Gökhan Kirkil

Kirkil, Gökhan

Name Variants

K.,Gokhan
G. Kirkil
Kirkil,Gokhan
Kirkil,G.
GÖKHAN KIRKIL
Kirkil, Gokhan
K., Gokhan
Kirkil, G.
Gökhan KIRKIL
Kirkil G.
Gökhan Kirkil
Gokhan, Kirkil
KIRKIL, Gökhan
K., Gökhan
KIRKIL, GÖKHAN
Kirkil, GÖKHAN
Kirkil, Gökhan

Job Title

Doç. Dr.

Email Address

gokhan.kirkil@khas.edu.tr

ORCID ID

0000-0001-9213-007X

Scopus Author ID

12039334900

Full item page

Scholarly Output

31

Articles

12

Citation Count

358

Supervised Theses

8 Scholarly Output Search Results

Now showing 1 - 10 of 27

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: 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: 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
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 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: 93
Resolved turbulence characteristics in large-eddy simulations nested within mesoscale simulations using the weather research and forecasting model
(2014) Kirkil, Gökhan; Kosovic, Branko; Kirkil, Gökhan
One-way concurrent nesting within the Weather Research and Forecasting Model (WRF) is examined for conducting large-eddy simulations (LES) nested within mesoscale simulations. Wind speed spectra and resolved turbulent stresses and turbulence kinetic energy from the nested LES are compared with data from nonnested simulations using periodic lateral boundary conditions. Six different subfilter-scale (SFS) stress models are evaluated using two different nesting strategies under geostrophically forced flow over both flat and hilly terrain. Neutral and weakly convective conditions are examined. For neutral flow over flat terrain turbulence appears on the nested LES domains only when using the two dynamic SFS stress models. The addition of small hills and valleys (wavelengths of 2.4 km and maximum slopes of ±10°) yields small improvements with all six models producing some turbulence on nested domains. Weak convection (surface heat fluxes of 10 Wm-2) further accelerates the development of turbulence on all nested domains. However considerable differences in key parameters are observed between the nested LES domains and their nonnested counterparts. Nesting of a finer LES within a coarser LES provides superior results to using only one nested LES domain. Adding temperature and velocity perturbations near the inlet planes of nested domains shows promise as an easy-to-implement method to accelerate turbulence generation and improve its accuracy on nested domains. © 2014 American Meteorological Society.
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.
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
The renewable energy transition in Turkey
(Kadir Has Üniversitesi, 2019) Kirkil, Gökhan; Ediger, Volkan S.; Kirkil, Gökhan
Energy is an integral part of human life. The energy demand has been increasing for many years. The primary energy source differed over time. Fossil fuels dominated the energy market since 19th century. Overuse of fossil fuels has created serious problems such as climate change. In order to tackle these problems renewable energy source (RES) utilization gained importance in the recent years. With the help of economic and technological improvements, the transition from fossil fuels to renewables has accelerated in recent decades. Many countries have already started to invest in RES instead of fossil fuels. Germany and China have some unique characteristics that can help to reshape Turkey's roadmap to the renewables. Turkey is a developing country with a population of about 80 million. Turkey's energy import dependency is 77%. According to the experts, Turkey has a good potential for RES. Due to its geographical position there is adequate potential of solar, wind, hydro and geothermal. Turkey has an opportunity to utilize its RES potential. If the important steps are taken, RES can become the primary source of Turkey's energy demand and would have positive effects on both the economy and sustainability. Key words: Renewable Energy Transition, Sustainability, Solar, Wind, Climate Change, Carbon Emissions
Citation Count: 0
Superior Conductivity of Transparent ZnO/MoS₂ Composite Films for Optoelectronic and Solar Cell Applications
(Mdpi, 2023) Kirkil, Gökhan; Kirkil, Gokhan; OEzugur Uysal, Bengue
The use of transparent conductive oxides in optoelectronics created a revolution where new-generation materials with high transmittance, low sheet resistance values, durability, and portability can be achieved without decreasing efficiency or increasing costs. Transparent ZnO/MoS2 sandwich-structured conductive composite films were produced in this study via the sol-gel method, which is considered the most efficient method due to its simple process and low cost. The crystal structure properties of ZnO/MoS2 were characterized via X-ray diffraction (XRD) patterns. The crystal sizes of ZnO films doped with different amounts of MoS2 were determined. A UV-visible absorption spectrometer was used to perform the spectroscopic analysis of the film. The area under the absorption curve and the full width of the half-maxima of absorbance data were calculated. Using these values, the optimum amount of MoS2 was determined for the best additive distribution. In addition, in order to determine the best transparent conductive material, resistance values measured via the four-point probe method were compared for different MoS2 additive amounts. The optical and electrical characterizations of transparent ZnO/MoS2 conductive oxide films were investigated. According to the parameters obtained via UV-vis spectroscopy, XRD, and four-point probe measurements, the most effective dispersion that exhibits a low width ratio and high resonance ratio was found for ZnO/MoS2 with a doping amount of 4 mg, the crystallite size of the films was found to be within the range of 21.5 and 24.6 nm, and these observations demonstrated a figure-of-merit value of more than 4.8 x 10(-2) with respect to these sandwich-structured films. Compared to the values of previous studies on various transparent ZnO-doped conductive oxide materials, it is possible to claim that these new films have a structure that is very similar to the transparent conductivity characteristics of other films, and they may even be superior relative to some MoS2 amounts.