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H. Sadeghi

Publications and source records attributed to H. Sadeghi.

18 recordsLinked to original sources
arXiv2021

Influence of trapezoidal lobe strut on fuel mixing and combustion in supersonic combustion chamber

The fuel injection system is the main process in supersonic vehicles. In this article, injection of fuel jet through a trapezoidal strut in the coaxial supersonic air stream is investigated. The numerical method is applied for the simulation of three-dimensional trapezoidal lobe strut in supersonic flow. Hydrogen gas is injected from the end of the trapezoidal strut injector to augment fuel mixing and diffusion downstream by intensifying vortices inside the domain. The power of the vortices varies by the injector velocity and free stream condition. To simulate our model, RANS equations with the SST turbulence model are solved. Effects of free-stream velocity and fuel jet total pressure on fuel distribution and mixing are also investigated. Our results indicate that fuel mixing enhances about 18% when a rectangular strut is replaced by a trapezoidal one. An achieved finding disclosed that the strength of vortices is amplified by the presence of a trapezoidal strut injector.

physics.flu-dyn↗
arXiv2017

Arrested relaxation in an isolated molecular ultracold plasma

Spontaneous avalanche to plasma splits the core of an ellipsoidal Rydberg gas of nitric oxide. Ambipolar expansion first quenches the electron temperature of this core plasma. Then, long-range, resonant charge transfer from ballistic ions to frozen Rydberg molecules in the wings of the ellipsoid quenches the centre-of-mass ion/Rydberg molecule velocity distribution. This sequence of steps gives rise to a remarkable mechanics of self-assembly, in which the kinetic energy of initially formed hot electrons and ions drives an observed separation of plasma volumes. These dynamics adiabatically sequester energy in a reservoir of mass transport, starting a process that anneals separating volumes to form an apparent glass of strongly coupled ions and electrons. Short-time electron spectroscopy provides experimental evidence for complete ionization. The long lifetime of this system, particularly its stability with respect to recombination and neutral dissociation, suggests that this transformation affords a robust state of arrested relaxation, far from thermal equilibrium.

physics.plasm-ph↗
arXiv2015

Generalized P-Reducible $(α, β)$-Metrics with Vanishing S-curvature

In this paper, we study one of the open problems in Finsler geometry which presented by Matsumoto-Shimada about the existence of P-reducible metric which is not C-reducible. For this aim, we study a class of Finsler metrics called generalized P-reducible metrics that contains the class of P-reducible metrics. We prove that every generalized P-reducible $(α, β)$-metric with vanishing S-curvature reduces to a Berwald metric or C-reducible metric. It results that there is not any concrete P-reducible $(α,β)$-metric with vanishing S-curvature.

math.DG↗
arXiv2015

On Generalized Douglas-Weyl $(α, β)$-Metrics

In this paper, we study generalized Douglas-Weyl $(α, β)$-metrics. Suppose that an regular $(α, β)$-metric $F$ is not of Randers type. We prove that $F$ is a generalized Douglas-Weyl metric with vanishing S-curvature if and only if it is a Berwald metric. Moreover by ignoring the regularity, if $F$ is not a Berwald metric then we find a family of almost regular Finsler metrics which is not Douglas nor Weyl. As its application, we show that generalized Douglas-Weyl square metric or Matsumoto metric with isotropic mean Berwald curvature are Berwald metrics.

math.DG↗
arXiv2015

Thermoelectric properties of graphene/boron nitride heterostructures

Using density functional theory combined with a Green's function scattering approach, we examine the thermoelectric properties of hetero-nanoribbons formed from alternating lengths of graphene and boron nitride. In such structures, the boron nitride acts as a tunnel barrier, which weakly couples states in the graphene, to form mini-bands . In un-doped nanoribbons, the mini bands are symmetrically positioned relative to the Fermi energy and do not enhance thermoelectric performance significantly. In contrast, when the ribbons are doped by electron donating or electron accepting adsorbates, the thermopower S and electronic figure of merit are enhanced and either positive or negative thermopowers can be obtained. In the most favourable case, doping with the electron donor tetrathiafulvalene (TTF) increases the room-temperature thermopower to -284 μv/K and doping by the electron acceptor tetracyanoethylene (TCNE) increases S to 210 μv/K. After including both electron and phonon contributions to the thermal conductance, figures of merit ZT up to of order 0.9 are obtained.

cond-mat.mes-hall↗
arXiv2015

GOLLUM: a next-generation simulation tool for electron, thermal and spin transport

