80,269 research outputs found
Nonlinear softening as a predictive precursor to climate tipping
Approaching a dangerous bifurcation, from which a dynamical system such as
the Earth's climate will jump (tip) to a different state, the current stable
state lies within a shrinking basin of attraction. Persistence of the state
becomes increasingly precarious in the presence of noisy disturbances. We
consider an underlying potential, as defined theoretically for a saddle-node
fold and (via averaging) for a Hopf bifurcation. Close to a stable state, this
potential has a parabolic form; but approaching a jump it becomes increasingly
dominated by softening nonlinearities. If we have already detected a decrease
in the linear decay rate, nonlinear information allows us to estimate the
propensity for early tipping due to noise. We argue that one needs to extract
information about the nonlinear features (a "softening") of the underlying
potential from the time series to judge the probability and timing of tipping.
This analysis is the logical next step if one has detected a decrease of the
linear decay rate. If there is no discernable trend in the linear analysis,
nonlinear softening is even more important in showing the proximity to tipping.
After extensive normal form calibration studies, we check two geological time
series from paleo-climate tipping events for softening of the underlying well.
For the ending of the last ice age, where we find no convincing linear
precursor, we identify a statistically significant nonlinear softening towards
increasing temperature. The analysis has thus successfully detected a warning
of the imminent tipping event.Comment: 22 pages, 11 figures, changed title back, corrected smaller mistakes,
updated reference
Pattern formation in large domains
Pattern formation is a phenomenon that arises in a wide variety of physical, chemical
and biological situations. A great deal of theoretical progress has been made in
understanding the universal aspects of pattern formation in terms of amplitudes of
the modes that make up the pattern. Much of the theory has sound mathematical
justification, but experiments and numerical simulations over the last decade have
revealed complex two-dimensional patterns that do not have a satisfactory theoretical
explanation. This paper focuses on quasi-patterns, where the appearance of small
divisors causes the standard theoretical method to fail, and ends with a discussion
of other outstanding problems in the theory of two-dimensional pattern formation
in large domains
Life assessment of combustor liner using unified constitutive models
Hot section components of gas turbine engines are subject to severe thermomechanical loads during each mission cycle. Inelastic deformation can be induced in localized regions leading to eventual fatigue cracking. Assessment of durability requires reasonably accurate calculation of the structural response at the critical location for crack initiation. In recent years nonlinear finite element computer codes have become available for calculating inelastic structural response under cyclic loading. NASA-Lewis sponsored the development of unified constitutive material models and their implementation in nonlinear finite element computer codes for the structural analysis of hot section components. These unified models were evaluated with regard to their effect on the life prediction of a hot section component. The component considered was a gas turbine engine combustor liner. A typical engine mission cycle was used for the thermal and structural analyses. The analyses were performed on a CRAY computer using the MARC finite element code. The results were compared with laboratory test results, in terms of crack initiation lives
Constraining the Surface Inhomogeneity and Settling Times of Metals on Accreting White Dwarfs
Due to the short settling times of metals in DA white dwarf atmospheres, any
white dwarfs with photospheric metals must be actively accreting. It is
therefore natural to expect that the metals may not be deposited uniformly on
the surface of the star. We present calculations showing how the temperature
variations associated with white dwarf pulsations lead to an observable
diagnostic of the surface metal distribution, and we show what constraints
current data sets are able to provide. We also investigate the effect that
time-variable accretion has on the metal abundances of different species, and
we show how this can lead to constraints on the gravitational settling times.Comment: 4 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letters, updated reference
Structural assessment of a space station solar dynamic heat receiver thermal energy storage canister
The structural performance of a space station thermal energy storage (TES) canister subject to orbital solar flux variation and engine cold start up operating conditions was assessed. The impact of working fluid temperature and salt-void distribution on the canister structure are assessed. Both analytical and experimental studies were conducted to determine the temperature distribution of the canister. Subsequent finite element structural analyses of the canister were performed using both analytically and experimentally obtained temperatures. The Arrhenius creep law was incorporated into the procedure, using secondary creep data for the canister material, Haynes 188 alloy. The predicted cyclic creep strain accumulations at the hot spot were used to assess the structural performance of the canister. In addition, the structural performance of the canister based on the analytically determined temperature was compared with that based on the experimentally measured temperature data
Dynamics of localized spins coupled to the conduction electrons with charge/spin currents
The effects of the charge/spin currents of conduction electrons on the
dynamics of the localized spins are studied in terms of the perturbation in the
exchange coupling between them. The equations of motion for the
localized spins are derived exactly up to , and the equations for
the two-spin system is solved numerically. It is found that the dynamics
depends sensitively upon the relative magnitude of the charge and spin
currents, i.e., it shows steady state, periodic motion, and even chaotic
behavior. Extension to the multi-spin system and its implications including
possible ``spin current detector'' are also discussed.Comment: 5 pages, 4 figures, REVTe
Precursor of Color Superconductivity
We investigate possible precursory phenomena of color superconductivity at
finite temperature with an effective theory of QCD. It is found that the
fluctuation of the diquark pair field exists with a prominent strength even
well above the critical temperature . We show that such a fluctuaiton
forms a collective mode, the corresponding pole of which approaches the origin
as is lowered to in the complex energy plane. We discuss the possible
relevance of the precursor to the observables to be detected in heavy-ion
collisions.Comment: 4 pages, 5 figures, Talk presented at the XVIth International
Conference on Particles and Nuclei (PANIC02), Osaka, Japan, Sep.30 - Oct.4,
2002, Uses espcrc1.st
Identifying the transporters of different flavonoids in plants
We recently identified a new component of flavonoid transport pathways in Arabidopsis. The MATE protein FFT (Flower Flavonoid Transporter) is primarily found in guard cells and seedling roots, and mutation of the transporter results in floral and growth phenotypes. The nature of FFT’s substrate requires further exploration but our data suggest that it is a kaempferol diglucoside. Here we discuss potential partner H+-ATPases and possible redundancy among the close homologues within the large Arabidopsis MATE family
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