The Experts below are selected from a list of 279 Experts worldwide ranked by ideXlab platform
F G Mitri - One of the best experts on this subject based on the ideXlab platform.
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optical pulling force and torques on rayleigh semiconductor prolate and Oblate Spheroids in bessel tractor beams
Journal of Quantitative Spectroscopy & Radiative Transfer, 2017Co-Authors: F G MitriAbstract:Abstract Optical tractor Bessel beams are gaining increased interest where a negative attractive force acting in opposite direction of wave propagation is harnessed for particle manipulation in opto-fluidics, the manufacturing of periodic composite metamaterials and other related applications. Previous works considered the spherical geometry, however, it is of some importance to develop improved models to investigate objects of more complex shapes and study the tractor beam effect on them. The aim of this work is therefore directed toward this goal, where the dipole approximation method is used to compute the optical force, spin and orbital torques on a subwavelength semiconductor spheroid illuminated by a zeroth-order Bessel vector beam. Numerical computations for the Cartesian components of the optical radiation force on prolate and Oblate Spheroids with arbitrary orientation are performed, with emphasis on the emergence of a negative pulling force and its dependence on the half-cone angle of the beam, the aspect ratio of the spheroid, and its orientation in space. Moreover, the Cartesian components of the spin radiation torque are computed where a negative spin torque can arise, which causes a rotational twisting effect of the spheroid around its center of mass in either the counterclockwise or the clockwise (negative) direction of spinning. In addition, the axial component of the orbital radiation torque is computed which also shows sign reversal. The results of this analysis provide a priori information for the design and development of novel optical tweezers devices and tractor beams, with potential applications in the manipulation and handling of elongated particles.
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optical bessel tractor beam on active dielectric rayleigh prolate and Oblate Spheroids
Journal of The Optical Society of America B-optical Physics, 2017Co-Authors: F G MitriAbstract:Optical Bessel tractor beams, designed to produce a negative pulling force on a particle, are gaining increased attention for applications in noncontact remote sampling, particle manipulation, and handling, to name some examples. In the long-wavelength (Rayleigh) limit, known also as the electric dipole approximation, earlier investigations demonstrated that a zeroth-order Bessel beam incident upon a passive dielectric sphere (i.e., no radiating sources in its core) always acts as a repulsor beam, which causes the particle to be pushed away from the source in the forward direction of the linear momentum. In contrast to what has already been established, this work shows that the incident wave field can act as a tractor beam (where a small spheroid is pulled backwards towards the source due to a negative attractive force) in the dipole approximation (Rayleigh) limit, provided that the particle is made of an active material, i.e., a dielectric spheroid acting as an oscillating source for which the extinction energy efficiency is negative. Numerical computations for the Cartesian components of the optical radiation force on active prolate and Oblate Spheroids with arbitrary orientation are performed. Emphasis is placed on the emergence of the tractor beam behavior and its dependence upon the half-cone angle, the polarization type of the incident beam, the spheroid aspect ratio, as well as its orientation in space. The analysis is extended to calculate the Cartesian components of the spin radiation torque, which causes a rotation of the spheroid around its center of mass in either the counterclockwise or the clockwise (negative) direction of spinning. Unlike the case of a sphere, the optical spin torque arises for a nonabsorptive Oblate or prolate spheroid with arbitrary orientation in the field of a zeroth-order Bessel beam. Potential applications in optically engineered metamaterials, optical tractor beams, tweezers, particle manipulation, rotation, and handling would benefit from the results of this study.
Luca Brandt - One of the best experts on this subject based on the ideXlab platform.
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drag reduction in turbulent channel flow laden with finite size Oblate Spheroids
Journal of Fluid Mechanics, 2017Co-Authors: Niazi M Ardekani, Pedro Costa, Wimpaul Breugem, Francesco Picano, Luca BrandtAbstract:We study suspensions of Oblate rigid particles in a viscous fluid for different values of the particle volume fractions.Direct numerical simulations have been performed using a direct-forcing immer ...
