64 research outputs found
The Verbal-semantic Level of the Foreign Student Language Identity (Based on Internet Discourse)
This paper introduces characteristic features of the language identity of a foreign high school student as exemplified in the Internet discourse of National Research Tomsk Polytechnic University. The verbal-semantic language level has been investigated. The material for the study is based on foreign students' "live communication" texts from social networks. The analysis of lexical, grammatical, and syntactic levels of the Russian language in the student Internet discourse has shown the particularities of the lexical-semantic content of the collective high school bilingual student language identity and features of the student sociolect
Shell Model for Drag Reduction with Polymer Additive in Homogeneous Turbulence
Recent direct numerical simulations of the FENE-P model of non-Newtonian
hydrodynamics revealed that the phenomenon of drag reduction by polymer
additives exists (albeit in reduced form) also in homogeneous turbulence. We
introduce here a simple shell model for homogeneous viscoelastic flows that
recaptures the essential observations of the full simulations. The simplicity
of the shell model allows us to offer a transparent explanation of the main
observations. It is shown that the mechanism for drag reduction operates mainly
on the large scales. Understanding the mechanism allows us to predict how the
amount of drag reduction depends of the various parameters in the model. The
main conclusion is that drag reduction is not a universal phenomenon, it peaks
in a window of parameters like Reynolds number and the relaxation rate of the
polymer
Drag Reduction by Polymers in Turbulent Channel Flows: Energy Redistribution Between Invariant Empirical Modes
We address the phenomenon of drag reduction by dilute polymeric additive to
turbulent flows, using Direct Numerical Simulations (DNS) of the FENE-P model
of viscoelastic flows. It had been amply demonstrated that these model
equations reproduce the phenomenon, but the results of DNS were not analyzed so
far with the goal of interpreting the phenomenon. In order to construct a
useful framework for the understanding of drag reduction we initiate in this
paper an investigation of the most important modes that are sustained in the
viscoelastic and Newtonian turbulent flows respectively. The modes are obtained
empirically using the Karhunen-Loeve decomposition, allowing us to compare the
most energetic modes in the viscoelastic and Newtonian flows. The main finding
of the present study is that the spatial profile of the most energetic modes is
hardly changed between the two flows. What changes is the energy associated
with these modes, and their relative ordering in the decreasing order from the
most energetic to the least. Modes that are highly excited in one flow can be
strongly suppressed in the other, and vice versa. This dramatic energy
redistribution is an important clue to the mechanism of drag reduction as is
proposed in this paper. In particular there is an enhancement of the energy
containing modes in the viscoelastic flow compared to the Newtonian one; drag
reduction is seen in the energy containing modes rather than the dissipative
modes as proposed in some previous theories.Comment: 11 pages, 13 figures, included, PRE, submitted, REVTeX
Specific Heat of Liquid Helium in Zero Gravity very near the Lambda Point
We report the details and revised analysis of an experiment to measure the
specific heat of helium with subnanokelvin temperature resolution near the
lambda point. The measurements were made at the vapor pressure spanning the
region from 22 mK below the superfluid transition to 4 uK above. The experiment
was performed in earth orbit to reduce the rounding of the transition caused by
gravitationally induced pressure gradients on earth. Specific heat measurements
were made deep in the asymptotic region to within 2 nK of the transition. No
evidence of rounding was found to this resolution. The optimum value of the
critical exponent describing the specific heat singularity was found to be a =
-0.0127+ - 0.0003. This is bracketed by two recent estimates based on
renormalization group techniques, but is slightly outside the range of the
error of the most recent result. The ratio of the coefficients of the leading
order singularity on the two sides of the transition is A+/A- =1.053+ - 0.002,
which agrees well with a recent estimate. By combining the specific heat and
superfluid density exponents a test of the Josephson scaling relation can be
made. Excellent agreement is found based on high precision measurements of the
superfluid density made elsewhere. These results represent the most precise
tests of theoretical predictions for critical phenomena to date.Comment: 27 Pages, 20 Figure
The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) technology demonstration
