55 research outputs found
Doppler Effect of Nonlinear Waves and Superspirals in Oscillatory Media
Nonlinear waves emitted from a moving source are studied. A meandering spiral
in a reaction-diffusion medium provides an example, where waves originate from
a source exhibiting a back-and-forth movement in radial direction. The periodic
motion of the source induces a Doppler effect that causes a modulation in
wavelength and amplitude of the waves (``superspiral''). Using the complex
Ginzburg-Landau equation, we show that waves subject to a convective Eckhaus
instability can exhibit monotonous growth or decay as well as saturation of
these modulations away from the source depending on the perturbation frequency.
Our findings allow a consistent interpretation of recent experimental
observations concerning superspirals and their decay to spatio-temporal chaos.Comment: 4 pages, 4 figure
Hyperpolarizability effects in a Sr optical lattice clock
We report the observation of the higher order frequency shift due to the
trapping field in a Sr optical lattice clock. We show that at the magic
wavelength of the lattice, where the first order term cancels, the higher order
shift will not constitute a limitation to the fractional accuracy of the clock
at a level of . This result is achieved by operating the clock at
very high trapping intensity up to kW/cm and by a specific study of
the effect of the two two-photon transitions near the magic wavelength
Accurate spectroscopy of Sr atoms
We report the frequency measurement with an accuracy in the 100 kHz range of
several optical transitions of atomic Sr : at 689 nm, at 688 nm and at 679 nm. Measurements are performed with
a frequency chain based on a femtosecond laser referenced to primary frequency
standards. They allowed the indirect determination with a 70 kHz uncertainty of
the frequency of the doubly forbidden 5s^2^1S_0- 5s5p^3P_0 transition of
Sr at 698 nm and in a second step its direct observation. Frequency
measurements are performed for Sr and Sr, allowing the
determination of , and isotope shifts, as well as the
hyperfine constants.Comment: 12 pages, 16 figure
Measurement of the 3s3p 3P1 lifetime in magnesium using a magneto-optical trap
We demonstrate an accurate method for measuring the lifetime of
long-lived metastable magnetic states using a magneto-optical trap
(MOT). Through optical pumping, the metastable (3s3p) (3)P(1) level is
populated in a standard MOT. During the optical pumping process, a
fraction of the population is captured in the magnetic quadrupole field
of the MOT. When the metastable atoms decay to the (3s(2)) (1)S(0)
ground state they are recaptured into the MOT. In this system no
alternative cascading transition is possible. The lifetime of the
metastable level is measured directly as an exponential load time of the
MOT. We have experimentally tested our method by measuring the lifetime
of the (3s3p) (3)P(1) of (24)Mg. This lifetime has been measured
numerous times previously, but with quite different results. Using our
method we find the (3s3p) (3)P(1) lifetime to be (4.4 +/- 0.2) ms.
Theoretical values point toward a lower value for the lifetime
Forecasting the SST space-time variability of the Alboran Sea with genetic algorithms
We propose a nonlinear ocean forecasting technique based on a combination of
genetic algorithms and empirical orthogonal function (EOF) analysis. The method
is used to forecast the space-time variability of the sea surface temperature
(SST) in the Alboran Sea. The genetic algorithm finds the equations that best
describe the behaviour of the different temporal amplitude functions in the EOF
decomposition and, therefore, enables global forecasting of the future
time-variability.Comment: 15 pages, 3 figures; latex compiled with agums.st
On elliptic solutions of the cubic complex one-dimensional Ginzburg-Landau equation
The cubic complex one-dimensional Ginzburg-Landau equation is considered.
Using the Hone's method, based on the use of the Laurent-series solutions and
the residue theorem, we have proved that this equation has neither elliptic
standing wave nor elliptic travelling wave solutions. This result amplifies the
Hone's result, that this equation has no elliptic travelling wave solutions.Comment: LaTeX, 12 page
Branch-and-lift algorithm for deterministic global optimization in nonlinear optimal control
This paper presents a branch-and-lift algorithm for solving optimal control problems with smooth nonlinear dynamics and potentially nonconvex objective and constraint functionals to guaranteed global optimality. This algorithm features a direct sequential method and builds upon a generic, spatial branch-and-bound algorithm. A new operation, called lifting, is introduced, which refines the control parameterization via a Gram-Schmidt orthogonalization process, while simultaneously eliminating control subregions that are either infeasible or that provably cannot contain any global optima. Conditions are given under which the image of the control parameterization error in the state space contracts exponentially as the parameterization order is increased, thereby making the lifting operation efficient. A computational technique based on ellipsoidal calculus is also developed that satisfies these conditions. The practical applicability of branch-and-lift is illustrated in a numerical example. © 2013 Springer Science+Business Media New York
Spatial Modeling of Vesicle Transport and the Cytoskeleton: The Challenge of Hitting the Right Road
The membrane trafficking machinery provides a transport and sorting system for many cellular proteins. We propose a mechanistic agent-based computer simulation to integrate and test the hypothesis of vesicle transport embedded into a detailed model cell. The method tracks both the number and location of the vesicles. Thus both the stochastic properties due to the low numbers and the spatial aspects are preserved. The underlying molecular interactions that control the vesicle actions are included in a multi-scale manner based on the model of Heinrich and Rapoport (2005). By adding motor proteins we can improve the recycling process of SNAREs and model cell polarization. Our model also predicts that coat molecules should have a high turnover at the compartment membranes, while the turnover of motor proteins has to be slow. The modular structure of the underlying model keeps it tractable despite the overall complexity of the vesicle system. We apply our model to receptor-mediated endocytosis and show how a polarized cytoskeleton structure leads to polarized distributions in the plasma membrane both of SNAREs and the Ste2p receptor in yeast. In addition, we can couple signal transduction and membrane trafficking steps in one simulation, which enables analyzing the effect of receptor-mediated endocytosis on signaling
BioSimulators: a central registry of simulation engines and services for recommending specific tools
Computational models have great potential to accelerate bioscience, bioengineering, and medicine. However, it remains challenging to reproduce and reuse simulations, in part, because the numerous formats and methods for simulating various subsystems and scales remain siloed by different software tools. For example, each tool must be executed through a distinct interface. To help investigators find and use simulation tools, we developed BioSimulators (https://biosimulators.org), a central registry of the capabilities of simulation tools and consistent Python, command-line and containerized interfaces to each version of each tool. The foundation of BioSimulators is standards, such as CellML, SBML, SED-ML and the COMBINE archive format, and validation tools for simulation projects and simulation tools that ensure these standards are used consistently. To help modelers find tools for particular projects, we have also used the registry to develop recommendation services. We anticipate that BioSimulators will help modelers exchange, reproduce, and combine simulations
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