Abstract
The change in flow characteristics downstream of a circular cylinder (inner cylinder) surrounded by a permeable cylinder (outer cylinder) made of a high porosity screen was investigated in shallow water using Particle Image Velocimetry (PIV) technique. The diameter of the inner cylinder, outer cylinder and the water height were kept constant during the experiments as d = 50 mm, D = 100 mm and h w = 50 mm, respectively. The depth-averaged free stream velocity was also kept constant as U = 180 mm/s which corresponded to a Reynolds number of Re d = 9000 based on the inner cylinder diameter. It was shown that the outer permeable cylinder had a substantialeffect on the vortex formation and consequent vortex shedding downstream of the circular cylinder, especially in the near wake. The time averaged vorticity layers, streamlines and velocity vector field depict that the location of the interaction of vortices considerably changed by the presence of the outer cylinder. Turbulent statistics clearly demonstrated that in comparison to the natural cylinder, turbulent kinetic energy and Reynolds stresses decreased remarkably downstream of the inner cylinder. Moreover, spectra of streamwise velocity fluctuations showed that the vortex shedding frequency significantly reduced compared to the natural cylinder case.
Highlights
The phenomenon of Vortex Induced Vibration (VIV) around bluff bodies is of practical interest of many fields in engineering since its outputs have several undesirable effects such as structural vibrations, increase in drag and lift forces on the body
The velocity vector field of the inner-outer cylinder arrangement clearly demonstrates that the recirculation of vortices gets closer to the back stagnation point of the inner cylinder
The organized vortex structure from the inner cylinder is perturbed by an outer permeable cylinder having a porosity of β = 0.7
Summary
The phenomenon of Vortex Induced Vibration (VIV) around bluff bodies is of practical interest of many fields in engineering since its outputs have several undesirable effects such as structural vibrations, increase in drag and lift forces on the body. VIV is caused by the unsteady vortex shedding which results in a pressure difference between the leading and trailing edges of the body, so the net drag force exerted on the body increases. In recent years investigations have been focused on applications in offshore engineering since oil risers (steel pipes) are being broken like a wire by large amplitude VIV’s caused by the deep ocean currents. Ocean engineers are being busy with solving this problem because riser systems cost one-third of an entire offshore production unit. The effective control of vortex induced vibrations is essential in these types of engineering applications
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