Experimental investigation of the combined effect of micro-blowing and porous materials on boundary layer and sound generation

Abstract

The emergence of electrified aircraft propulsion systems introduces significant challenges in thermal management, as the electrical components generate considerable amounts of heat that must be dissipated efficiently. One promising concept to address this challenge is boundary layer micro-blowing, where excess heated air is injected into the boundary layer to improve aerodynamic performance while simultaneously contributing to cooling. A further extension of this strategy is the use of open porous materials to distribute the blowing more homogeneously. In addition to enhancing the aerodynamic effectiveness of micro-blowing, such materials may also reduce the additional aerodynamic noise typically associated with jet injection into the boundary layer. The aim of this thesis is to investigate experimentally the combined effect of boundary layer micro-blowing and porous materials on both boundary layer development and sound generation. To this end, a dedicated experimental campaign was carried out in the aeroacoustic wind tunnel at Brandenburg University of Technology Cottbus-Senftenberg. The work encompassed the conceptualisation and integration of a micro-blowing unit, the characterisation of potential porous materials, and the design of a detailed measurement methodology. Hot-wire anemometry was used to resolve the velocity distributions and turbulence characteristics of the boundary layer, while an acoustic measurement system based on a multichannel microphone array was used to assess the associated noise radiation. The results provide new insights into the influence of porous material geometry and surface features on the distribution of micro-blowing, the resulting modifications to the boundary layer, and the corresponding acoustic emission. Observed differences between inserts demonstrate that both structural anisotropy and manufacturing constraints can significantly impact aerodynamic and acoustic performance. Overall, the findings suggest that the combination of micro-blowing and porous materials holds promise as a strategy for thermal management in electrified aircraft engines, with potential benefits for both aerodynamic efficiency and noise reduction. This work establishes a foundation for the optimisation of combined micro-blowing–porous material strategies and their eventual integration into full-scale electrified propulsion systems, contributing to the development of quieter and more efficient future aircraft.