| Summary: | 碩士 === 國立中興大學 === 機械工程學系所 === 101 === Abstract
This study investigates the flow characteristics using Particle Image Velocimetry (PIV), Laser Doppler measurement system (LDV), flow visualization and fast Fourier transform (FFT). The Reynolds number is 1000, the inflow velocity is 5cm/s, and the diameter ratio of staggered cylinders is 2.0. The staggered cylinders are arranged at three vertical spacing (T/D=2, 1.5, 1) with the horizontal spacing varying within -3.5 ≤ L/D ≤ 3. The flow structures behind the staggered cylinders can be divided into two main categories. First of all, as the gap flow biases toward the large cylinder, it is defined as Mode 1; while the gap flow biases toward the small cylinder, the flow is defined as Mode 2. According to the spacing arranged, the flow can be divided into several kinds. For the Mode 1, while T/D=2, L/D=-3.5 ≤ L/D ≤ -1.5 and T/D=1.5, -3.5 ≤ L/D ≤ -2.5, the wide and narrow wakes of each cylinder shed nearly independently. For T/D=1, -3.5 ≤ L/D ≤ -1, the C vortex of small cylinder (wide wake) and the A vortex of large cylinder (narrow wake) merges and move downstream with B vortex. For -3.5 ≦ L / D ≦ -3, the wake behind small cylinder is completely suppressed. For T/D=1.5, -2.5 ≤ L/D ≤ -1, the wake flow of large cylindrical interacts strongly with that of small cylinder. For T/D=1, -2.5 ≤ L/D ≤ 1, the narrow wake is terminated by the wide wake. In case of Mode 1, the narrow wake of large cylinder is truncated by the wide wake of small cylinder; and vice versa in the case of Mode 2. Further, in cases of Mode 2, for T/D=2, -1 ≤ L/D ≤ 0, and T/D=1.5, -0.5 ≤ L/D ≤ 1, the inner shear layer of large cylinder rolls up a series of vortex Bn, induced by the formation of the vortices C and D of the narrow of small cylinder. For T/D=2, 0 ≤ L/D ≤ 3, the wide and the narrow wakes shed nearly independently. In cases of T/D=1.5, 1, 1 ≤ L/D ≤ 3, the biased angle of gap flow increases and the degree of squeezing of the narrow wake becomes more severely.
Keywords: staggered, blunt, tip clearance flow, double cylinder, unequal diameter
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