37,221 research outputs found
Dispersion of biased swimming microorganisms in a fluid flowing through a tube
Classical Taylor-Aris dispersion theory is extended to describe the transport
of suspensions of self-propelled dipolar cells in a tubular flow. General
expressions for the mean drift and effective diffusivity are determined exactly
in terms of axial moments, and compared with an approximation a la Taylor. As
in the Taylor-Aris case, the skewness of a finite distribution of biased
swimming cells vanishes at long times. The general expressions can be applied
to particular models of swimming microorganisms, and thus be used to predict
swimming drift and diffusion in tubular bioreactors, and to elucidate competing
unbounded swimming drift and diffusion descriptions. Here, specific examples
are presented for gyrotactic swimming algae.Comment: 20 pages, 4 figures. Published version available at
http://rspa.royalsocietypublishing.org/content/early/2010/02/09/rspa.2009.0606.short?rss=
Integration of a mean-torque diesel engine model into a hardware-in-the-loop shipboard network simulation using lambda tuning
This study describes the creation of a hardware-in-the-loop (HIL) environment for use in evaluating network architecture, control concepts and equipment for use within marine electrical systems. The environment allows a scaled hardware network to be connected to a simulation of a multi-megawatt marine diesel prime mover, coupled via a synchronous generator. This allows All-Electric marine scenarios to be investigated without large-scale hardware trials. The method of closing the loop between simulation and hardware is described, with particular reference to the control of the laboratory synchronous machine, which represents the simulated generator(s). The fidelity of the HIL simulation is progressively improved in this study. First, a faster and more powerful field drive is implemented to improve voltage tracking. Second, the phase tracking is improved by using two nested proportional–integral–derivative–acceleration controllers for torque control, tuned using lambda tuning. The HIL environment is tested using a scenario involving a large constant-power load step. This provides a very severe test of the HIL environment, and also reveals the potentially adverse effects of constant-power loads within marine power systems
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Did organic compounds in the Tagish Lake meteorite form via catalytic processes in the solar nebula and within parent bodies?
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Production of high molecular weight organic compounds on the surfaces of amorphous iron silicate catalysts: Implications for organic synthesis in the solar nebula
The high molecular weight organic products of Fischer-Tropsch/Haber-Bosch syntheses on the surfaces of Fe-silicate catalysts have been studied by GCMS
Neo-Hookean fiber composites undergoing finite out-of-plane shear deformations
The response of a neo-Hookean fiber composite undergoing finite out-of-plane
shear deformation is examined. To this end an explicit close form solution for
the out-of-plane shear response of a cylindrical composite element is
introduced. We find that the overall response of the cylindrical composite
element can be characterized by a fictitious homogeneous neo-Hookean material.
Accordingly, this macroscopic response is identical to the response of a
composite cylinder assemblage. The expression for the effective shear modulus
of the composite cylinder assemblage is identical to the corresponding
expression in the limit of small deformation elasticity, and hence also to the
expression for the Hashin-Shtrikman bounds on the out-of-plane shear modulus
The transition to irreversibility in sheared suspensions: An analysis based on a mesoscopic entropy production
We study the shear-induced diffusion effect and the transition to
irreversibility in suspensions under oscillatory shear flow by performing an
analysis of the entropy production associated to the motion of the particles.
We show that the Onsager coupling between different contributions to the
entropy production is responsible for the scaling of the mean square
displacement on particle diameter and applied strain. We also show that the
shear-induced effective diffusion coefficient depends on the volume fraction
and use Lattice-Boltzmann simulations to characterize the effect through the
power spectrum of particle positions for different Reynolds numbers and volume
fractions. Our study gives a thermodynamic explanation of the the transition to
irreversibility through a pertinent analysis of the second law of
thermodynamics.Comment: 17 pages, 3 figures, paper submitted tp phys rev
Wind models for the NSTS ascent trajectory biasing for wind load alleviation
New concepts are presented for aerospace vehicle ascent wind profile biasing. The purpose for wind biasing the ascent trajectory is to provide ascent wind loads relief and thus decrease the probability for launch delays due to wind loads exceeding critical limits. Wind biasing trajectories to the profile of monthly mean winds have been widely used for this purpose. The wind profile models presented give additional alternatives for wind biased trajectories. They are derived from the properties of the bivariate normal probability function using the available wind statistical parameters for the launch site. The analytical expressions are presented to permit generalizations. Specific examples are given to illustrate the procedures. The wind profile models can be used to establish the ascent trajectory steering commands to guide the vehicle through the first stage. For the National Space Transportation System (NSTS) program these steering commands are called I-loads
The Calcium Triplet metallicity calibration for galactic bulge stars
We present a new calibration of the Calcium II Triplet equivalent widths
versus [Fe/H], constructed upon K giant stars in the Galactic bulge. This
calibration will be used to derive iron abundances for the targets of the GIBS
survey, and in general it is especially suited for solar and supersolar
metallicity giants, typical of external massive galaxies. About 150 bulge K
giants were observed with the GIRAFFE spectrograph at VLT, both at resolution
R~20,000 and at R~6,000. In the first case, the spectra allowed us to perform
direct determination of Fe abundances from several unblended Fe lines, deriving
what we call here high resolution [Fe/H] measurements. The low resolution
spectra allowed us to measure equivalent widths of the two strongest lines of
the near infrared Calcium II triplet at 8542 and 8662 A. By comparing the two
measurements we derived a relation between Calcium equivalent widths and [Fe/H]
that is linear over the metallicity range probed here, -1<[Fe/H]<+0.7. By
adding a small second order correction, based on literature globular cluster
data, we derived the unique calibration equation [Fe/H], with a rms dispersion of 0.197 dex, valid across the
whole metallicity range -2.3<[Fe/H]<+0.7.Comment: Accepted for publication in A&
Dynamical equations for high-order structure functions, and a comparison of a mean field theory with experiments in three-dimensional turbulence
Two recent publications [V. Yakhot, Phys. Rev. E {\bf 63}, 026307, (2001) and
R.J. Hill, J. Fluid Mech. {\bf 434}, 379, (2001)] derive, through two different
approaches that have the Navier-Stokes equations as the common starting point,
a set of steady-state dynamic equations for structure functions of arbitrary
order in hydrodynamic turbulence. These equations are not closed. Yakhot
proposed a "mean field theory" to close the equations for locally isotropic
turbulence, and obtained scaling exponents of structure functions and an
expression for the tails of the probability density function of transverse
velocity increments. At high Reynolds numbers, we present some relevant
experimental data on pressure and dissipation terms that are needed to provide
closure, as well as on aspects predicted by the theory. Comparison between the
theory and the data shows varying levels of agreement, and reveals gaps
inherent to the implementation of the theory.Comment: 16 pages, 23 figure
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