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Öğe An experimental investigation of the flow control of a circular cylinder in near wake with parallel plates at Re=7500(Elsevier Science Inc, 2024) Durhasan, Tahir; Ekinci, Firat; Firat, Erhan; Akilli, HuseyinThe vortex street suppression in the near wake region of the cylinder with parallel plates was investigated using particle image velocimetry (PIV) at Reynolds number of Re = 7500. The rigid plates are designed as flat, parallel with the free-stream direction, and located on both sides of the cylinder. Two different plate lengths (0.5D and 1D based on the cylinder diameter, D) were examined at gap ratios of g/D = 0.1, 0.3, 0.5, 0.7, and 0.9, respectively. Proper Orthogonal Decomposition (POD) analyses were also employed to reveal vortex mechanism in the near wake region. The gap ratio significantly influences the interaction of the vortices. At low gap ratios, the plates directly affect the development of shear layers on both side of the cylinder. The shear layers elongate downstream with a reduction of the vortex shedding frequency. Moreover, Reynolds shear stress, vortex shed-ding instabilities and vortex street are suppressed in the near wake region with the help of using parallel plates. On the other hand, vortex shedding reinforces instabilities at higher gap ratios. Besides, the most effective gap ratio is different for each the plate lengths to suppress instabilities.Öğe Analysis of attack angle effect on flow characteristics around torpedo-like geometry placed near the free-surface via CFD(2021) Kılavuz, Alpaslan; Ozgoren, Muammer; Durhasan, Tahir; Sahın, Besir; Kavurmacıoglu, Levent Ali; Akıllı, Huseyin; Sarıgıguzel, FuadIn this study, the flow characteristics of torpedo-like geometry placed near the free-surface at various angles of attack were investigated numerically. The study was carried out at the Reynolds number of Re=4x104 between immersion ratios of 0.75?h/D?3.5 and angles of attack ?=0°,4°, 8°, and 12°. Large Eddy Simulation (LES) turbulence model was used along with the Volume of Fluid (VOF) multiphase model to investigate the effects of free-surface. Wake region had an asymmetrical structure near the free-surface as a result of the interaction. A jet-like flow region was observed between the geometry and the free-surface at lower immersion ratios due to the restriction of the flow area. This flow region had a downward movement towards the lowerpressure wake region. The drag coefficient, CD, values were increased with the decrease of immersion ratio. At angles of attack ?=8° and 12°, the flow separation occurring near the nose caused an additional restriction in the flow area and directed the jet-like flow toward free-surface. Variation of Froude numbers (Fr) depending on the immersion ratio is examined, and it is found that Fr number and corresponding drag coefficient have higher values for the lower immersion ratio. The free-surface effect was found negligible at h/D?2.5 for all cases.Öğe Dye visualization over double delta wing with various kink angles(Springer, 2019) Durhasan, Tahir; Karasu, IlyasIn this study, the aim is to exhibit vortical behaviors of flow on double delta wings having 70 degrees strake sweep angle and kink angles of 150 degrees, 160 degrees and 165 degrees using dye visualization technique in a water channel. Experiments were performed at Reynolds numbers based on the chord length Re=10,000 and 25,000 for angle of attack in the range 5 degrees to 35 degrees. The visualizations were performed for both end-view and cross-flow planes. The results revealed that the kink angle has a significant role on the interaction of vortices and the strake vortex breakdown locations. The interaction between the strake vortex and the wing vortex is dominant on the flow behaviors at alpha <= 10 degrees. The flow behavior is affected by the kink angle. Two interaction mechanisms which are spiral and enveloping are observed. The spiral interaction alternates to enveloping interaction with increasing Reynolds number. Moreover, the trajectory of the strake vortex core moves outboard with increasing Reynolds number at alpha=10 degrees. For alpha >= 15 degrees, Reynolds number is less effective on the strake vortex breakdown location and also the vortex breakdown locations move the apex gradually with increasing angle of attack. Wake-alike flow structure takes place after occurrence of the vortex breakdown since vortex core splits into disorganized small-scale vortices. On the other hand, development of the wing vortex is more complex than the strake vortex since it collapses near the vicinity of the junction.