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Search Results: 1 - 10 of 123724 matches for " Gregory T. Clement "
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On the limitations of optical characterization of intense ultrasound fields in the Raman-Nath regime
Gregory T. Clement
Physics , 2014,
Abstract: Accurate interpretation of optical measurement of acoustic fields generally assumes two criteria are met regarding the acoustooptic interaction. First, the light-sound interaction must be weak enough to be considered in the Raman-Nath regime. That is, only diffracting orders up to the first order are considered. Second, the effects of optical refraction that take place over the width of the acoustic signal are generally treated as insignificant. The quantitative conditions dictating where each criterion is applicable can be expressed in terms of the Cook-Klein parameter and the Raman-Nath parameter. However, these conditions, which are the basis for most optical methods of acoustic field characterization, are incomplete. This technical report demonstrates a third requirement that is also necessary in order to assure the validity of the approach in high pressure characterization.
Joint probability density functions of random trajectories through a box
Gregory T. Clement
Mathematics , 2013,
Abstract: A wide range of physical problems can be described by randomly-oriented linear trajectories, including any system of objects, organisms, particles, or rays that follow a linear path. Dependent upon the particular random variables that define translation and direction, a description of the probabilistic behavior through a static volume can reveal evidence on an object's physical properties. Yet more information may be available in cases where this behavior is a function of volume shape. In this study two types of random trajectories are considered as they pass through a box of arbitrary relative dimension. One type defines trajectories from uniform random selection of a spatial location paired with a directional vector; the other is formed from a uniformly distributed position vector on a surface. The joint probability distributions for trajectory length as a function of box position are formulated and then examined for different size boxes, and their physical representations discussed.
Randomized transmit and receive ultrasound tomography
Gregory T. Clement,Tomoo Kamakura
Physics , 2015,
Abstract: A tomographic method is considered that forms images from sets of spatially randomized source signals and receiver sensitivities. The method is designed to allow image reconstruction for an extended number of transmitters and receivers in the presence noise and without plane wave approximation or otherwise approximation on the size or regularity of source and receiver functions. An overdetermined set of functions are formed from the Hadamard product between a Gaussian function and a uniformly distributed random number set. It is shown that this particular type of randomization tends to produce well-conditioned matrices whose pseudoinverses may be determined without implementing relaxation methods. When the inverted sets are applied to simulated first-order scattering from a Shepp-Logan phantom, successful image reconstructions are achieved for signal-to-noise ratios (SNR) as low as 1. Evaluation of the randomization approach is conducted by comparing condition numbers with other forms of signal randomization. Image quality resulting from tomographic reconstructions is then compared with an idealized synthetic aperture approach, which is subjected to a comparable SNR. By root-mean-square-difference comparisons it is concluded that - provided a sufficient level of oversampling - the dynamic transmit and dynamic receive approach produces superior images, particularly in the presence of low SNR.
Ultrasound field measurement using a binary lens
Gregory T. Clement,Hideyuki Nomura,Tomoo Kamakura
Physics , 2014, DOI: 10.1109/TUFFC.2014.006800
Abstract: Field characterization methods using a scattering target in the absence of a point-like receiver have been well described in which scattering is recorded by a relatively large receiver located outside the field of measurement. Unfortunately, such methods are prone to artifacts due to averaging across the receiver surface. To avoid this problem while simultaneously increasing the gain of a received signal, the present study introduces a binary plate lens designed to focus spherically-spreading waves onto a planar region having a nearly-uniform phase proportional to that of the target location. The lens is similar to a zone plate, but modified to produce a bi-convex-like behavior, such that it focuses both planar and spherically spreading waves. A measurement device suitable for characterizing narrowband ultrasound signals in air is designed around this lens by coupling it to a target and planar receiver. A prototype device is constructed and used to characterize the field of a highly-focused 400 kHz air transducer along 2 radial lines. Comparison of the measurements with numeric predictions formed from nonlinear acoustic simulation showed good relative pressure correlation, with mean differences of 10% and 12% over center 3dB FWHM drop and 12% and 17% over 6dB.
Diffraction tomography on curved boundaries: A projection-based approach
G. T. Clement
Physics , 2013, DOI: 10.1088/0266-5611/30/12/125010
Abstract: An approach to diffraction tomography is investigated for two-dimensional image reconstruction of objects surrounded by an arbitrarily-shaped curve of sources and receivers. Based on the integral theorem of Helmholtz and Kirchhoff, the approach relies upon a valid choice of the Green's functions for selected conditions along the (possibly-irregular) boundary. This allows field projections from the receivers to an arbitrary external location. When performed over all source locations, it will be shown that the field caused by a hypothetical source at this external location is also known along the boundary. This field can then be projected to new external points that may serve as a virtual receiver. Under such a reformation, data may be put in a form suitable for image construction by synthetic aperture methods. Foundations of the approach are shown, followed by a mapping technique optimized for the approach. Examples formed from synthetic data are provided.
