10,957 research outputs found
On the basic equations for the second-order modeling of compressible turbulence
Equations for the mean and turbulent quantities for compressible turbulent flows are derived. Both the conventional Reynolds average and the mass-weighted, Favre average were employed to decompose the flow variable into a mean and a turbulent quality. These equations are to be used later in developing second order Reynolds stress models for high speed compressible flows. A few recent advances in modeling some of the terms in the equations due to compressibility effects are also summarized
Asymptotically Stable Walking of a Five-Link Underactuated 3D Bipedal Robot
This paper presents three feedback controllers that achieve an asymptotically
stable, periodic, and fast walking gait for a 3D (spatial) bipedal robot
consisting of a torso, two legs, and passive (unactuated) point feet. The
contact between the robot and the walking surface is assumed to inhibit yaw
rotation. The studied robot has 8 DOF in the single support phase and 6
actuators. The interest of studying robots with point feet is that the robot's
natural dynamics must be explicitly taken into account to achieve balance while
walking. We use an extension of the method of virtual constraints and hybrid
zero dynamics, in order to simultaneously compute a periodic orbit and an
autonomous feedback controller that realizes the orbit. This method allows the
computations to be carried out on a 2-DOF subsystem of the 8-DOF robot model.
The stability of the walking gait under closed-loop control is evaluated with
the linearization of the restricted Poincar\'e map of the hybrid zero dynamics.
Three strategies are explored. The first strategy consists of imposing a
stability condition during the search of a periodic gait by optimization. The
second strategy uses an event-based controller. In the third approach, the
effect of output selection is discussed and a pertinent choice of outputs is
proposed, leading to stabilization without the use of a supplemental
event-based controller
A multiple-scale model for compressible turbulent flows
A multiple-scale model for compressible turbulent flows is proposed. It is assumed that turbulent eddy shocklets are formed primarily by the 'collisions' of large energetic eddies. The extra straining of the large eddy, due to their interactions with shocklets, enhances the energy cascade to smaller eddies. Model transport equations are developed for the turbulent kinetic energies and the energy transfer rates of the different scale. The turbulent eddy viscosity is determined by the total turbulent kinetic energy and the rate of energy transfer from the large scale to the small scale, which is different from the energy dissipation rate. The model coefficients in the modeled turbulent transport equations depend on the ratio of the turbulent kinetic energy of the large scale to that of the small scale, which renders the model more adaptive to the characteristics of individual flow. The model is tested against compressible free shear layers. The results agree satisfactorily with measurements
On the Basic Equations for the Second-order Modeling of Compressible Turbulence
Equations for the mean and the turbulence quantities of compressible turbulent flows are derived in this report. Both the conventional Reynolds average and the mass-weighted Favre average were employed to decompose the flow variable into mean and turbulent quantities. These equations are to be used later in developing second-order Reynolds stress models for high-speed compressible flows. A few recent advances in modeling some of the terms in the equation due to compressibility effects are also summarized
Polar cap observations of thermospheric winds and temperatures at Sondre Stromfjord, Greenland
An agreement of averaged temperatures with mass spectrometer incoherent scatter radar looked reasonable for several nights, but for many nights there are differences: (1) midnight period of cooling, and (2) temperature increases associated with overhead crossings of the auroral belt. The observed rise of the temperature before dawn in conjunction with the high 6300A intensities suggests a connection between the two effects: soft particle precipitation most likely candidate but frictional heating perhaps also a possibility. A comparison with the thermospheric general circulation model calculations also needed. The technique for formulating neutral wind vectors performs well in most cases. The observed patterns show evidence for abatement in the midnight sector in the meridional wind component at the separatix between the two cells with a frequency of the order of 20 to 25%, also observed in radar observations at Sondre Stromfjord. The observed patterns for magnetically quiet conditions show flow characteristic of the auroral belt, westward in evening followed by the midnight surge. The observed patterns for active conditions show dominance either by the evening cell or the morning cell, but most often the former
The structure of atomic nitrogen adsorbed on Fe(100)
Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 × 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED I–V spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Å above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Å). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe4N, thus supporting the view of a “surface nitride” and providing a consistent picture of the structural and bonding properties of this surface phase
Characteristic Length Scale of Electric Transport Properties of Genomes
A tight-binding model together with a novel statistical method are used to
investigate the relation between the sequence-dependent electric transport
properties and the sequences of protein-coding regions of complete genomes. A
correlation parameter is defined to analyze the relation. For some
particular propagation length , the transport behaviors of the coding
and non-coding sequences are very different and the correlation reaches its
maximal value . and \omax are characteristic values for
each species. The possible reason of the difference between the features of
transport properties in the coding and non-coding regions is the mechanism of
DNA damage repair processes together with the natural selection.Comment: 4 pages, 4 figure
Zeeman-limited Superconductivity in Crystalline Al Films
We report the evolution of the Zeeman-mediated superconducting phase diagram
(PD) in ultra-thin crystalline Al films. Parallel critical field measurements,
down to 50 mK, were made across the superconducting tricritical point of films
ranging in thickness from 7 ML to 30 ML. The resulting phase boundaries were
compared with the quasi-classical theory of a Zeeman-mediated transition
between a homogeneous BCS condensate and a spin polarized Fermi liquid. Films
thicker than 20 ML showed good agreement with theory, but thinner films
exhibited an anomalous PD that cannot be reconciled within a homogeneous BCS
framework.Comment: 8 pages, 9 figure
A Search for Moderate-Redshift Survivors from the Population of Luminous Compact Passive Galaxies at High Redshift
From a search of a ~ 2400 square degree region covered by both the SDSS and
UKIDSS databases, we have attempted to identify galaxies at z ~ 0.5 that are
consistent with their being essentially unmodified examples of the luminous
passive compact galaxies found at z ~ 2.5. After isolating good candidates via
deeper imaging, we further refine the sample with Keck moderate-resolution
spectroscopy and laser-guide-star adaptive-optics imaging. For 4 of the 5
galaxies that so far remain after passing through this sieve, we analyze
plausible star-formation histories based on our spectra in order to identify
galaxies that may have survived with little modification from the population
formed at high redshift. We find 2 galaxies that are consistent with having
formed > 95% of their mass at z > 5. We attempt to estimate masses both from
our stellar population determinations and from velocity dispersions. Given the
high frequency of small axial ratios, both in our small sample and among
samples found at high redshifts, we tentatively suggest that some of the more
extreme examples of passive compact galaxies may have prolate morphologies.Comment: 9 pages, 5 figures, ApJ in press; error in Table 1 corrected, some
new references adde
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