2,298 research outputs found
Optimal model parameters for multi-objective large-eddy simulations
A methodology is proposed for the assessment of error dynamics in large-eddy simulations. It is demonstrated that the optimization of model parameters with respect to one flow property can be obtained at the expense of the accuracy with which other flow properties are predicted. Therefore, an approach is introduced which allows to assess the total errors based on various flow properties simultaneously. We show that parameter settings exist, for which all monitored errors are "near optimal," and refer to such regions as "multi-objective optimal parameter regions." We focus on multi-objective errors that are obtained from weighted spectra, emphasizing both large- as well small-scale errors. These multi-objective optimal parameter regions depend strongly on the simulation Reynolds number and the resolution. At too coarse resolutions, no multi-objective optimal regions might exist as not all error-components might simultaneously be sufficiently small. The identification of multi-objective optimal parameter regions can be adopted to effectively compare different subgrid models. A comparison between large-eddy simulations using the Lilly-Smagorinsky model, the dynamic Smagorinsky model and a new Re-consistent eddy-viscosity model is made, which illustrates this. Based on the new methodology for error assessment the latter model is found to be the most accurate and robust among the selected subgrid models, in combination with the finite volume discretization used in the present study
Rapid generation of angular momentum in bounded magnetized plasma
Direct numerical simulations of two-dimensional decaying MHD turbulence in
bounded domains show the rapid generation of angular momentum in
nonaxisymmetric geometries. It is found that magnetic fluctuations enhance this
mechanism. On a larger time scale, the generation of a magnetic angular
momentum, or angular field, is observed. For axisymmetric geometries, the
generation of angular momentum is absent; nevertheless, a weak magnetic field
can be observed. The derived evolution equations for both the angular momentum
and angular field yield possible explanations for the observed behavior
Mechanical Flip-Chip for Ultra-High Electron Mobility Devices
Electrostatic gates are of paramount importance for the physics of devices
based on high-mobility two-dimensional electron gas (2DEG) since they allow
depletion of electrons in selected areas. This field-effect gating enables the
fabrication of a wide range of devices such as, for example, quantum point
contacts (QPC), electron interferometers and quantum dots. To fabricate these
gates, processing is usually performed on the 2DEG material, which is in many
cases detrimental to its electron mobility. Here we propose an alternative
process which does not require any processing of the 2DEG material other than
for the ohmic contacts. This approach relies on processing a separate wafer
that is then mechanically mounted on the 2DEG material in a flip-chip fashion.
This technique proved successful to fabricate quantum point contacts on both
GaAs/AlGaAs materials with both moderate and ultra-high electron mobility.Comment: 5 pages, 3 figure
Numerical solutions of the three-dimensional magnetohydrodynamic alpha-model
We present direct numerical simulations and alpha-model simulations of four
familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects:
selective decay, dynamic alignment, inverse cascade of magnetic helicity, and
the helical dynamo effect. The MHD alpha-model is shown to capture the
long-wavelength spectra in all these problems, allowing for a significant
reduction of computer time and memory at the same kinetic and magnetic Reynolds
numbers. In the helical dynamo, not only does the alpha-model correctly
reproduce the growth rate of magnetic energy during the kinematic regime, but
it also captures the nonlinear saturation level and the late generation of a
large scale magnetic field by the helical turbulence.Comment: 12 pages, 19 figure
A massive, distant proto-cluster at z=2.47 caught in a phase of rapid formation?
Numerical simulations of cosmological structure formation show that the
Universe's most massive clusters, and the galaxies living in those clusters,
assemble rapidly at early times (2.5 < z < 4). While more than twenty
proto-clusters have been observed at z > 2 based on associations of 5-40
galaxies around rare sources, the observational evidence for rapid cluster
formation is weak. Here we report observations of an asymmetric, filamentary
structure at z = 2.47 containing seven starbursting, submillimeter-luminous
galaxies and five additional AGN within a comoving volume of 15000 Mpc.
