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Öğe Effect of Permeability and Length of a Perforated Splitter Plate Downstream of the Circular Cylinder(Isfahan University of Technology, 2024) Sahin, S.; Durhasan, T.; Pınar, E.; Akıllı, H.Extensive research has been conducted on the flow control of bluff bodies to address negative impacts such as vibration, acoustic noise, and resonance caused by wake flow. The circular cylinder, due to its simple geometry, is frequently studied as a bluff body and is utilized in various engineering applications including cooling system pipes, electrical pylons, industrial flue systems, overpasses, satellite antennas, electrical cables, and marine drilling platforms. In this investigation, a perforated splitter plate was strategically positioned at different downstream locations to manage the wake flow of the cylinder. The experiments were conducted in a sophisticated, closed-loop water channel at the Fluid Mechanics Laboratory of Cukurova University, providing a controlled environment for precise flow analysis. To measure the instantaneous velocity vector field in the wake region of the cylinder at a Reynolds number (Re) of 5000 (based on the cylinder diameter, D), particle image velocimetry (PIV) was employed. Three different permeability values for the splitter plate (e=0.30, 0.50, 0.70) and three lengths (ls*=1, ls*=2, ls*=3) were tested, maintaining a constant gap (G/D=1) between the splitter plate’s leading edge and the cylinder surface. The splitter plates were aligned with the flow direction (?=0°). The permeable separator plates minimize the interaction of boundary layers formed around the cylinder, enhancing their effect in downstream regions where shear layer interaction is more pronounced. Consequently, this results in reduced fluctuations and a more stabilized wake flow downstream of the cylinder. © (2024), (Isfahan University of Technology). All rights reserved.Öğe Experimental investigation of free-surface effects on flow characteristics of a torpedo-like geometry having a cambered nose(Pergamon-Elsevier Science Ltd, 2022) Sarigiguzel, F.; Kilavuz, A.; Ozgoren, M.; Durhasan, T.; Sahin, B.; Kavurmacioglu, L. A.; Akilli, H.In this study, the free-surface effect in the wake region of a torpedo-like geometry having a cambered nose was investigated experimentally via Particle Image Velocimetry (PIV) and dye flow visualization methods. The Reynolds number was taken as Re = 2 x 10(4) and 4 x 10(4) while keeping the angle of attack (alpha = 0 degrees). The torpedo-like geometry is submerged at various positions from the free-surface in the range of 0.50 <= h/D <= 3.50 in which (h) is the distance from the water free-surface to the central plane of the geometry, and D is the diameter of the midsection of the torpedo-like geometry. The effect of the free-surface of water on the flow properties is also measured and presented comparatively in terms of normalized contours of instantaneous vorticity, time-averaged streamwise velocity, vorticity contours, Reynolds stress correlation, turbulent kinetic energy, velocity fluctuations in streamwise and cross-streamwise, vortex shedding frequency, spectral density distribution as well as pointwise variation. It is observed that the flow structures become asymmetric when the body is located near the free-surface for the submersion ratio of h/D <= 1.0. With the largest submersion ratio of h/D = 3.50, the flow characteristics are almost identical and symmetrical on both sides of the geometry.Öğe Flow characteristics comparison of PIV and numerical prediction results for an unmanned underwater vehicle positioned close to the free surface(Elsevier Sci Ltd, 2022) Kilavuz, A.; Ozgoren, M.; Kavurmacioglu, L. A.; Durhasan, T.; Sarigiguzel, F.; Sahin, B.; Akilli, H.In the present paper, flow characteristics of an Unmanned Underwater Vehicle (UUV) with a commonly used Myring profile were investigated numerically and experimentally using Computational Fluid Dynamics (CFD) and the Particle Image Velocimetry (PIV) technique under the influence of free surface. The 3-D and two-phase flow simulation generated using the Volume of Fluid (VOF) were carried out using the Large Eddy Simulation (LES) turbulence model for high accuracy in both near free-surface and almost uniform flow conditions. Due to the presence of the free-surface effect, dynamics and unsteady instantaneous flow characteristics such as force and moment coefficients, streamlines topology, and pressure values on the body surface along with vorticity structures were found to be very chaotic and have irregular motion in the wake while the followable variation trend of the time-averaged properties was obtained to show critical immersion ratio. The immersion ratios of 0.75 <= h/D <= 3.50 were examined at Reynolds numbers Re = 2.0 x 104 and 4.0 x 104. Jet-like flow between the UUV body and the free surface of the water was detected at the immersion ratio of h/D = 0.75, which caused a substantial asymmetry in flow structures, resulting in highest drag and lift values. Increased surface disturbance at Re = 4.0 x 104 caused air introduction into via jet-like flow in h/D = 0.75, which caused positive lift. Hy-drodynamic coefficients and isosurfaces shown that the free-surface effect decreased significantly up to h/D = 1.50 at constant Reynolds numbers. Further investigation of time-averaged velocity components, streamlines, vorticity and turbulence statistics revealed that h/D = 1.50 acted as a transitional immersion ratio as the flow structure changed significantly with Reynolds numbers. The utilized CFD approach yielded especially excellent agreement with the PIV measurements with the discrepancy which varies from 1% to 15% in near wake for streamwise velocity components to simulate the essential unmeasured flow features needed in the research and development process of UUVs when they move below the free surface.