85,429 research outputs found
Cue competition affects temporal dynamics of edge-assignment in human visual cortex
No full textEdge-assignment determines the perception of relative depth across an edge and the shape of the closer side. Many cues determine edge-assignment, but relatively little is known about the neural mechanisms involved in combining these cues. Here, we manipulated extremal edge and attention cues to bias edge-assignment such that these two cues either cooperated or competed. To index their neural representations, we flickered figure and ground regions at different frequencies and measured the corresponding steady-state visual-evoked potentials (SSVEPs). Figural regions had stronger SSVEP responses than ground regions, independent of whether they were attended or unattended. In addition, competition and cooperation between the two edge-assignment cues significantly affected the temporal dynamics of edge-assignment processes. The figural SSVEP response peaked earlier when the cues causing it cooperated than when they competed, but sustained edge-assignment effects were equivalent for cooperating and competing cues, consistent with a winner-take-all outcome. These results provide physiological evidence that figure-ground organization involves competitive processes that can affect the latency of figural assignment
Edge detection filter based on Mumford-Shah green function
No full textIn this paper, we propose an edge detection algorithm based on the Green function associated with Mumford-Shah (M-S) segmentation model. This Green function has a singularity at its center. A regularization method is therefore proposed here to obtain an edge detection filter known here as Bessel filter. This filter is robust in the presence of noise and its implementation is simple. It is demonstrated here that this filter detects edges particularly in the case of curved boundaries and sharp corners, more accurately than popular filters in the recent literature. A mathematical argument is also provided to prove that the gradient magnitude of the convolved image with this filter has local maxima in discontinuities of the original image. The Bessel filter enjoys better overall performance (the product of the detection performance and localization indices) in Canny-like criteria than the state of art filters in the literature. Quantitative and qualitative evaluations of the edge detection algorithms investigated in this paper on synthetic and real world benchmark images confirm the theoretical results presented here, indicating the superiority of the Bessel filter over the popular edge detection filters. The numerical complexity of the algorithm proposed here is as low as any convolution-based edge detection algorithm
Fully localised edge states in boundary layers
No full textInvestigation of the laminar-turbulent boundary is performed in a boundary-layer flow. Constant homogeneous suction is applied at the wall in order to prevent the spatial growth of the layer, leading to the parallel Asymptotic Suction Boundary Layer (ASBL). Edge tracking is performed in a large computational domain allowing for full spatial localisation of the structures on the laminar-turbulent separatrix. The obtained dynamics of the state goes through calm and bursting phases. During the latter the structure grows in size, shedding vortices downstream of its core which viscously decay during the calm phases. Comparison with the computation in spatially growing boundary layer is made. The influence of the Reynolds number and the path leading from the edge state to turbulent flow are considered
Three-dimensional edge waves in plates
No full textThis paper describes the propagation of three-dimensional symmetric waves localized near the traction-free edge of a semi-infinite elastic plate with either traction-free or fixed faces. For both types of boundary conditions, we present a variational proof of the existence of the low-order edge waves. In addition, for a plate with traction-free faces and zero Poisson ratio, the fundamental edge wave is described by a simple explicit formula, and the first-order edge wave is proved to exist. Qualitative variational predictions are compared with numerical results, which are obtained using expansions in three-dimensional Rayleigh–Lamb and shear modes. It is also demonstrated numerically that for any non-zero Poisson ratio in a plate with traction-free faces, the eigenfrequencies related to the first-order wave are complex valued
Scale invariant filtering design and analysis for edge detection
No full textExisting edge detection filters work well on straight edges but make significant errors near sharp corners by producing rounded corners. This is due to the fact that the edge maps produced by these filters are scale variant. We enhance Canny’s optimality criteria to incorporate detection performance near corners as an explicit design objective. The resulting optimal filter, termed “Bessel integral filter” can be derived analytically and exhibits superior performance over recent alternatives, both in terms of numerical accuracy and experimental fidelity. A noise free localization index is also derived here to account for the detection accuracy of discontinuities forming sharp corners in the absence of noise. We prove here that edges detected by the filters which are not optimal with respect to this noise free localization index are scale variant. However the Bessel integral filter proposed here is optimal with respect to the noise free localization index and therefore it is a scale invariant filte
