Browsing by Author "Kirkil, Gökhan"

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Citation Count: 0
A Comparative Study of Energy Models for Turkish Electricity Market Using LEAP
(IEEE Computer Society, 2019) Kirkil, Gökhan; Kirkil, Gökhan; Çelebi, Emre
Fossil fuel thermal power plants constitute a large part of the Turkish electricity generation capacity. Turkish government has been developing several energy policy documents to evaluate how various renewable energy sources of the country can be utilized optimally in the generation of electricity within the next 30 years. This study considers three scenarios in the transition to renewable energy for Turkey; the business as usual (BAU), energy conservation (EC) and renewable energy (REN) scenarios. EC scenario considers the use of energy-efficient appliances and imposing a carbon tax, whereas REN scenario considers increasing the share of the renewable energy sources as much as possible in the power generation mix. These scenarios were evaluated in terms of cost and environmental impact. The LEAP (Long-range Energy Alternatives Planning Model) was used in the research. The REN scenario has been shown to be the optimal energy policy option for Turkey in terms of cost and environmental impact.
Decarbonization pathways for Turkish power system using the leap model Leap modeli kullanılarak Türkiye elektrik sistemi için dekarbonizasyon yolları
(Kadir Has Üniversitesi, 2021) Yücekaya, Ahmet Deniz; Kirkil, Gökhan; Yücekaya, Ahmet Deniz
The negative impact of GHG released into the atmosphere on global warming cannot be ignored. Fossil-fueled power plants constitute a large part of Turkey's electricity production, as every country has a growing economy. Therefore, the electricity generation sector accounts for a significant portion of GHG emissions in Turkey. In addition to national bindings such as the Paris Agreement and the Kyoto Protocol, it is known that the Republic of Turkey aims to make not only electricity but also energy production greener in the coming years, in line with its own efforts. For this purpose, there are different modeling studies in the literature. This thesis aims to model Turkey's electricity generation sector in 2017, reveal the current situation, and then analyze how a greener and sustainable energy transformation will be possible with different scenarios and different main factors. In this direction, Turkey's electricity generation sector was modeled using the LEAP tool, then the decarbonization scenarios created within the openENTRANCE project were adapted to Turkey's data, and the numerical results of the scenarios were compared. As a result, it has been revealed that social awareness, adaptation to new technologies, and incentives of decision-makers are all critical factors in this regard.
Decarbonization potentials in the Turkish energy intensive industries
(Kadir Has Üniversitesi, 2021) Kirkil, Gökhan; Kirkil, Gökhan
Biyoçeşitlilik ve iklim krizi ile mücadelede uluslararası kuruluşların önderliğinde küresel ısınmayı minimumda tutmak üzere bir dizi önlem alınmaktadır. Önlemlerden biri olan sera gazı emisyonlarının azaltılması için öncelikle enerji sektöründe yoğun karbonlu kaynaklardan düşük karbonlulara geçiş sağlanmaktadır. Lakin sadece enerji geçişinin emisyonları azaltmak için yeterli olmadığı bilinmektedir. Bu nedenle küresel emisyon salınımında enerjiden sonra ikinci sırada gelen sanayi sektöründe karbonsuzlaştırma yöntemlerine ağırlık verilmektedir. Çoğu gelişmiş ülkenin çalışmalarında Enerji Yoğun Endüstri olarak sınıflandırılan, üretim sürecinde yoğun enerji harcayan ve yoğun emisyona sebep olan alt sektörlere özel karbonsuzlaştırma önerileri kapsamlı bir şekilde ele alınmaktadır. Türkiye Sanayisi ise teknolojik araştırma ve geliştirmeleri enerji verimliliğine odaklanarak gerçekleştirmektedir. Karbonsuzlaştırma seçenekleri enerji verimliliğini de içine alan geniş bir yelpaze sunduğu için Türk Sanayisi tarafından da benimsenmelidir. Bu çalışma Türkiye'de üretim yapan enerji yoğun endüstrilerde karbonsuzlaştırma potansiyelini sunmayı amaçlamıştır. Çalışmada öncelikle karbonsuzlaştırma kavramı ve iklim krizi ile mücadalede ön saflarda yer alan uluslarası kuruluşların karbonsuzlaştırmaya bakış açısı incelenmiştir. Ardından küresel çapta uygulanan karbonsuzlaştırma seçenekleri araştırılıp enerji yoğun endüstrilere özel çözümler saptanmıştır. Daha sonra Türkiye'de enerji yoğun endüstriler alt sektör bazında incelenmiş ve uygun olan karbonsuzlaştırma önerileri sunulmuştur. Bu önerilerin hayata geçmesi için hükümetin, kuruluş ve sanayicilerin iklim krizi ile mücadelede küresel hedeflere uygun, ortak bir yaklaşım benimsemesi gerekmektedir. Ancak bu sayede sanayide karbonsuzlaştırma seçeneklerinin değerlendirilmesi mümkün olabilecektir.
