Browsing by Publisher "MDPI AG"
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Item Natural convection and entropy generation in nanofluid filled entrapped trapezoidal cavities under the influence of magnetic field(MDPI AG, 2016) Selimefendigil F.; Öztop H.F.; Abu-Hamdeh N.In this article, entropy generation due to natural convection in entrapped trapezoidal cavities filled with nanofluid under the influence of magnetic field was numerically investigated. The upper (lower) enclosure is filled with CuO-water (Al2O3-water) nanofluid. The top and bottom horizontal walls of the trapezoidal enclosures are maintained at constant hot temperature while other inclined walls of the enclosures are at constant cold temperature. Different combinations of Hartmann numbers are imposed on the upper and lower trapezoidal cavities. Numerical simulations are conducted for different values of Rayleigh numbers, Hartmann number and solid volume fraction of the nanofluid by using the finite element method. In the upper and lower trapezoidal cavities magnetic fields with different combinations of Hartmann numbers are imposed. It is observed that the averaged heat transfer reduction with magnetic field is more pronounced at the highest value of the Rayleigh number. When there is no magnetic field in the lower cavity, the averaged Nusselt number enhances as the value of the Hartmann number of the upper cavity increases. The heat transfer enhancement rates with nanofluids which are in the range of 10% and 12% are not affected by the presence of the magnetic field. Second law analysis of the system for various values of Hartmann number and nanoparticle volume fractions of upper and lower trapezoidal domains is performed. © 2016 by the authors.Item An investigation of the perceptions and practices of nursing students regarding spirituality and spiritual care(MDPI AG, 2016) Kalkim A.; Midilli T.S.; Baysal E.The aim of this research was to determine Turkish nursing students’ knowledge, practices and perceptions of spirituality and spiritual care and to investigate the relationship between their perceptions and their demographics. This study was a descriptive survey conducted at a nursing school providing degree-level education in the city of Manisa, in the western part of Turkey. The sample of the study consisted of the 400 nursing students. A nursing student sociodemographic form, a form on nursing students’ knowledge and practices of spirituality and spiritual care, and the Spirituality and Spiritual Care Rating Scale were used to collect the data. Half of the students could meet patients’ or individuals’ spiritual needs, and the spiritual care that they gave was most frequently listening, empathy, and psychological support. The research findings were that nursing students’ perceptions of spirituality and spiritual care were “sufficiently” although not “very sufficiently” defined. Being female, being in the second year of education and seeing spiritual care education as necessary were determinants of their perceptions of spirituality and spiritual care. © 2016 by the authors; licensee MDPI, Basel, Switzerland.Item A new algorithm for the numerical solution of telegraph equations by using Fibonacci polynomials(MDPI AG, 2016) Bahşi A.K.; Yalçinbaş S.In this study, we present a numerical scheme to solve the telegraph equation by using Fibonacci polynomials. This method is based on the Fibonacci collocation method which transforms the equation into a matrix equation, and the unknown of this equation is a Fibonacci coefficients matrix. Some numerical examples with comparisons are included to demonstrate the validity and applicability of the proposed method. The results show the efficiency and accuracy of this paper.Item A new perturbation approach to optimal polynomial regression(MDPI AG, 2016) Pakdemirli M.A new approach to polynomial regression is presented using the concepts of orders of magnitudes of perturbations. The data set is normalized with the maximum values of the data first. The polynomial regression of arbitrary order is then applied to the normalized data. Theorems for special properties of the regression coefficients as well as some criteria for determining the optimum degrees of the regression polynomials are posed and proven. The new approach is numerically tested, and the criteria for determining the best degree of the polynomial for regression are discussed. © 2016 by the author; licensee MDPI, Basel, Switzerland.Item Nonlinear vibration of a nanobeam on a pasternak elastic foundation based on non-local euler-bernoulli beam theory(MDPI AG, 2016) Togun N.; Baǧdatli S.M.In this study, the non-local Euler-Bernoulli beam theory was employed in the nonlinear free and forced vibration analysis of a