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Publication Open Access A new consistent hybrid algorithm for solution of the PDF equations of turbulent reactive flow(American Institute of Physics (AIP) Publishing, 2013) Department of Mechanical Engineering; Sheikhsarmast, Reza Mokhtarpoor; Inmas, Shabrina Virta; Muradoğlu, Metin; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 46561This paper presents a newly developed consistent hybrid finite-volume (FV)/particle algorithm for solution of joint PDF (JPDF) model equation of turbulent reacting flows. In this approach, the open source FV package of OpenFOAM is employed to solve the Favre-averaged mean mass and momentum equations using pressure-based PISO algorithm while a particle-based Monte Carlo algorithm is used to solve the fluctuating velocity-turbulence frequency-compositions JPDF transport equation. In the earlier hybrid method [2, 3], a density-based FV algorithm was used to solve the mean flow equations but it has been found to be too dissipative and yet not very robust for incompressible or nearly incompressible flows mainly due to stiffness of the compressible flow equations in the low Mach number limit. In the this work, the density-based FV algorithm is first replaced with a pressure-based PISO algorithm to tackle this problem and then applied for simulation of the Sydney swirl stabilized bluff-body flame SM1. All the equations solved by the FV and particle algorithms are directly derived from the modeled JPDF transport equation so the present method is completely consistent at the level of governing equations. The position and velocity correction algorithms [3] are used to enforce full constancy at the numerical solution level. The results are found to be in a good agreement with the available experimental data and the recent computational results of De Meester et al. [1].Publication Open Access Adaptive Q control for tapping-mode nanoscanning using a piezoactuated bimorph probe(American Institute of Physics (AIP) Publishing, 2007) Department of Mechanical Engineering; Günev, İhsan; Varol, Aydın; Karaman, Sertaç; Başdoğan, Çağatay; Master Student; Faculty Member; Department of Mechanical Engineering; College of Engineering; N/A; N/A; N/A; 125489A new approach, called adaptive Q control, for tapping-mode atomic force microscopy (AFM) is introduced and implemented on a homemade AFM setup utilizing a laser Doppler vibrometer and a piezoactuated bimorph probe. In standard Q control, the effective Q factor of the scanning probe is adjusted prior to the scanning depending on the application. However, there is a trade-off in setting the effective Q factor of an AFM probe. The Q factor is either increased to reduce the tapping forces or decreased to increase the maximum achievable scan speed. Realizing these two benefits simultaneously using standard Q control is not possible. In adaptive Q control, the Q factor of the probe is set to an initial value as in standard Q control, but then modified on the fly during scanning when necessary to achieve this goal. In this article, we present the basic theory behind adaptive Q control, the electronics enabling the online modification of the probe's effective Q factor, and the results of the experiments comparing three different methods: scanning (a) without Q control, (b) with standard Q control, and (c) with adaptive Q control. The results show that the performance of adaptive Q control is superior to the other two methods.Publication Open Access Broadband and band-limited random vibration energy harvesting using a piezoelectric patch on a thin plate(Society of Photo-optical Instrumentation Engineers (SPIE), 2014) Erturk, Alper; Department of Mechanical Engineering; Arıdoğan, Mustafa Uğur; Başdoğan, İpek; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179940This paper presents analytical modeling and case studies of broadband and band-limited random vibration energy harvesting using a piezoceramic patch attached on a thin plate. The literature of vibration-based energy harvesting has been mostly focused on resonant cantilevered structures. However, cantilevered beam-type harvesters have limited broadband vibration energy harvesting capabilities unless they are combined with a nonlinear component. Moreover, cantilever arrangements cannot always be mounted on thin structures (which are basic components of marine, aerospace, and ground transportation systems) without significantly affecting the host system's design and overall dynamics. A patch-based piezoelectric energy harvester structurally integrated to a thin plate can be a proper alternative to using resonant cantilevers for harvesting energy from thin structures. Besides, plate-like structures have more number of vibration modes compared to beam structures, offering better broadband performance characteristics. In this paper, we present analytical modelling of patch-based piezoelectric energy harvester attached on a thin plate for random vibrations. The analytical model is based on electromechanically-coupled distributed-parameter formulation and validated by comparing the electromechanical frequency response functions (FRFs) with experimental results. Example case studies are then presented to investigate the expected power output of a piezoceramic patch attached on an aluminum plate for the case of random force excitations. The effect of bandwidth of random excitation on the mean power and shunted mean-square vibration response are explored with a focus on the number of vibration modes covered in the frequency range of input power spectral density (PSD).Publication Open Access Broadly tunable continuous-wave solid-state red source