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Search Results: 1 - 10 of 471 matches for " Jamshid Ghanbari "
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Cooling Flows of Self-Gravitating, Rotating, Viscous Systems
Mohsen Shadmehri,Jamshid Ghanbari
Physics , 2001, DOI: 10.1086/340894
Abstract: We obtain self-similar solutions that describe the dynamics of a self-gravitating, rotating, viscous system. We use simplifying assumptions; but explicitly include viscosity and the cooling due to the dissipation of energy. By assuming that the turbulent dissipation of energy is as power law of the density and the speed v_{rms} and for a power-law dependence of viscosity on the density, pressure, and rotational velocity, we investigate turbulent cooling flows. It has been shown that for the cylindrically and the spherically cooling flows the similarity indices are the same, and they depend only on the exponents of the dissipation rate and the viscosity model. Depending on the values of the exponents, which the mechanisms of the dissipation and viscosity determine them, we may have solutions with different general physical properties. The conservation of the total mass and the angular momentum of the system strongly depends on the mechanisms of energy dissipation and the viscosity model.
Radiative Cooling Flows of Self-Gravitating Filamentary Clouds
Mohsen Shadmehri,Jamshid Ghanbari
Physics , 2000, DOI: 10.1023/A:1013177820161
Abstract: We study the dynamics of a self-gravitating cooling filamentary cloud using a simplified model. We concentrate on the radial distribution and restrict ourselves to quasi-hydrostatic, cylindrically symmetric cooling flows. For a power-law dependence of cooling function on the temperature, self-similar solutions which describe quasi-hydrostatic cooling flows are derived. We consider obtically thin filaments with a constant mass per unit length and the solutions are parameterized by their line masses. There is no polytropic relation between the density and the pressure. The filament experiences radiative condensation, irrespective of the \gamma, the gas specific heat ratio. So, the filament becomes denser due to the quasi-hydrostatic flows and the density at the center increases in proportion to (t_{0}-t)^{-1}, where t denotes the time. The term, t_{0}, denotes an epoch at which the central density increases infinitely. We also found that the radius of the filament decreases in proportion to (t_{0}-t)^{0.5}.
Self-Similar Equilibria of Self-Gravitating, Magnetized, Rotating, Isothermal Systems
Mohsen Shadmehri,Jamshid Ghanbari
Physics , 2001, DOI: 10.1086/321555
Abstract: The self-similar equilibrium models of self-gravitating, rotating, isothermal systems are investigated analytically. In these models the rotation velocity is constant and the density varies as $\frac{f(\theta, \phi)}{r^2}$, where $r$ and $\theta$ are the spherical radius and the co-latitude, respectively. The nonaxisymmetric solutions contain three free parameters, one of the parameters depends on the rotation velocity. These parameters determine the overall shape of the density distribution. By assuming that the dominant component of the magnetic field is purely toroidal and the ratio of the purely toroidal magnetic pressure to the gas pressure, $\alpha$, is spatially constant, the axisymmetric solutions generalized so as the effect of magnetic field could be studied. We find that the equilibria of axially symmetric systems yield ellipsoids or spheres only when the ratio of rotation velocity to the sound speed is taken to be $\sqrt{2\alpha}$.
Linear Thermal Instability and Formation of Clumpy Gas Clouds Including the Ambipolar Diffusion
Mohsen Nejad-Asghar,Jamshid Ghanbari
Physics , 2003, DOI: 10.1046/j.1365-2966.2003.07053.x
Abstract: Thermal instability is one of the most important processes in the formation of clumpy substructure in magnetic molecular clouds. On the other hand, ambipolar diffusion, or ion-neutral friction, has long been thought to be an important energy dissipation mechanism in these clouds. Thus, we would interested to investigate the effect of ambipolar diffusion on the thermal instability and formation of clumps in the magnetic molecular clouds. For this purpose, in the first step, we turn our attention to the linear perturbation stage. In this way, we obtain a non-dimensional characteristic equation which reduces to the prior characteristic equation in the absence of the magnetic field and ambipolar diffusion. With numerical manipulation of this characteristic equation, we conclude that there are solutions where the thermal instability allows compression along the magnetic field but not perpendicular to it. We infer that this aspect might be an evidence in formation of observed disc-like (oblate) clumps in magnetic molecular clouds.
Shear tensor and dynamics of relativistic accretion disks around rotating black holes
Mahboobe Moeen Moghaddas,Jamshid Ghanbari,Ahmad Ghodsi
Physics , 2012,
Abstract: In this paper we solve the hydrodynamical equations of optically thin, steady state accretion disks around Kerr black holes. Here, fully general relativistic equations are used. We use a new method to calculate the shear tensor in the LNRF (Locally Non-Rotating Frame), BLF (Boyer-Lindquist Frame) and FRF (Fluid Rest Frame). We show that two components of shear tensor in the FRF are nonzero (in previous works only one nonzero component was assumed). We can use these tensors in usual transonic solutions and usual causal viscosity, but we derive solutions analytically by some simplifications. Then we can calculate the four velocity and density in all frames such as the LNRF, BLF and FRF.
