37,568 research outputs found
Computational convergence of the path integral for real dendritic morphologies
Neurons are characterised by a morphological structure unique amongst biological cells, the core of which is the dendritic tree. The vast number of dendritic geometries, combined with heterogeneous properties of the cell membrane, continue to challenge scientists in predicting neuronal input-output relationships, even in the case of sub-threshold dendritic currents. The Green’s function obtained for a given dendritic geometry provides this functional relationship for passive or quasi-active dendrites and can be constructed by a sum-over-trips approach based on a path integral formalism. In this paper, we introduce a number of efficient algorithms for realisation of the sum-over-trips framework and investigate the convergence of these algorithms on different dendritic geometries. We demonstrate that the convergence of the trip sampling methods strongly depends on dendritic morphology as well as the biophysical properties of the cell membrane. For real morphologies, the number of trips to guarantee a small convergence error might become very large and strongly affect computational efficiency. As an alternative, we introduce a highly-efficient matrix method which can be applied to arbitrary branching structures
Video Interpolation using Optical Flow and Laplacian Smoothness
Non-rigid video interpolation is a common computer vision task. In this paper
we present an optical flow approach which adopts a Laplacian Cotangent Mesh
constraint to enhance the local smoothness. Similar to Li et al., our approach
adopts a mesh to the image with a resolution up to one vertex per pixel and
uses angle constraints to ensure sensible local deformations between image
pairs. The Laplacian Mesh constraints are expressed wholly inside the optical
flow optimization, and can be applied in a straightforward manner to a wide
range of image tracking and registration problems. We evaluate our approach by
testing on several benchmark datasets, including the Middlebury and Garg et al.
datasets. In addition, we show application of our method for constructing 3D
Morphable Facial Models from dynamic 3D data
A New Vehicle Localization Scheme Based on Combined Optical Camera Communication and Photogrammetry
The demand for autonomous vehicles is increasing gradually owing to their
enormous potential benefits. However, several challenges, such as vehicle
localization, are involved in the development of autonomous vehicles. A simple
and secure algorithm for vehicle positioning is proposed herein without
massively modifying the existing transportation infrastructure. For vehicle
localization, vehicles on the road are classified into two categories: host
vehicles (HVs) are the ones used to estimate other vehicles' positions and
forwarding vehicles (FVs) are the ones that move in front of the HVs. The FV
transmits modulated data from the tail (or back) light, and the camera of the
HV receives that signal using optical camera communication (OCC). In addition,
the streetlight (SL) data are considered to ensure the position accuracy of the
HV. Determining the HV position minimizes the relative position variation
between the HV and FV. Using photogrammetry, the distance between FV or SL and
the camera of the HV is calculated by measuring the occupied image area on the
image sensor. Comparing the change in distance between HV and SLs with the
change in distance between HV and FV, the positions of FVs are determined. The
performance of the proposed technique is analyzed, and the results indicate a
significant improvement in performance. The experimental distance measurement
validated the feasibility of the proposed scheme
Slip-velocity of large neutrally-buoyant particles in turbulent flows
We discuss possible definitions for a stochastic slip velocity that describes
the relative motion between large particles and a turbulent flow. This
definition is necessary because the slip velocity used in the standard drag
model fails when particle size falls within the inertial subrange of ambient
turbulence. We propose two definitions, selected in part due to their
simplicity: they do not require filtration of the fluid phase velocity field,
nor do they require the construction of conditional averages on particle
locations. A key benefit of this simplicity is that the stochastic slip
velocity proposed here can be calculated equally well for laboratory, field,
and numerical experiments. The stochastic slip velocity allows the definition
of a Reynolds number that should indicate whether large particles in turbulent
flow behave (a) as passive tracers; (b) as a linear filter of the velocity
field; or (c) as a nonlinear filter to the velocity field. We calculate the
value of stochastic slip for ellipsoidal and spherical particles (the size of
the Taylor microscale) measured in laboratory homogeneous isotropic turbulence.
The resulting Reynolds number is significantly higher than 1 for both particle
shapes, and velocity statistics show that particle motion is a complex
non-linear function of the fluid velocity. We further investigate the nonlinear
relationship by comparing the probability distribution of fluctuating
velocities for particle and fluid phases
Large-eddy simulation and wall modelling of turbulent channel flow
We report large-eddy simulation (LES) of turbulent channel flow. This LES neither resolves nor partially resolves the near-wall region. Instead, we develop a special near-wall subgrid-scale (SGS) model based on wall-parallel filtering and wall-normal averaging of the streamwise momentum equation, with an assumption of local inner scaling used to reduce the unsteady term. This gives an ordinary differential equation (ODE) for the wall shear stress at every wall location that is coupled with the LES. An extended form of the stretched-vortex SGS model, which incorporates the production of near-wall Reynolds shear stress due to the winding of streamwise momentum by near-wall attached SGS vortices, then provides a log relation for the streamwise velocity at the top boundary of the near-wall averaged domain. This allows calculation of an instantaneous slip velocity that is then used as a ‘virtual-wall’ boundary condition for the LES. A Kármán-like constant is calculated dynamically as part of the LES. With this closure we perform LES of turbulent channel flow for Reynolds numbers Re_τ based on the friction velocity u_τ and the channel half-width δ in the range 2 × 10^3 to 2 × 10^7. Results, including SGS-extended longitudinal spectra, compare favourably with the direct numerical simulation (DNS) data of Hoyas & Jiménez (2006) at Re_τ = 2003 and maintain an O(1) grid dependence on Re_τ
Steady-State Ab Initio Laser Theory for N-level Lasers
We show that Steady-state Ab initio Laser Theory (SALT) can be applied to
find the stationary multimode lasing properties of an N-level laser. This is
achieved by mapping the N-level rate equations to an effective two-level model
of the type solved by the SALT algorithm. This mapping yields excellent
agreement with more computationally demanding N-level time domain solutions for
the steady state
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