The effect of shroud on vortex shedding mechanism of cylinder

In the present study, flow characteristics were investigated experimentally using particle image velocimetry technique (PIV) in a gap between a solid cylinder and a shroud to reveal the effect of shroud diameter (D s ) and porosity (ß) on the vortex shedding mechanism of the cylinder. Porosity (varied from ß = 0.3 to 0.7) and diameter ratio (D/D s = 0.4, 0.5 and 0.6) were main parameters examined at a Reynolds number of Re = 5000. For the porosity values of ß ? 0.5, it is observed that vortex formation of the cylinder occurs only in the gap and shroud produces its own wake flow patterns. Penetrating flow through the shroud extends the shear layers on the both sides of the shroud through the downstream direction and prevents the interaction of shear layers in the near wake region. The diameter ratio and the porosity are impactful on the wake flow patterns in outer region of the shroud since they are determinant of the penetrating flow rate. Force measurements were also performed in the air tunnel in order to reveal the effect of shroud on the drag coefficient of cylinder. It is found that the drag coefficient of the cylinders are reduced significantly by shrouds when compared with that obtained from the bare cylinder case. However, the drag coefficient of the cylinder together with the shroud is higher than the bare cylinder for all cases since the shrouds enlarge the area exposed to the flow. © 2019 Elsevier Ltd114R087The authors greatly acknowledge the contribution of the Scientific and Technological Research Council of Turkey to the funding of this research under contract no. 114R087. They wishes also to thank Prof. Dr. Yahya Erkan Akansu for providing the force measurement system

PIV measurement downstream of perforated cylinder in deep water

The flow structure of perforated circular cylinders was thoroughly scrutinized by using the technique of high-image-density Particle Image Velocimetry (PIV). The perforated circular cylinder diameter (D=100 mm), was kept constant during the experimental investigation and corresponding Reynolds number was Re=10000 based on the cylinder diameter. Turbulent statistics e.g., planar turbulent kinetic energy, stream-wise Reynolds normal stress, transverse Reynolds normal stress and Reynolds shear stress were computed in the wake region in order to reveal the differences among various porosities in the range of 0.25?ß?0.80. It would be noted that by increasing porosity, ß the flow fluctuations are substantially reduced in the wake region according to the PIV results. As a result, the prevention of Karman Vortex Street was accomplished by the use of perforated cylinders because of elongated and fragmented shear layers and reduced magnitudes of vortices. © 2018 Elsevier Masson SASNational Council for Scientific Research: 109R001The authors greatly acknowledge the contribution of the Scientific and Technological Research Council of Turkey to the funding of this research under contract no. 109R001

The passive control of unsteady flow structure downstream of a circular cylinder in shallow water

ASMEASME 2013 International Mechanical Engineering Congress and Exposition, IMECE 2013 --15 November 2013 through 21 November 2013 -- San Diego, CA --In the present study, it was aimed to suppress the vortex shedding occurred in the near wake of a circular cylinder (inner cylinder) by perforated cylinder (outer cylinder) in shallow water flow The inner cylinder (Di) and outer cylinder (Do) have fixed diameters, such as Di=50 mm and Do=100 mm, respectively. The effect of porosity, ß, was examined using four different porosity ratios, 0.3, 0.5, 0.6 and 0.8. In order to investigate the effect of arc angle of outer cylinder, ?, four different arc angles, ?=360°, 180°, 150° and 120° were used. The experiments were implemented in a recirculating water channel using the particle image velocimetry, PIV technique. The depth-averaged free-stream velocity was kept constant as U?=100 mm/s which corresponded to a Reynolds number of Re=5000 based on the inner cylinder diameter. The results demonstrated that the suppression of vortex shedding is substantially achieved by perforated outer cylinder for arc angle of ?=360° at ß=0.6. Turbulence Kinetic Energy statistics show that porosity, ß, is highly effective on the flow structure. In comparison with the values obtained from the case of the bare cylinder, at porosity ß=0.6, turbulence characteristics are reduced by %80. Also, the point, which the values of maximum TKE, shift to a farther downstream compared to the case of bare cylinder. Copyright © 2013 by ASME

The effects of perforated cylinders on the vortex shedding on the suppression of a circular cylinder

