Advances in aeroacoustic wind tunnel testing techniques for aircraft noise research

Extract

To ensure meaningful evaluation and prediction of configuration and flight effects on aircraft noise generation and propagation, reliable representation and measurement of relevant aerodynamic flow conditions as well as of acoustic characteristics must be possible. Acoustic wind tunnels, with models mounted in a quiet test section airstream surrounded by an anechoic working chamber, have now been established as primary tools for noise-model research work and should next be exploited also for the direct support of specific quiet aircraft projects. The special advantages of such tunnels in ensuring a more sheltered and controlled environment than outdoor mobile-model facilities and flight testing include capability of continuous operation, repeatable test conditions, high productivity, good measurement accuracy, testing flexibility, and the precise alleviation of reflections from neighbouring surfaces. Of course the recent experience on noise testing under forward speed conditions and on associated techniques is still very limited at both model scale and full scale, as compared with extensive and continuous aerodynamic testing over half a century. However, many of the problem areas associated with subsonic tunnel design and application for noise-model testing, as identified over five years ago, have now been clarified. Also, the special treatments or limitations involved have become quantifiable in many respects, as discussed later under the convenient main headings of tunnel test section requirements (section 2), tunnel circuit design (section 3), special measurement and analysis techniques (section 4) and model scale simulation (section 5).