1,721 research outputs found
Pneumatic boot for helicopter rotor deicing
Pneumatic deicer boots for helicopter rotor blades were tested. The tests were conducted in the 6 by 9 ft icing research tunnel on a stationary section of a UH-IH helicopter main rotor blade. The boots were effective in removing ice and in reducing aerodynamic drag due to ice
Analysis of dynamic inlet distortion applied to a parallel compressor model
An investigation of surge was conducted by using a parallel compressor model of the J85-13 compressor implement on an analog computer. Surges were initiated by various types of dynamic disturbances in inlet pressure. The compressor model was less sensitive to disturbances of short duration, high frequency, and long duration where the compressor discharge pressure could react. Adding steady distortion to dynamic disturbances reduced the amount of dynamic disturbance required to effect surge. Steady and unsteady distortions combined linearly to reduce surge margin
Dependence of electronic polarization on octahedral rotations in TbMnO3 from first principles
The electronic contribution to the magnetically induced polarization in
orthorhombic TbMnO3 is studied from first principles. We compare the cases in
which the spin cycloid, which induces the electric polarization via the
spin-orbit interaction, is in either the b-c or a-b plane. We find that the
electronic contribution is negligible in the first case, but much larger, and
comparable to the lattice-mediated contribution, in the second case. However,
we how that this behavior is an artifact of the particular pattern of
octahedral rotations characterizing the structurally relaxed Pbnm crystal
structure. To do so, we explore how the electronic contribution varies for a
structural model of rigidly rotated MnO6 octahedra, and demonstrate that it can
vary over a wide range, comparable with the lattice-mediated contribution, for
both b-c and a-b spirals. We introduce a phenomenological model that is capable
of describing this behavior in terms of sums of symmetry-constrained
contributions arising from the displacements of oxygen atoms from the centers
of the Mn-Mn bonds.Comment: 8 pages, 5 figures, 3 table
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 figure
On the calculation of the bandgap of periodic solids with MGGA functionals using the total energy
During the last few years, it has become more and more clear that functionals of the meta generalized gradient approximation (MGGA) are more accurate than GGA functionals for the geometry and energetics of electronic systems. However, MGGA functionals are also potentially more interesting for the electronic structure, in particular, when the potential is nonmultiplicative (i.e., when MGGAs are implemented in the generalized Kohn-Sham framework), which may help to get more accurate bandgaps. Here, we show that the calculation of bandgap of solids with MGGA functionals can also be done very accurately in a non-self-consistent manner. This scheme uses only the total energy and can, therefore, be very useful when the self-consistent implementation of a particular MGGA functional is not available. Since self-consistent MGGA calculations may be difficult to converge, the non-self-consistent scheme may also help to speed up the calculations. Furthermore, it can be applied to any other types of functionals, for which the implementation of the corresponding potential is not trivial
Boundary layer thickness effect on boattail drag
A combined experimental and analytical program was conducted to investigate the effects of boundary layer changes on the flow over high angle boattail nozzles. The tests were run on an isolated axisymmetric sting mounted model. Various boattail geometries were investigated at high subsonic speeds over a range of boundary layer thicknesses. In general, boundary layer effects were small at speeds up to Mach 0.8. However, at higher speeds significant regions of separated flow were present on the boattail. When separation was present large reductions in boattail drag resulted with increasing boundary layer thickness. The analysis predicts both of these trends
Is LaOFFeAs an electron-phonon superconductor ?
In this paper we calculate the electron-phonon coupling of the
newly-discovered superconductor LaOFFeAs from first-principles,
using Density Functional Perturbation Theory. For pure LaOFeAs, the calculated
electron-phonon coupling constant and logarithmic-averaged
frequency , give a maximum of 0.8 K, using the
standard Migdal-Eliashberg theory.
For the doped compounds, we predict even smaller coupling constants, due
to the strong suppression of the electronic Density of States at the Fermi
level. To reproduce the experimental , a 5-6 times larger coupling
constant would be needed.
Our results indicate that electron-phonon coupling is not sufficient to
explain superconductivity in the newly-discovered LaOFFeAs
superconductor, probably due to the importance of strong correlation effects
Dynamic analysis of a photovoltaic power system with battery storage capability
A photovolataic power system with a battery storage capability is analyzed. A dual battery current control concept is proposed, which enables the battery to either supply or accept power depending upon system environment and load conditions. A simulation of the power system, including the battery current control, is developed and evaluated. The evaulation demonstrate the visbility of the battery control concept of switch the battery from a charge to discharge mode and back as required by load and environmental conditions. An acceptable system operation is demonstrated over the entire insolation range. Additionally, system sensitivity, bandwidth, and damping characteristics of the battery control are shown to be acceptable for a projected hardware implementation
All-electron theory of the coupling between laser-induced coherent phonons in bismuth
Using first principles, all-electron calculations and dynamical simulations
we study the behavior of the A_1g and E_g coherent phonons induced in Bi by
intense laser pulses. We determine the potential landscapes in the laser heated
material and show that they exhibit phonon-softening, phonon-phonon coupling,
and anharmonicities. As a consequence the E_g mode modulates the A_1g
oscillations and higher harmonics of both modes appear, which explains recent
isotropic reflectivity measurements. Our results offer a unified description of
the different experimental observations performed so far on bismuth.Comment: 3 figure
Single-Dirac-Cone topological surface states in TlBiSe2 class of Topological Insulators
We have investigated several strong spin-orbit coupling ternary chalcogenides
related to the (Pb,Sn)Te series of compounds. Our first-principles calculations
predict the low temperature rhombohedral ordered phase in TlBiTe2, TlBiSe2, and
TlSbX2 (X=Te, Se, S) to be topologically Kane-Mele Z2 = -1 nontrivial. We
identify the specific surface termination that realizes the single Dirac cone
through first-principles surface state computations. This termination minimizes
effects of dangling bonds making it favorable for photoemission (ARPES)
experiments. Our analysis predicts that thin films of these materials would
harbor novel 2D quantum spin Hall states, and support odd-parity topological
superconductivity. For a related work also see arXiv:1003.2615v1. Experimental
ARPES results will be published elsewhere.Comment: Accepted for publication in Phys. Rev. Lett. (2010). Submitted March
201
- …