2,809 research outputs found
Construction of Reduced Order Models for Fluid Flows Using Deep Feedforward Neural Networks
We present a numerical methodology for construction of reduced order models,
ROMs, of fluid flows through the combination of flow modal decomposition and
regression analysis. Spectral proper orthogonal decomposition, SPOD, is applied
to reduce the dimensionality of the model and, at the same time, filter the POD
temporal modes. The regression step is performed by a deep feedforward neural
network, DNN, and the current framework is implemented in a context similar to
the sparse identification of non-linear dynamics algorithm, SINDy. A discussion
on the optimization of the DNN hyperparameters is provided for obtaining the
best ROMs and an assessment of these models is presented for a canonical
nonlinear oscillator and the compressible flow past a cylinder. Then, the
method is tested on the reconstruction of a turbulent flow computed by a large
eddy simulation of a plunging airfoil under dynamic stall. The reduced order
model is able to capture the dynamics of the leading edge stall vortex and the
subsequent trailing edge vortex. For the cases analyzed, the numerical
framework allows the prediction of the flowfield beyond the training window
using larger time increments than those employed by the full order model. We
also demonstrate the robustness of the current ROMs constructed via deep
feedforward neural networks through a comparison with sparse regression. The
DNN approach is able to learn transient features of the flow and presents more
accurate and stable long-term predictions compared to sparse regression
AMPTE/CCE‐SCATHA simultaneous observations of substorm‐associated magnetic fluctuations
This study examines substorm-associated magnetic field fluctuations observed by the AMPTE/CCE and SCATHA satellites in the near-Earth tail. Three tail reconfiguration events are selected, one event on August 28, 1986, and two consecutive events on August 30, 1986. The fractal analysis was applied to magnetic field measurements of each satellite. The result indicates that (1) the amplitude of the fluctuation of the north-south magnetic component is larger, though not overwhelmingly, than the amplitudes of the other two components and (2) the magnetic fluctuations do have a characteristic timescale, which is several times the proton gyroperiod. In the examined events the satellite separation was less than 10 times the proton gyroradius. Nevertheless, the comparison between the AMPTE/CCE and SCATHA observations indicates that (3) there was a noticeable time delay between the onsets of the magnetic fluctuations at the two satellite positions, which is too long to ascribe to the propagation of a fast magnetosonic wave, and (4) the coherence of the magnetic fluctuations was low in the August 28, 1986, event and the fluctuations had different characteristic timescales in the first event of August 30, 1986, whereas some similarities can be found for the second event of August 30, 1986. Result 1 indicates that perturbation electric currents associated with the magnetic fluctuations tend to flow parallel to the tail current sheet and are presumably related to the reduction of the tail current intensity. Results 2 and 3 suggest that the excitation of the magnetic fluctuations and therefore the trigger of the tail current disruption is a kinetic process in which ions play an important role. It is inferred from results 3 and 4 that the characteristic spatial scale of the associated instability is of the order of the proton gyroradius or even shorter, and therefore the tail current disruption is described as a system of chaotic filamentary electric currents. However, result 4 suggests that the nature of the tail current disruption can vary from event to event
Effect of next-nearest neighbor coupling on the optical spectra in bilayer graphene
We investigate the dependence of the optical conductivity of bilayer graphene
(BLG) on the intra- and inter-layer interactions using the most complete model
to date. We show that the next nearest-neighbor intralayer coupling introduces
new features in the low-energy spectrum that are highly sensitive to sample
doping, changing significantly the ``universal'' conductance. Further, its
interplay with interlayer couplings leads to an anisotropy in conductance in
the ultraviolet range. We propose that experimental measurement of the optical
conductivity of intrinsic and doped BLG will provide a good benchmark for the
relative importance of intra- and inter-layer couplings at different doping
levels.Comment: 5 pages, 5 figure
Photon recycling in Fabry-Perot micro-cavities based on SiN waveguides
We present a numerical analysis and preliminary experimental results on
one-dimensional Fabry-Perot micro-cavities in SiN waveguides. The
Fabry-Perot micro-cavities are formed by two distributed Bragg reflectors
separated by a straight portion of waveguide. The Bragg reflectors are composed
by a few air slits produced within the SiN waveguides. In order to
increase the quality factor of the micro-cavities, we have minimized, with a
multiparametric optimization tool, the insertion loss of the reflectors by
varying the length of their first periods (those facing the cavity). To explain
the simulation results the coupling of the fundamental waveguide mode with
radiative modes in the Fabry-Perot micro-cavities is needed. This effect is
described as a recycling of radiative modes in the waveguide. To support the
modelling, preliminary experimental results of micro-cavities in SiN
waveguides realized with Focused Ion Beam technique are reported.Comment: 5 pages, 5 figure
Can Disruption of Basal Ganglia-Thalamocortical Circuit in Wilson Disease Be Associated with Juvenile Myoclonic Epilepsy Phenotype?