Coexistence and decoupling of bulk and edge states in disordered two-dimensional topological insulators
No full textWe investigate the scattering and localization properties of edge and bulk states in a disordered two-dimensional topological insulator when they coexist at the same Fermi energy. Due to edge-bulk backscattering (which is not prohibited a priori by topology or symmetry), Anderson disorder makes the edge and bulk states localized indistinguishably. Two methods are proposed to effectively decouple them and to restore robust transport. The first kind of decouple is from long-range disorder since edge and bulk states are well separated in k space. The second one is from an edge gating, owing to the edge nature of edge states in real space. The latter can be used to electrically tune a system between an Anderson insulator and a topologically robust conductor, i.e., a realization of a topological transistor.Physics, Condensed MatterSCI(E)3ARTICLE5null9
The effect of leading edge serrations on dynamic stall
No full textAn investigation into the effects of dynamic stall was carried out on six aerofoil profiles with sinusoidal leading edges having two amplitudes and three different wavelengths. The study also investigated the effect of wavelength on the static performance of the aerofoil as well as the static hysteresis performance of these profiles. Compared to a baseline model, it was found that a reduction in wavelength increased the maximum lift and the static stall angle. The maximum baseline lift was not reached in any of the cases. The static hysteresis performance of the sinusoidal leading edge profiles was found to be significantly better than the baseline with virtually no static hysteresis recorded. The dynamic study revealed that the sinusoidal profiles improved the performance of the aerofoil by increasing the maximum percentage of lift generated as well as by reducing the size of the hysteresis loop
Edge waves and resonance on elastic structures: An overview
No full textCopyright @ 2011 SAGE PublicationsOver 50 years have elapsed since the first experimental observations of dynamic edge phenomena on elastic structures, yet the topic remains a diverse and vibrant source of research activity. This article provides a focused history and overview of such phenomena with a particular emphasis on structures such as strips, rods, plates and shells. Within this context, some of the recent research highlights are discussed and the contents of this special issue of Mathematics and Mechanics of Solids on dynamical edge phenomena are introduced
Direct numerical simulation of turbulent flow past a trailing edge and the associated noise generation
No full textDirect numerical simulations (DNS) are conducted of turbulent flow passing an infinitely thin trailing edge (TE). The objective is to investigate the turbulent flow field in the vicinity of the TE and the associated broadband noise generation. To generate a turbulent boundary layer a short distance from the inflow boundary, high amplitude lifted streaks and disturbances that can be associated with coherent outer layer vortices are introduced at the inflow boundary. A rapid increase in skin friction and a decrease in boundary layer thickness and pressure fluctuations is observed at the trailing edge. It is demonstrated that the behaviour of the hydrodynamic field in the vicinity of the TE can be predicted with reasonable accuracy using triple deck theory if the eddy viscosity is accounted for. Point spectra of surface pressure difference are shown to vary considerably towards the trailing edge, with a significant reduction of amplitude occurring in the low frequency range.The acoustic pressure obtained from the DNS is compared with predictions from two- and three-dimensional acoustic analogies and the classical trailing edge theory of Amiet. For low frequencies, two dimensional theory succeeds in predicting the acoustic pressure in the far field with reasonable accuracy due to a significant spanwise coherence of the surface pressure difference and predominantly two dimensional sound radiation. For higher frequencies, however, the full three dimensional theory is required for an accurate prediction of the acoustic far field. DNS data are used to test some of the key assumptions invoked by Amiet for the derivation of the classical trailing edge theory. Even though most of the approximations are shown to be reasonable, they collectively lead to a deviation from the DNS results, in particular for higher frequencies. Moreover, because the three dimensional acoustic analogy does not provide significantly improved results, it is suggested that some of the discrepancies can be attributed to the approach of evaluating the far field sound using a Kirchhoff-type integration of the surface pressure difference
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