Yazar "Donmez, O." seçeneğine göre listele
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Öğe Exact Solutions of the Gardner Equation and their Applications to the Different Physical Plasmas(SPRINGER, 2016) Daghan, D.; Donmez, O.Traveling wave solution of the Gardner equation is studied analytically by using the two dependent (G (')/G,1/G)-expansion and (1/G ('))-expansion methods and direct integration. The exact solutions of the Gardner equations are obtained. Our analytic solutions are applied to the unmagnetized four-component and dusty plasma systems consisting of hot protons and electrons to investigate dynamical features of the solitons and shock waves produced in these systems. A wide variety of parameters of the plasma is used, and the basic features of the Gardner solitons that are beyond the existing study in literature are found. It is observed that the analytic solutions from (G (')/G,1/G)-expansion and (1/G ('))-expansion methods only produce shock waves but the solitary waves are found from the analytic solutions derived from the direct integration. It is also noted that the superhot electrons and relative mass density of the electrons significantly effect the soliton's amplitude, width, and position. We have also numerically proved that the combination of every value of nomalized density mu (1) or temperature ratio sigma (1) with the other sets of plasma parameters creates a region where the solutions have similar physical properties. The time-dependent behavior of the soliton is also studied, and a periodic motion of soliton along the phase variable eta is found during the evolution. The investigations and the limits presented in this study may be helpful for studying and understanding the nonlinear properties of the solitary and shock waves seen in various physical and astrophysical plasma systems.Öğe Investigating the effect of integration constants and various plasma parameters on the dynamics of the soliton in different physical plasmas(AMER INST PHYSICS, 2015) Daghan, D.; Donmez, O.The nonlinear dynamics and propagation of ion acoustic waves in a relativistic and ideal plasmas, which have the pressure variation of electrons and ions and degenerate electrons, are investigated using the analytic solution of KdV type equations performed applying (G'/G)-expansion and (G'/G, 1/G)-expansion methods. The effects of various parameters, such as phase velocity of the ion acoustic wave, the ratio of ion temperature to electron temperature, normalized speed of light, electron and ion streaming velocities, arbitrary and integration constants, on the soliton dynamics are studied. We have found that dim and hump solitons and their amplitudes, widths and dynamics strongly depend on these plasma parameters and integration constants. The source term mu plays also a vital role in the formation of the solitons. Moreover, it is also found that the observed solitary wave solution can be excited from hump soliton to dip soliton. This dramatic change of the solitons can occur due to the various values of the integration constants and ion streaming velocities. Finally, it is important to note that the analytic solutions of the nonlinear equation, reported in this study, could be used to explain the structures of solitons in the astrophysical space and in laboratory plasmas. (C) 2015 AIP Publishing LLC.Öğe NUMERICAL SIMULATION OF ASTROPHYSICAL JETS: INTERACTION WITH SURROUNDING MEDIUM(SPRINGER/PLENUM PUBLISHERS, 2017) Donmez, O.; Aktas, A.; Ilter, U.Propagation of astrophysical jets inside an ambient medium transports a large amount of energy to surrounding materials as a consequence of interactions. These interactions have a crucial effect on the evolution and dynamics of the jets. They can cause the formation of the jet's head, which dissipates its energy. In this paper, we have numerically modeled the evolution of jet's dynamics to understand the effects of the critical parameters (Mach numbers, jet velocity, densities, pressures of the accelerated the jet and medium, sound speeds, and Lorentz factor) on the head of the jet, jet-cocoon, vortexes and shocks. When the jet propagates inside the overdense region, we observe clear evidence for deceleration of the jet and find a more complex structure. In the underdense cases, almost no back-flows and cocoons are developed. We have also modeled the pulsed type jets propagating into the overdense region and found a very rich internal structure of the jet, such as cocoon, knots, vortexes, etc. They could explain the structure of jets seen in Herbig-Haro bows and XZ Tauri proto-jet.