1,502 research outputs found
Charge Transport Scalings in Turbulent Electroconvection
We describe a local-power law scaling theory for the mean dimensionless
electric current in turbulent electroconvection. The experimental system
consists of a weakly conducting, submicron thick liquid crystal film supported
in the annulus between concentric circular electrodes. It is driven into
electroconvection by an applied voltage between its inner and outer edges. At
sufficiently large voltage differences, the flow is unsteady and electric
charge is turbulently transported between the electrodes. Our theoretical
development, which closely parallels the Grossmann-Lohse model for turbulent
thermal convection, predicts the local-power law . and are dimensionless
numbers that are similar to the Rayleigh and Prandtl numbers of thermal
convection, respectively. The dimensionless function , which is
specified by the model, describes the dependence of on the aspect ratio
. We find that measurements of are consistent with the theoretical
model.Comment: 12 pages, 7 figures, Submitted to Phys. Rev. E. See also
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Thermal Conductivities of Unidirectional Materials
In this paper the composite thermal conductivities of unidirec tional composites are studied and expressions are obtained for pre dicting these conductivities in the directions along and normal to the filaments. In the direction along the filament an expression is presented based on the assumption that the filaments and matrix are connected in parallel. In the direction normal to the filaments composite thermal conductivity values are obtained first by utiliz ing the analogy between the response of a unidirectional composite to longitudinal shear loading and to transverse heat transfer; second by replacing the filament-matrix composite with an idealized ther mal model. The results of the shear loading analogy agree reason ably well with the results of the thermal model particularly at filament contents below about 60%. These results were also com pared to experimental data reported in the literature and good agreement was found between the data and those theoretical re sults that were derived for circular filaments arranged in a square packing array.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67863/2/10.1177_002199836700100206.pd
Differential segregation and modification of mRNA during spermiogenesis in Marsilea vestita
AbstractWe are interested in the mechanisms that underlie cell fate determination in the endosporic male gametophytes of the fern, Marsilea vestita. Synchronous development is initiated by placing dry spores into water and involves the translation of stored mRNAs, with little transcription. Nine division cycles produce 32 spermatids surrounded by 7 sterile cells, and then each spermatid differentiates into a multiciliate gamete. Here, we focus on changes in the distribution of particular proteins, mRNAs, and patterns of polyadenylation as essential prerequisites for cell fate determination and gametogenesis. Earlier, we showed that α- and β-tubulin proteins become concentrated in spermatogenous initials, and that centrin mRNA is translated only in spermatogenous initials. In situ hybridizations reveal that centrin, cyclin B, and β-tubulin mRNAs are present in both sterile and spermatogenous cells, but that transcripts encoding RNA helicase and PRP-19 (a spliceosome component) become localized in spermatogenous cells. The targeted destruction of these two transcripts by RNAi treatments does not affect the numbers of division cycles, but the gametophytes exhibit anomalous patterns of cytokinesis, and a subsequent failure of spermatid differentiation. Thus, cell fate determination in the gametophyte involves localized translation, and the localization of mRNAs for proteins involved in transcript processing. We found differences in polyadenylation levels in sterile and spermatogenous cells that match the distribution of cytoplasmic poly(A) polymerase (PAP), which, in immunolocalizations, is abundant in spermatogenous cells, but undetectable in sterile cells. The activation of translation in spermatogenous initials, but not in sterile cells, may be under the control of mRNA processing enzymes, which become localized either as proteins or mRNAs in the spermatogenous subdomains before any divisions occur
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Elasticity tailoring of a composite D-Spar: Progress report for calendar year 1998
There are many potential benefits to be gained from the aeroelastic behavior of a wind-turbine blade with bend-twist coupling. However, the ability to manufacture blades with sufficient coupling to provide the desired benefits has yet to be established. This report investigates the feasible (or practical) range of the coupling coefficient that can be obtained on a uniform cross-section composite D-spar, which could be the backbone of a wind-turbine-blade. The most critical parameters are identified and studied across a range of possible values. Various features, such as the geometry, skin thickness, ply distribution, ply materials, and ply orientations, are evaluated for their effect on twist-bend coupling of a D-spar. It is found that sufficient coupling can be built into the D-spar shape, but that carbon-fiber composite plies angled between 15 and 30 degrees to the longitudinal axis may be required
Localized states in sheared electroconvection
Electroconvection in a thin, sheared fluid film displays a rich sequence of
bifurcations between different flow states as the driving voltage is increased.
We present a numerical study of an annular film in which a radial potential
difference acts on induced surface charges to drive convection. The film is
also sheared by independently rotating the inner edge of the annulus. This
simulation models laboratory experiments on electroconvection in sheared
smectic liquid crystal films. The applied shear competes with the electrical
forces, resulting in oscillatory and strongly subcritical bifurcations between
localized vortex states close to onset. At higher forcing, the flow becomes
chaotic via a Ruelle-Takens-Newhouse scenario. The simulation allows flow
visualization not available in the physical experiments, and sheds light on
previously observed transitions in the current-voltage characteristics of
electroconvecting smectic films.Comment: To be published in EuroPhysics Letters, 6 pages, 6 figures: final
versio
Gravitational Waves from Core Collapse Supernovae
We present the gravitational wave signatures for a suite of axisymmetric core
collapse supernova models with progenitors masses between 12 and 25 solar
masses. These models are distinguished by the fact they explode and contain
essential physics (in particular, multi-frequency neutrino transport and
general relativity) needed for a more realistic description. Thus, we are able
to compute complete waveforms (i.e., through explosion) based on
non-parameterized, first-principles models. This is essential if the waveform
amplitudes and time scales are to be computed more precisely. Fourier
decomposition shows that the gravitational wave signals we predict should be
observable by AdvLIGO across the range of progenitors considered here. The
fundamental limitation of these models is in their imposition of axisymmetry.
Further progress will require counterpart three-dimensional models.Comment: 10 pages, 5 figure
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