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Öğe Dynamical evolution of rotating accretion using different boundary conditions: State after stable accretion disk created(WORLD SCIENTIFIC PUBL CO PTE LTD, 2007) Donmez, OrhanThe 2D time-dependent solution of the thin and stable accretion disk with two-armed spiral shock waves in a closed binary system have been presented on the equatorial plane around the Schwarzschild black hole in Donmez (2004).(2) The subject of this paper is to study the influence of two different boundary conditions, far away from a black hole called the outer boundary, on an accretion disk around the black hole during the time evolution. We have started with a stable accretion disk after the point where two-armed spiral shock waves were created (Donmez, 2004).(2) The initial data which is also called the freezing boundary is used as a first boundary condition. As a second one, we use the outflow boundary condition. In both cases, the accretion disk is created and gases on the disk made closed trajectories. As a stable tori close to the black hole is created by using the first boundary, freezing condition, which has a similar to 10M radius where M is the mass of black hole, and the other boundary, outflow, creates stable two-armed spiral shock waves. The last stable circular orbit around the Schwarzschild black hole for this type of accretion disk is located around 11M in the case of the freezing boundary condition. The results of these simulations show that the tori and spiral shock waves are created in each case using freezing and the outflow boundary, respectively, and it also suggests that spiral waves are a robust feature of accretion disks in binary systems, and that these spiral shocks can indeed transfer the gravitational energy to the radiation energy observed by different X-ray satellites.Öğe Explicit solutions of the nonlinear partial differential equations(PERGAMON-ELSEVIER SCIENCE LTD, 2010) Daghan, Durmus; Donmez, Orhan; Tuna, AdnanThe convection and diffusion process or their mixed states are the Important phenomena in the different physical systems. In order to understand these physical processes, the nonlinear differential equations, Fisher. Burger-Fisher. Benjamin-Bona-Mahony-Burgers (BBMB) and Modified Benjamin-Bona-Mahony (MBBM) are solved to obtain the traveling wave solutions using (G'/G)-expansion method. In this study we give the exact solutions of these equations which describe the dynamics of turbulence created by the Interaction of matters. Our solutions are reduced to the well-known solutions in the literature assigning some special values to the constants in the solutions of these equations. Moreover, we have reached the new exact solutions for these equations mentioned above. We have also analyzed and plotted the results using different integration constants to understand the behavior of solutions. (C) 2009 Elsevier Ltd. All rights reserved.Öğe Numerical and experimental analysis of a salt gradient solar pond performance with or without reflective covered surface(ELSEVIER SCI LTD, 2008) Bezir, Nalan C.; Donmez, Orhan; Kayali, Refik; Oezek, NuriAn experimental salt gradient solar pond having a surface area of 3.5 x 3.5 m(2) and depth of 2 m has been built. Two covers, which are collapsible, have been used for reducing the thermal energy loses from the surface of the solar pond during the night and increasing the thermal efficiency of the pond solar energy harvesting during daytime. These covers having reflective properties can be rotated between 0 degrees and 180 degrees by an electric motor and they can be fixed at any angle automatically. A mathematical formulation which calculates the amount of the solar energy harvested by the covers has been developed and it is adapted into a mathematical model capable of giving the temporal temperature variation at any point inside or outside the pond at any time. From these calculations, hourly air and daily soil temperature values calculated from analytical functions are used. These analytic functions are derived by using the average hourly and daily temperature values for air and soil data obtained from the local meteorological station in Isparta region. The computational modeling has been carried out for the determination of the performance of insulated and uninsulated solar ponds having different sizes with or without covers and reflectors. Reflectors increase the performance of the solar ponds by about 25%. Finally, this model has been employed for the prediction of temperature variations of an experimental salt gradient solar pond. Numerical results are in good agreement with the experiments. (C) 2008 Elsevier Ltd. All rights reserved.