Large-Scale Jet Noise Testing, Reduction and Methods Validation "EXEJET": 5. Analysis of jet-airfoil interaction noise by microphone array techniques

Abstract

With the constant increase of the size of modern engines to get higher and higher bypassratios, the issue of engine integration is a still more important industrial challenge, especially at take-off and landing. This paper is focused on the identification of jet noise sources and jet-airfoil interaction sources by using microphone array data. The measurements were carried-out in the anechoic, opened test section wind tunnel of Onera, Cepra19. The microphone array technique relies on the Lighthill’s and Ffowcs-Williams & Hawkings’ acoustic analogy equation. The output is the cross-spectrum of the source term of the analogy, defined as the optimal solution to a regularized minimal error equation using the measured microphone cross-spectra as reference. In spite of the heavy computational costs of this technique, a large test matrix could be analyzed. The analysis of isolated jet noise data in subsonic regime shows the contribution of the conventional mixing noise source in the low frequency range, as expected, and of uniformly distributed, decorrelated noise sources in the jet flow at higher frequencies. In underexpanded supersonic regime, a shock-associated noise source is clearly identified, too. An additional contribution is detected in the vicinity of the nozzle exit both in supersonic and subsonic regimes. In presence of an airfoil, the distribution of the noise sources is deeply modified. In particular, a strong noise source is localized on the flap. For high Strouhal numbers, higher than about 2 (based on the jet mixing velocity and diameter), a significant contribution from the jet flow, particularly in the shear-layer near the flap, is observed, too. Indications of acoustic reflection effects on the airfoil are also discerned.