based on intracavity-doubled Cr (4+): forsterite laser(Society of Photo-optical Instrumentation Engineers (SPIE), 2002) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851This work describes the development and characterization of a continuous-wave (cw) room-temperature intracavity-doubled Cr4+:forsterite laser which produces broadly tunable red radiation. Such a source is potentially important in spectroscopy, display technologies, and medical applications. In the experiments, a 2-cm-long Cr4+:forsterite crystal was placed in an astigmatically compensated x-cavity which was end-pumped by a 1064-nm Nd:YAG laser. The crystal which had a small-signal pump absorption of 68% was maintained at 20 degreesC. An intracavity Brewster-cut SF10 prism was used to tune the output of the laser. Intracavity frequency doubling was achieved by using a periodically poled lithium niobate (PPLN) crystal which had 8 different poling periods. The PPLN crystal was placed inside the resonator between a curved folding mirror and the curved output coupler. The transmission of the output coupler was 2.6% at 1260 nm. The PPLN temperature was maintained at 188 degreesC. By translating the PPLN crystal through sections with different poling periods, second harmonic generation was obtained in the wavelength region between 613 and 655 run. With an incident pump power of 6.8 W at 1064 rim, the Cr4+:forsterite laser produced 245 mW of cw output power at 1260 nm and intracavity frequency doubling yielded 45 mW at 630 nm.Publication Open Access Dielectric resonators based on amorphous silicon(Society of Photo-optical Instrumentation Engineers (SPIE), 2001) Department of Physics; Serpengüzel, Ali; Faculty Member; Department of Physics; College of Sciences; 27855In this paper, we report on the design, modeling, fabrication, and characterization of dielectric microresonators based on hydrogenated amorphous silicon nitride and hydrogenated amorphous silicon oxide. The microresonators were modelled using the transfer matrix method (TMM). Quarter wavelength thick stacks of hydrogenated amorphous silicon nitride and hydrogenated amorphous silicon oxide were consecutively deposited using low temperature plasma enhanced chemical vapor deposition (PECVD). For the characterization of the dielectric microresonators the intrinsic photoluminescence of the amorphous silicon nitride is used. The photoluminescence is enhanced by at least an order of magnitude at the resonance wavelength of 710 nm. The minimum resonance linewidth is 6 nm, corresponding to a quality factor of 118. The maximum rejection bandwidth of the distributed Bragg reflector (DBR) is 150 nm. The enhancement and inhibition of the photoluminescence is understood by the modified photon density of states of the dielectric microresonator. The linewidth of the photoluminescence is also narrowed with respect to the linewidth of the bulk amorphous silicon nitride, again due to the presence of the electromagnetic modes of the microresonator.Publication Open Access Droplet resonator based optofluidic microlasers(Society of Photo-optical Instrumentation Engineers (SPIE), 2014) Brzobohaty, Oto; Jezek, Jan; Pilat, Zdenek; Zemanek, Pavel; Anand, Suman; McGloin, David; Department of Physics; Kiraz, Alper; Aas, Mehdi; Karadağ, Yasin; Jonas, Alexandr; Faculty Member; PhD Student; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; 22542; N/A; N/A; N/AAn SU-8 polymer microdisk resonator coated with a palladium (Pd) layer and coupled to a single-mode optical waveguide is used to as a hydrogen (H-2) gas sensor. In the presence of H2 a red shift is observed in the spectral positions of the microdisk whispering gallery modes (WGMs) due to the expansion in the Pd lattice. H-2 concentrations below the flammable limit (4%) down to 0.3% could be detected in nitrogen atmosphere at room temperature. For H-2 concentrations between 0.3 1%, WGM spectral positions shifted linearly with H-2 concentration at a rate of 32 pm/%H-2. Average response time of the devices was measured to be 50 s for 1% H-2. The proposed device concept can also be used to detect different chemical gases by using appropriate sensing layers.Publication Open Access Energy scaling of a multipass-cavity mode-locked femtosecond bulk laser with a carbon nanotube saturable absorber(Society of Photo-optical Instrumentation Engineers (SPIE), 2013) Ozharar, S.; Choi, S. Y.; Kim, K.; Rotermund, F.; Griebner, U.; Petrov, V.; Department of Physics; Toker, Işınsu Baylam; Çankaya, Hüseyin; Sennaroğlu, Alphan; Researcher; Faculty Member; Department of Physics; College of Sciences; N/A; N/A; 23851In the design of mode-locked lasers, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) have emerged as important alternatives to semiconductor saturable absorber mirrors (SESAMs) due to their favorable optical characteristics, low cost, and relatively simple fabrication scheme. Therefore, it is of great interest to explore the limits of energy scaling in solid-state lasers mode-locked with SWCNT-SAs. Due to their unique wavelength range for biomedical applications, a room-temperature Cr4+:forsterite laser operating near 1.3 mu m was used in the mode-locking experiments. The laser was end-pumped with a continuous-wave Yb-fiber laser at 1064 nm. Furthermore, a q-preserving multipass-cavity (MPC) was added to the short resonator to lower the pulse repetition rate to 4.51 MHz and to scale up the output pulse energy at low average power. The SWCNT-SA was fabricated with SWCNTs grown by the high-pressure CO conversion (HiPCO) technique. With dispersion compensation optics, the net group delay dispersion of the resonator was estimated to be around -4440 fs(2). When