98,121 research outputs found
Mesh refinement in a two-dimensional large eddy simulation of a forced shear layer
A series of large eddy simulations are made of a forced shear layer and compared with experimental data. Several mesh densities were examined to separate the effect of numerical inaccuracy from modeling deficiencies. The turbulence model that was used to represent small scale, 3-D motions correctly predicted some gross features of the flow field, but appears to be structurally incorrect. The main effect of mesh refinement was to act as a filter on the scale of vortices that developed from the inflow boundary conditions
Time-accurate simulations of a shear layer forced at a single frequency
Calculations are presented for the forced shear layer studied experimentally by Oster and Wygnanski, and Weisbrot. Two different computational approaches are examined: Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). The DNS approach solves the full three dimensional Navier-Stokes equations for a temporally evolving mixing layer, while the LES approach solves the two dimensional Navier-Stokes equations with a subgrid scale turbulence model. While the comparison between these calculations and experimental data was hampered by a lack of information on the inflow boundary conditions, the calculations are shown to qualitatively agree with several aspects of the experiment. The sensitivity of these calculations to factors such as mesh refinement and Reynolds number is illustrated
Evidence of Counter-Streaming Ions near the Inner Pole of the HERMeS Hall Thruster
NASA is continuing the development of a 12.5-kW Hall thruster system to support a phased exploration concept to expand human presence to cis-lunar space and eventually to Mars. The development team is transitioning knowledge gained from the testing of the government-built Technology Development Unit (TDU) to the contractor-built Engineering Test Unit (ETU). A new laser-induced fluorescence diagnostic was developed to obtain data for validating the Hall thruster models and for comparing the behavior of the ETU and TDU. Analysis of TDU LIF data obtained during initial deployment of the diagnostics revealed evidence of two streams of ions moving in opposite directions near the inner front pole. These two streams of ions were found to intersect the downstream surface of the front pole at large oblique angles. This data points to a possible explanation for why the erosion rate of polished pole covers were observed to decrease over the course of several hundred hours of thruster operation
An assessment and application of turbulence models for hypersonic flows
The current approach to the Accurate Computation of Complex high-speed flows is to solve the Reynolds averaged Navier-Stokes equations using finite difference methods. An integral part of this approach consists of development and applications of mathematical turbulence models which are necessary in predicting the aerothermodynamic loads on the vehicle and the performance of the propulsion plant. Computations of several high speed turbulent flows using various turbulence models are described and the models are evaluated by comparing computations with the results of experimental measurements. The cases investigated include flows over insulated and cooled flat plates with Mach numbers ranging from 2 to 8 and wall temperature ratios ranging from 0.2 to 1.0. The turbulence models investigated include zero-equation, two-equation, and Reynolds-stress transport models
What do emulsification failure and Bose-Einstein condensation have in common?