nanobeam resting on an elastic foundation of the Pasternak type. The analysis considered the effects of the small-scale of the nanobeam on the frequency. By utilizing Hamilton's principle, the nonlinear equations of motion, including stretching of the neutral axis, are derived. Forcing and damping effects are considered in the analysis. The linear part of the problem is solved by using the first equation of the perturbation series to obtain the natural frequencies. The multiple scale method, a perturbation technique, is applied in order to obtain the approximate closed solution of the nonlinear governing equation. The effects of the various non-local parameters, Winkler and Pasternak parameters, as well as effects of the simple-simple and clamped-clamped boundary conditions on the vibrations, are determined and presented numerically and graphically. The non-local parameter alters the frequency of the nanobeam. Frequency-response curves are drawn. © 2016 by the authors; licensee MDPI, Basel, Switzerland.Item Use of the genetic algorithm-based fuzzy logic controller for load-frequency control in a two area interconnected power system(MDPI AG, 2017) Cam E.; Gorel G.; Mamur H.The use of renewable energy resources has created some problems for power systems. One of the most important of these is load frequency control (LFC). In this study, in order to solve the LFC problem, modern control methods were applied to a two area multi source interconnected power system. A photovoltaic solar power plant (PV-SPP) was also connected, in order to identify the harmful effects on the frequency of the system. A new Genetic-based Fuzzy Logic (GA-FL) controller was designed to control the frequency of the system. For comparison, conventional proportional-integral-derivative (PID), fuzzy logic (FL), and Genetic Algorithm (GA)-PID controllers were also designed. The new control method exhibited a better performance than the conventional and other modern control methods, because of the low overshoot and short settling time. All simulations were realized with the Matlab-Simulink program. © 2017 by the authors.Item Recent developments in enzyme, DNA and immuno-based biosensors(MDPI AG, 2018) Asal M.; Özen Ö.; Şahinler M.; Polatoğlu İ.Novel sensitive, rapid and economical biosensors are being developed in a wide range of medical environmental and food applications. In this paper, we review some of the main advances in the field over the past few years by discussing recent studies from literature. A biosensor, which is defined as an analytical device consisting of a biomolecule, a transducer and an output system, can be categorized according to the type of the incorporated biomolecule. The biomolecules can be enzymes, antibodies, ssDNA, organelles, cells etc. The main biosensor categories classified according to the biomolecules are enzymatic biosensors, immunosensors and DNA-based biosensors. These sensors can measure analytes produced or reduced during reactions at lower costs compared to the conventional detection techniques. Numerous types of biosensor studies conducted over the last decade have been explored here to reveal their key applications in medical, environmental and food industries which provide comprehensive perspective to the readers. Overviews of the working principles and applications of the reviewed sensors are also summarized. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.Item MHD free convection and entropy generation in a corrugated cavity filled with a porous medium saturated with nanofluids(MDPI AG, 2018) Chamkha A.J.; Selimefendigil F.MHD free convection inside a triangular-wave-shaped corrugated porous cavity with Cu-water nanofluid is numerically studied with the finite element method. The influences of the Grashof number (104 ≤ Gr ≤ 106), Hartmann number (0 ≤ Ha ≤ 50), Darcy number (10-4 ≤ Da ≤ 10-1) and solid volume fraction of the nanoparticle (0 ≤ ϕ ≤ 0.05) on the convective flow features are examined. It is observed that increasing the Grashof number and Darcy number enhances the heat transfer, while the effect is opposite for the Hartmann number. As the corrugation frequency of the triangular wave increases, the local and averaged heat transfer rates decrease, which is more effective for higher values of Grashof and Darcy numbers. Normalized total entropy generation increases as the Darcy number and solid volume fraction of the nanoparticles increase and decreases as the Hartmann number increases both for flat and corrugated wall configurations. © 2018 by the authors.Item Forced convection of pulsating nanofluid flow over a backward facing step with various particle shapes(MDPI AG, 2018) Chamkha A.J.; Selimefendigil F.In this study, numerical analysis of forced convective