arXiv subjects

Seyed Mojtaba Rezaei Sani

Publications and source records attributed to Seyed Mojtaba Rezaei Sani.

5 recordsLinked to original sources

arXiv2022

Controlling Flow Separation over a Curved Ramp Using Vortex Generator Microjets

Introducing a fluid microjet into the boundary layer to increase fluid momentum and hence delay separation is a method for actively controlling a flow separation region. The present work numerically analyzed the control of a separation bubble behind a ramp. For this purpose, we first verified the numerical results for a flow (without a jet) over the ramp against reliable experimental studies from the literature. Next, the effects of introducing a microjet to the flow were also verified. A jet was then placed at three different distances above the ramp to study its impact on various parameters, including velocities, Reynolds stresses, pressure, vorticity, streamlines, and the separation bubble size. As the jet was moved further back, the jet-induced upwash region grew considerably. Finally, the effects of using three identical jets were studied and compared against those of a single jet. The results indicated that using a three-jet array shrank the separation bubble. Using an array with d/D = 15 can limit laterally the separation bubble about 2.75 times smaller than a single jet in the z-direction. Also, the employment of the jet managed to decrease the length of the separation zone in the x-direction up to 78%, in the case of Lx/L1 = 0.0143 and d/D = 10.

physics.flu-dyn↗

arXiv2022

A Comparison of Various Turbulence Models for Analysis of Fluid Microjet Injection into the Boundary Layer over a Flat Surface

The present work studied various models for predicting turbulence in the problem of injecting a fluid microjet into the boundary layer of a turbulent flow. For this purpose, the one-equation Spalart-Allmaras (SA), two-equation k-$ε$ and k-$ω$, multi-equation transition k-kL-$ω$, transition shear stress transport (SST), and Reynolds stress models were used for solving the steady flow. Moreover, the transition SST, scale-adaptive simulation (SAS), and detached eddy simulation (DES) models were used for the transient flow. A comparison of the results indicated that the steady solution methods performed sufficiently well for this problem. Furthermore, it was found that the four-equation transition SST model was the most accurate method for the prediction of turbulence in this problem. This model predicted the velocity along the x-axis in near- and far-jet locations with about 1% and 5% errors, respectively. It also outperformed the other methods in predicting Reynolds stresses, especially at the center (with an about 5% error).

physics.flu-dyn↗

arXiv2017

$d^0$ half-metallic ferromagnetism in CaN and CaAs pnictides: An ab initio study

Conventional magnetism occurs in systems which contain transition metals or rare earth ions with partially filled $d$ or $f$ shells. It is theoretically predicted that compounds of groups IA and IIA with IV and V, in some structural phases, are ferromagnetic half-metals which made them new candidates for spintronics applications. Employing density functional theory (DFT) we investigate magnetism in binary compounds CaN and CaAs. Regarding the structure of analogous magnetic materials and experimental results of CaAs synthesis, we have considered two cubic structures: rocksalt (RS) and zincblende (ZB), and four hexagonal structures: NiAs, wurtzite (WZ), anti-NiAs, and NaO. The calculated results show that CaN in cubic, NiAs, and wurtzite structures, and CaAs only in zincblende phase have ferromagnetic ground states with a magnetic moment of $1μ_B$. Electronic structure analysis of these materials indicates that magnetism originates from anion $p$ states. Existence of flat $p$ bands and consequently high density of states at the Fermi level of magnetic structures gives rise to Stoner spin splitting and spontaneous ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2017

Calculation of dispersion equations for uniaxial dielectric-magnetic mediums

In this overview paper, we investigate the dispersion of electromagnetic waves for dissipative and non-dissipative dielectric-magnetic uniaxial mediums. Changing the sign of one component of dielectric permittivity (ε) or magnetic permeability (μ) from positive to negative will lead to sixteen different cases (real or imaginary) for a non-dissipative medium and eight different cases for dissipative one. In a non-dissipative medium, dispersion relations follow the elliptic/hyperbolic relations. This rule completely vanishes regarding the dissipative medium. While the number of dispersion cases in a dissipative medium is lower than a non-dissipative one, there are cases which are not allowed in non-dissipative mediums but allowed in dissipative mediums.

physics.optics↗

arXiv2014

Density functional investigation of spin polarization in bulk and thin films of nitrogen intercalated Cu3N

It has been reported theoretically that the intercalation of nitrogen in the voids of the rather open cubic structure of bulk Cu3N build up a magnetic structure. In an extended effort to study this system, we have investigated spin polarization in bulk and thin films of nitrogen intercalated Cu3N (Cu3N2) structure by means of first-principles calculations based on Kohn-Sham density functional theory and ultrasoft pseudopotentials technique. Contrary to the previous study, the results show that after an accurate structural relaxation of the system, magnetism in the bulk structure vanishes. This effect is due to the migration of the intercalated nitrogen atom from the body center of the cell to the nearness of one of the cell faces. Similar study for the thin films of 5, 7, 9 and 11 monolayers thickness was performed and it was found that initial relaxation of structures with 7 and 11 monolayers show a net magnetic moment of 2.6 μB. By a more extended survey of the energy surfaces, the film with 7 monolayers loses its magnetic moment similar to the bulk structure but the film with 11 monolayers maintains its magnetic moment. It is possibly a new quantum size effect that keeps the intercalated nitrogen atom of the middlemost cell at the body center site. Electron density map of this film clearly confirms the spin polarization upon the intercalated atom.

cond-mat.mtrl-sci↗