Citation Count: 3
Detached eddy simulation of shallow mixing layer development between parallel streams
(Elsevier Science Bv, 2015) Kirkil, Gökhan
Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a smooth flat bed and medium size dunes. The study discusses the vertical non-uniformity in the mixing layer structure and provides a quantitative characterization of the growth of the large-scale quasi two-dimensional (2D) coherent structures with the distance from the splitter plate. Results show that in streamwise sections situated between 75D (D is the channel depth) and 1501) from the splitter plate the width of the mixing layer close to the free surface is 20-30% more than the width in the near-bed region. This is mostly because of the tilting of the mixing layer interface on the low-speed side toward the low speed stream as the free surface is approached. Power spectra of the horizontal velocity components near the free surface show the presence of a -3 subrange corresponding to inverse energy cascade in two-dimensional turbulence at streamwise locations situated more than 10D from the splitter plate consistent with the presence of large-scale quasi 2D horizontal eddies and the transfer of energy (inverse energy cascade) from the smaller scales toward these eddies. Consistent with visualizations of the mass transport of a passive scalar within the mixing layer close to the free surface the estimated streamwise length of the quasi 2D mixing layer eddies is about 2.5-3.0 times larger than the local width of the mixing layer. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a much more rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness. (C) 2014 International Association for Hydro-environment Engineering and Research Asia Pacific Division. Published by Elsevier B.V. All rights reserved.
Citation Count: 0
Development of a parallel 3D navier–stokes solver for sediment transport calculations in channels
(MDPI AG, 2020) Kirkil, Gökhan
We propose a method to parallelize a 3D incompressible Navier–Stokes solver that uses a fully implicit fractional-step method to simulate sediment transport in prismatic channels. The governing equations are transformed into generalized curvilinear coordinates on a non-staggered grid. To develop a parallel version of the code that can run on various platforms, in particular on PC clusters, it was decided to parallelize the code using Message Passing Interface (MPI) which is one of the most flexible parallel programming libraries. Code parallelization is accomplished by “message passing” whereby the computer explicitly uses library calls to accomplish communication between the individual processors of the machine (e.g., PC cluster). As a part of the parallelization effort, besides the Navier–Stokes solver, the deformable bed module used in simulations with loose beds are also parallelized. The flow, sediment transport, and bathymetry at equilibrium conditions were computed with the parallel and serial versions of the code for the case of a 140-degree curved channel bend of rectangular section. The parallel simulation conducted on eight processors gives exactly the same results as the serial solver. The parallel version of the solver showed good scalability.
Citation Count: 80
Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder
(Amer Inst Physics, 2015) Kirkil, Gökhan; Constantinescu, George
The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder's base and the variation of the mean flow and turbulence statistics in the near wake in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers Re-D the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (Re-D = 16 000 subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (Re-D = 5 * 10(5) supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed which is a problem of great practical relevance (e.g. for pier scour studies). (C) 2015 AIP Publishing LLC.