Öğe Influence of free-surface on wake flow characteristics of a torpedo-like geometry(Springer Japan Kk, 2022) Kilavuz, A.; Durhasan, T.; Ozgoren, M.; Sarigiguzel, F.; Sahin, B.; Kavurmacioglu, L. A.; Akilli, H.In the present work, the flow topologies of a generalized torpedo-like geometry were investigated experimentally via Particle Image Velocimetry (PIV) and dye visualization. The study was conducted at length based on the Reynolds number of Re = 20 x 10(3) and 40 x 10(3). The torpedo-like geometry was positioned at ratios of immersion between 0.50 <= h/D <= 3.50 to investigate the free-surface effect on the present results, comparatively. PIV measurements provided ensemble-averaged velocity fluctuations, turbulent kinetic energy and Reynolds stress correlation with spectral analysis of the vortex-shedding mechanism. It is observed that different vortex shedding mechanism occurs depending on the immersion ratio. At h/D = 0.5, wake flow is characterized by the lower shear layer while upper shear layer dominates it at h/D = 0.75 and 1.00. The influence of the free-surface on flow characteristics is found to be negligible at h/D > 2.00 for both Reynolds numbers. Alternating vortex shedding occurs and the wake regions at h/D = 3.5 became nearly symmetrical. The size of the wake zone is moved closer to the stern of the torpedo-like geometry at Re = 40 x 10(3) and causes a smaller recirculating region. Spectral analysis of the streamwise velocity revealed a decreasing trend of Strouhal number with increasing immersion ratios. The changing of the Strouhal number showed a significant increase at h/D = 0.75 for Re = 20 x 10(3) an ever-decreasing trend for Re = 40 x 10(3).Öğe Passive Flow Control over an Airfoil by Control Rod at Low Reynolds Number(Isfahan Univ Technology, 2020) Durhasan, T.In the present study, the flow control mechanism of SD7062 airfoil by a rod illustrated using Particle image velocimetry (PIV) technique at pre-stall angles of attack at Reynolds number of Re = 30000. The rod was installed on the suction surface of the airfoil at different chordwise locations. Diameter of the rod was normalized with the chord length of the airfoil and three diameter ratios (d / c = 0.017, 0.033 and 0.044) were examined at angles of attack of alpha = 6 degrees, 8 degrees and 10 degrees. Formation of laminar separation bubble for the baseline airfoil and the effect of rod on the laminar separation bubble were investigated in detail. It is observed that the height of boundary layer was reduced up to 22% by proper rod location and diameter ratio. Moreover, the rod suppressed the unsteady vortices over the suction surface of airfoil significantly. Therefore, the peak magnitudes of turbulent statistics were also decreased up to 30% by the rod.Öğe PIV measurement downstream of perforated cylinder in deep water(Elsevier Science Bv, 2018) Durhasan, T.; Pinar, E.; Ozkan, G. M.; Aksoy, M. M.; Akilli, H.; Sahin, B.The flow structure of perforated circular cylinders was thoroughly scrutinized by using the technique of high-image-density Particle Image Velocimetry (Ply). The perforated circular cylinder diameter (D = 100 mm), was kept constant during the experimental investigation and corresponding Reynolds number was Re = 10 000 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 <= beta <= 0.80. It would be noted that by increasing porosity, beta 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. (C) 2018 Elsevier Masson SAS. All rights reserved.Öğe The effect of shroud on vortex shedding mechanism of cylinder(Elsevier Sci Ltd, 2019) Durhasan, T.; Pinar, E.; Ozkan, G. M.; Akilli, H.; Sahin, B.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 (beta) on the vortex shedding mechanism of the cylinder. Porosity (varied from beta = 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 beta <= 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.Öğe The impacts of the free-surface and angle of attack on the flow structures around a torpedo-like geometry(Elsevier, 2022) Kilavuz, A.; Sarigiguzel, F.; Ozgoren, M.; Durhasan, T.; Sahin, B.; Kavurmacioglu, L. A.; Akilli, H.This study presents the hydrodynamic characteristics around a torpedo-like geometry under the free-surface effect at different angles of attack using preliminary dye visualization and Particle Image Velocimetry (PIV). The optimized torpedo-like geometry is placed at submersion ratios between 0.5 <= h/D <= 3.50 where h is the distance to the free-surface from its centerline and D is the diameter. Throughout the experiments, angles of attack were taken as 0 degrees <=alpha <= 12 degrees for two Reynolds numbers, Re & nbsp;=& nbsp;2.0 & nbsp;x & nbsp;10(4)& nbsp;and 4.0 & nbsp;x & nbsp;10(4). The PIV method provided instantaneous vorticity and time-averaged velocity components, vorticity, streamline topology, fluctuating velocity components, Reynolds stress correlation, and the turbulent kinetic energy. This study focused on the stern section and the wake structures at & nbsp;h/D=1.0 & nbsp;. It is demonstrated that a jet-like flow region occurred between the model and the free-surface for all angles of attack at small submersion ratios of 0.5 <= h/D <= 1.0 while it is observed at & nbsp;h/D=1.5 & nbsp;for & nbsp;alpha=8 degrees, a jet-like flow region occurred between the model and the free-surface. The impact of the jet-like flow was more noticeable for & nbsp;alpha=4 degrees & nbsp;and 8 degrees with & nbsp;velocity fluctuations & nbsp;in lower magnitudes. The nose section partially pierced the free-surface for h/D & nbsp;=& nbsp;0.5, 0.75, and 1.0 in the range of 4 degrees <=alpha <= 12 degrees and it prevented the wake region from connecting with the free-surface by directing the separated flow region toward the stern section. Pointwise variations of the turbulence data extracted from vertical lines within the wake region for all cases revealed that the effect of the free-surface on the turbulence statistics was negligible beyond h/D=2.0.