We have developed an efficient simulation tool 'GOLLUM' for the computation of electrical, spin and thermal transport characteristics of complex nanostructures. The new multi-scale, multi-terminal tool addresses a number of new challenges and functionalities that have emerged in nanoscale-scale transport over the past few years. To illustrate the flexibility and functionality of GOLLUM, we present a range of demonstrator calculations encompassing charge, spin and thermal transport, corrections to density functional theory such as LDA+U and spectral adjustments, transport in the presence of non-collinear magnetism, the quantum-Hall effect, Kondo and Coulomb blockade effects, finite-voltage transport, multi-terminal transport, quantum pumps, superconducting nanostructures, environmental effects and pulling curves and conductance histograms for mechanically-controlled-break-junction experiments.

cond-mat.mes-hall↗
arXiv2013

The Astrophysical S-factor of the 12C(α,γ)16O Reaction at Solar Energies

The astrophysical S-factor of the 4He-12C radiative capture is calculated in the potential model at the energy range 0.1-2.0 MeV. Radiative capture 12C(α,γ)16O is extremely relevant for the fate of massive stars and determines if the remnant of a supernova explosion becomes a black hole or a neutron star. Because this reaction occurs at low-energies the experimental measurements is very difficult and perhaps impossible. In this paper, radiative capture of the 12C(α,γ)16O reaction at very low-energies is taken as a case study. In comparison with other theoretical methods and available experimental data, excellent agreement is achieved for the astrophysical S-factor of this process.

nucl-th↗
arXiv2013

Strain Gradient Elasticity Solution for Functionally Graded Micro-cylinders

In this paper, strain gradient elasticity formulation for analysis of FG (Functionally Graded) micro-cylinders is presented. The material properties are assumed to obey a power law in radial direction. The governing differential equation is derived as a fourth order ODE. A power series solution for stresses and displacements in FG micro-cylinders subjected to internal and external pressures is obtained. Numerical examples are presented to study the effect of the characteristic length parameter and FG power index on the displacement field and stress distribution in FG cylinders. It is shown that the characteristic length parameter has a considerable effect on the stress distribution of FG micro-cylinders. Also, increasing material length parameter leads to decrease of the maximum radial and tangential stresses in the cylinder. Furthermore, it is shown that the FG power index has a significant effect on the maximum radial and tangential stresses.

cond-mat.mtrl-sci↗
arXiv2013

Axisymmetric thermoelectroelastic analysis of a piezoelectric half-space

In this study, an analytical solution is presented for thermo-electro-elastic analysis of piezoelectric semi-infinite bodies. For this purpose, governing equations are derived for a transversely isotropic piezoelectric material under axisymmetric thermo-electro-mechanical loading condition. A general closed form analytical solution is presented for the complementary and particular parts of the components of displacement vector and also electric potential function. Then, boundary conditions are imposed and in that case an explicit solution is obtained for piezoelectric semi-infinite bodies. Results show that when a piezoelectric half-space is subjected to constant/ramp surface thermal loading the maximum absolute value of radial stress occurs at the surface of the body. Whereas, the maximum absolute value of stress in normal direction of the half-space surface occurs not on the surface but somewhere near the surface of the body. Moreover, the peak values for these stress curves in the case of combined loadings are closest to the half space surface and the farthest in the case of ramp decaying thermal loading. In contrast to the constant/ramp surface thermal loading no clear maximum point for electric field distribution for the case of combined loadings can be seen in the normal direction to the half-space boundary.

cond-mat.mtrl-sci↗
arXiv2013

Finsler Metrics with Bounded Cartan Torsions

The norm of Cartan torsion plays an important role for studying of immersion theory in Finsler geometry. Indeed, Finsler manifold with unbounded Cartan torsion can not be isometrically imbedded into any Minkowski space. In this paper, we find two subclasses of (?, ?)-metrics which have bounded Cartan torsion. Then, we give two subclasses of (?, ?)-metrics whose bound on the Cartan torsions are independent of the norm of ?.

math.DG↗
arXiv2011

On the formation and decay of a molecular ultracold plasma

Double-resonant photoexcitation of nitric oxide in a molecular beam creates a dense ensemble of $50f(2)$ Rydberg states, which evolves to form a plasma of free electrons trapped in the potential well of an NO$^+$ spacecharge. The plasma travels at the velocity of the molecular beam, and, on passing through a grounded grid, yields an electron time-of-flight signal that gauges the plasma size and quantity of trapped electrons. This plasma expands at a rate that fits with an electron temperature as low as 5 K, colder that typically observed for atomic ultracold plasmas. The recombination of molecular NO$^+$ cations with electrons forms neutral molecules excited by more than twice the energy of the NO chemical bond, and the question arises whether neutral fragmentation plays a role in shaping the redistribution of energy and particle density that directs the short-time evolution from Rydberg gas to plasma. To explore this question, we adapt a coupled rate-equations model established for atomic ultracold plasmas to describe the energy-grained avalanche of electron-Rydberg and electron-ion collisions in our system. Adding channels of Rydberg predissociation and two-body, electron- cation dissociative recombination to the atomic formalism, we investigate the kinetics by which this relaxation distributes particle density and energy over Rydberg states, free electrons and neutral fragments. The results of this investigation suggest some mechanisms by which molecular fragmentation channels can affect the state of the plasma.