DANTEC DYNAMICS GmbH;LAVISION;MIT s.r.o.;TSI GmbH11th International Conference on Experimental Fluid Mechanics, EFM 2016 --15 November 2016 through 18 November 2016 -- --The aim of this study is the control of unsteady vortical flow occurred downstream of a circular cylinder located in shallow water flow using concentrically located outer perforated cylinder. The porosities, ß have been changed between 0.1 and 0.8 in the present study. The increments of porosity ß were taken as 0.05 in the range of 0.1 and 0.8 with a hole diameter of d=10 mm. The ratio of inner cylinder diameter to outer cylinder diameter, Di/Do was selected as 0.25, 0.3, 0.4, 0.5 and 0.6 the inner cylinder diameter is Di=50mm where the outer cylinder diameter is Dd=100mm. Experiments were performed at a constant depth of the water level as h=50mm (half of the outer cylinder diameter). Free stream velocity was taken as U=100 mm/s corresponding to a Reynolds number of Re Do=10000 based on the outer cylinder diameter. It has been observed that the inner circular cylinder was highly affected by the existence of surrounding outer perforated cylinders. It is observed that the intensity of Reynolds shear stress correlating, is completely attenuated in the region both downstream of concentric cylinder and between the concentric cylinders. It is determined from the experiments that porosity, ß =0.55 is the most effective parameter for control of flow structure that is occurred from the inner cylinder. © The Authors, published by EDP Sciences, 2017

Vortex street suppression of a circular cylinder using perforated semi-circular fairing in shallow water

In this study, the effect of perforated fairing on vortex street suppression of a circular cylinder was investigated experimentally in shallow water. In order to investigate the effect of arc angle, ? and porosity, ß which are the main parameters of the study, three different arc angles (? = 120°, 150° and 180°) and six different porosities (ß = 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) were examined. Perforated fairing was concentrically located with respect to the circular cylinder along its downstream direction. Turbulent statistics (turbulent kinetic energy, TKE and Reynolds shear stress, ) in the wake region were obtained by employing particle image velocimetry (PIV) technique at a Reynolds number of ReD = 5 × 103 based on the circular cylinder diameter, D. The results depicted that the flow structure downstream of bare cylinder was significantly affected by the presence of perforated fairing for the porosity, ß values, in the range of ß = 0.3–0.6. It is found that the wake region of the cylinder was elongated substantially along the main flow direction and the vortex shedding frequency, was reduced substantially. Moreover, opposing shear layers lost their strength considerably compared with the bare cylinder case. The peak magnitude of Reynolds shear stress, was reduced up to 75% for the arc angle of ? = 180° and the location of peak magnitude of Reynolds shear stress, moved further downstream regions for all cases. Compared to the bare cylinder case, the most effective flow control was obtained for the case having ß = 0.6 porosity and ? = 180° arc angle. © 2016109R001This study was supported by the funding of the Scientific and Technological Research Council of Turkey under contract no. 109R001

Flow structure around perforated cylinders in shallow water

The experimental investigations were carried out in order to have detailed information on the flow structure around perforated cylinders using high-image density Particle Image Velocimetry technique in shallow water flow. The depth-averaged free-stream velocity was kept constant as U?=100mm/s corresponding to the Reynolds number of Re=10000 based on the perforated cylinder diameter. In order to analyze the effect of porosity, ß on the flow structure, the porosities in the range of 0.1?ß?0.8 with an increment of 0.1 were used and the results were compared with the bare cylinder case by means of velocity and vorticity contours, turbulent kinetic energy, Reynolds shear stress and streamline topologies. It was concluded that the porosity, ß had a substantial effect on the control of large-scale vortical structures downstream of the cylinder in which the shear layers were elongated, fluctuations were significantly attenuated and formation of Karman Vortex Street was successfully prevented by the use of perforated cylinders. © 2015 Elsevier Ltd

Control of the flow in the annular region of a shrouded cylinder with splitter plate

DANTEC DYNAMICS GmbH;LAVISION;MIT s.r.o.;TSI GmbH11th International Conference on Experimental Fluid Mechanics, EFM 2016 --15 November 2016 through 18 November 2016 -- --In the present study, the flow control with a splitter plate was studied considering the annular region of a shrouded cylinder. The effect of splitter plate angle, which was defined according to the cylinder centreline is investigated experimentally in deep water using Particle image Velocimetry (PIV) technique and flow visualization by dye injection method. The range of splitter plate angle was selected within 60° ? ? ? 180o with an increment of 30°. The porosity of the shroud which is a perforated cylinder was selected as ß=0.7 in order to have larger fluid entrainment through the cylinder. The results were compared with the no-plate case and showed that the splitter plate located in the annular region of shrouded cylinders is effective on reducing the turbulence levels just behind the cylinder base, as well as the near wake of the perforated shroud. © The Authors, published by EDP Sciences, 2017