Modelling airframe noise

Reduction of aircraft noise is a major challenge for the aerospace industry and one of the main pillars of the EU's ambitious Clean Sky research programme. Computational aeroacoustics (CAA) analysis is an important design tool for development of low-noise concepts.

One of the most difficult aspects is modelling near-field turbulence. Software that resolves turbulence is useful, but computationally heavy and time consuming. EU-funded scientists set out to exploit the benefits of computationally efficient Reynolds-averaged Navier–Stokes (RANS) equations for CAA in the project 'Computational aero-acoustic analysis of low-noise airframe devices with the aid of stochastic method' (CALAS).

Analysis focused on two low-noise concepts for high lift and main landing gear configurations of regional aircraft. The team developed a stochastic source modelling method for broadband noise generation based on steady RANS computations using computational fluid dynamics techniques. The same method was used to formulate the turbulent flow-generated noise source, which was then incorporated into the CAA analysis to predict its contribution to far-field noise.

The stochastic method was first tested and demonstrated in CAA analysis of the baseline configuration of a high-lift wing concept (flap side-edge or FSE double-flap wing). The baseline results were then compared to results on a low-noise FSE configuration with an add-on wing tip fence. Outcomes demonstrated that the low-noise configuration enables an overall far-field noise reduction of 3–7 decibels (dB).

Scientists then analysed a baseline main landing gear configuration and three low-noise concepts. Only the acoustic liner patched on the rear wall of the landing gear bay was able to reduce noise compared to baseline. This configuration did so by about 1.8 dBA. dBA refers to so-called A-weighted sound levels often used for regulatory noise limits. This sound level scale is potentially better correlated with the relative risk of noise-induced hearing loss.

The CALAS project demonstrated the effectiveness of the stochastic noise source modelling method based on cost-effective RANS equations in industrial consideration of broadband noise for CAA. The methods will support rapid development of low-noise aircraft concepts by the European aeronautics industry. Within the current project, insights gained regarding low-noise configurations point to important design issues for future concepts.