Öğe Numerical simulation of the disk dynamics around the black hole: Bondi-Hoyle accretion(WORLD SCIENTIFIC PUBL CO PTE LTD, 2014) Koyuncu, Fahrettin; Donmez, OrhanWe have solved the General Relativistic Hydrodynamic (GRH) equations using the high resolution shock capturing scheme (HRSCS) to find out the dependency of the disk dynamics to the Mach number, adiabatic index, the black hole rotation parameter and the outer boundary of the computational domain around the non-rotating and rotating black holes. We inject the gas to computational domain at upstream and downstream regions at the same time with different initial conditions. It is found that variety of the mass accretion rates and shock cone structures strongly depend on Mach number and adiabatic index of the gas. The shock cones on the accretion disk are important physical mechanisms to trap existing oscillation modes, thereupon these trapped modes may generate strong X-rays observed by different X-ray satellites. Besides, our numerical approach also show that the shock cones produces the flip-flop oscillation around the black holes. The flip-flop instabilities which are monitored in our simulations may explain the erratic spin behavior of the compact objects (the black holes and neutron stars) seen from observed data.Öğe NUMERICAL TREATMENT OF THIN ACCRETION DISK DYNAMICS AROUND ROTATING BLACK HOLES(WORLD SCIENTIFIC PUBL CO PTE LTD, 2010) Yildiran, Deniz; Donmez, OrhanWe have set up two different initial conditions depending on the values of thermodynamical variables around the black hole. In the first setup, the computational domain is filled with constant parameters without injecting gas from the outer boundary. In the second, the computational domain is filled with the matter which is then injected from the outer boundary. The matter is assumed to be at rest far from the black hole. Both cases are modeled over a wide range of initial parameters such as the black hole angular momentum, adiabatic index, Mach number and asymptotic velocity of the fluid. It has been found that initial values and setups play an important role in determining the types of the shock cone and in designating the events on the accretion disk. The continuing injection from the outer boundary presents a tail shock to the steady state accretion disk. The opening angle of shock cone grows as long as the rotation parameter becomes larger. A more compressible fluid (bigger adiabatic index) also presents a bigger opening angle, a spherical shock around the rotating black hole, and less accumulated gas in the computational domain. While results from [J. A. Font, J. M. A. Ibanez and P. Papadopoulos, Mon. Not. R. Astron. Soc. 305 (1999) 920] indicate that the tail shock is warped around for the rotating hole, our study shows that it is the case not only for the warped tail shock but also for the spherical and elliptical shocks around the rotating black hole. The warping around the rotating black hole in our case is much smaller than the one by [J. A. Font, J. M. A. Ibanez and P. Papadopoulos, Mon. Not. R. Astron. Soc. 305 (1999) 920], due to the representation of results at the different coordinates. Contrary to the nonrotating black hole, the tail shock is slightly warped around the rotating black hole. The filled computational domain without any injection leads to an unstable accretion disk. However much of it reaches a steady state for a short period of time and presents quasi-periodic oscillation (QPO). Furthermore, the disk tends to loose mass during the whole dynamical evolution. The time-variability of these types of accretion flowing close to the black hole may clarify the light curves in SgrA*.Öğe Quasi-periodic oscillations and frequencies in an accretion disk and comparison with the numerical results from non-rotating black hole computed by the grh code(WORLD SCIENTIFIC PUBL CO PTE LTD, 2007) Donmez, OrhanThe shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed (quasi Periodic Oscillations (QPOs) in X-ray binaries. We present the set of characteristics frequencies associated with accretion disk around the rotating and nonrotating black holes for one particle case. These persistent frequencies are results of the rotating pattern in an accretion disk. We compare the frequency's from two different numerical results for fluid flow around the non-rotating black hole with one particle case. The numerical results are taken from Refs.1 and 2 using fully general relativistic hydrodynamical code with non-selfgravitating disk. While the first numerical result has a relativistic tori around the black hole, the second one includes one-armed spiral shock wave produced from star-disk interaction. Some physical modes presented in the QPOs can be excited in numerical simulation of relativistic tori and spiral waves on the accretion disk. The results of these different dynamical structures on the accretion disk responsible for QPOs are discussed in detail.