Electricty demand and supply scenario analysis for nigeria using long range energy alternatives planning
(Kadir Has Üniversitesi, 2017) Kirkil, Gökhan; Kirkil, Gökhan
Electricity demand and supply forecasts are very important tools for determining solutions to the problems in the electricity sector such as power outages. The Long Range Energy Alternatives Planning (LEAP) energy model was used for this study because of its low data requirements. The forecast for electricity demand and supply for Nigeria was carried out from 2010-2040. Three scenarios were generated which include Business as Usual (BAU) Energy Conservation (EC) and Renewable Energy (REN). The three scenarios were analyzed based on their electricity demand and supplies environmental impact and costs. The BAU scenario assumed that trends in the future will follow past trends. The EC scenario was generated based on efficient usage of electricity and reduction of transmission and distribution losses. in the EC scenario efficient electrical appliances will phase out the non-efficient ones which reduced the electricity demand significantly. On the other hand the REN scenario is based on the concept of harnessing renewable energy resources such as wind solar hydro and biomass for electricity generation. Results of the cost analysis of the three scenarios shows that the most competitive scenario in terms of cost is the EC scenario which has the least capital cost (44.2 billion USD less than the BAU scenario) and fixed costs (15 billion USD less than the BAU scenario) of the three scenarios but has the second largest quantity of Green House Gas (GHG) emissions of 1004.8 million tons of CO2eq. The REN scenario has the least GHG emissions among the three scenarios at 114.79 million tons of CO2eq but is the most expensive scenario to implement because of its high capital (56.3 billion USD more the BAU) and fixed costs (4.1 billion USD more than the BAU scenario). The EC scenario has 28.96 % less carbon emissions than the BAU scenario (1414.5 million tons of CO2eq) and has the least cost among the three scenarios. As a result of the current economic challenge faced by Nigeria and a growing electricity demand the EC scenario is the most realistic and suitable scenario to be implemented among the scenarios that were generated.
Energy transition scenario analysis for Turkey using long range energy alternatives planning (LEAP)
(Kadir Has Üniversitesi, 2019) Kirkil, Gökhan; Kirkil, Gökhan
Fossil fuel thermal power plants constitute a large part of the Turkish electricity generation capacity. The Turkish government has been developing several energy policy documents to evaluate how various renewable energy sources of the country can be utilized optimally in the generation of electricity for the next 30 years. The study considers three scenarios in the transition to renewable energy for Turkey; the business as usual (BAU), energy conservation (EE) and renewable energy (REN) scenarios were modeled with the help of the LEAP (Long-range Energy Alternatives Planning) software. EE scenario considers the use of energy-efficient appliances across all sectors of demand while emphasizing on more efficiency in electricity production activities, whereas REN scenario considers increasing the share of the renewable energy sources as much as possible in the power generation mix. These scenarios were evaluated in terms of cost and environmental impact. The optimized energy efficiency scenario has been shown to be the optimal energy policy option for Turkey in terms of cost and environmental impact. Keywords: renewable energy, energy transition, energy efficiency, LEAP, scenario analysis
Enerji kullanımında Avrupa Yeşil Mutabakatı'nın karbonsuzlaşma hedeflerinin incelenmesi
(2023) Ediger, Şevket Volkan; Ediger, Şevket Volkan; Kirkil, Gökhan
İklim değişikliğinin nedeni insanların fosil yakıtları kullanarak atmosferdeki sera gazlarını arttırması ve Dünya'nın enerji dengesini bozmasıdır. Çözüm ise zor ve dikkatli bir karbonsuzlaşma sürecini gerektirmektedir. Sera gazı salımının azaltılması, yenilenebilir enerji kaynaklarının kullanımının arttırılması ve enerji verimliliğinin arttırılması en çok kabul gören karbonsuzlaşma hedefleridir. AB, 2019 yılında Avrupa Yeşil Mutabakatı adında iddialı ve kapsamlı bir karbonsuzlaşma politikası açıklayarak 2050 yılında ilk karbonsuzlaşmış kıta olacaklarını duyurdu. Fosil yakıt kaynaklarından yenilenebilir enerji kaynaklarına doğru enerji geçişini sağlamaya yönelik birçok strateji, hedef, ekonomik mekanizmalar oluşturuldu. AB'nin 2050 yılında tamamen karbonsuzlaşmış bir kıta olabilmesi için yaklaşık son üç yüzyıldır yüksek oranlarda fosil bağımlısı olmuş enerji sektörlerinde enerji geçişini sağlaması en zor olan kısımdır. Bu enerji geçişi ve karbondan arındırma süreci için en zorlayıcı sorulardan biri: 'Enerji kullanımında Avrupa Yeşil Mutabakatı'nın karbonsuzlaşma hedeflerine ulaşılması mümkün mü?'