physics.chem-ph↗
arXiv2010

Triton Photodisintegration with Effective Field Theory

Effective field theory (EFT) has been recently used for the calculation of neutron-deuteron radiative capture at very low energies.We present here the use of EFT to calculate the two-body photodisintegration of the triton, considering the three-body force. The calculated cross section shows sharp rising from threshold to maximum about 0.88 mb at 13 MeV and decreasing slightly to about 0.81 mb at 19 MeV, in agreement with the experimental data. Our results are in good agreement with the experimental data and the other calculations using modern realistic two- and three-nucleon forces, like AV18/UrbanaIX potential.

nucl-th↗
arXiv2009

Application of EFT at Thermal Energies

We have been evaluated some observables of n-d systems by using pionless Effective Field Theory(\EFTNoPion) and insertion of the three-body force up to next-to-next to leading order(N$^2$LO). The evaluated data has been compared with experimental and the three-nucleon calculation of the total cross section with modern realistic two- and three-nucleon forces AV18/UrbIX potential models calculations.

nucl-th↗
arXiv2009

Triton Electric Form Factor at Low-Energies

Making use of the Effective Field Theory(EFT) expansion recently developed by the authors, we compute the charge form factor of triton up to next-to-next-to-leading order (N$^2$LO). The three-nucleon forces(3NF) is required for renormalization of the three-nucleon system and it effects are predicted for process and is qualitatively supported by available experimental data. We also show that, by including higher order corrections, the calculated charge form factor and charge radius of $^3$H are in satisfactory agreement with the experimental data and the realistic Argonne $v_{18}$ two-nucleon and Urbana IX potential models calculations. This method makes possible a high precision few-body calculations in nuclear physics. Our result converges order by order in low energy expansion and also cut-off independent.

nucl-th↗
arXiv2007

Neutron-Deuteron System and Photon Polarization Parameter at Thermal Neutron Energies

Effective Field Theory(EFT) is, the unique, model independent and systematic low-energy version of QCD for processes involving momenta below the pion mass. A low-energy photo-nuclear observable in three-body systems, photon polarization parameter at thermal neutron energies is calculated by using pionless EFT up to next-to-next to leading order(N$^2$LO). In order to make a comparative study of this model, we compared our results for photon polarization parameter with the realistic Argonne $v_{18}$ two-nucleon and Urbana IX or Tucson-Melbourne three-nucleon interactions. Three-body currents give small but significant contributions to some of the observables in the neutron-deuteron radiative capture cross section at thermal neutron energies. In this formalism the three-nucleon forces are needed up to N$^2$LO for cut-off independent results. Our result converges order by order in low energy expansion and also cut-off independent at this order.

nucl-th↗
arXiv2006

Effective field theory calculation of nd radiative capture at thermal energies

The cross section for the thermal neutron capture by the deuteron is calculated with pionless Effective Field Theory(EFT). No new Three-Nucleon forces are needed up to next-to-next-to-leading order in order to achieve cut-off independent results, besides those fixed by the triton binding energy and Nd scattering length in the triton channel. The cross-section is accurately determined to be $σ_{tot}=[0.503\pm 0.003]mb$. At zero energies, the magnetic $M1$-transition gives the dominant contribution and is calculated up to next-to-next-to-leading order (N$^2$LO). Close agreement between the available experimental data and the calculated cross section is reached. We demonstrate convergence and cutoff independence order by order in the low-energy expansion.

nucl-th↗
arXiv2005

$Nd\to{^3H}γ$ with Effective Field Theory

The cross section of neutron-deuteron radiative capture $n d\to {^3H}γ$ is calculated at energies relevant to Big-Bang nucleosynthesis ($ 20 \leq E \leq 200 $ Kev) with pionless Effective Field Theory. At these energies, magnetic transition M1 gives the dominant contribution. The M1 amplitude is calculated up to next-to-next-to leading order(N$^2$LO) with insertion of three-body force. Results are in good agreement within few percent theoretical uncertainty in comparison with available calculated data below E=200 Kev.

nucl-th↗