Directivity pattern of flow-induced noise from a wall-mounted, finite length circular cylinder

Sound emanating from a wall-mounted, finite length cylinder immersed in cross flow presents a significant engineering problem and is relevant to a range of applications including aircraft landing gear and automobile appendages. However, despite its frequent occurrence in industry, there exists little experimental data on the noise created by such objects. To characterise this type of flow-induced noise source, acoustic directivity measurements have been taken in an anechoic wind tunnel at the University of Adelaide for wall-mounted cylinders of circular cross section. The aspect ratio (the cylinder length to diameter ratio, L/D) and orientation of the cylinders were varied to determine the influence of these parameters on noise directivity. Furthermore, the results were compared with the radiation pattern of the two dimensional case as well as with a dipole source (of equivalent power) at the assumed origin. The experimental data give further insight into the characteristics of the sound generated from wall-mounted, finite length cylinders in cross flow. INTRODUCTION AND BACKGROUND Flow-induced noise radiating from a slender cylindrical body is important to a range of applications including railpantographs, landing gear and automobile appendages. Curle (1955) further developed Lighthill’s acoustic analogy (Lighthill 1952) to include solid surfaces immersed in cross flow and related the radiated sound field to the fluctuating pressure on the surface. Phillips (1956) showed that for a circular cylinder in cross flow, this pressure is dominated by lift fluctuations at the vortex shedding frequency. Phillips (1956) developed Eq. 1 to describe the far field variation of mean square pressure, where ρ is the density, U is the free stream velocity, St(=fD/U) is the Strouhal number of the fluctuating force based on the cylinder diameter, D, L is the cylinder length, s is the spanwise correlation length, x is the observer distance from the source, ao is the speed of sound, is the root mean square of the sectional lift coefficient and θ is the angle between the oncoming flow and the direction of the observer relative to the cylinder.