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Kaveh Karimzadeh Ziarati - One of the best experts on this subject based on the ideXlab platform.
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Numerical investigation of the mixed convection of a magnetic nanofluid in an annulus between two vertical concentric cylinders under the influence of a non-uniform external magnetic field
Journal of Thermal Analysis and Calorimetry, 2019Co-Authors: Mohamad Hamed Hekmat, Marzie Babaie Rabiee, Kaveh Karimzadeh ZiaratiAbstract:In the present study, the hydrothermal behavior of a magnetic nanofluid (4 vol% magnetite–water) in the annular space between two vertical circular cylinders under the influence of a linear magnetic field with Negative Gradient is numerically investigated. In particular, the effects of the Grashof number (1000 ≤ Gr ≤ 50,000), Reynolds number (20 ≤ Re ≤ 200), and the annulus radius ratio (1.5 ≤ D _o/ D _i ≤ 3.5) on the hydrothermal characteristics of the magnetic nanofluid are presented in detail. The flow is assumed to be steady, incompressible, and viscous. The two-phase mixture model is used to simulate the flow and heat transfer of the magnetic nanofluid. The three-dimensional governing equations are discretized using the finite volume scheme, while the SIMPLE algorithm is employed to couple the velocity and pressure. Moreover, the second-order upwind scheme is used to discretize the convective terms of the momentum and energy equations. The results show that the skin friction coefficient on the inner and outer walls of the annuli increases and decreases, respectively, by enhancing the Grashof number as well as the ratio of the radius. It is found that the Nusselt number increases by increasing the Grashof number. In the presence of an external magnetic field with the Negative Gradient, the skin friction coefficient on the inner and outer walls of the annulus is enhanced by decreasing the Reynolds number.
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effects of nanoparticles volume fraction and magnetic field Gradient on the mixed convection of a ferrofluid in the annulus between vertical concentric cylinders
Applied Thermal Engineering, 2019Co-Authors: Mohamad Hamed Hekmat, Kaveh Karimzadeh ZiaratiAbstract:Abstract In this paper, the thermomagnetic convection of magnetite-water ferrofluid is numerically investigated in the three-dimensional space between two vertical concentric cylinders under the influence of an external magnetic field. The main objective is to study the effect of volume fraction of the magnetic nanoparticles ( 0.01 ≤ α p ≤ 0.06 ) with the diameter of 10 nm and positive and Negative Gradients of the magnetic field ( - 5 × 10 5 A / m 2 ≤ G ≤ 5 × 10 5 A / m 2 ) on the hydrothermal behavior of the ferrofluid flow. For this purpose, the two-phase mixture model and the finite volume method are used to simulate the hydrodynamics and thermal behavior of the ferrofluid flow. The simulations are performed for a Reynolds number of 20, Grashof number of 10000, and outer to inner diameter ratio of 2.5. The numerical results indicate that the Kelvin force increases by increasing the volume fraction, decreasing the ferrofluid temperature and increasing the magnetic field Gradient. In the presence of an external magnetic field with Negative Gradient, the skin friction coefficient on the inner wall is enhanced by increasing the volume fraction and the magnetic field Gradient but on the outer wall, it decreases by increasing them. Besides, the Nusselt number increases with the increase in the volume fraction and the magnetic field Gradient. In addition, positive Gradient magnetic fields result in reducing the Nusselt number and friction coefficient on the inner wall, while they increase the friction coefficient on the outer wall.
Zvi Shiller - One of the best experts on this subject based on the ideXlab platform.