Öğe Experimental study on passive flow control of circular cylinder via perforated splitter plate(Techno-Press, 2021) Sahin, Serdar; Durhasan, Tahir; Pinar, Engin; Akilli, HuseyinPresent experimental investigation aims to reduce the shedding of vortex in the near wake region of a circular cylinder using a perforated splitter plate. Perforated plates were placed in the wake region of the cylinder and aligned with the streamwise direction. The length of the plates was equal to the diameter of the cylinder. Different plate porosities and locations were examined and obtained results were compared to the baseline cylinder. Flow measurements downstream of the cylinder were performed in a water channel by employing a particle image velocimetry technique (PIV) at a Reynolds number of Re=5×103. It is observed that the effect of the porosity on the flow characteristics of the cylinder depends on the location of the plate. The strength of shear layers and flow fluctuations in the near wake region of the cylinder are considerably diminished by the perforated splitter plate. It is found that the porosity of ?=0.3 is the most effective control element for gap ratio of G/D=0.5. On the other hand, proper gap ratio is determined as G/D=2 for porosity of ?=0.7. It is concluded in the present study that the perforated splitter plate could be used as alternative passive flow control technique in order to reduce vortex shedding of the cylinder. Copyright © 2021 Techno-Press, Ltd.Öğe Flow Characteristics over Double Delta Wings at Low Reynolds Numbers(Asce-Amer Soc Civil Engineers, 2020) Karasu, Ilyas; Durhasan, TahirWithin the scope of this study, a quantitative investigation was carried out employing the particle image velocimetry (PIV) technique to determine vortical flow characteristics of 70 degrees/40 degrees, 70 degrees/50 degrees, and 70 degrees/55 degrees double delta wings (DDWs). Experiments were performed in four chordwise sections (x/c=0.5, 0.6, 0.8, and 1) at two Reynolds numbers (R=1x104 and 2.5x104) and different angles of attack (alpha=5 degrees, 10 degrees, 15 degrees, and 25 degrees). It was observed that the secondary vortex is dominant on flow characteristics of double delta wings at low Reynolds numbers. The strengths of vortices increase with increases of both the wing sweep angle and Reynolds number. Furthermore, the location of wing vortex breakdown moves farther downstream as the wing sweep angle increased. Depending on the angle of attack, Reynolds number, and the wing sweep angle, either dual (strake and wing) or triple (strake, wing, and secondary) leading-edge vortices were observed. At a Reynolds number value R=1x104 and angles of attack in the range of alpha <= 15 degrees, the interactions of the vortices influence the flow structure significantly. Dominant frequencies of the vortices are detected near the shear layer formed between the strake vortex and secondary vortex, generally. Moreover, the spectral analyses indicated that bursting of the vortices causes complicated flow patterns over double delta wings, including small-scale vortices. It was found that the transverse Reynolds normal stress concentrates at chordwise sections where secondary vortices are dominant, whereas the locations of peak magnitude of vertical Reynolds normal stress coincide with the strake vortex cores.