. Bu soruyu cevaplamak için, AB'nin kurucu antlaşmaları, direktifleri, enerji stratejileri ve 2020 hedefleri doğrultusunda 2030 ve 2050 hedefleri analiz edildi. Literatürde yapılan araştırmalar AB'nin ve AB ülkelerinin hedeflerini, politikalarını birçok farklı metotla incelemiş olmalarına rağmen en çok enerji tüketime sahip olan Almanya, Fransa, İtalya ve İspanya bir arada incelenmemiştir. Bu amaçla, bu tez en çok enerji tüketimine sahip dört AB ülkesinin stratejilerini ve hedeflerini ile bu hedeflere ulaşma başarılarını analiz edip karşılaştırmalı bir şekilde değerlendirmiştir. Çalışmanın sonunda enerji dönüşümünde ve karbonsuzlaşmada AB'nin ve seçilen dört ülkenin 2030 hedeflerinden yenilenebilir enerji ve sera gazı azaltım hedeflerine ulaşabileceğinden ama enerji verimliliği hedeflerine ulaşamayacağın ve benzer bir sonucun 2050 hedefleri için de gerçekleşeceği ileri sürülmektedir.
Citation Count: 0
Exploring non-linear relationships between perceived interactivity or interface design and acceptance of collaborative web-based learning
(Springer, 2023) Kirkil, Gökhan; Kirkil, Gokhan
The novelty of this study is in developing a conceptual model for predicting the non-linear relationships between human-computer interaction factors and ease of use and usefulness of collaborative web-based learning or e-learning. Ten models (logarithmic, inverse, quadratic, cubic, compound, power, s-curve, growth, exponential, and logistic) were examined as functions of effects compared to linear relationships to see which was the most appropriate, based on R-2, adjusted R-2 and SEE values. To answer the addressed questions, the researcher surveyed 103 students from Kadir Has University about the perceived interface and interactivity of e-learning. The results show that most of the hypotheses formulated for this purpose have been proven. Our analysis shows that cubic models (the relationship between ease of use and usefulness, visual design, course environment, learner-interface interactivity, and course evaluation system and ease of use), quadratic models (the relationship between visual design, and system quality and usefulness, course structure and content, course environment, and system quality and ease of use), logarithmic model (the relationship between course evaluation system and usefulness), and s-curve models (learner-interface interactivity, navigation, and course structure and content and usefulness) performed better in the description for the correlations.
Citation Count: 0
Flow and Turbulence Structure Around an In-Stream Rectangular Cylinder with Scour Hole
(TSINGHUA UNIV, 2013) Kirkil, Gökhan; Constantinescu, George
An eddy resolving technique is used to reveal the unsteady dynamics of the coherent structures present in the flow field around an in-stream vertical cylinder with a scour hole at a channel Reynolds number of 240000. Such an investigation is important as most of the erosion around obstacles present in alluvial streams takes place after a scour hole of sufficiently large dimensions to stabilize the large-scale oscillations of the horseshoe vortex (HV) system has formed. The cylinder has a rectangular section and is placed perpendicular to the incoming flow. The geometry of the scour hole is obtained from an experiment. The mechanisms driving the bed erosion during the advanced stages of the scour process around the vertical plate are discussed. Simulation results demonstrate the critical role played by these large-scale turbulent eddies and their interactions in driving the local scour. Results show that important changes in the structure of the wake (e. g. the wake loses its undular shape due to suppression of the anti-symmetrical shedding of the roller vortices) and the nature of the interactions between the necklaces vortices of the HV system and the eddies present inside the detached shear layers (DSLs) occurs as the scour process proceeds. This means that information on the vortical structure of the flow at the initiation of the scour process or during its initial stages are insufficient to understand the local scour mechanisms.
