Evolution mechanisms of near-field noise in cylinders with varying aspect ratios

This study constructs a model of near-field noise for cylinders with a single free end using Large Eddy Simulation and the sound potential fluctuation equation. This model is employed to explore the evolution mechanisms of near-field noise at a Reynolds number (Re) of 14000. The results indicate that the flowfield structure of finite-length cylinders predominantly consists of three layers: a top flowfield dominated by washdown flows, a middle flowfield governed by periodic vortex shedding from the cylinder surface, and a bottom flowfield dominated by ground boundary layers. The proportion of these three flowfield layers on the cylinder surface determines their contribution rate to aerodynamic noise. In cylinders with a short aspect ratio, the direct interaction of washdown flows with the ground, coupled with boundary layers and vortex shedding, generates significant noise at the cylinder base. For cylinders with a large aspect ratio, the total height of the washdown flows and ground boundary layers is considerably less than the height of vortex shedding on the cylinder surface, leading to vortex shedding dominating the aerodynamic noise generation mechanisms. As the aspect ratio increases further, there is a slight variation in the maximum near-field sound pressure levels. This study provides new insights into the flow-acoustic coupling for active control of aerodynamic noise in cylindrical structures.