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drag reduction in turbulent channel flow laden with finite size Oblate Spheroids
arXiv: Fluid Dynamics, 2016Co-Authors: Niazi M Ardekani, Pedro Costa, Wimpaul Breugem, Francesco Picano, Luca BrandtAbstract:We study suspensions of Oblate rigid particles in a viscous fluid for different values of the particle volume fractions. Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle-particle and particle-wall interactions. With respect to the single phase flow, we show that in flows laden with Oblate Spheroids the drag is reduced and the turbulent fluctuations attenuated. In particular, the turbulence activity decreases to lower values than those obtained by only accounting for the effective suspension viscosity. To explain the observed drag reduction we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of Oblate particles. These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.
Dominique Thévenin - One of the best experts on this subject based on the ideXlab platform.
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effect of particle density in turbulent channel flows with resolved Oblate Spheroids
Computers & Fluids, 2019Co-Authors: Amir Eshghinejadfard, Seyed Ali Hosseini, Dominique ThéveninAbstract:Abstract The present paper studies the effect of particle density in a turbulent channel flow laden with resolved Oblate Spheroids at a frictional Reynolds number of Re τ = 180 . Direct numerical simulations are performed by using the lattice Boltzmann method (LBM) for solving the flow field. Particle-fluid interactions are modeled by the immersed boundary method (IBM). Simulations are done for dense regimes of heavy and neutrally-buoyant particles with a volume fraction of 5%. The particle-fluid density ratios are 8.0 and 1.0 for heavy and neutrally-buoyant particles, respectively. Results show that particle inertia can significantly modify the fluid and particle statistics. Heavy particles cause a higher reduction of the fluid streamwise velocity with respect to the single-phase flow. Turbulence is attenuated by both particle types but the reduction is stronger with heavy particles than that with neutrally-buoyant ones. Moreover, increasing the density of particles is found to create smaller but more energetic vortices. Quadrant analysis shows that the contribution of ejection and sweep events in the Reynolds shear stress reduce on increasing the particle density. The local volume fractions of the two particle types are also seen to be different. While the volume fraction of neutrally-buoyant particles reach a constant value at a certain distance from the walls, the concentration of heavy Oblate Spheroids increases gradually all the way up to channel center. Both particle types show preferential orientation near the walls, where the symmetry axis is normal to the wall. However, this preferential orientation is less pronounced when increasing the particle inertia. Finally, the translational velocity fluctuations of heavy particles are found to be higher in the streamwise direction, but lower in the wall-normal direction.
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Lattice Boltzmann simulation of resolved Oblate Spheroids in wall turbulence
Journal of Fluid Mechanics, 2018Co-Authors: Amir Eshghinejadfard, Lihao Zhao, Dominique ThéveninAbstract:The present study focuses on the behaviour of fully resolved Oblate Spheroids in turbulent channel flows using the lattice Boltzmann method (LBM). Mean and maximum drag reductions of 1.3 % and 4.4 %, respectively, are observed at a solid-phase volume fraction of 10 % by using Oblate Spheroids of aspect ratio $\unicode[STIX]{x1D706}=1/3$ and equivalent diameter $D_{eq}/H=1/6.5$ . The behaviour of Oblate Spheroids and spheres are found to be different in the near-wall region, where Oblate Spheroids, in contrast to spheres, do not augment the near-wall turbulence. Strong reduction of spanwise and wall-normal fluid velocity fluctuations in the buffer layer by using Oblate Spheroids is observed. It is also seen that reduction of Reynolds shear stress cannot solely guarantee the occurrence of drag reduction, as spherical particles increase the drag in spite of the reduction in Reynolds shear stress. Moreover, in drag-reduced particle-laden flows, although the spanwise spacing of streaks increases, vortices are stronger and smaller. Thus, in addition to the streak cycle of turbulence regeneration, the vortices generated by particles contribute to higher vortex strengths. By examining the role of the Oblate spheroid concentration, it is found to mainly influence the streak spacing, with a minor effect on the strength of vortices. Drag reduction in Oblate-spheroid-laden flows is thus attributed to the reduction of fluid velocity fluctuations in the transverse directions and enhancement of the spanwise streak spacing. Higher vortex strength has less effect on the initiation of drag reduction but can influence the level of drag reduction and explains the lower drag-reduction effect of rigid particles compared to polymers. By quadrant analysis, sweeps and ejections are seen to have less contribution to the Reynolds shear stress by using Oblate Spheroids. With respect to the particle data, Oblate Spheroids tend to orient with their symmetry axis mainly along the wall-normal direction. Very close to the wall, spanwise orientation is dominant and wall-normal alignment of the symmetry axis is reduced. Furthermore, Oblate Spheroids move faster than the fluid, before reaching a similar streamwise velocity in the channel centre. In contrast, spheres move slower than the fluid in the buffer region, as well as showing a local concentration peak near the wall. It is also seen that the angular velocity of Oblate Spheroids decays very rapidly whereas spheres have a higher rotation rate which shows a smoother reduction towards the centre.
