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    Flow around a cylinder surrounded by a permeable cylinder in shallow water
    (Springer, 2012) Özkan, Göktürk M.; Oruç, Vedat; Akıllı, Hüseyin; Şahin, Beşir
    The change in flow characteristics downstream of a circular cylinder (inner cylinder) surrounded by an outer permeable cylinder was investigated in shallow water using particle image velocimetry technique. The diameter of the inner cylinder and the water height were kept constant during the experiments as d = 50 mm and h w = 25 mm, respectively. The depth-averaged free-stream velocity was also kept constant as U = 170 mm/s which corresponded to a Reynolds number of Red = 8,500 based on the inner cylinder diameter. In order to examine the effect of diameter and porosity of the outer cylinder on flow characteristics of the inner cylinder, five different outer cylinder diameters (D = 60, 70, 80, 90 and 100 mm) and four different porosities (β = 0.4, 0.5, 0.6 and 0.7) were used. It was shown that both porosity and outer cylinder diameter had a substantial effect on the flow characteristics downstream of the circular cylinder. Turbulent statistics clearly demonstrated that in comparison with the bare cylinder (natural case), turbulent kinetic energy and Reynolds stresses decreased remarkably when an outer cylinder was placed around the inner cylinder. Thereby, the interaction of shear layers of the inner cylinder has been successfully prevented by the presence of outer cylinder. It was suggested by referring to the results that the outer cylinder having 1.6 ≤ D/d ≤ 2.0 and 0.4 ≤ D/d ≤ 0.6 should be preferred to have a better flow control in the near wake since the peak magnitude of turbulent kinetic energy was considerably low in comparison with the natural case and it was nearly constant for these mentioned porosities β, and outer cylinder to inner cylinder diameter ratios D/d.
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    PIV measurements on the passive control of flow past a circular cylinder
    (Elsevier, 2016) Oruç, Vedat; Akıllı, Hüseyin; Şahin, Beşir
    The passive control of water flow downstream of a circular cylinder with 50 mm diameter is experimentally investigated in this study using particle image velocimetry (PIV) technique. The experiments were carried out for three different Reynolds number (based on cylinder diameter) values of 3350, 6850 and 10,200. The flow structure was substantially vortical with large-scale downstream of the bare cylinder due to strong interactions of the shear layers separating from the cylinder surface. For the passive controlling of flow, a drop-shaped mesh surrounding the cylinder was used to suppress the formation of Karman vortex street in the wake region of the cylinder. Time averaged flow structures such as velocity distribution, vorticity, streamline topology, Reynolds shear stress and turbulent kinetic energy were referred for interpreting the experimental results. The spectral analysis of the measured velocity data was also performed to have information about the vortex shedding frequency of the unsteady flow in the cylinder wake. It was confirmed that flow was successfully controlled by paying attention to the absence of large-scale vortical flow structure (although small-scale vortices, which were not strong compared to the former one, were still present in the shear layers) as well as elongation in vorticity layers, attenuation in both Reynolds shear stress and turbulent kinetic energy distributions related to the fluctuating velocity components measured in the wake region.