Research Outputs
Permanent URI for this communityhttps://hdl.handle.net/20.500.14288/2
Browse
35 results
Search Results
Publication Metadata only Aerothermal load and drag force analysis of the electromagnetically launched projectiles under rarefied gas conditions(IEEE-Inst Electrical Electronics Engineers Inc, 2015) Şengil, Nevsan; Department of Physics; Şengil, Uluç; Master Student; Department of Physics; Graduate School of Social Sciences and Humanities; N/AElectromagnetically launched projectiles fly with hypersonic speeds in different regions of the Earth's atmosphere. Because of their hypersonic speeds, these projectiles are designed to withstand extreme thermal loads. Drag forces should also be considered to maximize the operational range. To reduce the thermal loads and drag forces, the geometric shape of these projectiles should be carefully designed. We can utilize either experimental or numerical methods to calculate these heating effects and drag forces. Numerical methods are more economic in terms of monetary cost and time. However, we cannot use the same numerical method in different regions of the Earth's atmosphere. In this paper, we used direct simulation Monte Carlo method to calculate thermal loads and drag forces of four different projectile geometries in the rarefied part of the atmosphere. Simulation results show that thermal loads and drag forces vary considerably depending on the projectile geometry.Publication Open Access Analysis and optimization of diode end-pumped solid-state lasers: applicationto Nd3+ : YVO4 lasers at 1064 and 1342 nm(Society of Photo-optical Instrumentation Engineers (SPIE), 2005) Buhours, S.; Department of Physics; Department of Electrical and Electronics Engineering; Sennaroğlu, Alphan; Kurt, Adnan; Faculty Member; Teaching Faculty; Department of Physics; Department of Electrical and Electronics Engineering; College of Sciences; 23851; 194455We describe a systematic procedure that uses experimental and numerical methods to analyze the continuous-wave power performance of diode end-pumped solid-state lasers. For the general case, saturation, excited-state absorption, and thermal lensing effects are considered and integral equations are derived to study the evolution of the pump and laser beams in the gain medium. As an application of the method, we consider two different diode end-pumped Nd3+:YVO4 lasers operating at 1064 and 1342 nm. Experimental efficiency data were first analyzed to determine the stimulated emission cross sections and the resonator losses. The best-fit laser parameters were then used to calculate the optimum crystal length that maximizes the output power of the laser. The described method should prove useful in the design of a wide range of efficient diode-pumped solid-state lasers.Publication Metadata only Application of the nonlinear methods in pneumocardiogram signals(Springer, 2020) Akilli, Mahmut; Ozbek, Mustafa; Zeren, Tamer; Akdeniz, K. Gediz; Department of Physics; Yılmaz, Nazmi; Teaching Faculty; Department of Physics; College of Sciences; 178427In this work, the pneumocardiogram signals of nine rats were analysed by scale index, Boltzmann Gibbs entropy and maximum Lyapunov exponents. The scale index method, based on wavelet transform, was proposed for determining the degree of aperiodicity and chaos. It means that the scale index parameter is close to zero when the signal is periodic and has a value between zero and one when the signal is aperiodic. A new entropy calculation method by normalized inner scalogram was suggested very recently. In this work, we also used this method for the first time in an empirical data. We compared the both methods with maximum Lyapunov exponents and observed that using together the scale index and the entropy calculation method by normalized inner scalogram increases the reliability of the pneumocardiogram signal analysis. Thus, the analysis of the pneumocardiogram signals by those methods enables to compare periodical and/or nonlinear aspects for further understanding of dynamics of cardiorespiratory system.Publication Open Access Bistable behavior of a two-mode Bose-Einstein condensate in an optical cavity(Maik Nauka/Interperiodica Publishing, 2013) Safaei, S.; Tanatar, B.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We consider a two-component Bose-Einstein condensate in a one-dimensional optical cavity. Specifically, the condensate atoms are taken to be in two degenerate modes due to their internal hyperfine spin degrees of freedom and they are coupled to the cavity field and an external transverse laser field in a Raman scheme. A parallel laser also excites the cavity mode. When the pump laser is far detuned from its resonance atomic transition frequency, an effective nonlinear optical model of the cavity-condensate system is developed under the discrete mode approximation (DMA), while matter-field coupling has been considered beyond the rotating wave approximation. By analytical and numerical solutions of the nonlinear dynamical equations, we examine the mean cavity field and population difference (magnetization) of the condensate modes. The stationary solutions of both the mean cavity field and normalized magnetization demonstrate bistable behavior under certain conditions for the laser pump intensity and matter-field coupling strength.Publication Metadata only Canonical transformations in three-dimensional phase-space(2009) Hakioğlu, Tuğrul; Department of Physics; Department of Physics; Dereli, Tekin; Faculty Member; Other; Department of Physics; College of Sciences; College of Sciences; 201358; N/ACanonical transformation in a three-dimensional phase-space endowed with Nambu bracket is discussed in a general framework. Definition of the canonical transformations is constructed based on canonoid transformations. It is shown that generating functions, transformed Hamilton functions and the transformation itself for given generating functions can be determined by solving Pfaffian differential equations corresponding to that quantities. Types of the generating functions are introduced and all of them are listed. Infinitesimal canonical transformations are also discussed. Finally, we show that the decomposition of canonical transformations is also possible in three-dimensional phase space as in the usual two-dimensional one.Publication Open Access Collective excitations of a laser driven atomic condensate in an optical cavity(Maik Nauka/Interperiodica Publishing, 2013) Öztop, B.; Türeci, H. E.