pulsating nanofluid flow over a backward-facing step with different nanoparticle shapes was performed by the finite volume method. The effects of the Strouhal number (between 0.1 and 2), solid nanoparticle volume fraction (between 0 and 0.04) and nanoparticle shapes (spherical, blade and cylindrical) on the heat transfer and fluid flow were examined with the aid of numerical simulation. It was observed that the average Nusselt number is a decreasing function of the Strouhal number for the considered range, and it enhances for higher solid particle fractions. Using nanofluids with spherical particles is advantageous in pulsating flow, whereas cylindrically-shaped particles are preferred in steady flow configurations. Average Nusselt number enhancements up to 30.24% and 27.95% are achieved with cylindrical- and spherical-shaped particles at the highest volume fraction. © 2018 by the authors.Item Numerical analysis for thermal performance of a photovoltaic thermal solar collector with SiO2-water nanofluid(MDPI AG, 2018) Chamkha A.J.; Selimefendigil F.Numerical analysis of a photovoltaic-thermal (PV/T) unit with SiO2-water nanofluid was performed. The coupled heat conduction equations within the layers and convective heat transfer equations within the channel of the module were solved by using the finite volume method. Effects of various particle shapes, solid volume fractions, water inlet temperature, solar irradiation and wind speed on the thermal and PV efficiency of the unit were analyzed. Correlation for the efficiencies were obtained by using radial basis function neural networks. Cylindrical shape particles were found to give best performance in terms of efficiency enhancements. Total efficiency enhances by about 7.39% at the highest volume fraction with cylindrical shape particles. Cylindrical shape particle gives 3.95% more enhancement as compared to spherical ones for the highest value of solid particle volume fraction. Thermal and total efficiency enhance for higher values of solid particle volume fraction, solar irradiation and lower values of convective heat transfer coefficient and inlet temperature. The performance characteristics of solar PV-thermal unit with radial basis function artificial neural network are found to be in excellent agreement with the results obtained from computational fluid dynamics modeling. © 2018 by the authors.Item MHD mixed convection and entropy generation in a lid-driven triangular cavity for various electrical conductivity models(MDPI AG, 2018) Chamkha A.J.; Selimefendigil F.; Oztop H.F.In this study, effects of different electrical conductivity models for magneto- hydrodynamic mixed convection of nanofluids in a lid-driven triangular cavity was numerically investigated with a finite element method. Effects of Richardson number and Hartmann number on the convective heat transfer characteristics were analyzed for various electrical conductivity models of nanofluids. Average Nusselt number decreases for higher Hartmann and Richardson numbers. Discrepancies in the local and average heat transfer exist between different electrical conductivity models, which is higher for higher values of Richardson number and Hartmann number. The total entropy generation rate was found reduced with higher values of Richardson number and Hartmann number while discrepancies exist between various electrical conductivity models. When the magnetic field is imposed, different behaviors of entropy generation rate versus solid particle volume fraction curve is obtained and it is dependent upon the range of solid particle volume fraction. © 2018 by the authors.Item Investigation of fracturing and adhesion behavior of hydroxapatite coating formed by aminoacetic acid-sodium aminoacetate buffer systems(MDPI AG, 2018) Aydın İ.; Kırman M.Biomaterials utilized in implantation can be categorized into 4 main categories, as ceramics, polymers, metals and composites. Ceramic-based biomaterials are opted for, particularly in the field of orthopedics. These materials, also named as bioceramics, are usually employed by coating them onto the base material, inasmuch as they are far from the mechanical values of bone. In this study, a hydroxyapatite coating that is fully compatible with human blood plasma was applied on Ti6Al4V alloy through a biomimetic technique using aminoacetic acid-sodium aminoacetate buffer system for the first time in the literature, and examinations related thereto were carried out. The surface of the base material Ti6Al4V alloy was activated with various chemicals. Subsequent to activating the surface, a coating process whereby the base material was kept in simulated body fluid for 24, 48, 72, 96 h was carried out. Ultimate microhardness (indentation) tests were performed to determine the average indentation depths in maximum load, vickers hardness and elasticity modulus of the coatings obtained by using the biomimetic method, while scratch tests were performed to measure the surface bonding strengths of the coating layers. Furthermore, the fracture toughness values of the coating were calculated. The results obtained through the study are evaluated and discussed. