2,874 research outputs found
Experimental verification and practical application of torquewhirl theory of rotordynamic instability
A theory developed by Vance in 1978 to explain the destabilizing effect of torque on a whirling rotor was experimentally verified. The measurements made on a specially designed test apparatus are described. New computer models were also developed to investigate the effect of torque on rotordynamic stability of multidisk flexible rotor bearing systems. The effect of torque was found to be most pronounced when the system is already marginally stable from other influences. The modifications required to include torque in a typical shaft transfer matrix are described, and results are shown which identify the type of rotor design most sensitive to load torque
Instantonic approach to triple well potential
By using a usual instanton method we obtain the energy splitting due to
quantum tunneling through the triple well barrier. It is shown that the term
related to the midpoint of the energy splitting in propagator is quite
different from that of double well case, in that it is proportional to the
algebraic average of the frequencies of the left and central wells.Comment: Revtex, 11 pages, Included one eps figur
Exclusive Measurement of the Nonmesonic Weak Decay of ^{5}_{\Lambda}He Hypernucleus
We performed a coincidence measurement of two nucleons emitted from the
nonmesonic weak decay (NMWD) of ^{5}_{\Lambda}He formed via the
^{6}Li(\pi^+,K^+) reaction. The energies of two nucleons and the pair number
distributions in the opening angle between them were measured. In both np and
nn pairs, we observed a clean back-to-back correlation coming from the two-body
decay of \Lambda p --> n p and \Lambda n --> n n, respectively. The ratio of
the nucleon pair numbers was N_{nn}/N_{np}=0.45 \pm 0.11(stat)\pm 0.03(syst) in
the kinematic region of cos(theta_{NN}) < -0.8. Since each decay mode was
exclusively detected, the measured ratio should be close to the ratio of
\Gamma(\Lambda p --> np)/\Gamma(\Lambda n --> nn).
The ratio is consistent with recent theoretical calculations based on the
heavy meson/direct quark exchange picture.Comment: Submitted to Phys. Rev. lett., 4 pages, 3 figure
H I content in Coma cluster substructure
Galaxy clusters are some of largest structures in the universe. These very
dense environments tend to be home to higher numbers of evolved galaxies that
what is found in lower density environments. It is well known that dense
environments can influence the evolution of galaxies through the removal of the
neutral gas (HI) reservoirs which fuel star formation. It is unclear which
environment has a stronger effect: the local environment (i.e. the substructure
within the cluster), or the cluster itself. Using the new HI data from the
Westerbork Coma Survey, we explore the average HI content of galaxies across
the cluster comparing galaxies that reside in substructure to those that do
not. We apply to the Dressler-Shectman test to our newly compiled redshift
catalogue of the Coma cluster to search for substructure. With so few of the
Coma galaxies directly detected in HI, we use the HI stacking technique to
probe average HI content below what can be directly detected. Using the
Dressler-Shectman test, we find 15 substructures within the footprint of the
Westerbork Coma Survey. We compare the average HI content for galaxies within
substructure to those not in substructure. Using the HI stacking technique, we
find that the Coma galaxies (for which are not detected in HI) are more than
10--50 times more HI deficient than expected which supports the scenario of an
extremely efficient and rapid quenching mechanism. By studying the galaxies
that are not directly detected in HI, we also find Coma to be more HI deficient
than previously thought.Comment: 18 pages (+ 21 page appendix), 23 figures, accepted for publication
in A&
Large enhancement of the thermopower in NaCoO at high Na doping
Research on the oxide perovskites has uncovered electronic properties that
are strikingly enhanced compared with those in conventional metals. Examples
are the high critical temperatures of the cuprate superconductors and the
colossal magnetoresistance in the manganites. The conducting layered cobaltate
displays several interesting electronic phases as is varied
including water-induced superconductivity and an insulating state that is
destroyed by field. Initial measurements showed that, in the as-grown
composition, displays moderately large thermopower and
conductivity . However, the prospects for thermoelectric cooling
applications faded when the figure of merit was found to be small at this
composition (0.60.7). Here we report that, in the poorly-explored
high-doping region 0.75, undergoes an even steeper enhancement. At the
critical doping 0.85, (at 80 K) reaches values 40 times
larger than in the as-grown crystals. We discuss prospects for low-temperature
thermoelectric applications.Comment: 6 pages, 7 figure
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