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    Control of the flow past a sphere in a turbulent boundary layer using O-ring
    (Techno-Press, 2022) Okbaz, Abdulkeri; Ozgoren, Muammer; Canpolat, Cetin; Sahin, Besir; Akilli, Huseyin
    This research work presents an experimental study's outcomes to reveal the impact of an O-ring on the flow control over a sphere placed in a turbulent boundary layer. The investigation is performed quantitatively and qualitatively using particle image velocimetry (PIV) and dye visualization. The sphere model having a diamater of 42.5 mm is located in a turbulent boundary layer flow over a smooth plate for gap ratios of 0 <= G/D <= 1.5 at Reynolds number of 5 x 10(3). Flow characteristics, including patterns of instantaneous vorticity, streaklines, time-averaged streamlines, velocity vectors, velocity fluctuations, Reynolds stress correlations, and turbulence kinetic energy (< TKE >), are compared and discussed for a naked sphere and spheres having O-rings. The boundary layer velocity gradient and proximity of the sphere to the flat plate profoundly influence the flow dynamics. At proximity ratios of G/D=0.1 and 0.25, a wall jet is formed between lower side of the sphere and flat plate, and velocity fluctuations increase in regions close to the wall. At G/D=0.25, the jet flow also induces local flow separations on the flat plate. At higher proximity ratios, the velocity gradient of the boundary layer causes asymmetries in the mean flow characteristics and turbulence values in the wake region. It is observed that the O-ring with various placement angles (theta) on the sphere has a considerable alteration in the flow structure and turbulence statistics on the wake. At lower placement angles, where the O-ring is closer to the forward stagnation point of the sphere, the flow control performance of the O-ring is limited; however, its impact on the flow separation becomes pronounced as it is moved away from the forward stagnation point. At G/D=1.50 for O-ring diameters of 4.7 (2 mm) and 7 (3 mm) percent of the sphere diameter, the-ring exhibits remarkable flow control at theta=50 degrees and theta=55 degrees before laminar flow separation occurrence on the sphere surface, respectively. This conclusion is yielded from narrowed wakes and reductions in turbulence statistics compared to the naked sphere model. The O-ring with a diameter of 3 mm and placement angle of 50 degrees exhibits the most effective flow control. It decreases, in sequence, streamwise velocity fluctuations and length of wake recovery region by 45% and 40%, respectively, which can be evaluated as source of decrement in drag force.
  • Küçük Resim Yok
    Öğe
    Control of the flow past a sphere in a turbulent boundary layer using O-ring
    (Techno-Press, 2022) Okbaz, Abdulkeri; Ozgoren, Muammer; Canpolat, Cetin; Sahin, Besir; Akilli, Huseyin
    This research work presents an experimental study's outcomes to reveal the impact of an O-ring on the flow control over a sphere placed in a turbulent boundary layer. The investigation is performed quantitatively and qualitatively using particle image velocimetry (PIV) and dye visualization. The sphere model having a diamater of 42.5 mm is located in a turbulent boundary layer flow over a smooth plate for gap ratios of 0 <= G/D <= 1.5 at Reynolds number of 5 x 10(3). Flow characteristics, including patterns of instantaneous vorticity, streaklines, time-averaged streamlines, velocity vectors, velocity fluctuations, Reynolds stress correlations, and turbulence kinetic energy (< TKE >), are compared and discussed for a naked sphere and spheres having O-rings. The boundary layer velocity gradient and proximity of the sphere to the flat plate profoundly influence the flow dynamics. At proximity ratios of G/D=0.1 and 0.25, a wall jet is formed between lower side of the sphere and flat plate, and velocity fluctuations increase in regions close to the wall. At G/D=0.25, the jet flow also induces local flow separations on the flat plate. At higher proximity ratios, the velocity gradient of the boundary layer causes asymmetries in the mean flow characteristics and turbulence values in the wake region. It is observed that the O-ring with various placement angles (theta) on the sphere has a considerable alteration in the flow structure and turbulence statistics on the wake. At lower placement angles, where the O-ring is closer to the forward stagnation point of the sphere, the flow control performance of the O-ring is limited; however, its impact on the flow separation becomes pronounced as it is moved away from the forward stagnation point. At G/D=1.50 for O-ring diameters of 4.7 (2 mm) and 7 (3 mm) percent of the sphere diameter, the-ring exhibits remarkable flow control at theta=50 degrees and theta=55 degrees before laminar flow separation occurrence on the sphere surface, respectively. This conclusion is yielded from narrowed wakes and reductions in turbulence statistics compared to the naked sphere model. The O-ring with a diameter of 3 mm and placement angle of 50 degrees exhibits the most effective flow control. It decreases, in sequence, streamwise velocity fluctuations and length of wake recovery region by 45% and 40%, respectively, which can be evaluated as source of decrement in drag force.
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    Control of the flow past a sphere near a flat wall using passive jet
    (Pergamon-Elsevier Science Ltd, 2019) Okbaz, Abdulkerim; Ozgoren, Muammer; Dogan, Sercan; Canpolat, Cetin; Akilli, Huseyin; Sahin, Besir
    In the present investigation, interactions between the vortices shedding from a single sphere and the boundary layer flow over a flat plate with various gap ratios have been investigated at Re = 5000 using the techniques of dye visualization and Particle Image Velocimetry. A passive jet has been created to control the flow past the sphere by a hole drilled into the sphere from the forward stagnation point to the rear of the sphere with various diameters. The results show that the complex flow downstream of the sphere, which is occurred from the combination of 'Carman vortex street and the boundary layer over the flat plate can be controlled by a passive jet. Because the jet flow interrupts periodicity of the vortices shedding from the sphere by supplying fluid flow into the wake with relatively high velocity. Presence of the hole significantly attenuates the magnitudes of the turbulence characteristics. Furthermore, the magnitudes of the turbulence characteristics decrease with increasing the hole diameter depending on the location of the sphere in the boundary layer. However, a larger hole diameter can result in higher turbulence levels in the region of the passive jet. Hence, the effectiveness of the each case (the diameter of the hole) changes with the location of the sphere over the flat plate.
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    An overview of renewable electric power capacity and progress in new technologies in the world
    (Pergamon-Elsevier Science Ltd, 2015) Bilgili, Mehmet; Ozbek, Arif; Sahin, Besir; Kahraman, Ali
    In recent years, the atmosphere has been exposed to a substantial increase in concentration of the trace gases. In 2012, energy-related CO2 emissions reached a record of 31.6 gigatonnes which corresponds to 1.4% comparing to 2011. According to the International Energy Agency (IEA), an energy revolution is needed to achieve a 50% reduction of global CO2 emissions relative to current levels by 2050. In this revolution, energy efficiency, energy storage, new transport technologies, nuclear energy and CO2 capture and storage should play a crucial role. Furthermore, renewable energy technologies comprise a large potential for emission reductions which takes second place after energy efficiency improvements. Until 2035, renewable energy will also have greater impact than other potential alternatives including both CO2 capture and storage or nuclear energy. For this reason, renewable energy industries, markets, and policy frameworks are developed rapidly in recent years, and they will play a substantial role in the world's future. This paper presents an overview of renewable electric power capacity worldwide and discusses the current status, technologies, industry and recent developments of renewable energy sources (RESs) in detail. In addition, the top countries in terms of total installed renewable electric power capacity are also identified. (C) 2015 Elsevier Ltd. All rights reserved.