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.Item Forced convection of Fe 3 O 4 -water nanofluid in a bifurcating channel under the effect of variable magnetic field(MDPI AG, 2019) Selimefendigil F.; Oztop H.F.; Sheremet M.A.; Abu-Hamdeh N.In this study, forced convection of Fe 3 O 4 –water nanofluid in a bifurcating channel was numerically studied under the influence of variable magnetic. Galerkin residual finite element method was used for numerical simulations. Effects of various values of Reynolds number (between 100 and 500), Hartmann number (between 0 and 3), and solid nanoparticle volume fraction (between 0% and 4%) on the convective heat transfer characteristics were analyzed. It was observed that location and size of the re-circulation zones established in the walls of the bifurcating channel strongly influenced by the variable magnetic field and Reynolds number. Average Nusselt number versus Hartmann number showed different characteristics for hot walls of the vertical and horizontal branching channels. The average Nusselt number enhancements were in the range of 12–15% and 9–12% for hot walls of the branching channel in the absence and presence of magnetic field (at Hartmann number of 3). © 2019 by the authors.Item Real-time monitoring of indoor air quality with internet of things-based e-nose(MDPI AG, 2019) Taştan M.; Gökozan H.Today, air pollution is the biggest environmental health problem in the world. Air pollution leads to adverse effects on human health, climate and ecosystems. Air is contaminated by toxic gases released by industry, vehicle emissions and the increased concentration of harmful gases and particulate matter in the atmosphere. Air pollution can cause many serious health problems such as respiratory, cardiovascular and skin diseases in humans. Nowadays, where air pollution has become the largest environmental health risk, the interest in monitoring air quality is increasing. Recently, mobile technologies, especially the Internet of Things, data and machine learning technologies have a positive impact on the way we manage our health. With the production of IoTbased portable air quality measuring devices and their widespread use, people can monitor the air quality in their living areas instantly. In this study, e-nose, a real-time mobile air quality monitoring system with various air parameters such as CO2, CO, PM10, NO2 temperature and humidity, is proposed. The proposed e-nose is produced with an open source, low cost, easy installation and doit- yourself approach. The air quality data measured by the GP2Y1010AU, MH-Z14, MICS-4514 and DHT22 sensor array can be monitored via the 32-bit ESP32 Wi-Fi controller and the mobile interface developed by the Blynk IoT platform, and the received data are recorded in a cloud server. Following evaluation of results obtained from the indoor measurements, it was shown that a decrease of indoor air quality was influenced by the number of people in the house and natural emissions due to activities such as sleeping, cleaning and cooking. However, it is observed that even daily manual natural ventilation has a significant improving effect on air quality. © 2019 by the authors.Item HA coating on Ti6Al7Nb alloy using an electrophoretic deposition method and surface properties examination of the resulting coatings(MDPI AG, 2019) Aydin I.; Bahçepinar A.I.; Kirman M.; Çipiloğlu M.A.Ti and its alloys, which are commonly used in biomedical applications, are often preferred due to their proximity to the mechanical properties of bone. In order to increase the biocompatibility and bioactivities of these materials, biomaterials based on ceramic are used in coating operations. In this study, by using an electrophoretic deposition method, instead of on the Ti6Al4V alloy which is commonly used in the literature, a hydroxyapatite (HA) coating operation was applied on the surface of the Ti6Al7Nb alloy, and the surface properties of the coatings were examined. Ti6Al7Nb is a new-generation implant on which there have not been many studies. The voltage values which were used in the coating operation were 50, 100, 150 and 200 V, and the time parameter was stabilized at 1 min. In our method, when preparing the solution, HA, ethanol, and polyvinyl alcohol (PVA) were used. At the end of the study, by using an electron microscope (SEM) the