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optimal obstacle avoidance based on the hamilton jacobi bellman equation
International Conference on Robotics and Automation, 1994Co-Authors: S Sundar, Zvi ShillerAbstract:This paper presents a method for generating shortest paths in cluttered environments, based on the Hamilton-Jacobi-Bellman (HJB) equation. Formulating the shortest obstacle avoidance problem as a time optimal control problem, the shortest paths are generated by following the Negative Gradient of the return function, which satisfies the HJB equation. A method to generate near-optimal paths is also presented, based on a psuedo return function. Paths generated by this method are guaranteed to reach the goal, at which the psuedo return function is shown to have a unique minimum. The computation required to generate the near-optimal paths is substantially lower than those of traditional potential field methods, making it applicable to on-line obstacle avoidance. Examples with circular obstacles demonstrate close correlation between the near-optimal and optimal paths, and the advantages of the proposed approach over traditional potential field methods. >
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optimal obstacle avoidance based on the hamilton jacobi bellman equation
International Conference on Robotics and Automation, 1994Co-Authors: S Sundar, Zvi ShillerAbstract:This paper presents a method for generating shortest paths in cluttered environments, based on the Hamilton-Jacobi-Bellman (HJB) equation. Formulating the shortest obstacle avoidance problem as a time optimal control problem, the shortest paths are generated by following the Negative Gradient of the return function, which satisfies the HJB equation. A method to generate near-optimal paths is also presented, based on a psuedo return function. Paths generated by this method are guaranteed to reach the goal, at which the psuedo return function is shown to have a unique minimum. The computation required to generate the near-optimal paths is substantially lower than those of traditional potential field methods, making it applicable to on-line obstacle avoidance. Examples with circular obstacles demonstrate close correlation between the near-optimal and optimal paths, and the advantages of the proposed approach over traditional potential field methods. >
Habib Aminfar - One of the best experts on this subject based on the ideXlab platform.
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two phase mixture model simulation of the hydro thermal behavior of an electrical conductive ferrofluid in the presence of magnetic fields
Journal of Magnetism and Magnetic Materials, 2012Co-Authors: Habib Aminfar, Mousa Mohammadpourfard, F MohseniAbstract:Abstract This paper presents a numerical investigation of the hydro-thermal behavior of a ferrofluid (sea water and 4 vol% Fe 3 O 4 ) in a rectangular vertical duct in the presence of different magnetic fields, using two-phase mixture model and control volume technique. Considering the electrical conductivity of the ferrofluid, in addition to the ferrohydrodynamics principles, the magnetohydrodynamics principles have also been taken into account. Three cases for magnetic field have been considered to study mixed convection of the ferrofluid: non-uniform axial field (Negative and positive Gradient), uniform transverse field and another case when both fields are applied simultaneously. The results indicate that Negative Gradient axial field and uniform transverse field act similarly and enhance both the Nusselt number and the friction factor, while positive Gradient axial field decreases them. It is also concluded that, under the influence of both fields by increasing the intensity of uniform transverse field the effect of non-uniform axial fields decrease.
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a 3d numerical simulation of mixed convection of a magnetic nanofluid in the presence of non uniform magnetic field in a vertical tube using two phase mixture model
Journal of Magnetism and Magnetic Materials, 2011Co-Authors: Habib Aminfar, Mousa Mohammadpourfard, Yousef Narmani KahnamoueiAbstract:Abstract In this paper, results of applying a non-uniform magnetic field on a ferrofluid (kerosene and 4 vol% Fe 3 O 4 ) flow in a vertical tube have been reported. The hydrodynamics and thermal behavior of the flow are investigated numerically using the two phase mixture model and the control volume technique. Two positive and Negative magnetic field Gradients have been examined. Based on the obtained results the Nusselt number can be controlled externally using the magnetic field with different intensity and Gradients. It is concluded that the magnetic field with Negative Gradient acts similar to Buoyancy force and augments the Nusselt number, while the magnetic field with positive Gradient decreases it. Also with the Negative Gradient of the magnetic field, pumping power increases and vice versa for the positive Gradient case.
Sanjiv K Tiwari - One of the best experts on this subject based on the ideXlab platform.
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vertical magnetic field Gradient in the photospheric layers of sunspots
Astronomy and Astrophysics, 2017Co-Authors: Jayant Joshi, A Lagg, J Hirzberger, S K Solanki, Sanjiv K TiwariAbstract:Aims. We investigate the vertical Gradient of the magnetic field of sunspots in the photospheric layer. Methods. Independent observations were obtained with the Solar Optical Telescope/Spectropolarimeter (SOT/SP) on board the Hinode spacecraft and with the Tenrife Infrared Polarimeter-2 (TIP-2) mounted at the German Vacuum Tower Telescope (VTT). We apply state-of-the-art inversion techniques to both data sets to retrieve the magnetic field and the corresponding vertical Gradient along with other atmospheric parameters in the solar photosphere. Results. In the sunspot penumbrae we detected patches of Negative vertical Gradients of the magnetic field strength, i.e., the magnetic field strength decreases with optical depth in the photosphere. The Negative Gradient patches are located in the inner and partly in the middle penumbrae in both data sets. From the SOT/SP observations we found that the Negative Gradient patches are restricted mainly to the deep photospheric layers and are concentrated near the edges of the penumbral filaments. Magnetohydrodynamic (MHD) simulations also show Negative Gradients in the inner penumbrae, also at the locations of filaments. In the observations and the simulation Negative Gradients of the magnetic field vs. optical depth dominate at some radial distances in the penumbra. The Negative Gradient with respect to optical depth in the inner penumbrae persists even after averaging in the azimuthal direction in the observations and, to a lesser extent, in the MHD simulations. If the Gradients in the MHD simulations are determined with respect to geometrical height, then the azimuthal averages are always positive within the sunspot (above log τ = 0), corresponding to magnetic field increasing with depth, as generally expected. Conclusions. We interpret the observed localized presence of Negative vertical Gradient of the magnetic field strength in the observations as a consequence of stronger field from spines expanding with height and closing above the weaker field inter-spines. The presence of the Negative Gradients with respect to optical depth after azimuthal averaging can be explained by two different mechanisms: the high corrugation of equal optical depth surfaces and the cancellation of polarized signal due to the presence of unresolved opposite polarity patches in the deeper layers of the penumbra.
