20,611 research outputs found
Reduced gravity liquid configuration simulator
Reduced gravity liquid configuration simulator to study propellant behavior in rocket fuel tank
Quantum-field-theoretical approach to phase-space techniques: Symmetric Wick theorem and multitime Wigner representation
In this work we present the formal background used to develop the methods
used in earlier works to extend the truncated Wigner representation of quantum
and atom optics in order to address multi-time problems. The truncated Wigner
representation has proven to be of great practical use, especially in the
numerical study of the quantum dynamics of Bose condensed gases. In these
cases, it allows for the simulation of effects which are missed entirely by
other approximations, such as the Gross-Pitaevskii equation, but does not
suffer from the severe instabilities of more exact methods. The numerical
treatment of interacting many-body quantum systems is an extremely difficult
task, and the ability to extend the truncated Wigner beyond single-time
situations adds another powerful technique to the available toolbox. This
article gives the formal mathematics behind the development of our "time-Wigner
ordering" which allows for the calculation of the multi-time averages which are
required for such quantities as the Glauber correlation functions which are
applicable to bosonic fields.Comment: Submitted to PR
Direct indication of particle size in fluidized beds
Differential pressure measurements indicate particle size and particle size distribution in fluidized beds. The technique is based on the relationship between bed particle size and the intensity and frequency of fluctuations. By measuring the fluctuations, an estimate of average particle size of the fluid-bed material can be made
Weld sequence optimization: the use of surrogate models for solving sequential combinatorial problems
The solution of combinatorial optimization problems usually involves the consideration of many possible design configurations. This often makes such approaches computationally expensive, especially when dealing with complex finite element models. Here a surrogate model is proposed that can be used to reduce substantially the computational expense of sequential combinatorial finite element problems. The model is illustrated by application to a weld path planning problem
Effects of ultrasound on Transforming Growth Factor-beta genes in bone cells
Therapeutic ultrasound (US) is a widely used form of biophysical stimulation that is increasingly applied to promote fracture healing. Transforming growth factor-beta (TGF-beta), which is encoded by three related but different genes, is known to play a major part in bone growth and repair. However, the effects of US on the expression of the TGF-beta genes and the physical acoustic mechanisms involved in initiating changes in gene expression in vitro, are not yet known. The present study demonstrates that US had a differential effect on these TGF-beta isoforms in a human osteoblast cell line, with the highest dose eliciting the most pronounced up-regulation of both TGF-beta1 and TGF-beta3 at 1 hour after treatment and thereafter declining. In contrast, US had no effect on TGF-beta2 expression. Fluid streaming rather than thermal effects or cavitation was found to be the most likely explanation for the gene responses observed in vitro
Occupation number and fluctuations in the finite-temperature Bose-Hubbard model
We study the occupation numbers and number fluctuations of ultra-cold atoms
in deep optical lattices for finite temperatures within the Bose-Hubbard model.
Simple analytical expressions for the mean occupation number and number
fluctuations are obtained in the weak-hopping regime using an interpolation
between results from different perturbation approaches in the Mott-insulator
and superfluid phases. These analytical results are compared to exact one
dimensional numerical calculations using a finite temperature variant of the
Density-Matrix Renormalisation Group (DMRG) method and found to have a high
degree of accuracy. We also find very good agreement in the crossover
``thermal'' region. With the present approach the magnitude of number
fluctuations under realistic experimental conditions can be estimated and the
properties of the finite temperature phase diagram can be studied.Comment: 4 pages, 1 eps figure, submitted to PR
Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone
Background: Evolutionary processes, including selection and differential fitness, shape the introgression of genetic material across a hybrid zone, resulting in the exchange of some genes but not others. Differential introgression of molecular or phenotypic markers can thus provide insight into factors contributing to reproductive isolation. We characterized patterns of genetic variation across a hybrid zone between two tidal marsh birds, Saltmarsh (Ammodramus caudacutus) and Nelson’s (A. nelsoni) sparrows (n = 286), and compared patterns of introgression among multiple genetic markers and phenotypic traits.
Results: Geographic and genomic cline analyses revealed variable patterns of introgression among marker types. Most markers exhibited gradual clines and indicated that introgression exceeds the spatial extent of the previously documented hybrid zone. We found steeper clines, indicating strong selection for loci associated with traits related to tidal marsh adaptations, including for a marker linked to a gene region associated with metabolic functions, including an osmotic regulatory pathway, as well as for a marker related to melanin-based pigmentation, supporting an adaptive role of darker plumage (salt marsh melanism) in tidal marshes. Narrow clines at mitochondrial and sex-linked markers also offer support for Haldane’s rule. We detected patterns of asymmetrical introgression toward A. caudacutus, which may be driven by differences in mating strategy or differences in population density between the two species.
Conclusions: Our findings offer insight into the dynamics of a hybrid zone traversing a unique environmental gradient and provide evidence for a role of ecological divergence in the maintenance of pure species boundaries despite ongoing gene flow
Phase-space analysis of bosonic spontaneous emission
We present phase-space techniques for the modelling of spontaneous emission
in two-level bosonic atoms. The positive-P representation is shown to give a
full and complete description and can be further developed to give exact
treatments of the interaction of degenerate bosons with the electromagnetic
field in a given experimental situation. The Wigner representation, even when
truncated at second order, is shown to need a doubling of the phase-space to
allow for a positive-definite diffusion matrix in the appropriate Fokker-Planck
equation and still fails to agree with the full quantum results of the
positive-P representation. We show that quantum statistics and correlations
between the ground and excited states affect the dynamics of the emission
process, so that it is in general non-exponential.Comment: 16 pages, 6 figure
Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone
Background: Evolutionary processes, including selection and differential fitness, shape the introgression of genetic material across a hybrid zone, resulting in the exchange of some genes but not others. Differential introgression of molecular or phenotypic markers can thus provide insight into factors contributing to reproductive isolation. We characterized patterns of genetic variation across a hybrid zone between two tidal marsh birds, Saltmarsh (Ammodramus caudacutus) and Nelson’s (A. nelsoni) sparrows (n = 286), and compared patterns of introgression among multiple genetic markers and phenotypic traits.
Results: Geographic and genomic cline analyses revealed variable patterns of introgression among marker types. Most markers exhibited gradual clines and indicated that introgression exceeds the spatial extent of the previously documented hybrid zone. We found steeper clines, indicating strong selection for loci associated with traits related to tidal marsh adaptations, including for a marker linked to a gene region associated with metabolic functions, including an osmotic regulatory pathway, as well as for a marker related to melanin-based pigmentation, supporting an adaptive role of darker plumage (salt marsh melanism) in tidal marshes. Narrow clines at mitochondrial and sex-linked markers also offer support for Haldane’s rule. We detected patterns of asymmetrical introgression toward A. caudacutus, which may be driven by differences in mating strategy or differences in population density between the two species.
Conclusions: Our findings offer insight into the dynamics of a hybrid zone traversing a unique environmental gradient and provide evidence for a role of ecological divergence in the maintenance of pure species boundaries despite ongoing gene flow
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