Analysis and Modelling of Side Edge Noise from Wing Tips and Flaps

Kurzfassung

This thesis deals with the elaboration of a semi-empirical prediction scheme for the acoustic emissions from flap side-edges and wing tips. More specifically, a flap side-edge is recognized as one of the free end of a deployed trailing-edge flap as part of conventional high-lift devices. In comparison, wing tips refer to the free end of an airfoil of some type without it being included as part of a high-lift device. In the present effort, a flow-based acoustic modelling approach is adopted which differs from standard geometry-based prediction schemes available in the literature. The geometry-based method provides predictions of the radiated acoustic pressure whose parametric dependencies are determined by some proportionality relationship to characteristic geometric dimensions of the flap side-edge or wing tip. On the other end, the flow-based approach established those relationship based on an analysis of the specific flap side-edge or wing tip flow field. The advantage of the last approach is an increased prediction flexibility, especially when coupled with computational fluid dynamics to gather the necessary input flow quantities. It thus extends the applicability range of the prediction scheme to better deal with unconventional aircraft configurations. A particular challenge in the establishment of such a flow-based prediction approach lies in the definition of metrics closely related to the sound source mechanisms. To this end, extensive experimental aeroacoustics investigations were performed at two cantilever flap models (1:1 and 1:1.6 scales) as well as at a high-lift wing (1:6.33 scale) to provide the necessary flow field and acoustic database for the development of the prediction scheme. Because of it's central importance to the noise radiation problem, much attention is also given to the characterization of the tip vortex and its formation process. Near field flow data was gathered using the seven-hole probe measurement technique at the 1:1.6 scale flap model. This data enabled a quantification of the tip mean flow field as a function of flap loading, i.e. vs. flap deployment angle, and the identification of regions dominated by large mean flow gradients along the flap edges. These regions are hypothesized to be centers of high turbulence activity and sound production, characterized by local vortex characteristic length and velocity scales. Those scales are found to relate linearly with variations in flap loading, i.e. its lift coefficient, or tip vortex circulation. Through linear regression analysis, parametric relationships were devised which should simplify greatly the task of comparing results from similar experiments done in different wind tunnel environment and using different wing models. Furthermore a wing's lift coefficient or its tip vortex circulation are parameters which can be easily obtained from RANS computations. The acoustic investigations were performed using both standard free-field microphone measurements of the far-field sound as well as the phased microphone array technique. In particular, a custom-made traversable small aperture phased array was utilized to performed measurements of the acoustic directivity of the 1:1.6 scale cantilever flap model. The acoustic data for this configuration are found to be broadband in nature with a maximum noise level at a frequency of approximately 0.8 kHz. Measured noise intensities are proportional to the 5:5th power of the characteristic velocity for the low-frequency part of the spectrum and proportional to the 6:5th power of the characteristic velocity for its high-frequency part. The directivity measurement results indicate that the measured distinct rear-arc radiation maximum of the low frequencies spectral data is not consistent 3with the classical mechanism of edge scattering of flow turbulence which would imply a cardiod directivity with a maximum in upstream direction. The sound radiation can be attributed to a mixture of classical edge scattering in combination with unsteady force fluctuations on the airfoil as a consequence of vortex unsteadiness and sound wave diffraction. At high frequencies, spectral data scale according to a proportionality law with a 6:5th power exponent of the characteristic velocity, which is close to the typical dipole-like source radiation. It was shown that the maximum sound pressure level, found at 3:15 kHz, is related to the merging of the primary tip vortex (forming on the tip face) with the secondary vortex (forming on the airfoil's suction side), subjecting the flow field to strong sudden local fluctuations. This suggests as source mechanism, a combination of classical edge scattering and quadrupole-like sound generation due to intense and highly unsteady force fluctuations on the airfoil as a result of vortex unsteadiness during the merging process. The directivity patterns indicate also a possible shielding of the acoustic source by the wing leading to rear-arc maximum in radiation. Based on the above findings an empirical prediction scheme is proposed which is validated against the measured noise radiation from a large set of wind tunnel models of various complexity, geometries and scale. The results provided herein demonstrate that a knowledge of the bulk characteristics of the flow field about a flap side-edge or wing tip is sufficient for the prediction of the sound emission in a wide range of cases.