214 research outputs found
Optimization of the transmitter-receiver separation in the ground-penetrating radar
Cataloged from PDF version of article.The finite-difference time-domain method is applied to simulate three-dimensional subsurface-scattering problems, involving a ground-penetrating-radar (GPR) model consisting of two transmitters and a receiver. The receiving antenna is located in the middle of the twos identical transmitters, which are fed 180degrees out of phase. This configuration implies the existence of a symmetry plane in the middle of two transmitters and the cancellation of the direct signals coupled from the transmitters at the receiver location. The antenna polarizations and their separations are arbitrary. The transmitter-receiver-transmitter configured GPR model is optimized in terms of the scattered energy observed at the receiver by varying the antenna separation. Many simulation results are used to demonstrate the effects of the antenna separation and the optimal separation encountered for a specific target and GPR scenario
Modeling of ground-penetrating-radar antennas with shields and simulated absorbers
Cataloged from PDF version of article.A three-dimensional (3-D) finite-difference time-domain
(FDTD) scheme is employed to simulate ground-penetrating
radars. Conducting shield walls and absorbers are used to reduce
the direct coupling to the receiver. Perfectly matched layer (PML)
absorbing boundary conditions are used for matching the multilayered
media and simulating physical absorbers inside the FDTD
computational domain. Targets are modeled by rectangular prisms
of arbitrary permittivity and conductivity. The ground is modeled
by homogeneous and lossless dielectric media
Signal-processing techniques to reduce the sinusoidal steady-state error in the FDTD method
Cataloged from PDF version of article.Techniques to improve the accuracy of the finite-difference
time-domain (FDTD) solutions employing sinusoidal excitations
are developed. The FDTD computational domain is considered
as a sampled system and analyzed with respect to the aliasing
error using the Nyquist sampling theorem. After a careful examination
of how the high-frequency components in the excitation
cause sinusoidal steady-state errors in the FDTD solutions, the use
of smoothing windows and digital low-pass filters is suggested to
reduce the error. The reduction in the error is demonstrated for
various cases
Three-dimensional FDTD modeling of a ground-penetrating radar
Cataloged from PDF version of article.The finite-difference time-domain (FDTD) method
is used to simulate three-dimensional (3-D) geometries of realistic
ground-penetrating radar (GPR) scenarios. The radar unit is modeled
with two transmitters and a receiver in order to cancel the
direct signals emitted by the two transmitters at the receiver. The
transmitting and receiving antennas are allowed to have arbitrary
polarizations. Single or multiple dielectric and conducting buried
targets are simulated. The buried objects are modeled as rectangular
prisms and cylindrical disks. Perfectly-matched layer absorbing
boundary conditions are adapted and used to terminate the
FDTD computational domain, which contains a layered medium
due to the ground–air interface
An efficient and accurate technique for the incident-wave excitations in the FDTD method
Cataloged from PDF version of article.An efficient technique to improve the accuracy
of the finite-difference time-domain (FDTD) solutions employing
incident-wave excitations is developed. In the separate-field
formulation of the FDTD method, any incident wave may be
efficiently introduced to the three-dimensional (3-D) computational
domain by interpolating from a one-dimensional (1-D)
incident-field array (IFA), which is a 1-D FDTD grid simulating
the propagation of the incident wave. By considering the FDTD
computational domain as a sampled system and the interpolation
operation as a decimation process, signal-processing techniques
are used to identify and ameliorate the errors due to aliasing.
The reduction in the error is demonstrated for various cases. This
technique can be used for the excitation of the FDTD grid by any
incident wave. A fast technique is used to extract the amplitude
and the phase of a sampled sinusoidal signal
Dynamic point-region quadtrees for particle simulations
Cataloged from PDF version of article.We propose an algorithm for dynamically updating point-region (PR) quadtrees. Our algorithm is optimized for simultaneous update of data points comprising a quadtree. The intended application area focuses on simulating continuum phenomena, such as crowds, fluids, and smoke. We minimize the number of tree updates by making use of small changes in the positions of data points. We compare the efficiency of the proposed algorithm with two other approaches for updating a quadtree. One of these techniques creates the tree from scratch at each time-step. The second technique subsequently deletes a data point from the tree and reinserts it in its updated position. We achieve significant performance gains with our method in both cases. (C) 2012 Elsevier Inc. All rights reserved
Emergency crowd simulation for outdoor environments
Cataloged from PDF version of article.We simulate virtual crowds in emergency situations caused by an incident, such as a fire, an explosion, or a terrorist attack. We use a continuum dynamics-based approach to simulate the escaping crowd, which produces more efficient simulations than the agent-based approaches. Only the close proximity of the incident region, which includes the crowd affected by the incident, is simulated. We use a model-based rendering approach where a polygonal mesh is rendered for each agent according to the agent's skeletal motion. To speed up the animation and visualization, we employ an offline occlusion culling technique. We animate and render a pedestrian model only if it is visible according to the static visibility information computed. In the pre-processing stage, the navigable area is decomposed into a grid of cells and the from-region visibility of these cells is computed with the help of hardware occlusion queries. (C) 2009 Elsevier Ltd. All rights reserved
Investigation of transition frequencies of two acoustically coupled bubbles using a direct numerical simulation technique
The theoretical results regarding the ``transition frequencies'' of two
acoustically interacting bubbles have been verified numerically. The theory
provided by Ida [Phys. Lett. A 297 (2002) 210] predicted the existence of three
transition frequencies per bubble, each of which has the phase difference of
between a bubble's pulsation and the external sound field, while
previous theories predicted only two natural frequencies which cause such phase
shifts. Namely, two of the three transition frequencies correspond to the
natural frequencies, while the remaining does not. In a subsequent paper [M.
