10,521 research outputs found
Fast High Resolution Echelle Spectroscopy Of A Laboratory Plasma
An echelle diffraction grating and a multianode photomultiplier tube are paired to construct a high resolution (R=lambda/delta lambda approximate to 2.5x10(4)) spectrograph with fast time response for use from the UV through the visible. This instrument has analyzed the line shape of C III impurity ion emission at 229.687 nm over the lifetime (approximate to 100 mu s) of the hydrogen plasmas produced at SSX. The ion temperature and line of sight average velocity are inferred from the observed thermal broadening and Doppler shift of the line. The time resolution of these measurements is about 1 mu s, sufficient to observe the fastest magnetohydrodynamic activity
LES of additive and non-additive pulsatile flows in a model arterial stenosis
Transition of additive and non-additive pulsatile flows through a simple 3D model of arterial stenosis is investigated by using a large eddy simulation (LES) technique. We find in both the pulsatile cases that the interaction of the two shear layers, one of which separates from the nose of the stenosis and the another one from its opposite wall, causes recirculation in the flow downstream of the stenosis where the nature of the transient flow becomes turbulent. The strength of this recirculation is found to be quite high from the non-additive pulsations when the flow Reynolds numbers, Re ≥ 1500, for which both the pressure and shearing stresses take on an oscillating form at the post-stenotic region. Potential medical consequences of these results are discussed in the paper. In addition, some comparisons of the non-additive pulsatile results are given with those of both the additive pulsatile and steady flows. The capability of using LES to simulate the pulsatile transitional flow is also assessed, and the present results show that the smaller (subgrid) scales (SGS) contributes about 78% energy dissipation to the flow when the Reynolds number is taken as 2000. The level of SGS dissipation decreases as the Reynolds number is decreased. The numerical results are validated with the experimental data available in literature where a quite good agreement is found
Dynamics of Neural Networks with Continuous Attractors
We investigate the dynamics of continuous attractor neural networks (CANNs).
Due to the translational invariance of their neuronal interactions, CANNs can
hold a continuous family of stationary states. We systematically explore how
their neutral stability facilitates the tracking performance of a CANN, which
is believed to have wide applications in brain functions. We develop a
perturbative approach that utilizes the dominant movement of the network
stationary states in the state space. We quantify the distortions of the bump
shape during tracking, and study their effects on the tracking performance.
Results are obtained on the maximum speed for a moving stimulus to be
trackable, and the reaction time to catch up an abrupt change in stimulus.Comment: 6 pages, 7 figures with 4 caption
Dynamical Synapses Enhance Neural Information Processing: Gracefulness, Accuracy and Mobility
Experimental data have revealed that neuronal connection efficacy exhibits
two forms of short-term plasticity, namely, short-term depression (STD) and
short-term facilitation (STF). They have time constants residing between fast
neural signaling and rapid learning, and may serve as substrates for neural
systems manipulating temporal information on relevant time scales. The present
study investigates the impact of STD and STF on the dynamics of continuous
attractor neural networks (CANNs) and their potential roles in neural
information processing. We find that STD endows the network with slow-decaying
plateau behaviors-the network that is initially being stimulated to an active
state decays to a silent state very slowly on the time scale of STD rather than
on the time scale of neural signaling. This provides a mechanism for neural
systems to hold sensory memory easily and shut off persistent activities
gracefully. With STF, we find that the network can hold a memory trace of
external inputs in the facilitated neuronal interactions, which provides a way
to stabilize the network response to noisy inputs, leading to improved accuracy
in population decoding. Furthermore, we find that STD increases the mobility of
the network states. The increased mobility enhances the tracking performance of
the network in response to time-varying stimuli, leading to anticipative neural
responses. In general, we find that STD and STP tend to have opposite effects
on network dynamics and complementary computational advantages, suggesting that
the brain may employ a strategy of weighting them differentially depending on
the computational purpose.Comment: 40 pages, 17 figure
Two-dimensional Vesicle dynamics under shear flow: effect of confinement
Dynamics of a single vesicle under shear flow between two parallel plates is
studied using two-dimensional lattice-Boltzmann simulations. We first present
how we adapted the lattice-Boltzmann method to simulate vesicle dynamics, using
an approach known from the immersed boundary method. The fluid flow is computed
on an Eulerian regular fixed mesh while the location of the vesicle membrane is
tracked by a Lagrangian moving mesh. As benchmarking tests, the known vesicle
equilibrium shapes in a fluid at rest are found and the dynamical behavior of a
vesicle under simple shear flow is being reproduced. Further, we focus on
investigating the effect of the confinement on the dynamics, a question that
has received little attention so far. In particular, we study how the vesicle
steady inclination angle in the tank-treading regime depends on the degree of
confinement. The influence of the confinement on the effective viscosity of the
composite fluid is also analyzed. At a given reduced volume (the swelling
degree) of a vesicle we find that both the inclination angle, and the membrane
tank-treading velocity decrease with increasing confinement. At sufficiently
large degree of confinement the tank-treading velocity exhibits a
non-monotonous dependence on the reduced volume and the effective viscosity
shows a nonlinear behavior.Comment: 12 pages, 8 figure
Dynamical regimes and hydrodynamic lift of viscous vesicles under shear
The dynamics of two-dimensional viscous vesicles in shear flow, with
different fluid viscosities and inside and
outside, respectively, is studied using mesoscale simulation techniques.
Besides the well-known tank-treading and tumbling motions, an oscillatory
swinging motion is observed in the simulations for large shear rate. The
existence of this swinging motion requires the excitation of higher-order
undulation modes (beyond elliptical deformations) in two dimensions.
Keller-Skalak theory is extended to deformable two-dimensional vesicles, such
that a dynamical phase diagram can be predicted for the reduced shear rate and
the viscosity contrast . The simulation results
are found to be in good agreement with the theoretical predictions, when
thermal fluctuations are incorporated in the theory. Moreover, the hydrodynamic
lift force, acting on vesicles under shear close to a wall, is determined from
simulations for various viscosity contrasts. For comparison, the lift force is
calculated numerically in the absence of thermal fluctuations using the
boundary-integral method for equal inside and outside viscosities. Both methods
show that the dependence of the lift force on the distance of
the vesicle center of mass from the wall is well described by an effective
power law for intermediate distances with vesicle radius .
The boundary-integral calculation indicates that the lift force decays
asymptotically as far from the wall.Comment: 13 pages, 13 figure
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