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The interaction of a turbulent flow with the leading edge of a blade is a main noise source mechanism for fans and wind turbines. Motivated by the silent flight of owls, the present paper describes an experimental study performed to explore the noise-reducing effect of comb-like extensions, which are fixed to the leading edge of a low-speed airfoil. The measurements took place in an aeroacoustic wind tunnel using the microphone array technique, while the aerodynamic performance of the modified airfoils was captured simultaneously. It was found that the comb structures lead to a noise reduction at low frequencies, while the noise at high frequencies slightly increases. The most likely reasons for this frequency shift are that the teeth of the combs break up large incoming turbulent eddies into smaller ones or that they shift turbulent eddies away from the airfoil surface, thereby reducing pressure fluctuations acting on the airfoil. The aerodynamic performance does not change significantly.

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Hard Disk Drive (HDD) system enclosures in a data center require effective cooling systems to avoid HDD overheating. These systems often rely on air cooling because of their cost effciency and maintainability. Air cooling systems typically consist of an array of axial fans which push or pull the air through the system. These fans emit high level tonal noise particularly at high tip speed ratios. High-capacity HDDs, on the other hand, are sensitive to high acoustic noise, which consequently increases the risk of read/write error and deteriorates drive performance. Therefore, cooling fan noise adversely affects the function of the HDD enclosure systems which emphasizes the need to understand the noise sources and develop methods to mitigate HDD noise exposure.

This study aims to complete and validate a recently developed reduced-order model for the fast prediction of aerodynamic performance (P -Q) curve of electronics cooling fans, taking into account the real fan geometry and the tip clearance effect. The effect of tip clearance on tip vortex generation and fan performance was revealed by an experimentally validated 3D computational fluid dynamics (CFD) method. Six different tip clearance ratios ranging from 0% to 12.7% were investigated. The tip clearance was found to have a strong effect on tip vortex generation, which significantly affects the aerodynamic performance of the fan. Fan efficiency increased by up to 7% by reducing the tip clearance ratio by 2.5%. The tip clearance effect was successfully included by the analytical method combined with a correlation equation developed based on the CFD study. The results show that the combined reduced-order model can produce reasonably accurate predictions for fan P -Q curve with errors less than 7.1% compared to CFD results, while having a wide valid range of tip clearance ratio up to 10%. The precision of this model was further validated against experimental results for eight commercial fans. The computational speed of this model is more than three orders of magnitude faster than a steady-state CFD study, making it highly appropriate for fast analysis of fan performance and thermal-flow co-design.