In this study, two kinds of circular cylinders which cut grooves to the cylinder surface for the purpose of cylinder drag reduction were produced. One of which is the circular cylinder which set 2 grooves in the cylinder upstream side, and other is the circular cylinder which provided 6 grooves every 60 degrees. Experiments were performed using a wind tunnel varying an attack angel α = 0 to 60 degrees in the range of Reynolds number Re= 1×10 4 to 1.2×10 5 . The relationship between a Strouhal number and Reynolds number, the pressure distribution on the surface of cylinder, the flow feature around the cylinder, and drag reduction effect were investigated. As the results of experiments, in the case of 2 groove cylinder, the Strouhal number of the cylinder with grooves increases from 0.20 to 0.28-0.30, the base pressure coefficient rises from -1.4 to -0.8, then the drag coefficient decreases from 1.3 to 0.55 at α + θ f < 80 degrees, in the range of Re > 4×10 4 . However, it became clear that the drag reduction effect was lost as an attack angle α is attached.In order to compensate this weak point, the 6 groove cylinder was proposed, and the characteristic test was performed. It was obtained that the cylinder with the deep depth of groove is not influenced by the direction of wind. The value of drag coefficient was about 56% of value of the drag coefficient of smooth cylinder. It was shown that the wake width of the proposed cylinders narrows from the wake width of the smooth cylinder.
In order to understand the aspect of the mutual interference flow from two circular cylinders, the visual observation experiment was performed. The cylinder setting conditions were three kinds of distance ratios (L/d=1.5, 2.5 and 5.5), and seven kinds of arrangement angles (α=0, 15, 30, 45, 60, 75 and 90 degrees). The oscillating conditions were four kinds of amplitude ratios (2a/d=0.25, 0.5, 0.75 and 1.0), and the oscillation frequency ratio f/f K in 24 steps. The Reynolds number was about 640. As the result of experiment, even if the distance ratio was the same, the vortex shedding characteristics changed with arrangement angles. The mutual interference will become remarkable if the distance ratio is small. In the arrangement angle, 30 degrees and 45 degrees are carrying out mutual interference most. Even when a forced in-line oscillation was performed under the conditions in which two circular cylinders are carrying out mutual interference, it was found that a lock-in phenomenon occurs. The vortex shedding features were obtained and flow pattern distributions were shown. The lock-in characteristics were investigated and the lock-in ranges have been presented in each distance ratio. Four kinds of typical flow patterns at the time of the lock-in of staggered arrangement oscillating two circular cylinders were shown.
In this study, the flow features of vortex shedding from a pair of parallel arranged circular cylinders of unequal diameter oscillating along the direction of the flow were observed by visualizing water flow experiment at the ranges of the frequency ratio f/f K =0~7, amplitude ratio 2a/d=0.125, 0.25, 0.5 and 0.75, gap ratio G/d=0.25, 0.75 and 1.75, diameter ratio d'/d=0.25 and 0.5 and Reynolds number Re=500. The variations of mean vortex shedding frequency from oscillating cylinder were investigated. As a result of the experiments, the occurrence of the lock-in phenomenon and its range were shown. It was obtained that the states of the interference flow at the time of lock-in by the cylinder oscillation. Some representative flow patterns were obtained. It was shown that the stage number of flow pattern was depended on the gap ratio G/d.
scite is a Brooklyn-based startup that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.