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Öğe Comparison of Flow Characteristics of Different Sphere Geometries Under the Free Surface Effect(E D P SCIENCES, 2013) Özgören, Muammer; Doğan, Sercan; Okbaz, Abdulkerim; Aksoy, Muharrem Hilmi; Şahin, Beşir; Akıllı, HakanComparison of the experimental results of turbulent flow structures between a smooth sphere and a sphere with a vent hole, roughened, and o-ring is presented in the presence of a free-surface. Dye visualization and particle image velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake for Reynolds number Re = 5000 based on the sphere diameter with a 42.5mm in an open water channel. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from point of flow physics for the different sphere locations in the range of 0 <= h/D <= 2.0 where h was the space between the top point of the sphere and the free surface. The ratio of ventilation hole to sphere diameter was 0.15, o-ring was located at 55 degrees with a 2 mm from front stagnation point of the sphere and roughened surface was formed by means of totally 410 circular holes with a 3 mm diameter and around 2 mm depth in an equilateral triangle arrangement. The flow characteristics of instantaneous velocity vectors, vorticity contours, time-averaged streamline patterns, Reynolds stress correlations and streamwise and cross-stream velocity fluctuations for both the smooth and passively controlled sphere were interpreted.Öğe Comparison of flow structures in the downstream region of a cylinder and sphere(ELSEVIER SCIENCE INC, 2011) Özgören, Muammer; Pınar, Engin; Şahin, Beşir; Akıllı, HüseyinAn experimental investigation of flow structures downstream of a circular cylinder and sphere immersed in a free-stream flow is performed for Re = 5000 and 10,000 using qualitative and quantitative flow visualization techniques. The obtained results are presented in terms of time-averaged velocity vectors, patterns of streamlines, vorticity, Reynolds stress correlations and turbulent kinetic energy distributions. Flow data reveal that the size of wake flow region, the location of singular and double points, the peak values of turbulence quantities, such as Reynolds stress correlations, vorticity fluctuations and turbulent kinetic energy vary as a function of models' geometry and Reynolds Numbers. The concentration of small scale vortices is more dominant in the wake of the sphere than that of the cylinder. The maximum value of turbulent kinetic energy (TKE) occurs close to the saddle point for the cylinder case while two maximum values of TKE occur along shear layers for the sphere one because of the 3-D flow behavior. (C) 2011 Elsevier Inc. All rights reserved.Öğe Effect of different wall functions on the prediction of flow and heat transfer characteristics in plate fin and tube heat exchangers(Selçuk Üniversitesi Mühendislik-Mimarlık Fakültesi, 2005) Akkoca, Azize; Şahin, Beşir; Tutar, MustafaIn the present study, effects of wall functions available in the Fluent software on the fluid flow and heat transfer characteristics of a plate fin and tube heat exchanger are investigated in the range of 500 ? Re ? 6000 for a non-dimensional fin spacing of Fs = 0.233 and the results are compared with experimental data. As it is well-known, wall functions are used to bridge the viscosity-affected region between the wall and the fully turbulent region in the flow domain. Both Standard k-? and RNG k-? turbulence models are employed in order to predict the flow and heat transfer characteristics inside the flow passage of the plate fin and tube heat exchanger comprised of four-row of staggered tube layout with wavy fin configuration. The test heat exchanger model is selected from the experimental work exists in the literature and the best wall function that has close agreement with the experimental data is chosen as Enhanced Wall Treatment.Öğe Experimental Study of Flow Structures around Side-by-Side Spheres(AMER CHEMICAL SOC, 2013) Pınar, Engin; Şahin, Beşir; Özgören, Muammer; Akıllı, HüseyinThe flow characteristics of a single and the two side-by-side spheres immersed in a uniform flow for Re = 5 x 10(3) are studied qualitatively and quantitatively employing dye visualization and a particle image velocimetry. The current study focuses on the analysis of the flow behavior using instantaneous and time-mean flow data in plan-view and cross-sectional planes. Flow features and distributions of velocity vectors, V*, at the critical locations; patterns of velocity fluctuations, u* and nu*; contours of sectional streamlines, ; patterns of vorticity, omega*; and correlations of Reynolds stress, are interpreted. The gap ratios between two side-by-side spheres varying within the range of 1.0 <= G/D <= 2.50 influence the instability of flow structures substantially.