STRUCTURE AND NOISE CHARACTERISTICS OF TURBULENT FLOW OVER A SPHEREGiwoong Yun ResumoNumerical simulations of flow around a sphere are conducted at two different Reynolds numbers (Re = 3700 and 10^{4} ) based on the freestream velocity and the sphere diameter. The numerical method used for obtaining the flow over a sphere is based on an immersed boundary method in a cylindrical coordinate system. At Re = 3700, the shear layer is elongated in the streamwise direction to form a cylindrical vortex sheet and its instability appears at x/d ~ 2. The flow behind the sphere is nearly laminar at x/d < 1 and contains few vortices there. On the other hand, for Re = 10^{4} the shear-layer instability occurs right behind the sphere in the form of vortex tube and the flow becomes turbulent in the near wake. Therefore, at Re = 10^{4}, the size of the recirculation region is smaller and the wake recovers more quickly than that at Re = 3700. It is shown using the particle tracing and vortex identification method that the shear-layer and wake instabilities are closely related to each other. In order to investigate the acoustic field around the sphere, the Curle's solution of the Lighthill acoustic analogy is applied. With the far-field and compact-source approximation, the acoustic source in flow around a sphere is regarded as a point source. Due to the three-dimensional vortical evolution in the wake, the acoustic field evolves in a complicated manner, which is very different from that from flow over a cylinder. |
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