Ehud Gavze - One of the best experts on this subject based on the ideXlab platform.
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the orientation dynamics of small prolate and Oblate Spheroids in linear shear flows
International Journal of Multiphase Flow, 2016Co-Authors: Ehud Gavze, Mark Pinsky, Alexander KhainAbstract:Abstract The present work deals with the stable orientation of Oblate and prolate Spheroids in general steady linear flows and with the mode of convergence to these stable orientations. The orientation dynamics is governed by the Jeffery equation. The stable orientations are either fixed points or limit cycles in the orientation space. The type of stable orientation depends on whether the eigenvalues of the linear part of Jeffery equation are real or complex. We define prolate and Oblate Spheroids to be equivalent if the aspect ratio of one is the reciprocal of the other. We show that, in a given flow, equivalent Oblate and prolate Spheroids possess the same number of fixed points and limit cycles of which only one is stable. If they possess only fixed points, then their corresponding stable fixed points are orthogonal. If they possess one fixed point and one limit cycle each, then the stable fixed point of one is orthogonal to the plane of the limit cycle of the other. The rate of convergence to these attractors is important to consideration of the orientations in time-space varying flow fields. We show that non-normal growth (NNG) of the distance to these attractors may delay the convergence by several characteristic shear time scales. We derive conditions for occurrence of NNG and explicit expressions for the maximal duration of the growth. We consider a specific case of which the vorticity is a stable orientation of prolate Spheroids. We analyze the conditions that imply monotonic or, conversely, non-monotonic convergence to this orientation due to NNG. We thereby find the corresponding conditions for convergence of the equivalent Oblate Spheroids to their attractors, normal to the vorticity. We show that the convergence is monotonic if the vorticity is parallel to the strain tensor’s largest eigenvector, but that NNG occurs if the vorticity is parallel to the strain tensor’s intermediate eigenvector. The NNG duration decreases with increasing vorticity-strain ratio and with the strain intermediate eigenvalue approaching the largest eigenvalue.
Niazi M Ardekani - One of the best experts on this subject based on the ideXlab platform.
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drag reduction in turbulent channel flow laden with finite size Oblate Spheroids
Journal of Fluid Mechanics, 2017Co-Authors: Niazi M Ardekani, Pedro Costa, Wimpaul Breugem, Francesco Picano, Luca BrandtAbstract:We study suspensions of Oblate rigid particles in a viscous fluid for different values of the particle volume fractions.Direct numerical simulations have been performed using a direct-forcing immer ...
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drag reduction in turbulent channel flow laden with finite size Oblate Spheroids
arXiv: Fluid Dynamics, 2016Co-Authors: Niazi M Ardekani, Pedro Costa, Wimpaul Breugem, Francesco Picano, Luca BrandtAbstract:We study suspensions of Oblate rigid particles in a viscous fluid for different values of the particle volume fractions. Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle-particle and particle-wall interactions. With respect to the single phase flow, we show that in flows laden with Oblate Spheroids the drag is reduced and the turbulent fluctuations attenuated. In particular, the turbulence activity decreases to lower values than those obtained by only accounting for the effective suspension viscosity. To explain the observed drag reduction we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of Oblate particles. These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.