Citation Count: 0
Flow structure in a mixing layer developing over flat bed at high reynolds numbers
(CRC Press/Balkema, 2016) Kirkil, Gökhan
Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a flat bed at a high Reynolds number (ReD = 160,000). The influence of Reynolds number on the development of the mixing layer as well as the vertical non-uniformity in the mixing layer structure is discussed. The numerical results show that as Reynolds number increases, the mixing layer grows rapidly but also stabilizes more quickly. The shift of the centerline of the mixing layer is toward the low-speed side is less for high Reynolds number simulation than low Reynolds number simulation. Mixing layer growth rate and its change in the vertical direction are compared with experiments and a simulation at ReD = 16,000. Passive scalar is introduced at the tip of the splitter plate close to the free surface to estimate the size of the mixing structures based on mass transport. © 2016 Taylor & Francis Group, London.
Citation Count: 0
Flow Structure in a Shallow Mixing Layer Developing over 2-D Dunes
(E D P Sciences, 2018) Kirkil, Gökhan
A high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel over two-dimensional (2D) dunes. The study discusses the vertical non-uniformity in the mixing layer and provides a quantitative characterization of the growth of the large-scale quasi 2D coherent structures with the distance from the splitter plate. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a very rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness. Results show that in streamwise sections situated after 100D (D is the channel depth) from the splitter plate, the width of the mixing layer close to the free surface stays constant. The tilting of the mixing layer interface toward the low speed stream is observed as the free surface is approached in all vertical sections.
Citation Count: 42
A framework for water and carbon footprint analysis of national electricity production scenarios
(Pergamon-Elsevier Science Ltd, 2017) Kirkil, Gökhan; Küçükvar, Murat; Onat, Nuri Cihat; Kirkil, Gökhan
While carbon footprint reduction potential and energy security aspects of renewable and non-renewable resources are widely considered in energy policy their effects on water resources are mostly overlooked. This research aims to develop a framework for water and carbon footprint analysis to estimate the current and future trends of water consumption and withdrawal by electricity production sectors for national energy development plans - alongside carbon emissions from various electricity sources. With this motivation the Turkish electric power industry is selected as a case study and a decision support tool is developed to determine the water consumption withdrawal and carbon emissions from energy mixes under three different scenarios namely Business-As-Usual (BAU) Official Governmental Plan (OGP) and Renewable Energy-Focused Development Plan (REFDP). The results indicate that water is used substantially even by renewable resources such as hydroelectricity and biomass which are generally considered to be more environmental friendly than other energy sources. The average water consumption of the OGP energy mix in 2030 is estimated to be about 8.1% and 9.6% less than that of the BAU and REFDP scenarios respectively. On the other hand it is found that the water withdrawal of the energy mix in 2030 under the REFDP scenario is about 46.3% and 16.9% less than that of BAU and OGP scenarios. Carbon emissions from BAU are projected to be 24% higher than OGP and 39% higher than REFDP in 2030. Carbon emissions and water usage are strongly correlated in BAU scenario as compared with OGP and REFDP thus carbon friendly energy sources will result in fewer water consumptions and withdrawals particularly under REFDP. (C) 2017 Elsevier Ltd. All rights reserved.
Citation Count: 0
GENeSYS-MOD Turkey: Quantitative Scenarios for Low Carbon Futures of the Turkish Energy System
(IEEE Computer Society, 2022) Kirkil, Gökhan; Celebi, E.; Yucekaya, A.D.; Kirkil, G.