microstructures of the coatings were examined; elemental analyses (EDS) of the coating surfaces were performed; and by using an X-radiation diffraction (XRD) method, the phases which the coatings contained and the concentration of these phases were determined, and the coating thickness, roughness, and hardness values were also determined. Also, by conducting a Scratch test, the strength of the surface combination was examined. At the end of the study, in each parameter, a successful HA coating was seen. By comparing parameters with each other, the ideal voltage value in this coating was determined. It was determined that the most suitable coating was obtained at 100 V voltage and 1 min deposition time. © 2019 by the authors.Item Pulsating flow of CNT–water nanofluid mixed convection in a vented trapezoidal cavity with an inner conductive T-shaped object and magnetic field effects(MDPI AG, 2020) Chamkha A.J.; Selimefendigil F.; Oztop H.F.Mixed convection of carbon-nanotube/water nanofluid in a vented cavity with an inner conductive T-shaped object was examined under pulsating flow conditions under magnetic field effects with finite element method. Effects of different parameters such as Richardson number (between 0.05 and 50), Hartmann number (between 0 and 30), cavity wall inclination (between 0◦ and 10◦), size (between 0.1 H and 0.4 H) and orientation (between −90◦ and 90◦) of the T-shaped object, and amplitude (between 0.5 and 0.9) and frequency (Strouhal number between 0.25 and 5) of pulsating flow on the convective flow features were studied. It was observed that the average Nusselt number enhanced with the rise of strength of magnetic field, solid nanoparticle volume fraction, and amplitude of the pulsation, while the effect was opposite for higher values of Ri number and cavity wall inclination angle. The presence of the T-shaped object and adjusting its size and orientation had significant impact on the main flow stream from inlet to outlet and recirculations around the T-shaped object and in the vicinity of hot wall of the cavity along with the magnetic field strength. Pulsating flow resulted in heat transfer enhancement as compared to steady flow case for all configurations. However, the amount of increment was different depending on the variation of the parameters of interest. Heat transfer enhancements were 41.85% and 20.81% when the size of the T-shaped object was increased from 0.1 H to 0.4 H. The T-shaped object can be utilized in the vented cavity as an excellent tool for convective heat transfer control. As highly conductive CNT particles were used in water, significant enhancements in the average Nusselt number between 97% and 108% were obtained both in steady flow and in pulsating flow cases when magnetic field was absent or present. © 2020 by the authors.Item Optimal perturbation iteration method for solving fractional model of damped burgers' equation(MDPI AG, 2020) Deniz S.; Konuralp A.; la Sen M.D.The newly constructed optimal perturbation iteration procedure with Laplace transform is applied to obtain the new approximate semi-analytical solutions of the fractional type of damped Burgers' equation. The classical damped Burgers' equation is remodeled to fractional differential form via the Atangana-Baleanu fractional derivatives described with the help of the Mittag-Leffler function. To display the efficiency of the proposed optimal perturbation iteration technique, an extended example is deeply analyzed. © 2020 by the authors.Item Effects of a rotating cone on the mixed convection in a double lid-driven 3D porous trapezoidal nanofluid filled cavity under the impact of magnetic field(MDPI AG, 2020) Chamkha A.J.; Selimefendigil F.; Oztop H.F.Effects of a rotating cone in 3D mixed convection of CNT-water nanofluid in a double lid-driven porous trapezoidal cavity is numerically studied considering magnetic field effects. The numerical simulations are performed by using the finite element method. Impacts of Richardson number (between 0.05 and 50), angular rotational velocity of the cone (between −300 and 300), Hartmann number (between 0 and 50), Darcy number (between 10−4 and 5 × 10−2), aspect ratio of the cone (between 0.25 and 2.5), horizontal location of the cone (between 0.35 H and 0.65 H) and solid particle volume fraction (between 0 and 0.004) on the convective heat transfer performance was studied. It was observed that the average Nusselt number rises with higher Richardson numbers for stationary cone while the effect is reverse for when the cone is rotating in clockwise direction at the highest supped. Higher discrepancies between the average Nusselt number is obtained for 2D cylinder and 3D cylinder configuration which is 28.5% at the highest rotational