2,642 research outputs found
On modelling transitional turbulent flows using under-resolved direct numerical simulations: The case of plane Couette flow
Direct numerical simulations have proven of inestimable help to our
understanding of the transition to turbulence in wall-bounded flows. While the
dynamics of the transition from laminar flow to turbulence via localised spots
can be investigated with reasonable computing resources in domains of limited
extent, the study of the decay of turbulence in conditions approaching those in
the laboratory requires consideration of domains so wide as to exclude the
recourse to fully resolved simulations. Using Gibson's C++ code ChannelFlow, we
scrutinize the effects of a controlled lowering of the numerical resolution on
the decay of turbulence in plane Couette flow at a quantitative level. We show
that the number of Chebyshev polynomials describing the cross-stream dependence
can be drastically decreased while preserving all the qualitative features of
the solution. In particular, the oblique turbulent band regime experimentally
observed in the upper part of the transitional range is extremely robust. In
terms of Reynolds numbers, the resolution lowering is seen to yield a regular
downward shift of the upper and lower thresholds Rt and Rg where the bands
appear and break down. The study is illustrated with the results of two
preliminary experiments.Comment: 20 pages, 9 figures. Accepted on August 24, 2010, to appear in TCF
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
Computer-Aided Geometry Modeling
Techniques in computer-aided geometry modeling and their application are addressed. Mathematical modeling, solid geometry models, management of geometric data, development of geometry standards, and interactive and graphic procedures are discussed. The applications include aeronautical and aerospace structures design, fluid flow modeling, and gas turbine design
D-wave correlated Critical Bose Liquids in two dimensions
We develop a description of a new quantum liquid phase of interacting bosons
in 2d which possesses relative D-wave two-body correlations and which we call a
D-wave Bose Liquid (DBL). The DBL has no broken symmetries, supports gapless
boson excitations residing on "Bose surfaces" in momentum space, and exhibits
power law correlations with continuously variable exponents. While the DBL can
be constructed for bosons in the 2d continuum, the state only respects the
point group symmetries of the square lattice. On the lattice the DBL respects
all symmetries and does not require a particular filling. But lattice effects
allow a second distinct phase, a quasi-local variant which we call a D-wave
Local Bose Liquid (DLBL). Remarkably, the DLBL has short-range boson
correlations and hence no Bose surfaces, despite sharing gapless excitations
and other critical signatures with the DBL. Moreover, both phases are metals
with a resistance that vanishes as a power of the temperature. We establish
these results by constructing a class of many-particle wavefunctions for the
DBL, which are time reversal invariant analogs of Laughlin's quantum Hall
wavefunction for bosons at . A gauge theory formulation leads to a
simple mean field theory, and an N-flavor generalization enables incorporation
of gauge field fluctuations to deduce the properties of the DBL/DLBL; various
equal time correlation functions are in qualitative accord with the properties
inferred from the wavefunctions. We also identify a promising Hamiltonian which
might manifest the DBL or DLBL, and perform a variational study comparing to
other competing phases. We suggest how the DBL wavefunction can be generalized
to describe an itinerant non-Fermi liquid phase of electrons on the square
lattice with a no double occupancy constraint, a D-wave metal phase.Comment: 33 pages, 17 figure
- …