9,809 research outputs found
Self-Similar Force-Free Wind From an Accretion Disk
We consider a self-similar force-free wind flowing out of an infinitely thin
disk located in the equatorial plane. On the disk plane, we assume that the
magnetic stream function scales as , where is the
cylindrical radius. We also assume that the azimuthal velocity in the disk is
constant: , where is a constant. For each choice of the
parameters and , we find an infinite number of solutions that are
physically well-behaved and have fluid velocity throughout the domain
of interest. Among these solutions, we show via physical arguments and
time-dependent numerical simulations that the minimum-torque solution, i.e.,
the solution with the smallest amount of toroidal field, is the one picked by a
real system. For , the Lorentz factor of the outflow increases
along a field line as \gamma \approx M(z/\Rfp)^{(2-\nu)/2} \approx R/R_{\rm
A}, where \Rfp is the radius of the foot-point of the field line on the disk
and R_{\rm A}=\Rfp/M is the cylindrical radius at which the field line
crosses the Alfven surface or the light cylinder. For , the Lorentz
factor follows the same scaling for z/\Rfp < M^{-1/(1-\nu)}, but at larger
distances it grows more slowly: \gamma \approx (z/\Rfp)^{\nu/2}. For either
regime of , the dependence of on shows that the rotation of
the disk plays a strong role in jet acceleration. On the other hand, the
poloidal shape of a field line is given by z/\Rfp \approx
(R/\Rfp)^{2/(2-\nu)} and is independent of . Thus rotation has neither a
collimating nor a decollimating effect on field lines, suggesting that
relativistic astrophysical jets are not collimated by the rotational winding up
of the magnetic field.Comment: 21 pages, 15 figures, accepted to MNRA
Theoretical analysis of perching and hovering maneuvers
Unsteady aerodynamic phenomena are encountered in a large number of modern aerospace and non-aerospace applications. Leading edge vortices (LEVs) are of particular interest because of their large impact on the forces and performance. In rotorcraft applications, they cause large vibrations and torsional loads (dynamic stall), affecting the performance adversely. In insect
flight however, they contribute positively by enabling high-lift flight. Identifying the conditions that result in LEV formation and modeling their effects on the flow is an important ongoing challenge. Perching (airfoil decelerates to rest) and hovering (zero freestream velocity) maneuvers are of special interest. In earlier work by the authors, a Leading Edge Suction Parameter (LESP) was developed to predict LEV formation for airfoils undergoing arbitrary variation in pitch and plunge at a constant freestream velocity. In this research, the LESP criterion is extended to situations where the freestream velocity is varying or zero. A point-vortex model based on this criterion is developed and results from the model are compared against those from a computational fluid dynamics (CFD) method. Abstractions of perching and hovering maneuvers are used to validate the low-order model's performance in highly unsteady vortex-dominated flows, where the time-varying freestream/translational velocity is small in magnitude compared to the other contributions to the velocity experienced by the leading edge region of the airfoil. Time instants of LEV formation, flow topologies and force coefficient histories for the various motion kinematics from the low-order model and CFD are obtained and compared. The LESP criterion is seen to be successful in predicting the start of LEV formation and the point-vortex method is effective in modeling the flow development and forces on the airfoil. Typical run-times for the low-order method are between 30-40 seconds, making it a potentially convenient tool for control/design applications
മത്സ്യവിപണനം (Marketing of fish and fishery products)
Fish production and marketing are two important
components of fisheries development. In 1996, India produced
5.1 million tonnes of fish of which 68% was marketed in fresh form, 12% in frozen form, 14% in dried form and the rest was converted into fish meal or used for other purposes
Significance of thermal fluctuations and hydrodynamic interactions in receptor-ligand mediated adhesive dynamics of a spherical particle in wall bound shear flow
The dynamics of adhesion of a spherical micro-particle to a ligand-coated
wall, in shear flow, is studied using a Langevin equation that accounts for
thermal fluctuations, hydrodynamic interactions and adhesive interactions.
Contrary to the conventional assumption that thermal fluctuations play a
negligible role at high Pclet numbers, we find that for particles
with low surface densities of receptors, rotational diffusion caused by
fluctuations about the flow and gradient directions aids in bond formation,
leading to significantly greater adhesion on average, compared to simulations
where thermal fluctuations are completely ignored. The role of wall
hydrodynamic interactions on the steady state motion of a particle, when the
particle is close to the wall, has also been explored. At high Pclet
numbers, the shear induced force that arises due to the stresslet part of the
Stokes dipole, plays a dominant role, reducing the particle velocity
significantly, and affecting the states of motion of the particle. The coupling
between the translational and rotational degrees of freedom of the particle,
brought about by the presence of hydrodynamic interactions, is found to have no
influence on the binding dynamics. On the other hand, the drag coefficient,
which depends on the distance of the particle from the wall, plays a crucial
role at low rates of bond formation. A significant difference in the effect of
both the shear force and the position dependent drag force, on the states of
motion of the particle, is observed when the Plet number is small.Comment: The manuscript has been accepted as an article in Physical Review E
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