7,260 research outputs found
Cavitation induced starvation for piston-ring/liner tribological conjunction
The study investigates the mechanism of ring-liner lubrication in the vicinity of the top and bottom dead centres of an internal combustion engine. Predicting lubricant transient behaviour is critical when the inlet reversal leads to thin films and inherent metal-to-metal interaction. It was found that the cavitation, which is located at the trailing edge of the contact before reversal, briefly survives after reversal as a confined bubble at the leading edge. This depletes the film promoting starvation. Several algorithms were compared. It is concluded that the lubricant film is thinner than initially thought
Modelling the closest double degenerate system RXJ0806.3+1527 and its decreasing period
In the hypothesis that the 5.4m binary RXJ0806.3+1527 consists of a low mass
helium white dwarf (donor) transferring mass towards its more massive white
dwarf companion (primary), we consider as possible donors white dwarfs which
are the result of common envelope evolution occurring when the helium core mass
of the progenitor giant was still very small (~ 0.2Msun), so that they are
surrounded by a quite massive hydrogen envelope (~1/100Msun or larger), and
live for a very long time supported by proton--proton burning. Mass transfer
from such low mass white dwarfs very probably starts during the hydrogen
burning stage, and the donor structure will remain dominated by the burning
shell until it loses all the hydrogen envelope and begins transferring helium.
We model mass transfer from these low mass white dwarfs, and show that the
radius of the donor decreases while they shed the hydrogen envelope. This
radius behavior, which is due to the fact that the white dwarf is not fully
degenerate, has two important consequences on the evolution of the binary: 1)
the orbital period decreases, with a timescale consistent with the period
decrease of the binary RXJ0806.3+1527; 2) the mass transfer rate is a factor of
about 10 smaller than from a fully degenerate white dwarf, easing the problem
connected with the small X-ray luminosity of this object. The possibility that
such evolution describes the system RXJ0806.3+1527 is also consistent with the
possible presence of hydrogen in the optical spectrum of the star, whose
confirmation would become a test of the model.Comment: 13 pages, 4 figures, accepted for publication on ApJ, main journa
Were most Low Mass X ray Binaries born in Globular Clusters?
We summarize the status of art of the secular evolution of low mass X-ray binaries (LMXBs) and take a close look at the orbital period distribution of LMXBs and of binary millisecond pulsars (MSP), in the hypothesis that this latter results from the LMXB evolution. The deficiency of systems below the period gap, which in cataclysmic binaries occurs between ~ 2 and 3 hr, points to a very different secular evolution of LMXBs with respect to their counterparts containing a white dwarf compact object. The presence of several ultrashort period LMXBs (some of which are also X-ray millisecond pulsars), the important fraction of binary MSPs at periods between 0.1 and 1 day, the periods (26 and 32hr) of two ``interacting'' MSPs in Globular Clusters are other pieces of the puzzle in the period distribution. We consider the possible explanations for these peculiarities, and point out that Grindlay's old proposal that all (most of) LMXBs in the field were originally born in globular clusters must be carefully reconsidered
Aspects of meson properties in dense nuclear matter
We investigate the modification of meson spectral densities in dense nuclear
matter at zero temperature. These effects are studied in a fully relativistic
mean field model which goes beyond the linear density approximation and also
includes baryon resonances. In particular, the role of N*(1520) and N*(1720) on
the rho meson spectral density is highlighted. Even though the nucleon-nucleon
loop and the nucleon-resonance loop contribute with the opposite sign, an
overall reduction of rho meson mass is still observed at high density.
Importantly, it is shown that the resonances cause substantial broadening of
the rho meson spectral density in matter and also induces non-trivial momentum
dependence. The spectral density of the a0 meson is also shown. We study the
dispersion relations and collective oscillations induced by the rho meson
propagation in nuclear matter together with the influence of the mixing of rho
with the a0 meson. The relevant expression for the plasma frequency is also
recovered analytically in the appropriate limit.Comment: 19 pages, 17 figure
Planetary nebulae in the elliptical galaxy NGC 821: kinematics and distance determination
Using a slitless spectroscopy method with the 8.2 m Subaru telescope and its
FOCAS Cassegrain spectrograph, we have increased the number of planetary nebula
(PN) detections and PN velocity measurements in the flattened elliptical galaxy
NGC 821. A comparison with the detections reported previously by the Planetary
Nebula Spectrograph (PN.S) group indicates that we have confirmed most of their
detections. The velocities measured by the two groups, using different
telescopes, spectrographs and slitless techniques, are in good agreement. We
have built a combined sample of 167 PNs and have confirmed the keplerian
decline of the line-of-sight velocity dispersion reported previously. We also
confirm misaligned rotation from the combined sample. A dark matter halo may
exist around this galaxy, but it is not needed to keep the PN velocities below
the local escape velocity as calculated from the visible mass. We have measured
the m(5007) magnitudes of 145 PNs and produced a statistically complete sample
of 40 PNs in NGC 821. The resulting PN luminosity function (PNLF) was used to
estimate a distance modulus of 31.4 mag, equivalent to 19 Mpc. We also
estimated the PN formation rate. NGC 821 becomes the most distant galaxy with a
PNLF distance determination. The PNLF distance modulus is smaller than the
surface brightness fluctuation (SBF) distance modulus by 0.4 mag. Our kinematic
information permits to rule out the idea that a shorter PNLF distance could be
produced by the contamination of the PNLF by background galaxies with emission
lines redshifted into the on-band filter transmission curve.Comment: Accepted for publication in ApJ; 16 figure
Multipartite entanglement in 2 x 2 x n quantum systems
We classify multipartite entangled states in the 2 x 2 x n (n >= 4) quantum
system, for example the 4-qubit system distributed over 3 parties, under local
filtering operations. We show that there exist nine essentially different
classes of states, and they give rise to a five-graded partially ordered
structure, including the celebrated Greenberger-Horne-Zeilinger (GHZ) and W
classes of 3 qubits. In particular, all 2 x 2 x n-states can be
deterministically prepared from one maximally entangled state, and some
applications like entanglement swapping are discussed.Comment: 9 pages, 3 eps figure
Dispatchable High-Power Wind Turbine Based on a Multilevel Converter with Modular Structure and Hybrid Energy Storage Integration
This paper presents a new multilevel converter solution with modular structure and hybrid energy-storage integration suitable to drive modern/future high-power medium-voltage wind turbines. The hybrid energy-storage integration means that part of the converter submodules are built with batteries and part of them with conventional capacitors. Since traditional wind turbines are non-dispatchable generators, the integration of an energy storage system could be beneficial in multiple ways as the wind power plant could provide stability support to the grid, improvement of the unit commitment and economic dispatch, and the power plant owner could increase his revenues in the electricity market. The capacitors of the proposed converter are responsible to transfer the power produced by the wind turbine to the grid, and the batteries are only charged/discharged with the mismatch between the power produced by the turbine and the power to be injected into the grid, considering a dispatchable operation where the power injected into the grid is different from the power generated by the turbine. The medium-voltage structure could be an interesting option to overcome problems related to high currents in modern/future high-power wind turbines resulting in more efficient, more compact and lighter solutions. Modular multilevel converters are suitable to handle medium-voltage levels and they allow for a straightforward integration of energy storage systems in a decentralized manner
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