562 research outputs found
Development of a hybrid multi-scale simulation approach for spray processes
This paper presents a multi-scale approach coupling a Eulerian interface-tracking method and a Lagrangian particle-tracking method to simulate liquid atomisation processes. This method aims to represent the complete spray atomisation process including the primary break-up process and the secondary break-up process, paving the way for high-fidelity simulations of spray atomisation in the dense spray zone and spray combustion in the dilute spray zone. The Eulerian method is based on the coupled level-set and volume-of-fluid method for interface tracking, which can accurately simulate the primary break-up process. For the coupling approach, the Eulerian method describes only large droplet and ligament structures, while small-scale droplet structures are removed from the resolved Eulerian description and transformed into Lagrangian point-source spherical droplets. The Lagrangian method is thus used to track smaller droplets. In this study, two-dimensional simulations of liquid jet atomisation are performed. We analysed Lagrangian droplet formation and motion using the multi-scale approach. The results indicate that the coupling method successfully achieves multi-scale simulations and accurately models droplet motion after the Eulerian–Lagrangian transition. Finally, the reverse Lagrangian–Eulerian transition is also considered to cope with interactions between Eulerian droplets and Lagrangian droplets.This work was supported by the Engineering and Physical Sciences Research Council of the UK (grant number EP/L000199/1)
Physics of puffing and microexplosion of emulsion fuel droplets
The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.This article has been made available through the Brunel Open Access Publishing Fund
Static and dynamic properties of frictional phenomena in a one-dimensional system with randomness
Static and dynamic frictional phenomena at the interface with random
impurities are investigated in a two-chain model with incommensurate structure.
Static frictional force is caused by the impurity pinning and/or by the pinning
due to the regular potential, which is responsible for the breaking of
analyticity transition for impurity-free cases. It is confirmed that the static
frictional force is always finite in the presence of impurities, in contrast to
the impurity-free system. The nature of impurity pinning is discussed in
connection with that in density waves. The kinetic frictional force of a steady
sliding state is also investigated numerically. The relationship between the
sliding velocity dependence of the kinetic frictional force and the strength of
impurity potential is discussed.Comment: RevTex, 14 pages, 6 PostScript figures, to appear in Phys. Rev.
Theoretical Study of Friction: A Case of One-Dimensional Clean Surfaces
A new method has been proposed to evaluate the frictional force in the
stationary state. This method is applied to the 1-dimensional model of clean
surfaces. The kinetic frictional force is seen to depend on velocity in
general, but the dependence becomes weaker as the maximum static frictional
force increases and in the limiting case the kinetic friction gets only weakly
dependent on velocity as described by one of the laws of friction. It is also
shown that there is a phase transition between state with vanishing maximum
static frictional force and that with finite one. The role of randomness at the
interface and the relation to the impurity pinning of the sliding
Charge-Density-Wave are discussed. to appear in Phys.Rev.B. abstract only. Full
text is available upon request. E-mail: [email protected]: 2 pages, Plain TEX, OUCMT-94-
Breakdown of a conservation law in incommensurate systems
We show that invariance properties of the Lagrangian of an incommensurate
system, as described by the Frenkel Kontorova model, imply the existence of a
generalized angular momentum which is an integral of motion if the system
remains floating. The behavior of this quantity can therefore monitor the
character of the system as floating (when it is conserved) or locked (when it
is not). We find that, during the dynamics, the non-linear couplings of our
model cause parametric phonon excitations which lead to the appearance of
Umklapp terms and to a sudden deviation of the generalized momentum from a
constant value, signalling a dynamical transition from a floating to a pinned
state. We point out that this transition is related but does not coincide with
the onset of sliding friction which can take place when the system is still
floating.Comment: 7 pages, 6 figures, typed with RevTex, submitted to Phys. Rev. E
Replaced 27-03-2001: changes to text, minor revision of figure
Dynamical frictional phenomena in an incommensurate two-chain model
Dynamical frictional phenomena are studied theoretically in a two-chain model
with incommensurate structure. A perturbation theory with respect to the
interchain interaction reveals the contributions from phonons excited in each
chain to the kinetic frictional force. The validity of the theory is verified
in the case of weak interaction by comparing with numerical simulation. The
velocity and the interchain interaction dependences of the lattice structure
are also investigated. It is shown that peculiar breaking of analyticity states
appear, which is characteristic to the two-chain model. The range of the
parameters in which the two-chain model is reduced to the Frenkel-Kontorova
model is also discussed.Comment: RevTex, 9 pages, 7 PostScript figures, to appear in Phys. Rev.
Anomalous pinning behavior in an incommensurate two-chain model of friction
Pinning phenomena in an incommensurate two-chain model of friction are
studied numerically. The pinning effect due to the breaking of analyticity
exists in the present model. The pinning behavior is, however, quite different
from that for the breaking of analyticity state of the Frenkel-Kontorova model.
When the elasticity of chains or the strength of interchain interaction is
changed, pinning force and maximum static frictional force show anomalously
complicated behavior accompanied by a successive phase transition and they
vanish completely under certain conditions.Comment: RevTex, 9 pages, 19 figures, to appear in Phys. Rev. B58 No.23(1998
Identification of plant genetic resources with high potential contribution to soil fertility enhancement in the Sahel, with special interest in fallow vegetation
The sandy soil in the Sahel is characterized as low inherent fertility, that is, having nutrient deficiency (total N and available P), low organic matter and high risk of erosion. Under the concept of integrated soil fertility management (ISFM), possible contribution of natural inhabitant plants to the improvement of soil fertility in the Sahel was evaluated. A broad variation in δ 15N values was observed among the plant species commonly found in cropland and fallow land of the Sahelian zone. Annual leguminous herbs, Cassia mimosoides (Caesalpiniaceae) and Alysicarpus ovalifolius (Papilionaceae), had low δ 15N values, showing their higher dependency on biological nitrogen fixation. They will be efficiently utilized as an extensive means of soil fertility management, for example, through more encouraged incorporation into the fallow vegetation. Ctenium elegans, Eragrostis tremula and Schizachyrium exile, greatly dominating annual grass species in the fallow land, though their δ 15N values were high, would contribute to the soil fertility by supplying a significant amount of organic matte
Effect of Joule heating in current-driven domain wall motion
It was found that high current density needed for the current-driven domain
wall motion results in the Joule heating of the sample. The sample temperature,
when the current-driven domain wall motion occurred, was estimated by measuring
the sample resistance during the application of a pulsed-current. The sample
temperature was 750 K for the threshold current density of 6.7 x 10^11 A/m2 in
a 10 nm-thick Ni81Fe19 wire with a width of 240 nm. The temperature was raised
to 830 K for the current density of 7.5 x 10^11 A/m2, which is very close to
the Curie temperature of bulk Ni81Fe19. When the current density exceeded 7.5 x
10^11 A/m2, an appearance of a multi-domain structure in the wire was observed
by magnetic force microscopy, suggesting that the sample temperature exceeded
the Curie temperature.Comment: 13 pages, 4 figure
Impurity Scattering from -layers in Giant Magnetoresistance Systems
The properties of the archetypal Co/Cu giant magnetoresistance (GMR)
spin-valve structure have been modified by the insertion of very thin
(sub-monolayer) -layers of various elements at different points within
the Co layers, and at the Co/Cu interface. Different effects are observed
depending on the nature of the impurity, its position within the periodic
table, and its location within the spin-valve. The GMR can be strongly enhanced
or suppressed for various specific combinations of these parameters, giving
insight into the microscopic mechanisms giving rise to the GMR.Comment: 5 pages, 2 figure
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