Adaption of structured porous media for flow and noise control

Abstract

Aerodynamically generated sound is a prevalent issue in engineering, posing significant challenges for aviation and energy industries. This thesis addresses two self-noise mechanisms relevant to these sectors: turbulent boundary layer trailing edge noise and the vortex shedding tones of bluff bodies in crossflow. The literature review underscores the potential of bio-inspired porous media on aerodynamic and bluff body noise radiations. Although the current approach is successful due to its mitigation of self-noise emissions, the prevailing application methodology of porous media is inherently complex. The research efforts on a structured porous material, which brings many advantages, are light. They not only streamline experimental procedures and enhance replicability but also open new avenues of contributions due to their ease of modification, characterisation and optical clarity. In the first chapter, a Design of Experiments approach was harnessed to fully capture the effects of the structured porous characteristics on turbulent boundary layer trailing edge noise. Investigated were streamwise and spanwise separation distances between pores, pore size and porous coverage. Results show acceptable prediction accuracies are obtained for all the response variables, proving the approach’s feasibility. A minimum spanwise and streamwise spacing of pores, in combination with minimum pore diameter and an intermediate level of porous coverage, presents the optimal configuration to reduce aeroacoustic emission. The second chapter investigates the application of a simplistic pore arrangement for trailing edge noise reduction using several 3D-printed trailing edges, each with a single row of pores parallel to the trailing edge. The approach stimulated a phase-cancellation between the pressure waves emanating from two sources: the array of pores and the acoustic scattering at the trailing edge dipole source. The results show that the trailing edges can achieve low-noise radiation, and a study of the near-wake and boundary layer established that this configuration will not alter the boundary layer significantly. Finally, the effects of modifications, based on internal flow field data, to a Structured Porous- Coated Cylinder on the vortex shedding tones and wake development were investigated. Modifications of the existing structure were designed to affect the regions of internal flow within the porous layers to explore how the alteration would impact the vortex shedding attenuating capacity. The acoustic data confirms the ability of the Structured Porous-Coated Cylinder to reduce turbulent shedding noise over its bare cylinder counterparts. The modified variants proved the importance of streamwise communication in the internal structure. Furthermore, the stagnation regions can be filled, which retains the acoustic performance. Lastly, the removal of the ability for the flow to travel in the spanwise direction within the internal structure has been shown to improve the acoustic far-field radiation over the original Structured Porous-Coated Cylinder.