| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 102 === This research studies characteristics of the near wake region by the vertical incompressible flow past a finite-length horizontal cylinder with a cap at the tip. The diameter, D, of the cylinder is 0.64 cm and the length, L = 15 cm, of the cylinder is placed into the test section of the water tunnel. The sizes of three caps are 2D (= 1.28 cm),3D (= 1.92 cm) and 4D (= 2.56 cm) respectively. Experiments were carried out in a vertical closed-loop water tunnel with the Reynolds numbers of 250, 560 and 1080. Flow structures were observed by the flow visualization method and Particle Image Velocimetry (PIV) technique was used to measure velocity fields. Experimental results show that when the size of the cap increases, the region which is influenced by the downwash effect decreases. The vortex shedding starts to take place in the wake region at the location with a distance of 1D, 2D or 3D from the cylinder tip along the cylinder length direction for the cases with the cap size of 4D, 3D or 2D respectively. The bifurcation line forms in the wake region and is away from the horizontal cylinder about the distance of 3D, 2D or 1.5D for the cases with the cap size of 4D, 3D or 2D respectively. The maximum values of average vorticity and average shear strain, which are estimated by using the time-averaged velocity field, both increase with higher Reynolds number, or with the smaller cap size.
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