Fan broadband interaction noise prediction using a synthetic turbulence method

Kurzfassung

To address growing concerns regarding community noise due to air traffic, noise regulations are becoming increasingly stricter. Therefore, aircraft and engine manufacturers are motivated to develop more silent products. As fan noise is known to be a dominant source during critical flight phases, its reduction is a prerequisite. In recent years, fan tones were intensively studied and effective noise abatement measures were implemented. As a result, the focus now shifts towards fan broadband noise. RANS-informed synthetic turbulence methods are increasingly applied for studying this noise mechanism. They are computationally efficient without simplifying the flow or blade geometries. In this thesis, the fRPM-fan method, which is an fRPM-based synthetic turbulence method, was applied to three different transonic fans. Extensive parameter studies were performed to further the understanding of rotor-stator-interaction noise. The 2D simulation approach was used to study the influence of cyclostationary effects for the NASA SDT fan. The key parameters, which are necessary to prescribe synthesized turbulence, were separately studied with respect to cyclostationarity. The cyclostationary realization of the turbulent length scale proved to be critical. It caused a drop in noise levels at lower frequencies. Instead of a circumferential averaging of the turbulent length scale, a spectral averaging technique was proposed. This new method ensures that the turbulent length scale is dictated by the energetically most dominant turbulence component. A constant simulation using a spectrally averaged instead of a circumferentially averaged turbulent length scale was thus capable of reproducing the sound power levels of a cyclostationary simulation. The relevance of background versus wake turbulence was studied for the next-generation ASPIRE fan. For a purely 2D approach relying on inputs from a q3D URANS simulation on a streamline, the background turbulence was found to be dominant over wake turbulence. This finding was reversed when a correction technique was applied to account for the difference in flow and turbulence characteristics between a q3D and a 3D CFD simulation. Neglecting the radial dimension in the former lead to significantly lowered wake turbulence levels. This study essentially showed that strip-based approaches can give questionable results and using inputs from a 3D CFD simulation is preferential. Furthermore, it highlighted that the contribution of background turbulence may need to be considered for modern fan designs and for test rigs featuring non-negligible ingestion turbulence levels. The 2D and 3D fRPM-fan methods were compared for the ACAT1 fan. While both approaches reproduced experimental trends with respect to the operating conditions, the 2D approach overestimated sound levels. A 2D--3D correction was proposed to address two main physical aspects: 1.) The transverse velocity frequency spectrum, which is critical for fan broadband noise, differs for 2D and 3D turbulence. 2.) Only supercritical gusts produce sound capable of propagating in a duct. Due to the absence of the third dimension, this mechanism cannot be captured by the 2D simulation. Throughout this thesis, the fRPM-fan method was continuously improved in terms of accuracy and efficiency. The accuracy of the fRPM-fan method was assessed with respect to experimental data. A good agreement was achieved. Lastly, the computational cost of a 2D fRPM-fan simulation was decreased by a factor of 3.4. In fact, a comparison of the computational effort of the fRPM-fan method with other fan noise prediction methods showed that the method is competitive.