57,698 research outputs found
Radiation force on relativistic jets in active galactic nuclei
Radiative deceleration of relativistic jets in active galactic nuclei as the
result of inverse Compton scattering of soft photons from accretion discs is
discussed. The Klein-Nishina (KN) cross section is used in the calculation of
the radiation force due to inverse Compton scattering. Our result shows that
deceleration due to scattering in the KN regime is important only for jets
starting with a bulk Lorentz factor larger than 1000. When the bulk Lorentz
factor satisfies this condition, particles scattering in the Thomson regime
contribute positively to the radiation force (acceleration), but those
particles scattering in the KN regime are dominant and the overall effect is
deceleration. In the KN limit, the drag due to Compton scattering, though less
severe than in the Thomson limit, strongly constrains the bulk Lorentz factor.
Most of the power from the deceleration goes into radiation and hence the
ability of the jet to transport significant power (in particle kinetic energy)
out of the subparsec region is severely limited. The deceleration efficiency
decreases significantly if the jet contains protons and the proton to electron
number density ratio satisfies the condition where is the minimum Lorentz factor of
relativistic electrons (or positrons) in the jet frame and is the
proton to electron mass ratio.Comment: 10 pages including 8 figures; accepted for publication in MNRA
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Modelling and simulation on the tool wear in nanometric cutting
Tool wear is a significant factor affecting the machined surface quality. In this paper, a Molecular Dynamics (MD) simulation approach is proposed to model the wear of the diamond tool in nanometric cutting. It includes the effects of the cutting heat on the workpiece property. MD simulation is carried out to simulate the nanometric cutting of a single crystal silicon plate with the diamond tip of an Atomic Force Microscope (AFM). The wear mechanism is investigated by the calculation of the temperature, the stress in the diamond tip, and the analysis of the relationship between the temperature and sublimation energy of the diamond atoms and silicon atoms. Microstrength is used to characterize the wear resistance of the diamond tool. The machining trials on an AFM are performed to validate the results of the MD simulation. The results of MD simulation and AFM experiments all show that the thermo-chemical wear is the basic wear mechanism of the diamond cutting tool
The dynamics of bistable liquid crystal wells
A planar bistable liquid crystal device, reported in Tsakonas et al. [27], is modelled within the Landau-de Gennes theory for nematic liquid crystals. This planar device consists of an array of square micron-sized wells. We obtain six different classes of equilibrium profiles and these profiles are classified as diagonal or rotated solutions. In the strong anchoring case, we propose a Dirichlet boundary condition that mimics the experimentally imposed tangent boundary conditions. In the weak anchoring case, we present a suitable surface energy and study the multiplicity of solutions as a function of the anchoring strength. We find that diagonal solutions exist for all values of the anchoring strength W ≥ 0 while rotated solutions only exist for W ≥ Wc > 0, where Wc is a critical anchoring strength that has been computed numerically. We propose a dynamic model for the switching mechanisms based on only dielectric effects. For sufficiently strong external electric fields, we numerically demonstrate diagonal to rotated and rotated to diagonal switching by allowing for variable anchoring strength across the domain boundary
Thermodynamical quantities of lattice full QCD from an efficient method
I extend to QCD an efficient method for lattice gauge theory with dynamical
fermions. Once the eigenvalues of the Dirac operator and the density of states
of pure gluonic configurations at a set of plaquette energies (proportional to
the gauge action) are computed, thermodynamical quantities deriving from the
partition function can be obtained for arbitrary flavor number, quark masses
and wide range of coupling constants, without additional computational cost.
Results for the chiral condensate and gauge action are presented on the
lattice at flavor number , 1, 2, 3, 4 and many quark masses and coupling
constants. New results in the chiral limit for the gauge action and its
correlation with the chiral condensate, which are useful for analyzing the QCD
chiral phase structure, are also provided.Comment: Latex, 11 figures, version accepted for publicatio
Bound States and Critical Behavior of the Yukawa Potential
We investigate the bound states of the Yukawa potential , using different algorithms: solving the Schr\"odinger
equation numerically and our Monte Carlo Hamiltonian approach. There is a
critical , above which no bound state exists. We study the
relation between and for various angular momentum quantum
number , and find in atomic units, , with , ,
, and .Comment: 15 pages, 12 figures, 5 tables. Version to appear in Sciences in
China
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An investigation on the mechanics of nanometric cutting and the development of its test-bed
The mechanics of machining at a very small depth of cut (100 nm or less) is not
well understood. The chip formation physics, cutting forces generation, resulting
temperatures and the size effects significantly affect the efficiency of the process
and the surface quality of the workpiece. In this paper, the cutting mechanics
at nanometric scale are investigated in comparison with conventional cutting
principles. Molecular Dynamics (MD) is used to model and simulate the nanometric
cutting processes. The models and simulated results are evaluated and
validated by the cutting trials on an atomic force microscope (AFM).
Furthermore, the conceptual design of a bench-type ultraprecision machine tool
is presented and the machine aims to be a facility for nanometric cutting of threedimensional
MEMS devices. The paper concludes with a discussion on the potential
and applications of nanometric cutting techniques/equipment for the
predictabilty, producibility and productivity of manufacturing at the nanoscale
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