As the expected lifetime of both the luminous AGN and starburst phase of a
galaxy is ~100 Myr, we conclude that these sources were likely triggered in
rapid succession by environmental factors, or, alternatively, the duration of
these cosmologically rare phenomena is much longer than prior direct
measurements suggest. The stellar mass already built up in the structure is
and we estimate that the cluster mass will exceed that
of the Coma supercluster at . The filamentary structure is in line
with hierarchical growth simulations which predict that the peak of cluster
activity occurs rapidly at z > 2.Comment: 7 pages, 3 figures, 2 tables, accepted in ApJL (small revisions from
previous version
Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys - IV. Influence of mergers in the evolution of faint field galaxies from z~1
HST images of a sample of 285 galaxies with measured z from the CFRS and
Autofib-LDSS redshift surveys are analysed to derive the evolution of the
merger fraction out to z~1. We have performed visual and machine-based merger
identifications, as well as counts of bright pairs of galaxies with magnitude
differences less than 1.5 mag. We find that the pair fraction increases with z,
with up to ~20% of the galaxies being in physical pairs at z~0.75-1. We derive
a merger fraction varying with z as (1+z)^{3.2 +/- 0.6}, after correction for
line-of-sight contamination, in excellent agreement with the merger fraction
derived from the visual classification of mergers for which m = 3.4 +/- 0.6.
After correcting for seeing effects on the ground-based selection of survey
galaxies, we conclude that the pair fraction evolves as (1+z)^{2.7 +/- 0.6}.
This implies that an average L* galaxy will have undergone 0.8 to 1.8 merger
events from z=1 to 0, with 0.5 to 1.2 merger events occuring in a 2 Gyr time
span at z~0.9. This result is consistent with predictions from semi-analytical
models of galaxy formation. From the simple co-addition of the observed
luminosities of the galaxies in pairs, physical mergers are computed to lead to
a brightening of 0.5 mag for each pair on average, and a boost in star
formation rate of a factor of 2, as derived from the average [O II] equivalent
widths. Mergers of galaxies are therefore contributing significantly to the
evolution of both the luminosity function and luminosity density of the
Universe out to z~1.Comment: 14 pages, 6 PS figures included. Accepted for publication in MNRA
2D Metal-Insulator transition as a percolation transition
By carefully analyzing the low temperature density dependence of 2D
conductivity in undoped high mobility n-GaAs heterostructures, we conclude that
the 2D metal-insulator transition in this system is a density inhomogeneity
driven percolation transition due to the breakdown of screening in the random
charged impurity disorder background. In particular, our measured conductivity
exponent of approaches the 2D percolation exponent value of 4/3 at
low temperatures and our experimental data are inconsistent with there being a
zero-temperature quantum critical point in our system.Comment: 5 pages, 3 figure
Orientation of the Stripe Formed by the Two-Dimensional Electrons in Higher Landau Levels
Effect of periodic potential on the stripe phase realized in the higher
Landau levels is investigated by the Hartree-Fock approximation. The period of
the potential is chosen to be two to six times of the fundamental period of the
stripe phase. It is found that the stripe aligns perpendicularly to the
external potential in contrast to a naive expectation and hydrodynamic theory.
Charge modulation towards the Wigner crystallization along the stripe is
essential for the present unexpected new result.Comment: 5 pages, RevTex, two figures included in the tex
Final design proposal: Delta Group-Nood Rider 821(tm)
The Nood Rider 821 (trademark) twin-engine, prop passenger aircraft is described. It is argued that the aircraft is very economical to operate and maintain, offering competitive advantages in the air travel marketplace. The aircraft was designed to operate in 'Aeroworld', a fictional world where the passengers are ping pong balls and the distances between cites are on the order of thousands of feet
Vertical Field-Effect Transistor Based on Wavefunction Extension
We demonstrate a mechanism for a dual layer, vertical field-effect
transistor, in which nearly-depleting one layer will extend its wavefunction to
overlap the other layer and increase tunnel current. We characterize this
effect in a specially designed GaAs/AlGaAs device, observing a tunnel current
increase of two orders of magnitude at cryogenic temperatures, and we suggest
extrapolations of the design to other material systems such as graphene
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