Analysis of the Noise Shielding Characteristics of a NACA0012 2D Wing

Abstract

A laser-generated non-intrusive impulsive sound source is used to perform shield- ing experiments in the acoustic wind tunnel Braunschweig at a generic 2D wing with NACA0012 profile. The sound source was already shown in previous reports to have a monopole-like character with a nearly omni-directional sound radiation pattern, both in stagnant medium and in a flow field. In the current report, emphasis is first put on the analysis of the data post-processing steps required for the obtention of correct spectral and time domain data for the laser sound source. In particular, the subject of microphone corrections is reconsidered in details. A missing free-field correction of the results in,1 is corrected herein to reveal the correct time domain evolution of the generated pressure pulses. Further, non-linear propagation effects are considered. It is found that sound attenuation due to atmospheric absorption does have a non-negligible broadband impact on the far-field source noise levels, increasing in importance with mi- crophone distance. Other non-linear effects related to high-amplitude wave propagation i.e. wave steepening and higher rates of attenuation with far-field distance, could not be observed herein. Noise shielding results are presented for two Mach number values and three angles of attack. Except for a source location in the proximity of the trailing edge, the flow is found to have no measurable influence on the shielding characteristics of the NACA0012 wing for the experimental setup considered. For a source near the trailing edge, a reduced noise shielding is measured upstream of the trailing edge. It is postulated, that noise is generated through the interaction of the source sound wave with the trailing edge, and that it is amplified in the forward arc according to the mean flow field velocity.