Bipolar jets launched from magnetically diffusive accretion disks. I. Ejection efficiency vs field strength and diffusivity
Somayeh Sheikhnezami,Christian Fendt,Oliver Porth,Bhargav Vaidya,Jamshid Ghanbari
Physics , 2012, DOI: 10.1088/0004-637X/757/1/65
Abstract: We investigate the launching of jets and outflows from magnetically diffusive accretion disks. Using the PLUTO code we solve the time-dependent resistive MHD equations taking into account the disk and jet evolution simultaneously. The main question we address is which kind of disks do launch jets and which kind of disks do not? In particular, we study how the magnitude and distribution of the (turbulent) magnetic diffusivity affect mass loading and jet acceleration. We have applied a turbulent magnetic diffusivity based on \alpha-prescription, but have also investigate examples where the scale height of diffusivity is larger than that of the disk gas pressure. We further investigate how the ejection efficiency is governed by the magnetic field strength. Our simulations last for up to 5000 dynamical time scales corresponding to 900 orbital periods of the inner disk. As a general result we observe a continuous and robust outflow launched from the inner part of the disk, expanding into a collimated jet of super fast magneto-sonic speed. For long time scales the disk internal dynamics changes, as due to outflow ejection and disk accretion the disk mass decreases. For magneto-centrifugally driven jets we find that for i) less diffusive disks, ii) a stronger magnetic field, iii) a low poloidal diffusivity, or a iv) lower numerical diffusivity (resolution), the mass loading of the outflow is increased - resulting in more powerful jets with high mass flux. For weak magnetization the (weak) outflow is driven by the magnetic pressure gradient. We further investigate the jet asymptotic velocity and the jet rotational velocity in respect of the different launching scenarios. We find a lower degree of jet collimation than previous studies, most probably due to our revised outflow boundary condition.
A statistical study of the luminosity gap in galaxy groups
Saeed Tavasoli,Habib G. Khosroshahi,Ali Koohpaee,Hadi Rahmani,Jamshid Ghanbari
Physics , 2010, DOI: 10.1086/658122
Abstract: The luminosity gap between the two brightest members of galaxy groups and clusters is thought to offer a strong test for the models of galaxy formation and evolution. This study focuses on the statistics of the luminosity gap in galaxy groups, in particular fossil groups, e.g. large luminosity gap, in an analogy with the same in a cosmological simulation. We use spectroscopic legacy data of seventh data release (DR7) of SDSS, to extract a volume limited sample of galaxy groups utilizing modified friends-of-friends (mFoF) algorithm. Attention is paid to galaxy groups with the brightest group galaxy (BGG) more luminous than \Mr = -22. An initial sample of 620 groups in which 109 optical fossil groups, where the luminosity gap exceeds 2 magnitude, were identified. We compare the statistics of the luminosity gap in galaxy groups at low mass range from the SDSS with the same in the Millennium simulations where galaxies are modeled semi-analytically. We show that the BGGs residing in galaxy groups with large luminosity gap, i.e. fossil groups, are on average brighter and live in lower mass halos with respect to their counter parts in non-fossil systems. Although low mass galaxy groups are thought to have recently formed, we show that in galaxy groups with 15 galaxies brighter than $M_r\ge -19.5$, evolutionary process are most likely to be responsible for the large luminosity gap. We also examine a new probe of finding fossil group. In addition we extend the recently introduced observational probe based on the luminosity gap, the butterfly diagram, to galaxy groups and study the probe as a function of halo mass. This probe can, in conjunction with the luminosity function, help to fine tune the semi-analytic models of galaxies employed in the cosmological simulations.
A New Family of Nonlinear Fifth-Order Solvers for Finding Simple Roots  [PDF]
Behzad Ghanbari
Applied Mathematics (AM) , 2012, DOI: 10.4236/am.2012.36088
Abstract: In this paper, we present a new family of iterative methods for solving nonlinear equations. It is proved that the order of convergence of this family is five. Two functions and two derivative evaluations should be computed per iteration. To demonstrate convergence properties of the proposed family of methods, some numerical examples are given. Further numerical comparisons are made with several other existing fifth-order methods.
Experimental Study of Drying Process of COLZA Seeds in Fluidized Bed Dryer by Statistical Methods  [PDF]
Jamshid. Khorshidi, Hassan. Davari
Advances in Chemical Engineering and Science (ACES) , 2012, DOI: 10.4236/aces.2012.21016
Abstract: In this study the effect of initial parameters such as inlet gas temperature, initial particles temperature and gas velocity on temperature changes of solid particles and outlet gas temperature in a fluidized bed dryer was studied. For testing, an experimental setup was established. With combination of air and Colza seeds belonging to D groups of the Geldart classification (Geldart, 1986) fluidization regime was carried out. With five test series with maintaining the inlet gas temperature, solid particle temperature and outlet gas temperature during time were carefully measured. To analyze these data by using regression analysis to predict solid particle and outlet gas temperature, 2 correlations on initial pa-rameters were presented. The result has shown that temperature gradients in the beginning of fluidization, is very high and therefore the exponential functions in the regression model is used to predict the temperature changes.
Design and Construction of a Spiral Heat Exchanger  [PDF]
Jamshid Khorshidi, Salman Heidari
Advances in Chemical Engineering and Science (ACES) , 2016, DOI: 10.4236/aces.2016.62021
Abstract: In this article, the performance and applications of a Spiral Plate Heat Exchanger are demonstrated. Also, governing equation of heat transfer phenomena in such heat exchangers is discussed. Regarding the governing equations, a LAB-sized model of this type of heat exchanger was designed and constructed. Galvanized Iron sheets were used as the heat transfer surfaces. Two Galvanized Iron sheets were rolled together around a central core and, as a result, two separated channels were made. Also, a predesign simulation of the heat exchanger was done using the Fluent software to predict the performance of the heat exchanger. First the geometry was made using Gambit software environment then the model was analyzed through Fluent. Because of less fouling, easier cleaning and high heat transfer coefficient, Spiral Heat Exchanger is a good alternative to the other types of heat exchangers, especially when it’s going to handle high fouling flows or highly viscous fluids. Low fouling rate of the heat exchanger, reduces the need of cleaning and therefore the out of service will be decreased. In the constructed heat exchanger, Nusselt number increases as the mass flow rate increases. Average Nusselt number is about 100 that is very good.
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