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vertical magnetic field Gradient in the photospheric layers of sunspots
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: Jayant Joshi, A Lagg, J Hirzberger, S K Solanki, Sanjiv K TiwariAbstract:We investigate the vertical Gradient of the magnetic field of sunspots in the photospheric layer. Independent observations were obtained with the SOT/SP onboard the Hinode spacecraft and with the TIP-2 mounted at the VTT. We apply state-of-the-art inversion techniques to both data sets to retrieve the magnetic field and the corresponding vertical Gradient. In the sunspot penumbrae we detected patches of Negative vertical Gradients of the magnetic field strength, i.e.,the magnetic field strength decreases with optical depth in the photosphere. The Negative Gradient patches are located in the inner and partly in the middle penumbrae in both data sets. From the SOT/SP observations, we found that the Negative Gradient patches are restricted mainly to the deep photospheric layers and are concentrated near the edges of the penumbral filaments. MHD simulations also show Negative Gradients in the inner penumbrae, also at the locations of filaments. Both in the observations and simulation Negative Gradients of the magnetic field vs. optical depth dominate at some radial distances in the penumbra. The Negative Gradient with respect to optical depth in the inner penumbrae persists even after averaging in the azimuthal direction, both in the observations and, to a lesser extent, also in MHD simulations. We interpret the observed localized presence of the Negative vertical Gradient of the magnetic field strength in the observations as a consequence of stronger field from spines expanding with height and closing above the weaker field inter-spines. The presence of the Negative Gradients with respect to optical depth after azimuthal averaging can be explained by two different mechanisms: the high corrugation of equal optical depth surfaces and the cancellation of polarized signal due to the presence of unresolved opposite polarity patches in the deeper layers of the penumbra.
A Paggi - One of the best experts on this subject based on the ideXlab platform.
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temperature profiles of hot gas in early type galaxies
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: D W Kim, Liam Traynor, A Paggi, Ewan Osullivan, Craig S Anderson, D J Burke, R DabruscoAbstract:Using the data products of the Chandra Galaxy Atlas (Kim et al. 2019a), we have investigated the radial profiles of the hot gas temperature in 60 early type galaxies. Considering the characteristic temperature and radius of the peak, dip, and break (when scaled by the gas temperature and virial radius of each galaxy), we propose a universal temperature profile of the hot halo in ETGs. In this scheme, the hot gas temperature peaks at RMAX = 35 +/- 25 kpc (or ~0.04 RVIR) and declines both inward and outward. The temperature dips (or breaks) at RMIN (or RBREAK) = 3 - 5 kpc (or ~0.006 RVIR). The mean slope between RMIN (RBREAK) and RMAX is 0.3 +/- 0.1. Allowing for selection effects and observational limits, we find that the universal temperature profile can describe the temperature profiles of 72% (possibly up to 82%) of our ETG sample. The remaining ETGs (18%) with irregular or monotonically declining profiles do not fit the universal profile and require another explanation. The temperature Gradient inside RMIN (RBREAK) varies widely, indicating different degrees of additional heating at small radii. Investigating the nature of the hot core (HC with a Negative Gradient inside RMIN), we find that HC is most clearly visible in small galaxies. Searching for potential clues associated with stellar, AGN feedback, and gravitational heating, we find that HC may be related to recent star formation. But we see no clear evidence that AGN feedback and gravitational heating play any significant role for HC.
