Computational investigation of installation noise reduction in urban air mobility vehicles using structured porous surface treatments

Abstract

Urban Air Mobility Vehicles (UAMVs) offer a possible solution to the ever-increasing ground-based congestion by utilising the free airspace. However, since these vehicles are expected to operate close to densely populated areas, the noise generated during take-off, landing and low-altitude flight is a major concern, both with respect to certification and public perception. Due to the use of electric propulsion, the contribution of engine noise is reduced, and therefore aerodynamic noise from the rotors, airframe and their interactions becomes increasingly important. This thesis investigates the use of turbulence-resolving Computational Fluid Dynamics (CFD) and computational aeroacoustics to predict the flow and noise associated with UAMVs, alongside the impact of porous coatings for installation noise reduction. The simulations were carried out using OpenFOAM. The computational setup was first assessed using high-performance computing scaling tests, followed by 2-D and 3-D NACA 0012 validation cases. The 2-D Reynolds-Averaged Navier–Stokes (RANS) validation showed good agreement with NASA reference data, with maximum deviations of 2% in lift coefficient and 3.5% in drag coefficient when compared to computational reference codes. The 3-D Large-Eddy Simulation (LES) validation captured the main flow features of the blunt trailing-edge case, including separation, reattachment, transition and vortex shedding. The validated setup was then applied to low-Mach-number aeroacoustic simulations of a tripped NACA 0012 airfoil. The results showed that tripping was required to reproduce the experimental boundary-layer development, and that LES and Zonal LES were able to predict the main flow and acoustic behaviour. In contrast, the Improved Delayed Detached Eddy Simulation (IDDES) produced limited resolved turbulence in the near-wall region and significantly underpredicted the acoustic levels. The acoustic predictions also showed that Ffowcs Williams–Hawkings (FW-H) surface and closing-disc placement had a strong influence on the predicted acoustic spectra. The best placement and closing disc location were identified providing future guidance. Structured porous coatings were then investigated using both fully resolved geometry and reduced-order approaches. Fully resolved LES of a structured porous coated cylinder showed that the coating modified the near-cylinder flow through internal separation, circumferential pore flow, shear-layer diffusion and boundary-layer development on the inner cylinder. A Darcy–Forchheimer approximation was then developed to represent the porous coating at lower computational cost, and was shown to reproduce several of the main wake trends while also highlighting the sensitivity of the results to the selected model coefficients. Finally, the porous coating was assessed for modelled vortex impingement on both a cylinder and an airfoil leading edge. For the cylinder case, the coating diffused the incoming vortex, reduced near-wake turbulence intensity and reduced the Overall Sound Pressure Level by up to 5.8 dB. For the airfoil leading-edge case, the coating weakened the downstream vortex imprint, reduced surface-pressure fluctuations near the leading edge and reduced the Overall Sound Pressure Level at all observer angles, with a maximum reduction of 8.1 dB.