Ida, Phys. Rev. E 67 (2003) 056617], it was shown theoretically that transition
frequencies other than the natural frequencies may cause the sign reversal of
the secondary Bjerknes force acting between pulsating bubbles. In the present
study, we employ a direct numerical simulation technique that uses the
compressible Navier-Stokes equations with a surface-tension term as the
governing equations to investigate the transition frequencies of two coupled
bubbles by observing their pulsation amplitudes and directions of translational
motion, both of which change as the driving frequency changes. The numerical
results reproduce the recent theoretical predictions, validating the existence
of the transition frequencies not corresponding to the natural frequency.Comment: 18 pages, 8 figures, in pres
Endothelial function in patients with familial Mediterranean fever-related amyloidosis and association with cardiovascular events
Objectives. Secondary amyloidosis is the most important complication of FMF and endothelial function is more severely impaired. Elevated asymmetric dimethyl arginine (ADMA) may mediate the excess cardiovascular disease (CVD) risk of this group. We aimed to compare endothelial function characteristics, including ADMA, in patients with FMF-related amyloidosis and primary glomerulopathies and to define risk factors for a CVD event. Methods. We undertook a cross-sectional study with prospective follow-up including consecutive patients with FMF-related amyloidosis (n = 98) or other non-diabetic glomerulopathies (n = 102). All patients had nephrotic-range proteinuria and normal glomerular filtration rate. Flow-mediated dilatation (FMD) was assessedand ADMA levels, CRP and pentraxin 3 (PTX3) were determined. Patients were followed for cardiovascular events. Results. Amyloidosis patients secondary to FMF showed higher levels of ADMA, CRP and PTX3 and lower FMD as compared with patients with other glomerulopathies. Cardiovascular events (n = 54) were registered during 3 years of follow-up. Increased ADMA levels and lower FMD were observed in patients with cardiovascular risk in both groups, but especially in individuals with amyloidosis.Conclusion. Patients with FMF-related amyloidosis have increased CVD event risk, probably related to the high ADMA levels, elevated inflammatory markers and decreased FMD measures observed in these patients
Availability and affordability of blood pressure-lowering medicines and the effect on blood pressure control in high-income, middle-income, and low-income countries: an analysis of the PURE study data.
BACKGROUND: Hypertension is considered the most important risk factor for cardiovascular diseases, but its control is poor worldwide. We aimed to assess the availability and affordability of blood pressure-lowering medicines, and the association with use of these medicines and blood pressure control in countries at varying levels of economic development. METHODS: We analysed the availability, costs, and affordability of blood pressure-lowering medicines with data recorded from 626 communities in 20 countries participating in the Prospective Urban Rural Epidemiological (PURE) study. Medicines were considered available if they were present in the local pharmacy when surveyed, and affordable if their combined cost was less than 20% of the households' capacity to pay. We related information about availability and affordability to use of these medicines and blood pressure control with multilevel mixed-effects logistic regression models, and compared results for high-income, upper-middle-income, lower-middle-income, and low-income countries. Data for India are presented separately because it has a large generic pharmaceutical industry and a higher availability of medicines than other countries at the same economic level. FINDINGS: The availability of two or more classes of blood pressure-lowering drugs was lower in low-income and middle-income countries (except for India) than in high-income countries. The proportion of communities with four drug classes available was 94% in high-income countries (108 of 115 communities), 76% in India (68 of 90), 71% in upper-middle-income countries (90 of 126), 47% in lower-middle-income countries (107 of 227), and 13% in low-income countries (nine of 68). The proportion of households unable to afford two blood pressure-lowering medicines was 31% in low-income countries (1069 of 3479 households), 9% in middle-income countries (5602 of 65 471), and less than 1% in high-income countries (44 of 10 880). Participants with known hypertension in communities that had all four drug classes available were more likely to use at least one blood pressure-lowering medicine (adjusted odds ratio [OR] 2·23, 95% CI 1·59-3·12); p<0·0001), combination therapy (1·53, 1·13-2·07; p=0·054), and have their blood pressure controlled (2·06, 1·69-2·50; p<0·0001) than were those in communities where blood pressure-lowering medicines were not available. Participants with known hypertension from households able to afford four blood pressure-lowering drug classes were more likely to use at least one blood pressure-lowering medicine (adjusted OR 1·42, 95% CI 1·25-1·62; p<0·0001), combination therapy (1·26, 1·08-1·47; p=0·0038), and have their blood pressure controlled (1·13, 1·00-1·28; p=0·0562) than were those unable to afford the medicines. INTERPRETATION: A large proportion of communities in low-income and middle-income countries do not have access to more than one blood pressure-lowering medicine and, when available, they are often not affordable. These factors are associated with poor blood pressure control. Ensuring access to affordable blood pressure-lowering medicines is essential for control of hypertension in low-income and middle-income countries. FUNDING: Population Health Research Institute, the Canadian Institutes of Health Research, Heart and Stroke Foundation of Ontario, Canadian Institutes of Health Research Strategy for Patient Oriented Research through the Ontario SPOR Support Unit, the Ontario Ministry of Health and Long-Term Care, pharmaceutical companies (with major contributions from AstraZeneca [Canada], Sanofi Aventis [France and Canada], Boehringer Ingelheim [Germany amd Canada], Servier, and GlaxoSmithKline), Novartis and King Pharma, and national or local organisations in participating countries
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