This paper examines the quantitative scenarios for low-carbon futures of the Turkish energy system at aggregated (country level) and regionally disaggregated (NUTS-1 level) levels. We have employed four different storylines for the future European energy system. They are quantified and implemented for the European energy system (30 regions, mostly single countries, including Turkey) using the open-source global energy system model, GENeSYS-MOD v3.0. We have compared the results of all scenarios at aggregated and disaggregated levels and found that there are significant differences among them. Specifically, the hydrogen production (and its use) has increased considerably in the disaggregated model when compared to the aggregated level results. The major reason for these differences is found to be the better estimation of regional renewable capacity factors (wind and solar) in the disaggregated level compared to aggregated level. © 2022 IEEE.
Importance of LNG in Turkish natural gas supply security
(Kadir Has Üniversitesi, 2019) Kirkil, Gökhan; Ediger, Volkan S.; Kirkil, Gökhan
The Natural Gas has been increasing its share of usage day by day due to the technologic developments, low carbon emission, increasing natural gas reserve in the fossil-based energy market. Nowadays, the share of natural gas in the total energy mix is 24%. Hence, the natural gas has been seen as a transaction source because the countries are in the aim of zero carbon emission. LNG is the most increasing product in the natural gas market these days due to shale gas production areas are far away to consumption areas according to this LNG usage has increased. LNG has been increased usage areas of natural gas and secure countries natural gas supply with regard to flexible system integration, easy transportation, and small scale cargos like oil products. Furthermore, LNG is an essential value for Turkish natural gas market. In this study, the importance of LNG in Turkish natural gas supply security has examined. First of all, world natural gas and LNG market have analyzed, and developments and changes on the market have reviewed. Following to this, the Turkish natural gas market has been analyzed and business opportunities which LNG may make and advantages to the market have searched. In the mathematical model, future natural gas consumption, a share of LNG and pipe gas has been forecasted with three different scenarios. According to these forecasts, the determination has made with regard to the storage investments and what has to be done. Finally, what LNG is going to gain to the Turkish market and the importance of LNG in the Turkish natural gas supply is underlined. Keywords: LNG, Natural Gas, Supply Security, importance of LNG
Citation Count: 4
Large Eddy Simulation of Wind Flow over A Realistic Urban Area
(MDPI, 2020) Kirkil, Gökhan; Lin, Ching-Long
A high-resolution large eddy simulation (LES) of wind flow over the Oklahoma City downtown area was performed to explain the effect of the building height on wind flow over the city. Wind flow over cities is vital for pedestrian and traffic comfort as well as urban heat effects. The average southerly wind speed of eight meters per second was used in the inflow section. It was found that heights and distribution of the buildings have the greatest impact on the wind flow patterns. The complexity of the flow field mainly depended on the location of buildings relative to each other and their heights. A strong up and downflows in the wake of tall buildings as well as large-scale coherent eddies between the low-rise buildings were observed. It was found out that high-rise buildings had the highest impact on the urban wind patterns. Other characteristics of urban canopy flows, such as wind shadows and channeling effects, are also successfully captured by the LES. The LES solver was shown to be a powerful tool for understanding urban canopy flows; therefore, it can be used in similar studies (e.g., other cities, dispersion studies, etc.) in the future.
Citation Count: 1
Modeling of Wind Effects on Stratified Flows in Open Channels: A Model for the Istanbul Strait (Bosphorus)
(2016) Bilge, Ayşe Hümeyra; Kirkil, Gökhan; Burak, Selmin; İncegül, Metehan
Stratified flows in open channels arise as a result of density or surface level differences. If the channel is connected to a basin at one or both ends, strong winds originating from the basin cause the "wind setup" effect that increases the water level at the entrance of the channel. On the other hand, along the channel, persistent winds in the upper layer flow direction lead to an increase of the drift velocity and to a decrease in upper layer flow depth. The Istanbul Strait (Bosphorus) connecting the Black and the Marmara Seas, is characterized by a stratified flow caused by the surface level and salinity difference between these basins, consisting of a southward upper layer flow and a northward lower layer flow. Along the strait, there are three hydraulic control points; the north sill, a midway contraction reach and the south sill. Under wind effects, the northern and southern entrances of the strait behave as an estuary whereas the midway reach to the south of the contraction acts as as an open channel. In winter, when the sea level difference is relatively low, the wind setup due to southerly winds may cause a blockage and even reversal of the upper layer flow. On the other hand in spring when there is excessive river discharge, northerly winds increase the influx of Black Sea waters into the strait and may lead to a blockage of the lower layer. We claim that strong northerly winds may cause a decrease of the upper layer depth beyond the contraction and we propose a simple model for its estimation in terms of the wind and water flow speeds.