Öğe Simulation of astrophysical jet using the special relativistic hydrodynamics code(ELSEVIER SCIENCE INC, 2006) Donmez, Orhan; Kayali, RefikThis paper describes a multi-dimensional hydrodynamic code which can be used for the studies of relativistic astrophysical flows. The code solves the special relativistic hydrodynamic equations as a hyperbolic system of conservation laws based on high resolution shock capturing (HRSC) scheme. Two standard tests, one of which is the relativistic blast wave tested in our previous paper [O. Donmez, Astrophys. Space Sci. 293 (2004) 323-354], and the other is the collision of two ultra-relativistic blast waves tested in here, are presented to demonstrate that the code captures correctly and gives solution in the discontinuities, accurately. The relativistic astrophysical jet is modeled for the ultra-relativistic flow case. The dynamics of jet flowing is then determined by the ambient parameters such as densities, and velocities of the jets and the momentum impulse applied to the computational surface. We obtain solutions for the jet structure, propagation of jet during the time evolution, and variation in the Mach number on the computational domain at a fixed time. (c) 2006 Elsevier Inc. All rights reserved.Öğe Solving 1-D special relativistic hydrodynamics (SRH) equations using different numerical methods and results from different test problems(ELSEVIER SCIENCE INC, 2006) Donmez, OrhanIn this paper, we have solved I-D special relativistic hydrodynamical equations using different numerical methods in computational gas dynamics. Numerical solutions of these equations for smooth wave cases give better solution when we use Non-TVD (total variable diminishing) but solution of discontinuity wave produces some oscillation behind the shock. On the other hand, TVD type schemes give good approximation at discontinuity cases. Because TVD schemes completely remove the oscillations, they reduce locally the accuracy of the solution around the extrema. (c) 2006 Elsevier Inc. All rights reserved.Öğe Spherical-shell accretion onto the black hole-torus system(WORLD SCIENTIFIC PUBL CO PTE LTD, 2015) Donmez, OrhanThe general relativistic hydrodynamical simulation of the spherical-shell accretion onto the stable torus around non-rotating and rotating black holes isotropically falling from a finite distance are constructed for the first time. This type of accretion might be used to explain the dynamics of the torus. The accreted matter sonically, supersonically or highly supersonically interacts with a torus and forms a newly developed dynamical structure. This spherical-shell changes the angular momentum of the torus and mediates torus instabilities which cause the termination of the torus. The impact of the rest-mass density of the perturbation is also studied which found that the high density perturbation destroys the torus in a few dynamical times. It is also found that the dumping time of the matter is much larger for the torus around a rotating black hole. On the other hand, the Papaloizou-Pringle instability from the spherical-shell accretion appears to be much more softer than the former perturbations which are called the Bondi-Hoyle accretion and accretion of the bulk of gas. The Papaloizou-Pringle instability is damped in a short time scale immediately after their formation.Öğe The dynamical evolution of the black hole-torus system perturbed by a Bondi-Hoyle accretion(WORLD SCIENTIFIC PUBL CO PTE LTD, 2014) Donmez, OrhanThe existence of the black hole (BH)-torus system has been given a considerable attention to explain the variability of X-ray and Gamma-ray (gamma-ray) data. The perturbation of this system by a Bondi-Hoyle accretion leads to an instability which might be used to understand the dynamics of X-ray binaries and gamma-ray burst (GRB). The instability is of a Papaloizou-Pringle type and the fastest growing mode of this instability corresponds to m = 1. In this paper, we put forward a scenario in which the stable BH-torus system is perturbed by a matter which is coming from red giants due to the stellar winds. We model the perturbed BH-torus system to find out how the dynamics of the system changes depending on the rest-mass density of the initial perturbation and to estimate the maximum rest-mass density of the perturbation, rho(p), which creates a quasi-periodic oscillation without having a shock cone around the BH for fixed rho(c) (rho(c) is the maximum rest-mass density of the initial stable torus). We have found that the perturbation with a rest-mass density, rho(p) < 100 rho(atm) (rho(atm) is the rest-mass density of the atmosphere), for any Mach number is the best model for the formation of the oscillating torus around the BH. Otherwise, the shock cone appears in the downstream region of the accreted domain. It is also found that the instability is observed while rho(p) < 100 rho(atm).