speed. Even though there are very slight variations between the average Nu values for 3D cylinder and 3D cone case, there are significant variations in the local variation of the average Nusselt number. Higher enhancements in the average Nusselt number are achieved with CNT particles even though the magnetic field reduced the convection and the value is 84.3% at the highest strength of magnetic field. Increasing the permeability resulted in higher local and average heat transfer rates for the 3D porous cavity. In this study, the aspect ratio of the cone was found to be an excellent tool for heat transfer enhancement while 95% enhancements in the average Nusselt number were obtained. The horizontal location of the cone was found to have slight effects on the Nusselt number variations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.Item Throughput optimization of multichannel allocation mechanism under interference constraint for hybrid overlay/underlay cognitive radio networks with energy harvesting(MDPI AG, 2020) Karaca H.M.By harvesting energy from ambient radio frequency (RF) signals, significant progress has been achieved in wireless networks self-maintaining their life cycles. Motivated by this and improved spectrum reuse by combined use of overlay/underlay modes of cognitive radio networks (CRNs), this paper proposes a novel multi-channel (m-channel) allocation performance maximization algorithm for low-power mobiles. CRNs, called secondary transmitters (STs), can harvest energy from RF signals by nearby active primary transmitters (PTs). In the proposed scheme, PTs and STs are distributed as independent homogeneous Poisson point processes and contact their receivers at fixed distances. Each PT contains a guard zone to protect its intended receiver from ST interference, and provides RF energy to STs located in its harvesting zone. Prioritization of STs during opportunistic allocation of channels is critical as properties like energy level and harvesting capability improve channel distribution performance. A novel metric is proposed that prioritizes STs based on initial energy levels, harvesting capability, and number of channels through which they can transmit. For comparison, three algorithms were considered: a greedy mechanism for m-channel allocation of hybrid CRNs without harvesting, the proposed m-channel allocation schemes based on maximum independent sets (MIS), and the proposed metric of hybrid CRNs with harvesting capability. The simulations show that the proposed m-channel allocation method based on MIS outperforms the greedy algorithm. The proposed m-channel allocation using the proposed metric on hybrid CRNs with energy harvesting ability produced the best performance of the three methods, proving the superiority of the proposed algorithm. ©2020 by the authors. Licensee MDPI, Basel, Switzerland.Item Enzyme and biological activities of the water extracts from the plants aesculus hippocastanum, olea europaea and hypericum perforatum that are used as folk remedies in Turkey(MDPI AG, 2020) Sarikurkcu C.; Locatelli M.; Tartaglia A.; Ferrone V.; Juszczak A.M.; Ozer M.S.; Tepe B.; Tomczyk M.Phenolic compounds are secondary metabolites that are found ubiquitously in plants, fruits, and vegetables. Many studies have shown that regular consumption of these compounds could have a positive effect on our health. The aim of this study was to compare the phytochemical contents of the water extracts from three different plants used as folk remedies in Turkey: Aesculus hippocastanum, Olea europaea, and Hypericum perforatum. A liquid chromatography-electrospray tandem mass spectrometry (LC-ESI-MS/MS) analysis was performed to explore the phenolic profiles. The biological activities of these extracts were also evaluated in terms of their antioxidant activities (2,2-diphenyl-1-picrylhydrazyl DPPH, 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid ABTS, Ferric Reducing Antioxidant Power Assay FRAP, cupric ion reducing antioxidant capacity CUPRAC, β-carotene, phosphomolybdenum, and metal chelating) and enzyme inhibitory properties (against acetylcholinesterase, butyrylcholinesterase, and tyrosinase). The aqueous extract of H. perforatum showed the highest levels of total phenolic, flavonoid, and saponin contents. Protocatechuic acid, vanillic acid, verbascoside, hesperidin, hyperoside, apigenin 7-hexosides, and quercetin were the most common compounds found in this species. The results confirm that A. hippocastanum, O. europaea, and H. perforatum represent a potential source of natural-derived molecules with positive properties that could be used as valid starting point for new food supplements, and drugs in the pharmaceutical, cosmetic, and food industries. © 2020 by the authors.