| Summary: | 碩士 === 國立中興大學 === 機械工程學系 === 87 === The objective of this study is to investigate and realize the influence of cylinder wake on the cavity shear layer while a single cylinder is placed at different positions above the cavity. All the experiments were performed in a recirculating water channel. Quantitative velocity measurement and qualitative flow visualization were preformed by ways of fiber laser Doppler’s anemometry (FLDA) system and the laser sheet technique. The parameters in this experiment are described as follows: the cavity has a length/depth ratio 2 (L/D=2.0), the diameters of the cylinder in terms of d/D ranges within 0.33-0.8. The vertical distance (H/D) between cylinder and cavity ranges from 0.4 to 4.0, and the streamwise locations vary from XL.E/L=0 to XL.E/L=3.0. For all experimental conditions studied herein, the free stream velocity is U∞=20cm/s and the Reynolds number, based on cylinder diameter, is around Red=
U∞d/ν=4000-1640.
It is found that the periodic perturbation due to cylinder wake strongly affects and enhances the shear layer instabilities across the cavity when a single cylinder is situated above the cavity. When the cylinder moves downstream of the cavity but is maintained at the same height, the streamwise extent of flow accelerating region between cylinder and cavity will also move downstream. However, the amount of speed increment does not change apparently. When the cylinder is located near the upstream corner of the cavity and maintained at a height H/D=1.2-0.8 above the cavity, the cylinder wake will significantly affect the unstable cavity shear layer. In these situations, strong interaction between the periodic wake and the cavity shear layer will take place. When the velocity between cylinder and cavity goes beyond a critical value, the unstable oscillation of the cavity shear layer will jump to another oscillation mode. For these conditions, the oscillation within the cavity is multi-mode, and the competition between each mode will take place and transfer energy in between. When the cylinder is positioned more close to the upstream corner of the cavity, the cross-sectional velocity profile across the cavity does not follow the tanh-velocity profile. This implies that prediction of the oscillating mode within the cavity via linear stability theory is no longer valid. When the height between cylinder and cavity continuously reduces down to H/D=0.4, the blockage of the cylinder will reduce the streamwise averaged velocity. Thus, the unstable oscillation of cavity shear layer will diminish, and only a large-scale recirculation structure in the cavity can be observed.
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