108,783 research outputs found
Observing collapse in two colliding dipolar Bose-Einstein condensates
We study the collision of two Bose-Einstein condensates with pure dipolar
interaction. A stationary pure dipolar condensate is known to be stable when
the atom number is below a critical value. However, collapse can occur during
the collision between two condensates due to local density fluctuations even if
the total atom number is only a fraction of the critical value. Using full
three-dimensional numerical simulations, we observe the collapse induced by
local density fluctuations. For the purpose of future experiments, we present
the time dependence of the density distribution, energy per particle and the
maximal density of the condensate. We also discuss the collapse time as a
function of the relative phase between the two condensates.Comment: 6 pages, 7 figure
Modelling and control of the flame temperature distribution using probability density function shaping
This paper presents three control algorithms for the output probability density function (PDF) control of the 2D and 3D flame distribution systems. For the 2D flame distribution systems, control methods for both static and dynamic flame systems are presented, where at first the temperature distribution of the gas jet flames along the cross-section is approximated. Then the flame energy distribution (FED) is obtained as the output to be controlled by using a B-spline expansion technique. The general static output PDF control algorithm is used in the 2D static flame system, where the dynamic system consists of a static temperature model of gas jet flames and a second-order actuator. This leads to a second-order closed-loop system, where a singular state space model is used to describe the dynamics with the weights of the B-spline functions as the state variables. Finally, a predictive control algorithm is designed for such an output PDF system. For the 3D flame distribution systems, all the temperature values of the flames are firstly mapped into one temperature plane, and the shape of the temperature distribution on this plane can then be controlled by the 3D flame control method proposed in this paper. Three cases are studied for the proposed control methods and desired simulation results have been obtained
Why isolated streamer discharges hardly exist above the breakdown field in atmospheric air
We investigate streamer formation in the troposphere, in electric fields
above the breakdown threshold. With fully three-dimensional particle
simulations, we study the combined effect of natural background ionization and
of photoionization on the discharge morphology. In previous investigations
based on deterministic fluid models without background ionization, so-called
double-headed streamers emerged. But in our improved model, many electron
avalanches start to grow at different locations. Eventually the avalanches
collectively screen the electric field in the interior of the discharge. This
happens after what we call the `ionization screening time', for which we give
an analytical estimate. As this time is comparable to the streamer formation
time, we conclude that isolated streamers are unlikely to exist in fields well
above breakdown in atmospheric air.Comment: Changed citation information. 6 pages, 4 figures, Geophysical
Research Letters, Vol. 40, 2417-2422, 201
A Semi-Blind Source Separation Method for Differential Optical Absorption Spectroscopy of Atmospheric Gas Mixtures
Differential optical absorption spectroscopy (DOAS) is a powerful tool for
detecting and quantifying trace gases in atmospheric chemistry
\cite{Platt_Stutz08}. DOAS spectra consist of a linear combination of complex
multi-peak multi-scale structures. Most DOAS analysis routines in use today are
based on least squares techniques, for example, the approach developed in the
1970s uses polynomial fits to remove a slowly varying background, and known
reference spectra to retrieve the identity and concentrations of reference
gases. An open problem is to identify unknown gases in the fitting residuals
for complex atmospheric mixtures.
In this work, we develop a novel three step semi-blind source separation
method. The first step uses a multi-resolution analysis to remove the
slow-varying and fast-varying components in the DOAS spectral data matrix .
