3.5◦ and becomes stronger at increasing Mach number. The movement range of the shock comprises approx. 6% − 8% of the chord length and the oscillation frequency is approximately fbuffet ≈ 180Hz. With increasing Mach number, variations in the streamwise velocity distribution within the boundary layer aft the shock wave are also observable. That is, a stronger recirculation is present in pre-buffet conditions, e.g., Ma = 0.68 or Ma = 0.70, which decreases with a higher Mach number. It appears that this is a characteristic for the large-scale shock oscillation of the DRA-2303 airfoil. With respect to the modified test configurations, i.e., the introduction of artificial sound waves or the installation of a PTE, the obtained results can be summarized as follows: • The artificial sound waves emitted by a loudspeaker do not influence the general flow characteristics of a prebuffet or a fully developed buffet flow as barely any variations are noticeable compared to the reference case. Moreover, the modulation of the carrier signal mimicing an amplitude modulation of the sound pressure of the artificial UTW yields neither hardly any alteration of the buffet frequencies nor any relevant variations in the boundary layer. • The findings w.r.t. to the cavity noise evidence that it is indeed possible to trigger buffet-related shock oscillations in pre-buffet flow conditions and to modify the buffet frequency. The artificially introduced UTW emitted by the cavity excite the flow field yielding a modified buffet cycle. • The PTE allow a substantial attenuation of the transonic shock oscillations. Using the measurements simultaneous decomposition of the instantaneous shock position, boundary layer thickness, swirling strength within the boundary layer, and SPL of the UTW based on a 1D NA-MEMD likewise reveal a coupling between these quantities, which shows the aforementioned modulation of the vortical structures within the boundary layer and the sound pressure level. Additionally, we observe a broadband frequency range of UTW for all test cases which suggests that the UTW do not possess a dominant and distinct frequency. The obtained findings point to a self-sustained feedback model which closely relates to the one of B. Lee but with a feedback mechanism which is linked to the separated boundary layer. A detailed description of such a model requires further highly resolved investigations of the near-wall boundary layer.
Projektbezogene Publikationen (Auswahl)
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“Investigation of shock–acoustic-wave interaction in transonic flow,” Experiments in Fluids, vol. 59, no. 1, pp. 1–13, 2018
A. Feldhusen-Hoffmann, V. Statnikov, M. Klaas, and W. Schröder
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“Detection of small-scale/large-scale interactions in turbulent wallbounded flows,” Physical Review Fluids, vol. 5, no. 11, p. 114 610, 2020
E. Mäteling, M. Klaas, and W. Schröder,
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“Study on large-scale amplitude modulation of near-wall small-scale structures in turbulent wall-bounded flows,” in STAB/DGLR Symposium, 2020
E. Mäteling, M. Klaas, and W. Schroeder
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“Analysis of transonic buffet using dynamic mode decomposition,” Experiments in Fluids, vol. 62, no. 4, pp. 1–17, 2021
A. Feldhusen-Hoffmann, C. Lagemann, S. Loosen, P. Meysonnat, M. Klaas, and W. Schröder
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“On the damping effect of porous trailing edges for transonic buffet phenomena,” in STAB/DGLR Workshop, 2021
C. Lagemann, E. Mäteling, M. Klaas, and W. Schroeder