Citation Count: 0
A Numerical Study of Shallow Mixing Development over Flat Surface and Dunes
(TSINGHUA UNIV, 2013) Kirkil, Gökhan; Constantinescu, George
Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a smooth flat bed and dunes. The study discusses the vertical non-uniformity in the mixing layer structure and provides a quantitative characterization of the growth of the large-scale quasi two-dimensional (2D) coherent structures with the distance from the splitter plate. Results show that in streamwise sections situated between 75D (D is the channel depth) and 150D from the splitter plate the width of the mixing layer close to the free surface is 20-30% more than the width in the near-bed region in the case in which the channel bed is flat. This is mostly because of the tilting of the mixing layer interface on the low-speed side toward the low speed stream as the free surface is approached. Power spectra of the horizontal velocity components show the presence of a -3 subrange at streamwise locations situated more than 10D from the splitter plate consistent with the presence of large-scale quasi 2D horizontal eddies and the transfer of energy (inverse energy cascade) from the smaller scales toward these eddies. Consistent with visualizations of the mass transport of a passive scalar within the mixing layer close to the free surface the estimated streamwise length of the quasi 2D mixing layer eddies is about 2.5 to 3.0 times larger than the local width of the mixing layer. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a much more rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness.
Citation Count: 41
A numerical study of the laminar necklace vortex system and its effect on the wake for a circular cylinder
(Amer Inst Physics, 2012) Kirkil, Gökhan; Constantinescu, George
Large eddy simulation (LES) is used to investigate the structure of the laminar horseshoe vortex (HV) system and the dynamics of the necklace vortices as they fold around the base of a circular cylinder mounted on the flat bed of an open channel for Reynolds numbers defined with the cylinder diameter D smaller than 4460. The study concentrates on the analysis of the structure of the HV system in the periodic breakaway sub-regime which is characterized by the formation of three main necklace vortices. Over one oscillation cycle of the previously observed breakaway sub-regime the corner vortex and the primary vortex merge (amalgamate) and a developing vortex separates from the incoming laminar boundary layer (BL) to become the new primary vortex. Results show that while the classical breakaway sub-regime in which one amalgamation event occurs per oscillation cycle is present when the nondimensional displacement thickness of the incoming BL at the location of the cylinder is relatively large (delta*/D > 0.1) a new type of breakaway sub-regime is present for low values of delta*/D. This sub-regime which we call the double-breakaway sub-regime is characterized by the occurrence of two amalgamation events over one full oscillation cycle. LES results show that when the HV system is in one of the breakaway sub-regimes the interactions between the highly coherent necklace vortices and the eddies shed inside the separated shear layers (SSLs) are very strong. For the relatively shallow flow conditions considered in this study (H/D congruent to 1 H is the channel depth) at times the disturbances induced by the legs of the necklace vortices do not allow the SSLs on the two sides of the cylinder to interact in a way that allows the vorticity redistribution mechanism to lead to the formation of a new wake roller. As a result the shedding of large-scale rollers in the turbulent wake is suppressed for relatively large periods of time. Simulation results show that the wake structure changes randomly between time intervals when large-scale rollers are forming and are convected in the wake (von Karman regime) and time intervals when the rollers do not form. When the wake is in the von Karman regime the shedding frequency of the rollers is close to that observed for flow past infinitely long cylinders.