Öğe Investigation of flow characteristics around a sphere placed in a boundary layer over a flat plate(ELSEVIER SCIENCE INC, 2013) Özgören, Muammer; Okbaz, Abdulkerim; Doğan, Sercan; Şahin, Beşir; Akıllı, HüseyinFlow characteristics around a sphere located over a smooth flat plate were experimentally investigated using dye visualization and PIV technique. The sphere was embedded in a turbulent boundary layer with a thickness of 63 mm which was larger than the sphere diameter of D = 42.5 mm. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from the point of flow physics for different sphere locations in the range of 0 <= G/D <= 1.5 where G was the space between the bottom point of the sphere and the flat plate surface. Reynolds numbers with a range of 2500 <= Re <= 10000 based on the free-stream velocity while the velocity distributions over the plate surface are the developed turbulent boundary layer condition attained by using a tripwire. Distributions of velocity fluctuations, patterns of sectional streamlines, vorticity contours, velocity fields, turbulent kinetic energy and corresponding Reynolds stress correlations are obtained using PIV data. It was found that a jet-like flow stimulated the flow entrainment between the core and wake regions as a function of the sphere locations. The gap ratio has a strong influence on the flow structure of the wake-boundary layer interaction and the variation of the reattachment location of the separated flow from the plate surface. The time-averaged flow patterns yield asymmetric structures downstream of the sphere due to the effect of the boundary layer flow distribution. (C) 2012 Elsevier Inc. All rights reserved.Öğe Large Eddy Simulation of Free-Surface Effects on the Wake Structures Downstream of a Spherical Body(Permagon-Elsevıer Scıence Ltd, 2012) Hassanzadeh, Rahim; Şahin, Beşir; Özgören, MuammerThe wake structure downstream of a solid body, which moves near the free-surface, is under the influence of the free-surface motion. The rate of this influence depends on the submerging elevation or distance between the solid body and the free-surface. This investigation presents the interaction between the free-surface and the wake structure downstream of a spherical body at four different submerging elevations such as h/d=0.25, 0.50, 1.00 and 2.00 for the Reynolds number of 5000. For the present study, large eddy simulation (LES) using a dynamic Smagorinsky sub-grid stress (SGS) is applied on the hybrid grid system according to an advanced computer code based on the finite volume approach. Examination of obtained results reveals that the rate of interaction between the free-surface and the wake structure downstream of the spherical body is maximum for the case of h/d=0.25 and by increasing the submerging elevation, the rate of this interaction decreases. At the submerging elevation of h/d=2.00, the free-surface effects are minimum and beyond h/d=2.00 it may be assumed that these free-surface effects on the sphere wake structure disappear.Öğe Turbulent Shear Flow Downstream of a Sphere with and Without an O-Ring Located Over a Plane Boundary(E D P SCIENCES, 2012) Özgören, Muammer; Okbaz, Abdulkerim; Doğan, Sercan; Şahin, Beşir; Akıllı, HüseyinFlow-structure interaction of separated shear flow from the sphere and a flat plate was investigated by using dye visualization and the particle image velocimetry technique. Later, a passive control method was applied with 2mm o-ring located on the sphere surface at 55 degrees from front stagnation point. The experiments were carried out in open water channel for Reynolds number value of Re=5000. Flow characteristics have been examined in terms of the 2-D instantaneous and time-averaged velocity vectors, patterns of vorticity, streamlines, rms of velocity fluctuations and Reynolds stress variations and discussed from the point of flow physics, vortex formation, lengths of large-scale Karman Vortex Streets and Kelvin-Helmholtz vortices depending on the sphere locations over the flat plate. It is demonstrated that the gap flow occurring between the sphere bottom point and the flat plate surface has very high scouring effect until h/d=0.25 and then unsymmetrical flow structure of the wake region keeps up to h/D=1.0 for smooth sphere. For the sphere with o-ring, the wake flow structure becomes symmetrical at smaller gap ratios and reattachment point on the flat plate surface occurs earlier. Moreover, o-ring on the sphere diminishes peak magnitudes of the flow characteristics and thus it is expected that the flow-induced forces will be lessened both on the sphere and flat plate surface. Vortex formation lengths and maximum value occurring points become closer locations to the rear surface of the sphere with o-ring.
