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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 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.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).
