A comprehensive and consistent design-to-noise study of high-lift profiles and their noise reduction potential

Kurzfassung

The optimal design of future transport aircraft has to comply with further tightened environmental requirements. Therefore, the usage of mature design tools is mandatory that provide on the one hand side a prediction capability for the effect of even subtle design variations. On the other hand, predictions with short turnaround time of the order of 100 CPUh are necessary to enable a sufficient resolution of the design space at acceptable numerical effort. For aerodynamic analysis with Computational Fluid Dynamics (CFD), Reynolds Averaged Navier-Stokes Simulation (RANS) has matured as a state-of-the-art simulation tool in industry that provides the crucial short turnaround times and sufficient reliability of results. However, up to date no useful acoustic metric could be derived from the RANS flow variables that could be further utilized for aeroacoustics driven design (design-to-noise). Scale resolving simulation tools, on the other hand, require a simulation effort far beyond 100 CPUh and are of limited use if a large set of variants has to be studied. In some predecessor work Computational Aeroacoustics (CAA) simulation with vortex sound sources derived from RANS deduced synthetic turbulence has demonstrated a feasible way to model broadband noise spectra for problems such as trailing edge noise or high-lift slat noise. Successful simulations revealed that i) major noise source characteristics can be deduced from a space-time stochastic realization of synthetic turbulence and that ii) CAA turnaround times are comparable to that of RANS. Hence, the approach has some potential to bridge the current prediction gap by providing sufficiently efficient a meaningful acoustic metric from RANS turbulence data. In this work a 2-D design study has been carried out using 5 different high-lift profiles to demonstrate the applicability of the approach for design-to-noise applications. CAA and CFD simulations have been conducted using 3 different velocities (40m/s, 50m/s, and 61.5m/s). Acoustic results (narrow band spectra) and time averaged aerodynamic characteristics (cp-distribution) of the 2-D design study have been cross-compared for the F16 reference high-lift profile with a scale resolving approach. Good agreement for both data sets could be demonstrated. With the Very Long Chord Slat (VLCS) variant a geometry intended to lower the noise emissions of the F16 reference profile has been studied. Two novel Krüger flap designs have been evaluated concerning their noise reduction trends. Meaningful noise trends and clear noise reduction potentials could be identified. Conventional slat noise is found to scale with the fourth power of Mach number. For the F16 reference profile a gap/overlap slat setting variation study has been performed. The gap variation study revealed clearer noise reduction potential yielding about 3dB overall sound pressure level (OASPL) reduction towards the ground. The VLCS yields an OASPL noise reduction potential of up to 6dB towards the ground, as was found in previous experimental studies. The Krüger flap designs yield the clearest noise reduction potential. For the first considered Krüger design, OASPL noise reduction yields on average 6dB over the entire lower polar arc range. A second aerodynamically optimized Krüger design, however significantly loses part of the noise reduction potential, thus indicating the importance of an holistic approach towards high-lift design that incorporates besides aerodynamic also aeroacoustics characteristics.