The Coronagraph Instrument (CGI) on the Nancy Grace Roman Space Telescope will demonstrate the highcontrast technology necessary for visible-light exoplanet imaging and spectroscopy from space via direct imaging of Jupiter-size planets and debris disks. This in-space experience is a critical step toward future, larger missions targeted at direct imaging of Earth-like planets in the habitable zones of nearby stars. This paper presents an overview of the current instrument design and requirements, highlighting the critical hardware, algorithms, and operations being demonstrated. We also describe several exoplanet and circumstellar disk science cases enabled by these capabilities. A competitively selected Community Participation Program team will be an integral part of the technology demonstration and could perform additional CGI observations beyond the initial tech demo if the instrument performance warrants it
Paving the Way to Future Missions: the Roman Space Telescope Coronagraph Technology Demonstration
This document summarizes how far the Nancy Grace Roman Space Telescope Coronagraph Instrument (Roman CGI) will go toward demonstrating high-contrast imaging and spectroscopic requirements for potential future exoplanet direct imaging missions, illustrated by the HabEx and LUVOIR concepts. The assessment is made for two levels of assumed CGI performance: (i) current best estimate (CBE) as of August 2020, based on laboratory results and realistic end-to-end simulations with JPL-standard Model Uncertainty Factors (MUFs); (ii) CGI design specifications inherited from Phase B requirements. We find that the predicted performance (CBE) of many CGI subsystems compares favorably with the needs of future missions, despite providing more modest point source detection limits than future missions. This is essentially due to the challenging pupil of the Roman Space Telescope; this pupil pushes the coronagraph masks sensitivities to misalignments to be commensurate with future missions. In particular, CGI will demonstrate active low-order wavefront control and photon counting capabilities at levels of performance either higher than, or comparable to, the needs of future missions
Paving the Way to Future Missions: the Roman Space Telescope Coronagraph Technology Demonstration
This document summarizes how far the Nancy Grace Roman Space Telescope
Coronagraph Instrument (Roman CGI) will go toward demonstrating high-contrast
imaging and spectroscopic requirements for potential future exoplanet direct
imaging missions, illustrated by the HabEx and LUVOIR concepts. The assessment
is made for two levels of assumed CGI performance: (i) current best estimate
(CBE) as of August 2020, based on laboratory results and realistic end-to-end
simulations with JPL-standard Model Uncertainty Factors (MUFs); (ii) CGI design
specifications inherited from Phase B requirements. We find that the predicted
performance (CBE) of many CGI subsystems compares favorably with the needs of
future missions, despite providing more modest point source detection limits
than future missions. This is essentially due to the challenging pupil of the
Roman Space Telescope; this pupil pushes the coronagraph masks sensitivities to
misalignments to be commensurate with future missions. In particular, CGI will
demonstrate active low-order wavefront control and photon counting capabilities
at levels of performance either higher than, or comparable to, the needs of
future missions.Comment: 10 pages, 3 tables. Revised version v2: added some co-author
THE EFFECT OF DRAG-REDUCING ADDITIVES ON THE TURBULENT STRUCTURE IN CHANNEL FLOWS (BURSTS, EJECTIONS, COHERENT)
It is well known that small amounts of long-chain high molecular weight polymer molecules reduce viscous drag in wall bounded turbulent liquid flows. However, the mechanisms through which drag reduction is achieved are not understood largely because the rheology of the drag-reducing solutions is not well defined. In the present study, two fully developed, low concentration (less than 3 ppm) drag-reducing flows with 20 to 30% drag reduction were investigated and compared to a water flow. At these very low concentrations, the difference in the shear viscosity of the drag-reducing solutions and the solvent are minimal. A two-component laser Doppler velocimeter was used to make simultaneous measurements of the velocity components parallel and normal to the wall. Measurements of mean and root-mean-square (RMS) velocities as well as the uv turbulent shear stress confirmed that the additives modify the buffer region of the flow. The major influence occurred through damping of fluctuations normal to the wall. Eulerian burst detection techniques were developed through extensions of the quadrant two, VITA, u-level and modified u-level detection techniques. When properly used all of these techniques gave an accurate estimate of the average time between bursts for 8700 (LESSTHEQ) Re(,h) (LESSTHEQ) 17800 in water flows and in the two drag-reducing flows. In these very dilute drag-reducing flows, the average time between bursts increased about the same amount as the streak spacing. The modified u-level technique was shown to do the best job of detecting the entire ejection and burst event. Conditional velocity samples using the modified u-level technique showed that the ejection and burst event are closely correlated with the occurrence of slower than average fluid moving both away from the wall and toward the wall
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