Ideal bosons and classical ring polymers formed via self-assembly, are known
to have the same partition function, and so analogous phase transitions. In
ring polymers, the analogue of Bose-Einstein condensation occurs when a ring
polymer of macroscopic size appears. We show that a transition of the same
general form occurs within a whole class of systems with self-assembly, and
illustrate it with the emulsification failure of a microemulsion phase of
water, oil and surfactant. As with Bose-Einstein condensation, the transition
occurs even in the absence of interactions.Comment: 7 pages, 1 figure, typeset with EUROTeX, uses epsfi
Analysis and calibration of absorptive images of Bose-Einstein condensate at non-zero temperatures
We describe the method allowing quantitative interpretation of absorptive
images of mixtures of BEC and thermal atoms which reduces possible systematic
errors associated with evaluation of the contribution of each fraction. By
using known temperature dependence of the BEC fraction, the analysis allows
precise calibration of the fitting results. The developed method is verified in
two different measurements and compares well with theoretical calculations and
with measurements performed by another group.Comment: 17 pages, 8 figure
Low temperature heat capacity of Fe_{1-x}Ga_{x} alloys with large magneostriction
The low temperature heat capacity C_{p} of Fe_{1-x}Ga_{x} alloys with large
magnetostriction has been investigated. The data were analyzed in the standard
way using electron () and phonon () contributions. The
Debye temperature decreases approximately linearly with increasing
Ga concentration, consistent with previous resonant ultrasound measurements and
measured phonon dispersion curves. Calculations of from lattice
dynamical models and from measured elastic constants C_{11}, C_{12} and C_{44}
are in agreement with the measured data. The linear coefficient of electronic
specific heat remains relatively constant as the Ga concentration
increases, despite the fact that the magnetoelastic coupling increases. Band
structure calculations show that this is due to the compensation of majority
and minority spin states at the Fermi level.Comment: 14 pages, 6 figure
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Nanoindentation Of Si Nanostructures: Buckling And Friction At Nanoscales
A nanoindentation system was employed to characterize mechanical properties of silicon nanolines (SiNLs), which were fabricated by an anisotropic wet etching (AWE) process. The SiNLs had the linewidth ranging from 24 nm to 90 nm, having smooth and vertical sidewalls and the aspect ratio (height/linewidth) from 7 to 18. During indentation, a buckling instability was observed at a critical load, followed by a displacement burst without a load increase, then a full recovery of displacement upon unloading. This phenomenon was explained by two bucking modes. It was also found that the difference in friction at the contact between the indenter and SiNLs directly affected buckling response of these nanolines. The friction coefficient was estimated to be in a range of 0.02 to 0.05. For experiments with large indentation displacements, irrecoverable indentation displacements were observed due to fracture of Si nanolines, with the strain to failure estimated to be from 3.8% to 9.7%. These observations indicated that the buckling behavior of SiNLs depended on the combined effects of load, line geometry, and the friction at contact. This study demonstrated a valuable approach to fabrication of well-defined Si nanoline structures and the application of the nanoindentation method for investigation of their mechanical properties at the nanoscale.Microelectronics Research Cente
The ``Nernst Theorem'' and Black Hole Thermodynamics
The Nernst formulation of the third law of ordinary thermodynamics (often
referred to as the ``Nernst theorem'') asserts that the entropy, , of a
system must go to zero (or a ``universal constant'') as its temperature, ,
goes to zero. This assertion is commonly considered to be a fundamental law of
thermodynamics. As such, it seems to spoil the otherwise perfect analogy
between the ordinary laws of thermodynamics and the laws of black hole
mechanics, since rotating black holes in general relativity do not satisfy the
analog of the ``Nernst theorem''. The main purpose of this paper is to attempt
to lay to rest the ``Nernst theorem'' as a law of thermodynamics. We consider a
boson (or fermion) ideal gas with its total angular momentum, , as an
additional state parameter, and we analyze the conditions on the single
particle density of states, , needed for the Nernst formulation
of the third law to hold. (Here, and denote the single particle
energy and angular momentum.) Although it is shown that the Nernst formulation
of the third law does indeed hold under a wide range of conditions, some simple
classes of examples of densities of states which violate the ``Nernst theorem''
are given. In particular, at zero temperature, a boson (or fermion) gas
confined to a circular string (whose energy is proportional to its length) not
only violates the ``Nernst theorem'' also but reproduces some other
thermodynamic properties of an extremal rotating black hole.Comment: 20 pages, plain LaTeX fil
The Measure for the Multiverse and the Probability for Inflation
We investigate the measure problem in the framework of inflationary
cosmology. The measure of the history space is constructed and applied to
inflation models. Using this measure, it is shown that the probability for the
generalized single field slow roll inflation to last for e-folds is
suppressed by a factor , and the probability for the generalized
-field slow roll inflation is suppressed by a much larger factor
. Some non-inflationary models such as the cyclic model do not
suffer from this difficulty.Comment: 16 page
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