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We theoretically examine collective excitations of an optically driven atomic Bose-Einstein condensate, coupled to a high-finesse optical cavity. This open system has been recently used for the experimental demonstration of the Dicke superradiance of cavity photons, which is simultaneously and mutually triggered by spontaneous breaking of translational symmetry of the condensate into a crystalline order. We first develop a Hartree-Fock mean field dynamical model of the physical system. Using this model, we compute the dynamics of the cavity photons, the condensate density profile and the Dicke phase transition diagram. Both the imaginary-time and real-time evolution methods are used in the calculations. Collective excitations are determined by the solving Bogoliubov-de Gennes equations. The spectrum, softening of the modes and energetic hierarchy of excitations are determined.Publication Metadata only Corneal tissue welding of clear corneal cut with infrared laser irradiation at different wavelengths(Turkish Ophthalmological Society, 2009) Rasier, Rifat; Ozeren, Mediha; Artunay, Ozgur; Bahcecioglu, Halil; Seckin, Ismail; Kurt, Adnan; Gulsoy, Murat; Department of Physics; N/A; Sennaroğlu, Alphan; Kalaycıoğlu, Hamit; Faculty Member; PhD Student; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 23851; N/APurpose: The aim of the study is to investigate the potential of infrared lasers for cornea welding in order to seal corneal cuts done during cataract surgery Method: Clear corneal cuts were laser (1070-nm) and Tm: YAP laser (1980-nm) were applied on full thickness corneal tissue of freshly enucleated 40 bovine eyes. Optimal laser power, wavelength and exposure time parameters for immediate closure with minimal thermal damage was estimated through histological examination of welded samples with hematoxylin-eosin staining. Ten corneas with 1070-nm and ten corneas with 1980-nm wavelengths were compared with best paremeters. Results: Full thickness welding results to sample sizes were the following: 2 out of 7 with 809-nm diode laser; 2 out of 7 with 980-nm diode laser; 2 out of 3 with 1070-nm YLF; and 4 out of 6 with 1980-nm Tm: YAP laser. According to histologic examination 1070-nm and 1980-nm wavelengths which results had less carbonization and less change in substantia cornea compard; 5 out of 10 corneas were full thickness welded with 1070-nm and 4 out of 10 corneas were partial welded with 1980-nm. Discussion: Our study suggested that 809-nm diode laser welding in association with the topical application of ICG is a valid method for the closure of corneal tissues but 908-nm, 1070-nm diode laser and 1980-nm Tm: YAP laser welding without topical application of any chromophores are other promising options and have a great potential for corneal laser welding.Publication Metadata only D=3 anisotropic and d=2 tj models: phase diagrams, thermodynamic properties, and chemical potential shift(Springer, 2006) Hinczewski, M.; Department of Physics; Berker, Ahmet Nihat; Faculty Member; Department of Physics; College of Sciences; 179795The anisotropic d=3 tJ model is studied by renormalization-group theory, yielding the evolution of the system as interplane coupling is varied from the isotropic three-dimensional to quasi-two-dimensional regimes. Finite-temperature phase diagrams, chemical potential shifts, and in-plane and interplane kinetic energies and antiferromagnetic correlations are calculated for the entire range of electron densities. We find that the novel tau phase, seen in earlier studies of the isotropic d=3 tJ model, persists even for strong anisotropy. While the tau phase appears at low temperatures at 30-35% hole doping away from [n(i)]=1, at smaller hole dopings we see a complex lamellar structure of antiferromagnetic and disordered regions, with a suppressed chemical potential shift, a possible marker of incommensurate ordering in the form of microscopic stripes. An investigation of the renormalization-group flows for the isotropic two-dimensional tJ model also shows a clear pre-signature of the tau phase, which in fact appears with finite transition temperatures upon addition of the smallest interplane coupling.Publication Open Access Decoupling of multifrequency dipole antenna arrays for microwave imaging applications(Hindawi, 2010) Saenz, E.; Özbay, E.; Ederra, I.; Gonzalo, R.; Department of Physics; Güven, Kaan; Faculty Member; Department of Physics; College of Sciences; 52290The mutual coupling between elements of a multifrequency dipole antenna array is experimentally investigated by S-parameter measurements and planar near-field scanning of the radiated field. A multifrequency array with six dipoles is analyzed. In order to reduce the coupling between dipoles, a planar metasurface is placed atop the array acting as superstrate. The mutual coupling of the antenna elements in the absence and presence of the superstrate is presented comparatively. Between 3 and 20 dB mutual coupling reduction is achieved when the superstrate is used. By scanning the field radiated by the antennas and far-field measurements of the radiation pattern, it is observed that the superstrate confines the radiated power, increases the boresight radiation, and reduces the endfire radiation.Publication Metadata only Dispersive effects on optical information storage in Bose-Einstein condensates with ultraslow short pulses(2006) Tarhan, Devrim; Department of Physics; Department of Physics; Sennaroğlu, Alphan; Müstecaplıoğlu, Özgür Esat; Faculty Member; Faculty Member; Department of Physics; College of Sciences; College of Sciences; 23851; 1674We investigate the potential of atomic Bose–Einstein condensates as dynamic memory devices for coherent optical information processing. Specifically, the number of ultraslow pulses that can be simultaneously present within the storage time in the condensate has been analyzed. By modeling short-pulse propagation through the condensate, taking into account high-order dispersive properties, constraints on the information storage capacity are discussed. The roles of temperature, spatial inhomogeneity, the interatomic interactions, and the coupling laser on the pulse shape are pointed out. For a restricted set of parameters, it has been found that coherent optical information storage capacity would be optimized.