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.

Abstract. In order to understand the aspect of the mutual interference flow from two circular cylinders, the visual observation experiment was performed by use a water flow apparatus. The purpose of this study is accumulation of the basic image data for comparing with numerical computation or previous experimental results. In this report, the intervals of two circular cylinders were varied, the visualization experiment was performed, and the vortex shedding characteristics and the flow pattern in each case were investigated. The cylinder setting conditions were seven kinds (the position of the rear-side circular cylinder is changed). The cylinder diameter ratios were four kinds (D/d=1.0, 1.67, 2.5 and 5.0). The variation of Reynolds number was three kinds (Re=548.7, 1200 and 2500). The dye oozing streak method was used in this visualization experiment. Although the previous PIV experimental result and present result obtained the same flow feature, the aspect of an interference flow became clear by changing the color of tracer ink.

This paper investigates the application of proper orthogonal decomposition (POD) for data obtained from visualizations. Using the POD method, the flow field behind one and two cylinders in a staggered configuration was analyzed. The data processed by this method were obtained from experimental measurements of flow fields using the particle image velocimetry (PIV) method and visualization. The dominant frequencies of the flow pattern from these data were compared using constant temperature anemometry (CTA) measurements. Attention was mainly focused on the flow at three Reynolds numbers: 500, 1200, and 2500. Velocity and vortex fields were created from PIV measurements in the wind tunnel for Re = 500, and video images of flow fields were obtained from dye visualizations in the hydrodynamic tunnel. The components of velocity, vorticity (both of PIV), and change in grayscale (from visualization) were used as input data for POD analysis. A methodology for data processing from visualizations was developed for subsequent analysis using the POD method. A new technique has been found to identify structures in the wake of the cylinders in a staggered configuration by analyzing POD based on various types of input data. The measured fields of dominant frequencies from the CTA and a thorough analysis of the POD modes and their relative energy values for each type of data made it possible to identify the structures and mechanisms that occur in the wake of cylinders. This analysis facilitated a better understanding of the importance of these structures and mechanisms, which can then be used to control the flow behind the cylinders. Graphic abstract

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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.

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In this study, the flow features of vortex shedding from a pair of parallel arranged circular cylinders (same diameters and