The random flow fluctuations as well as

The aerodynamics of circular cylinders placed in a laminar cross-flow has extensively been studied as it involves some very interesting physics and is of significant importance in many engineering applications, including risers in marine engineering, buildings, bridges, tubular heat exchangers, power transmission lines, chimneys, towers and so on. The noise generation mechanism and ways to reduce noise from bluff bodies is also of great academic and industrial importance. While the aerodynamics of bluff bodies has been the subject of much experimental studies, there still exists a need for high-quality measurement of the quantities important for the understanding of the noise generation mechanism.
The broadband and tonal characteristics of the noise generated by circular cylinders depend on the Reynolds number of the flow 1. At low Reynolds numbers between 50 and ?10?^5, known as the subcritical flow regime, the boundary layer remains laminar, resulting in laminar separation, characterized by periodic shedding of vortices. At very low Reynolds numbers of below 50, the boundary layer separates to form a pair of vortices in the near-wake with no vortices shed, while at critical and fully turbulent regimes with Reynolds number of greater than? 10?^5, the boundary layer becomes turbulence before the separation point, causing strong random flow fluctuations as well as the periodic shedding structures. A complete review of the different flow regimes, vortex shedding structures and the Reynolds number dependency of the vortex shedding phenomenon can be found in Ref 2.
The sound generated as a result of the flow interaction with circular cylinders can be characterized as both tonal, due to the period aerodynamic forces acting on the cylinder as a results of the vortex shedding, and broadband, due to the presence of strong three-dimensional random fluctuations within the boundary layer and the near-wake. The sound generation due to the vortex shedding has received considerable attention since the pioneering research of Strouhal 3 in 1878, who made the first quantitative measurement on the sound generated from the flow of air over wires stretched between radial arms from a rotating shaft. Strouhal found that the frequency of the sound produced is independent of the wire tension or length and established the Strouhal number relationship (St=fD/U_?) to describe the vortex shedding frequency. The observation of the vortex shedding in the wake of a cylinder by Bernard 4 and von Karman 5 led von Kruger and Lauth 6 and Rayleigh 7 to associate the tones with the periodic vortex shedding. Rayleigh also showed that the vortex shedding Strouhal number is a function of Reynolds number. Later experiments 8-10 further confirmed the Reynolds number dependency of the vortex shedding frequency and that there is a minimum Reynolds number for the onset of vortex shedding. Kovasznay 11 found that periodic vortex shedding begins at a Reynolds number of 40 and Roshko 12 demonstrated that for Re?300, the vortex wake becomes irregular and turbulent.


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