Öğe On the development of quasi-periodic oscillations in Bondi-Hoyle accretion flows(WILEY-BLACKWELL, 2011) Donmez, O.; Zanotti, O.; Rezzolla, L.The numerical investigation of the Bondi-Hoyle accretion on to a moving black hole has a long history, both in Newtonian and in general-relativistic physics. By performing new two-dimensional and general-relativistic simulations on to a rotating black hole, we point out a novel feature, namely that quasi-periodic oscillations (QPOs) are naturally produced in the shock cone that develops in the downstream part of the flow. Because the shock cone in the downstream part of the flow acts as a cavity trapping pressure perturbations, modes with frequencies in the integer ratios of 2:1 and 3:1 are easily produced. The frequencies of these modes depend on the black hole spin and on the properties of the flow, and scale linearly with the inverse of the black hole mass. Our results may be relevant for explaining the detection of QPOs in Sagittarius A*, once such detection is confirmed by further observations. Finally, we report on the development of the flip-flop instability, which can affect the shock cone under suitable conditions; such an instability has been discussed before in Newtonian simulations but was never found in a relativistic regime.Öğe On the development of the Papaloizou-Pringle instability of the black hole-torus systems and quasi-periodic oscillations(OXFORD UNIV PRESS, 2014) Donmez, O.We present the numerical study of dynamical instability of a pressure-supported relativistic torus, rotating around the black hole with a constant specific angular momentum on a fixed space-time background, in case of perturbation by a matter coming from the outer boundary. Two-dimensional hydrodynamical equations are solved at equatorial plane using the high resolution shock capturing method to study the effect of perturbation on the stable systems. We have found that the perturbed torus creates an instability which causes the gas falling into the black hole in a certain dynamical time. All the models indicate an oscillating torus with certain frequency around their instant equilibrium. The dynamic of the accreted torus varies with the size of initial stable torus, black hole spin and other variables, such as Mach number, sound speed, cusp location of the torus, etc. The mass accretion rate is slightly proportional to the torus-to-hole mass ratio in the black hole-torus system, but it strongly depends on the cusp location of the torus. The cusp located in the equipotential surfaces of the effective potential moves outwards into the torus. The dynamical change of the torus increases the mass accretion rate and triggers the Papaloizou-Pringle instability. It is also observed that the growth of the m = 1 mode of the Papaloizou-Pringle instability occurs for a wide range of fluid and hydrodynamical parameters and a black hole spin. We have also computed the quasi-periodic oscillations from the oscillating relativistic torus.Öğe Relativistic simulation of flip-flop instabilities of Bondi-Hoyle accretion and quasi-periodic oscillations(OXFORD UNIV PRESS, 2012) Donmez, O.It is known from recent numerical calculations that BondiHoyle accretion creates a shock cone behind compact objects. This type of accretion leads to instabilities, which can explain certain astrophysical phenomena. In this paper, our main goal is to find the flip-flop behaviour of the shock cone in the relativistic region. In order to do so we have modelled the dynamics of a shock cone around non-rotating and rotating black holes at the equatorial plane in 2D. The effects of the various parameters on the shock cones and instabilities, such as the asymptotic velocity, sound speed, Mach number and adiabatic index, are studied. We have determined the mass accretion rate, shock opening angle, shock cone oscillation, quasi-periodic oscillations (QPOs), and growth rate of instabilities to reveal the disc properties and its radiation. We have discovered, for the first time, flip-flop instabilities around a black hole in the relativistic region by solving the general relativistic hydrodynamical equations. The flip-flop instabilities are found for sound speeds Cs, 8 < 0.2 with moderate Mach numbers ( M=3 and M=4 for Cs, 8 = 0.1 or M=7 and M=8 for Cs, 8 = 0.05). Our calculation clearly confirms that the shock cone should be detached from the black hole in the BondiHoyle accretion flow with G = 2 for non-rotating and rotating black holes. Results reveal that the flip-flopping shock cone not only creates a torque effect on the black hole but also produces continuous X-ray flares with a certain frequency. Furthermore, QPOs originate inside the shock cone and are stronger in regions that have a radius of a few gravitational radii away from the centre owing to the flip-flop oscillation. Finally, our results are compared with the results of numerical and theoretical calculations in Newtonian hydrodynamics, and it is found that they are in good agreement.