In this paper, we describe the multimodal MRI findings in a patient with Wilson disease and a seizure disorder, characterized by an electroclinical picture resembling juvenile myoclonic epilepsy. The brain structural MRI showed a deposition of ferromagnetic materials in the basal ganglia, with marked hypointensities in T2-weighted images of globus pallidus internus bilaterally. A resting-state fMRI study revealed increased functional connectivity in the patient, compared to control subjects, in the following networks: (1) between the primary motor cortex and several cortical regions, including the secondary somatosensory cortex and (2) between the globus pallidus and the thalamo-frontal network. These findings suggest that globus pallidus alterations, due to metal accumulation, can lead to a reduction in the normal globus pallidus inhibitory tone on the thalamo-(motor)-cortical pathway. This, in turn, can result in hyperconnectivity in the motor cortex circuitry, leading to myoclonus and tonic-clonic seizures. We suppose that, in this patient, Wilson disease generated a ‘lesion model’ of myoclonic epilepsy
BCI-FES training system design and implementation for rehabilitation of stroke patients
Author name used in this publication: Kai-yu TongAuthor name used in this publication: Suk-tak ChanRefereed conference paper2007-2008 > Academic research: refereed > Refereed conference paperVersion of RecordPublishe
Cerebral plasticity after subcortical stroke as revealed by cortico-muscular coherence
Author name used in this publication: Kai-Yu TongAuthor name used in this publication: Suk-Tak Chan2008-2009 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe
Shock-Boundary Layer Interactions in Supersonic Turbine Cascades
The physics of shock-boundary layer interactions in a supersonic turbine
cascade is investigated through a wall-resolved large eddy simulation. Special
attention is given to the characterization of the low-frequency dynamics of the
separation bubbles using flow visualization, spectral analysis, space-time
cross correlations, and flow modal decomposition. The mean flowfield shows
different shock structures formed on both sides of the airfoil. On the suction
side, an oblique shock impinges on the turbulent boundary layer, whereas a Mach
reflection interacts with the pressure side boundary layer. Instantaneous flow
visualizations illustrate elongated streamwise structures on the incoming
boundary layers and their interactions with the shocks and separation bubbles.
The passage of high-speed (low-speed) streaks through the recirculation bubbles
leads to the downstream (upstream) motion of the separation point on both
suction and pressure sides, resulting in spanwise modulation of the bubbles.
Space-time cross-correlations reveal that the near-wall streaks drive the
suction side separation bubble motion, which in turn promotes the oscillations
of the reattachment shock and shear layer flapping. Space-time correlations
also indicate the existence of a phase jump in the pressure fluctuations
along the separation bubble on the suction side. After this phase jump, a
downstream propagating pressure disturbance is observed, while prior to this
point, the pressure disturbances dominantly propagate in the upstream
direction. Finally, the organized motions in the shock-boundary layer
interactions and their corresponding characteristic frequencies are identified
using proper orthogonal decomposition.Comment: 40 pages, 19 figures. Submitted to Physical Review Fluid
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