Öğe FLOW CONTROL OF A CIRCULAR CYLINDER BY PERMEABLE SPLITTER PLATE WITH DIFFERENT POROSITIES AND ANGLE VALUES(Turkish Soc Thermal Sciences Technology, 2024) Sahin, Serdar; Durhasan, Tahir; Pinar, Engin; Akilli, HuseyinFlow control of bluff bodies has been studied extensively to eliminate adverse effects of wake flow such as vibration and acoustic noise or resonance. The circular cylinder has been studied as the bluff body since it is basic geometry and has been used in engineering applications such as heat exchanger tubes, power transmission lines, chimney stacks, bridges, radio telescopes, power lines, offshore drilling rigs etc. In this study, a permeable splitter plate was located at various downstream locations to control the wake flow of the cylinder. All experiments were carried out in a large-scale closed-loop water channel in the Fluid Mechanics Laboratory at Cukurova University. PIV was used to measure the instantaneous velocity vector field in the wake region of the cylinder at Reynolds number Re=5000, which is based on the cylinder diameter, D. Four different splitter plate angle values (0 =0 degrees; 15 degrees; 30 degrees; 45 degrees), three different porosity values (epsilon=0.30; 0.50; 0.70) were investigated. The porosity (epsilon) of the separator plates is defined as the ratio of the total hole area to the plate surface area. All lengths are nondimensionalized by dividing by the cylinder diameter and shown with the * index. The splitter plate length kept to constant during the experiment as ls*=1. The distance between the leading edge of the splitter plate and the cylinder (lg*) is variable due to the rotation of the separator plate at certain angles in the flow direction. To overcome this, the distance between the splitter plate rotation axis and the cylinder was taken as a parameter and shown with the **. The gap between splitter plate midpoint and cylinder (lg**) kept to constant during the experiments as lg**=1.5. When the plates are rotated, the cross-section parallel to the flow decreases, which increases the interaction between the boundary layers. Since the permeable separator plates prevent the interaction of the boundary layers formed in the flow around the cylinder, the effect of the permeable separator plates increases in the downstream regions where the interaction of the boundary layers increases. Thus, the fluctuations are reduced, and a more stabilized trail flow occurs downstream of the cylinder. It was observed that the vortex formation was delayed with the increase of the separator plate angle. In this study, the effect of the separator plate angle and the effect of the plate permeability were clearly observed.Öğe Flow topology downstream of the hollow square cylinder with slots(Pergamon-Elsevier Science Ltd, 2020) Durhasan, TahirFlow patterns downstream of the hollow square cylinder with slots are experimentally investigated to show effect of the slot ratio on the wake flow characteristics and turbulence statistics at Reynolds number of Re = 6000 using the particle image velocimetry (PIV) technique. Proper orthogonal decomposition (POD) analyses indicate that the self-issuing jet dramatically changes the mechanism of vortex shedding interaction. Penetration of the self-issuing jet into the wake region leads to increasing of the vortex formation length for low slot ratios. On the other hand, the self-issuing jet penetrates further downstream due to increasing momentum coefficient of the jet and splits the wake region in half for high slot ratios. Strouhal number increases with the increasing slot ratio and it attains approximately the twofold of the solid cylinder case, since the self-issuing jet accelerates the vortex shedding process. However, turbulence statistics are significantly reduced for all slot ratios. Distributions of the spectral density in the wake region of the cylinder indicate that the vortex shedding instability could be reduced considerably by the slotted cylinder. Drag coefficient, which is estimated from PIV data, reduces up to 42% in comparison with the solid cylinder model.