A k-space method for nonlinear wave propagation
Yun Jing,Greg. T. Clement
Physics , 2011, DOI: 10.1109/TUFFC.2012.2372
Abstract: A k-space method for nonlinear wave propagation in absorptive media is presented. The Westervelt equation is first transferred into k-space via Fourier transformation, and is solved by a modified wave-vector time-domain scheme [Mast et al., IEEE Tran. Ultrason. Ferroelectr. Freq. Control 48, 341-354 (2001)]. The present approach is not limited to forward propagation or parabolic approximation. One- and two-dimensional problems are investigated to verify the method by comparing results to the finite element method. It is found that, in order to obtain accurate results in homogeneous media, the grid size can be as little as two points per wavelength, and for a moderately nonlinear problem, the Courant-Friedrichs-Lewy number can be as small as 0.4. As a result, the k-space method for nonlinear wave propagation is shown here to be computationally more efficient than the conventional finite element method or finite-difference time-domain method for the conditions studied here. However, although the present method is highly accurate for weakly inhomogeneous media, it is found to be less accurate for strongly inhomogeneous media. A possible remedy to this limitation is discussed.
Financial Market Pricing of Earnings Quality: Evidence from a Multi-Factor Return Model  [PDF]
Gregory M. Dempster, Nathaniel T. Oliver
Open Journal of Business and Management (OJBM) , 2019, DOI: 10.4236/ojbm.2019.71021
Abstract: Although earnings quality has been an important part of literature in accounting and financial economics for some time, there are relatively few examples of empirical work designed to isolate the effects of variation in earnings quality on the returns to equity ownership in the marketplace. Building on the previous literature, we conduct a robust analysis of these effects by employing earnings restatements as a proxy for quality of earnings in a multi-factor return model. Our results indicate that material misstatements of earnings are priced risk factors that have persistent (long-run) impacts on equity returns. Applications to business practice are discussed in the light of these results.
Ito’s Formula for the Discrete-Time Quantum Walk in Two Dimensions  [PDF]
Clement Ampadu
Journal of Quantum Information Science (JQIS) , 2012, DOI: 10.4236/jqis.2012.22008
Abstract: Following Konno [1], it is natural to ask: What is the Ito’s formula for the discrete time quantum walk on a graph different than Z, the set of integers? In this paper we answer the question for the discrete time quantum walk on Z2, the square lattice.
On Some Questions of C. Ampadu Associated with the Quantum Random Walk  [PDF]
Clement Ampadu
Applied Mathematics (AM) , 2014, DOI: 10.4236/am.2014.519291
Abstract: We review (not exhaustively) the quantum random walk on the line in various settings, and propose some questions that we believe have not been tackled in the literature. In a sense, this article invites the readers (beginner, intermediate, or advanced), to explore the beautiful area of quantum random walks.
Assessment of Pollutants in Water and Sediment Samples in Lake Chad, Baga, North Eastern Nigeria  [PDF]
Joseph Clement Akan, Mohammed T. Abbagambo, Zaynab M. Chellube, Fanna Inna Abdulrahman
Journal of Environmental Protection (JEP) , 2012, DOI: 10.4236/jep.2012.311161
Abstract: The purpose of this research is to determine the levels of some physicochemical parameters in water and sediment samples from Kwantan turare in Lake Chad, Baga, Borno State, Nigeria. Water samples were collected from five point designated as S1 to S5 for the determination of biochemical oxygen demand (BOD), chemical oxygen demand (COD), dissolved oxygen (DO) total dissolved solid (TDS), total suspended solid (TSS) anions and trace element. Sediment samples were also collected for the determination of heavy metals and total organic carbon (TOC). These parameters were determined using approved standard procedures. The levels of heavy metals in the water and sediment samples were determined using atomic absorption spectrophotometer (AAS). The concentrations of DO, BOD, COD, TSS, TDS, TOC, nitrite, nitrate, Phosphate, Sulphate, and Chloride in the water samples from the five sampling point were higher than the WHO limits for the protection of fish and other aquatic life. The concentrations of Fe, Mn, Cu, Cd, Pb, Ni and Co in the water samples were higher than the WHO guideline limits, indicating severed pollution of this portion of Lake Chad. This high level of heavy metals in the water samples is expected owing to runoff of wastewater from agricultural activities within the study area. However, the concentrations of all the metals in the sediment samples were higher than the recommended values stipulated by WHO; indicating that the sediment samples from this portion of Lake Chad were also polluted by heavy metals. Results of Analysis of variance (ANOVA) revealed that the concentrations of all the parameters studied were statistically significant among the sampling points. The prolonged presence of heavy metals in the water and sediment samples from this portion of Lake Chad give course for concern with time. The results of the study implies that continuous monitoring has to be carried out to ascertain the long-term impact of anthropogenic inputs to take remedial measures so as to ensure the health of aquatic life.
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