The second step decomposes the preprocessed data in the first step
into a linear combination of the reference spectra plus a remainder, or
, where columns of matrix are known reference spectra,
and the matrix contains the unknown non-negative coefficients that are
proportional to concentration. The second step is realized by a convex
minimization problem ,
where the norm is a hybrid norm (Huber estimator) that helps to
maintain the non-negativity of . The third step performs a blind independent
component analysis of the remainder matrix to extract remnant gas
components. We first illustrate the proposed method in processing a set of DOAS
experimental data by a satisfactory blind extraction of an a-priori unknown
trace gas (ozone) from the remainder matrix. Numerical results also show that
the method can identify multiple trace gases from the residuals.Comment: submitted to Journal of Scientific Computin
Bipolar High Field Excitations in Co/Cu/Co Nanopillars
Current-induced magnetic excitations in Co/Cu/Co bilayer nanopillars
(50 nm in diameter) have been studied experimentally at low temperatures
for large applied fields perpendicular to the layers. At sufficiently high
current densities excitations, which lead to a decrease in differential
resistance, are observed for both current polarities. Such bipolar excitations
are not expected in a single domain model of spin-transfer. We propose that at
high current densities strong asymmetries in the longitudinal spin accumulation
cause spin-wave instabilities transverse to the current direction in bilayer
samples, similar to those we have reported for single magnetic layer junctions.Comment: 4 pages, 4 figures+ 2 additional jpg figures (Fig. 2d and Fig. 3)
high resolution figures and recent related articles are available at:
http://www.physics.nyu.edu/kentlab/news.htm
Microphase transitions of block copolymer/homopolymer under shear flow
Cell dynamics simulation is used to investigate the phase behavior of block
copolymer/homopolymer mixture subjected to a steady shear flow. Phase
transitions occur from transverse to parallel and then to perpendicular
lamellar structure with an increase of shear rate and this is the result of
interaction between the shear flow and the concentration fluctuation.
Rheological properties, such as normal stress differences and shear viscosity,
are all closely related with the direction of the lamellae. Furthermore, we
specifically explore the phase behavior and the order parameter under weak and
strong shear of two different initial states, and realize the importance of the
thermal history. It is necessary to apply the shear field at the appropriate
time if we want to get what we want. These results provide an easy method to
create ordered, defect-free materials in experiment and engineering technology
through imposing shear flow.Comment: 14 pages, 9 figure
Effects of the triaxial deformation and pairing correlation on the proton emitter 145Tm
The ground-state properties of the recent reported proton emitter 145Tm have
been studied within the axially or triaxially deformed relativistic mean field
(RMF) approaches, in which the pairing correlation is taken into account by the
BCS-method with a constant pairing gap. It is found that triaxiality and
pairing correlations play important roles in reproducing the experimental one
proton separation energy. The single-particle level, the proton emission orbit,
the deformation parameters beta = 0.22 and gamma = 28.98 and the corresponding
spectroscopic factor for 145Tm in the triaxial RMF calculation are given as
well.Comment: 17 pages, 7 figures and 1 table. accepted by Physical Review
Blood lactate clearance after maximal exercise depends on active recovery intensity
AIM: High-intensity exercise is time-limited by onset of fatigue, marked by accumulation of blood lactate. This is accentuated at maximal, all-out exercise that rapidly accumulates high blood lactate. The optimal active recovery intensity for clearing lactate after such maximal, all-out exercise remains unknown. Thus, we studied the intensity-dependence of lactate clearance during active recovery after maximal exercise.<p></p>
METHODS: We constructed a standardized maximal, all-out treadmill exercise protocol that predictably lead to voluntary exhaustion and blood lactate concentration >10 mM. Next, subjects ran series of all-out bouts that increased blood lactate concentration to 11.5±0.2 mM, followed by recovery exercises ranging 0% (passive)-100% of the lactate threshold.<p></p>
RESULTS: Repeated measurements showed faster lactate clearance during active versus passive recovery (P<0.01), and that active recovery at 60-100% of lactate threshold was more efficient for lactate clearance than lower intensity recovery (P<0.05). Active recovery at 80% of lactate threshold had the highest rate of and shortest time constant for lactate clearance (P<0.05), whereas the response during the other intensities was graded (100%=60%>40%>passive recovery, P<0.05).<p></p>
CONCLUSION: Active recovery after maximal all-out exercise clears accumulated blood lactate faster than passive recovery in an intensity-dependent manner, with maximum clearance occurring at active recovery of 80% of lactate threshold
- …