Öğe Kavisli Delikli Plaka ile Girdap Kaynaklı Türbülanslı Akışın Kontrolü(2020) Özkan, Göktürk Memduh; Durhasan, Tahir; Fidancı, Pınar EnginBu çalışmada, dairesel bir silindir art izinde oluşan girdap kaynaklı türbülanslı akışın, farklı ve kısmi geçirgenlik oranlarına sahip kavisli delikli plaka (kontrol elemanı) ile kontrolü incelenmiştir. Ele alınan parametre aralıklarında PIV ile hız ölçümleri yapılmış, akış alanında Reynolds kayma gerilmeleri hesaplanmış ve kontrol durumu ile yalın silindir durumunun kontur dağılımları nicel olarak karşılaştırılmıştır. Elde edilen sonuçlara göre, ele alınan Reynolds sayılarının akış kontrolüne benzer oranda etki ettiği anlaşılmıştır. Bununla birlikte, incelenen tüm geçirgenlik oranları ve yay açıları için silindir art izinde oluşan kayma gerilmelerinin azaldığı görülmüştür. 120o? ? ?180o yay açılarında ve ?=0,5 ve ? =0,6 geçirgenlik oranlarında, yalın silindire kıyasla maksimum Reynolds kayma gerilmelerinin %96’ya kadar düştüğü ve silindir art izindeki daimi olmayan akışın tamamen kontrol edildiği ortaya çıkarılmıştır.Öğe Numerical Investigation of Flow Structure around NACA 0018 with slot(2023) Asan, Omer Fethi; Guler, Emre; Aksoy, Muhammed Murat; Pınar, Engin; Durhasan, TahirWe numerically investigated NACA 0018 airfoil with slot at various angles of attack. The computational fluid dynamics (CFD) results were compared with the base airfoil to show the effectiveness of the airfoil. Reynolds number was kept constant as Re=3x105 to thoroughly understand how different locations of slot would provide passive flow control from aerodynamic perspective. We have used three different slot locations to reveal the effectiveness of the slot compared to the base airfoil for higher angles of attack. Our results show that the slotted design would yield to increase the lift and to delay the stall angle of base airfoil.Öğe Passive Flow Control around NACA 0018 Airfoil Using Riblet at Low Reynolds Number(2021) Güler, Emre; Durhasan, Tahir; Karasu, İlyas; Akbıyık, HürremIn this study, aerodynamic capabilities of NACA 0018 airfoil is numerically investigatedby installing riblet on the suction side of airfoil. Numerical results were obtained by ANSYS Fluentusing k-kl-kw transition model at Reynolds number of Re=100 000. Three different riblet airfoilconfiguration was performed at six different angles of attack (?=8°, 10°, 13°, 15°, 17° and 19°) andthese results compared with the clean model. For M1 model the riblet was located at chord wise sectionof x/c=0.3 while it installed at x/c=0.7 for M2 model. For M3 model two riblets were used and theywere located at both x/c=0.3 and x/c=0.7. Obtained numerical result show that the use of ribletremarkably affects the flow characteristics of airfoil. At ?=8° the CL/CD value of M1 model is increasedby 4.5% when compared to clean model. It is indicated that angle of attack at ?=10o , lift coefficient isincreased for all models with compared to clean model. Stall angle is delayed from ?=13° to ?=15° atM1 and M3 with compared to clean model and lift coefficient is increased about 37% because of therestriction of the laminar separation bubble and trailing edge separation.Öğe POD Analysis of Flow around Torpedo-Like Geometry with a Hemispherical Nose(2023) Kenan, Ömer; Yanıktepe, Bülent; Sekeroglu, Ertugrul; Ozgoren, Muammer; Akbudak, Ezgi; Durhasan, Tahir; Kılavuz, AlpaslanThere has been an increased interest in underwater vehicles for a wide range of applications over the past decade. In the current study, flow characteristics around a common, torpedo-like geometry with a Myring profile were investigated at a length-based Reynolds number of Re=20000 using the Particle Image Velocimetry (PIV). As a result of PIV and Proper Orthogonal Decomposition (POD) analyzes in this study; instantaneous streamline topology ?, instantaneous vorticity ?L?U_? contours, instantaneous streamwise velocity component u?U_? graphs, cross-streamwise velocity component v?U_? graphs, instantaneous vorticity ?L?U_? graphs, perpendicular to the flow direction, instantaneous vortex graphs and time-averaged TKE graphs were evaluated and compared. At the same time, as a result of PIV analysis, time-average streamline topology <?