29,077 research outputs found
Effect of wing pivot location on longitudinal aerodynamic characteristics of a variable sweep wing having an M planform
Wing pivot location effect on longitudinal aerodynamic characteristics of variable sweep wing having M planfor
Some configuration effects on static stability of airplanes at high angles of attack and low speeds
Vortex spectrum in superfluid turbulence: interpretation of a recent experiment
We discuss a recent experiment in which the spectrum of the vortex line
density fluctuations has been measured in superfluid turbulence. The observed
frequency dependence of the spectrum, , disagrees with classical
vorticity spectra if, following the literature, the vortex line density is
interpreted as a measure of the vorticity or enstrophy. We argue that the
disagrement is solved if the vortex line density field is decomposed into a
polarised field (which carries most of the energy) and an isotropic field
(which is responsible for the spectrum).Comment: Submitted for publication
http://crtbt.grenoble.cnrs.fr/helio/GROUP/infa.html
http://www.mas.ncl.ac.uk/~ncfb
Particle-particle and quasiparticle random phase approximations: Connections to coupled cluster theory
We establish a formal connection between the particle-particle (pp) random
phase approximation (RPA) and the ladder channel of the coupled cluster doubles
(CCD) equations. The relationship between RPA and CCD is best understood within
a Bogoliubov quasiparticle (qp) RPA formalism. This work is a follow-up to our
previous formal proof on the connection between particle-hole (ph) RPA and
ring-CCD. Whereas RPA is a quasibosonic approximation, CC theory is a correct
bosonization in the sense that the wavefunction and Hilbert space are exactly
fermionic. Coupled cluster theory achieves this goal by interacting the ph
(ring) and pp (ladder) diagrams via a third channel that we here call
"crossed-ring" whose presence allows for full fermionic antisymmetry.
Additionally, coupled cluster incorporates what we call "mosaic" terms which
can be absorbed into defining a new effective one-body Hamiltonian. The
inclusion of these mosaic terms seems to be quite important. The pp-RPA an d
qp-RPA equations are textbook material in nuclear structure physics but are
largely unknown in quantum chemistry, where particle number fluctuations and
Bogoliubov determinants are rarely used. We believe that the ideas and
connections discussed in this paper may help design improved ways of
incorporating RPA correlation into density functionals based on a CC
perspective
Quantum turbulence at finite temperature: the two-fluids cascade
To model isotropic homogeneous quantum turbulence in superfluid helium, we
have performed Direct Numerical Simulations (DNS) of two fluids (the normal
fluid and the superfluid) coupled by mutual friction. We have found evidence of
strong locking of superfluid and normal fluid along the turbulent cascade, from
the large scale structures where only one fluid is forced down to the vorticity
structures at small scales. We have determined the residual slip velocity
between the two fluids, and, for each fluid, the relative balance of inertial,
viscous and friction forces along the scales. Our calculations show that the
classical relation between energy injection and dissipation scale is not valid
in quantum turbulence, but we have been able to derive a temperature--dependent
superfluid analogous relation. Finally, we discuss our DNS results in terms of
the current understanding of quantum turbulence, including the value of the
effective kinematic viscosity
Proper and improper zero energy modes in Hartree-Fock theory and their relevance for symmetry breaking and restoration
We study the spectra of the molecular orbital Hessian (stability matrix) and
random-phase approximation Hamiltonian of broken-symmetry Hartree-Fock
solutions, focusing on zero eigenvalue modes. After all negative eigenvalues
are removed from the Hessian by following their eigenvectors downhill, one is
left with only positive and zero eigenvalues. Zero modes correspond to orbital
rotations with no restoring force. These rotations determine states in the
Goldstone manifold, which originates from a spontaneously broken continuous
symmetry in the wave function. Zero modes can be classified as improper or
proper according to their different mathematical and physical properties.
Improper modes arise from symmetry breaking and their restoration always lowers
the energy. Proper modes, on the other hand, correspond to degeneracies of the
wave function, and their symmetry restoration does not necessarily lower the
energy. We discuss how the RPA Hamiltonian distinguishes between proper and
improper modes by doubling the number of zero eigenvalues associated with the
latter. Proper modes in the Hessian always appear in pairs which do not double
in RPA. We present several pedagogical cases exemplifying the above statements.
The relevance of these results for projected Hartree-Fock methods is also
addressed
Flow Induced Organization and Memory of a Vortex Lattice
We report on experiments probing the evolution of a vortex state in response
to a driving current in 2H-NbSe crystals. By following the vortex motion
with fast transport measurements we find that the current enables the system to
reorganize and access new configurations. During this process the system
exhibits a long-term memory: if the current is turned off the vortices freeze
in place remembering their prior motion. When the current is restored the
motion resumes where it stopped. The experiments provide evidence for a
dynamically driven structural change of the vortex lattice and a corresponding
dynamic phase diagram that contains a previously unknown regime where the
critical current can be either or by applying an
appropriate driving current.Comment: 5 pages, 4figure
Engineering entanglement for metrology with rotating matter waves
Entangled states of rotating, trapped ultracold bosons form a very promising scenario for quantum metrology. In order to employ such states for metrology, it is vital to understand their detailed form and the enhanced accuracy with which they could measure phase, in this case generated through rotation. In this work, we study the rotation of ultracold bosons in an asymmetric trapping potential beyond the lowest Landau level (LLL) approximation. We demonstrate that while the LLL can identify reasonably the critical frequency for a quantum phase transition and entangled state generation, it is vital to go beyond the LLL to identify the details of the state and quantify the quantum Fisher information (which bounds the accuracy of the phase measurement). We thus identify a new parameter regime for useful entangled state generation, amenable to experimental investigation
The OLYMPUS Internal Hydrogen Target
An internal hydrogen target system was developed for the OLYMPUS experiment
at DESY, in Hamburg, Germany. The target consisted of a long, thin-walled,
tubular cell within an aluminum scattering chamber. Hydrogen entered at the
center of the cell and exited through the ends, where it was removed from the
beamline by a multistage pumping system. A cryogenic coldhead cooled the target
cell to counteract heating from the beam and increase the density of hydrogen
in the target. A fixed collimator protected the cell from synchrotron radiation
and the beam halo. A series of wakefield suppressors reduced heating from beam
wakefields. The target system was installed within the DORIS storage ring and
was successfully operated during the course of the OLYMPUS experiment in 2012.
Information on the design, fabrication, and performance of the target system is
reported.Comment: 9 pages, 13 figure
Thermodynamically consistent description of the hydrodynamics of free surfaces covered by insoluble surfactants of high concentration
In this paper we propose several models that describe the dynamics of liquid
films which are covered by a high concentration layer of insoluble surfactant.
First, we briefly review the 'classical' hydrodynamic form of the coupled
evolution equations for the film height and surfactant concentration that are
well established for small concentrations. Then we re-formulate the basic model
as a gradient dynamics based on an underlying free energy functional that
accounts for wettability and capillarity. Based on this re-formulation in the
framework of nonequilibrium thermodynamics, we propose extensions of the basic
hydrodynamic model that account for (i) nonlinear equations of state, (ii)
surfactant-dependent wettability, (iii) surfactant phase transitions, and (iv)
substrate-mediated condensation. In passing, we discuss important differences
to most of the models found in the literature.Comment: 31 pages, 2 figure
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