mode-locked with the SWCNT-SA, the resonator produced 10-nJ, 121-fs pulses at 1247 nm with a spectral bandwidth of 16 nm, corresponding to a time-bandwidth product of 0.37. To our knowledge, this represents the highest peak power (84 kW) generated to date from a bulk femtosecond solid-state laser, mode-locked by using a SWCNT-SA. The results also suggest that the peak power achieved in our experiments was limited only by the self-focusing in the Cr4+:forsterite gain medium and further increase in output energy should in principle be possible in other gain media mode-locked with SWCNT-SAs.Publication Open Access Enhanced sinterability, thermal conductivity and dielectric constant of glass-ceramics with PVA and BN additions(Multidisciplinary Digital Publishing Institute (MDPI), 2022) Akkasoğlu, Ufuk; Çiçek, Buğra; N/A; Department of Chemistry; Arıbuğa, Dilara; Balcı, Özge; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 295531With the rapid development of the microelectronics industry, many efforts have been made to improve glass-ceramics' sinterability, thermal conductivity, and dielectric properties, which are essential components of electronic materials. In this study, low-alkali borosilicate glass-ceramics with PVA addition and glass-BN composites were prepared and successfully sintered at 770 degrees C. The phase composition, density, microstructure, thermal conductivity, and dielectric constant were investigated. It was shown that PVA addition contributes to the densification process of glass-ceramics (~88% relative density, with closed/open pores in the microstructure) and improves the thermal conductivity of glass material from 1.489 to 2.453 W/K.m. On the other hand, increasing BN addition improves microstructures by decreasing porosities and thus increasing relative densities. A glass-12 wt. % BN composite sample exhibited almost full densification after sintering and presented apparent and open pores of 2.6 and 0.08%, respectively. A high thermal conductivity value of 3.955 W/K.m and a low dielectric constant of 3.00 (at 5 MHz) were observed in this material. Overall, the resulting glass-ceramic samples showed dielectric constants in the range of 2.40-4.43, providing a potential candidate for various electronic applications.Publication Open Access Impact of rehabilitation on fatigue in post-Covid-19 patients: a systematic review and meta-analysis(Multidisciplinary Digital Publishing Institute (MDPI), 2022) de Sire, Alessandro; Moggio, Lucrezia; Marotta, Nicola; Agostini, Francesco; Tasselli, Anna; Ferrante, Vera Drago; Curci, Claudio; Calafiore, Dario; Ferraro, Francesco; Bernetti, Andrea; Ammendolia, Antonio; Taşkıran, Özden Özyemişçi; Faculty Member; School of Medicine; 133091The post-COVID-19 syndrome may affect patients after the COVID-19 post-acute phase. In particular, the 69% of patients reported persistent fatigue at the discharge. To date, no clear data are available regarding the most effective rehabilitative approaches for the treatment of this condition. Thus, this systematic review aimed to evaluate the rehabilitation treatment's efficacy on fatigue in post-COVID-19 patients. We systematically searched PubMed, Scopus, and Web of Science databases to find longitudinal study designs presenting: post-COVID-19 patients as participants; a rehabilitative approach aimed to reduce post-COVID-19 syndrome as intervention; and fatigue intensity assessed through an evaluation tool that quantified the perceived exertion (i.e., fatigue severity scale, FSS; Borg Scale (BS); Borg Category Ratio 10, CR10; Checklist Individual Strength (CIS) fatigue scale; FACIT (Functional Assessment of Chronic Illness Therapy) fatigue scale). The present systematic review protocol was registered on PROSPERO (registration number CRD42021284058). Out of 704 articles, 6 studies were included. Nearly all patients showed COVID-19-related fatigue, and after the rehabilitation treatment, only 17% of subjects reported the persistency of symptoms. The overall effect size reported a -1.40 decrease in Borg Category Ratio 10 with a SE of 0.05 and a 95% CI between -1.50 and -1.30 (p < 0.001). The present systematic review and meta-analysis underlines the rehabilitation role in the fatigue reduction in patients affected by post-COVID-19 syndrome.Publication Open Access Magnetic actuated FR4 scanners for compact spectrometers - art. no. 699303(Society of Photo-optical Instrumentation Engineers (SPIE), 2008) Department of Electrical and Electronics Engineering; Ataman, Çağlar; Ürey, Hakan; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 8579A novel magnetic actuated polymer optical platform is integrated into a Michelson interferometer type Fourier transform infrared spectrometer. The proposed advantages of the novel platform over existing approaches, such as MEMS spectrometers, or bulky FTIR systems, include millimeter range dimensions providing a large clear aperture and enabling conventional machining for device fabrication, a controllable AC and/or DC motion both in rotational and translational modes, and low frequency operation. It has been demonstrated that the platform is capable of achieving 400 mu m DC deflection in ambient pressure in the translational mode, and a total optical scan angle exceeding 60 degrees in the resonant rotational mode. A Michelson type Fourier transform spectrometer was built using a retro-reflector bearing FR4 platform and a spectral resolution of 25cm(-1) is demonstrated with this setup. In addition, possible use of the same platform in various other spectrometer configurations and methods to improve the motion precision are discussed.
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