> and Time-averaged vorticity <?L?U_? > contours values are given. When the flow characteristics were compared by using the obtained POD method of the PIV results, it was determined that although the POD data were generally quite similar to the PIV results, the POD analysis results had a more uniform flow structure and vortex turbulence was lessened.Öğe Silindir Art İzindeki Daimi Olmayan Girdapların Kılavuz Plakalar Yardımı ile Bastırılması(2023) Ekinci, Fırat; Durhasan, Tahir; Akıllı, HüseyinDairesel kesitli silindirin art izindeki daimi olmayan girdapların kılavuz plakalar kullanılarak bastırılması deneysel olarak araştırılmıştır. Kılavuz plakalar yay şeklinde imal edilmiş ve üç farklı açıklık oranı (G/D=0,1, 0,3 ve 0,5) ve plaka açı değerleri 60°???90° aralığında incelenmiştir. Silindir art izindeki akış yapıları PIV ölçümü ile Reynolds sayısının 7500 değerinde elde edilmiştir. Akış ölçümünden elde edilen Reynolds kayma gerilmeleri, girdap konturları, hız bileşenleri ve girdap kopması frekans sonuçları ile yalın silindir durumu ile kıyaslanarak açıklanmıştır. Düşük plaka açı değerlerinde girdap kopma frekansları üzerinde daha etkin olduğu yüksek plaka açı değerlerinin ise girdap sönümlemede daha etkin olduğu gözlemlenmiştir. Reynolds kayma gerilmelerinin bastırılmasında açılık oranı G/D=0,1’in diğer açıklık oranları ile kıyaslandığında daha etkili olduğu gözlemlenmiştir.Öğe The Effect of Riblets on the Aerodynamic Performance of NACA 0018 Airfoil(2024) Güler, Emre; Pınar, Engin; Durhasan, TahirIn this numerical study, riblets on the airfoil were utilized to enhance the aerodynamic performance of NACA0018 airfoil. Riblets of identical height and base length are strategically placed on the suction surface of the airfoil with varying spacing ratios along the flow direction (x) and chord length (c), specifically x/c = 0.3 and 0.7. Four distinct riblet airfoil models are subjected to computational fluid dynamics (CFD) analysis within an angle of attack range from 0° to 21° at a Reynolds number of Re=1×105. The obtained results are systematically compared with the performance of the plain airfoil. Numerical analyses reveal the significant influence of the spacing ratio on flow control and the overall aerodynamic performance of the airfoil, establishing a direct relationship with riblet spacing. The presence of riblet structures is observed to increase the lift coefficient, concurrently delaying the stall angle up to 19°. Notably, the ribbed structures effectively mitigate the interaction between the laminar separation bubble and trailing edge separation, leading to a reduction in turbulent kinetic energy values.Öğe Time series analysis of sea surface temperature change in the coastal seas of Turkiye(Pergamon-Elsevier Science Ltd, 2024) Bilgili, Mehmet; Durhasan, Tahir; Pinar, EnginSea surface temperature (SST) is a crucial geophysical parameter in assessing heat exchange between the air and sea surface. Changes in SST and its accurate prediction play a pivotal role in explaining the global heat balance, determining atmospheric circulations, and constructing global climate models. This work aims to reveal a model for one-month-ahead forecasting of SST time series data along the Turkiye coasts, encompassing the Mediterranean, Aegean, Marmara, and Black Seas, and their long-term future forecast. A long short-term memory (LSTM) neural network and seasonal autoregressive integrated moving average (SARIMA) models are used for this purpose. The ECMWF ERA5 (0.5(o)x0.5 degrees) monthly SST dataset spanning the years 1970-2023 is used for model development. The results obtained from the LSTM and SARIMA models show that there will be an increasing trend in SSTs along these seacoasts until 2050. The SST measurements of 23.4 degrees C, 20.2 degrees C, 17.0 degrees C, and 16.6 degrees C recorded along the Mediterranean, Aegean, Marmara, and Black Seas in 2023 are expected to rise to 25.1 degrees C, 21.9 degrees C, 18.1 degrees C, and 18.8 degrees C, respectively, by 2050. These figures indicate an increase of 7.3%, 8.4%, 6.5%, and 13.3% in the SST values across these coastal seas over the next quarter century.
