1,720,961 research outputs found
Neutrally buoyant band gap design for underwater applications
No full textAbstract — A locally resonant sonic material (LRSM) design is
considered in two dimensions (2D) for obtaining a neutrally
buoyant band gap material in underwater applications. The
material is intended to have an overall effective mass density the
same as that of water, and to produce bandgaps in the 10kHz
region. The design problem is more constrained than a standard
LRSM design scenario, in that necessitating an overall desired
effective density requires a specific combination of material and
geometrical properties, whilst also giving the required bandgap
performance using only material properties and dimensions
likely to be achievable in practice. A design is described featuring
a proposed soft rubber-type layer encasing a Lucite core within a
Lucite matrix with has an overall material thickness of 90mm. A
-20dB band stop in the transmission coefficient magnitude over
approximately the 8-12kHz range is achieved. Results are
obtained by simulating the system using AFiDS, a staggered grid
Finite Difference Time Domain (FDTD) simulator
Experimental demonstration of improvements to operator splitting method using field II
No full textThis article describes a recent improvement in the
accuracy with respect to propagation distance of an operator
splitting approach used in conjunction with Field II for nonlinear
propagation modeling. In its initial implementation the method
used fixed parameters for propagation of virtual planes used in
the algorithm, and these were shown to produce simulated
nonlinear pulses which matched experimental nonlinear pulses
well at a selected propagation distance. However, when
propagating to several multiples of this distance the method was
found to degrade in accuracy with respect to experimental
measurements. In this paper initial results obtained by switching
from the fixed parameter approach for the plane propagation to
an adaptive parameter approach are given. Comparison with
experimental data shows a notable increase in accuracy over the
previous approach, suggesting that further work on developing
adaptive approaches for the plane propagation could prove
beneficial. The results are only preliminary and a more
comprehensive study of different experimental data sets and
adaptive parameter selection schemes should be made before any
definitive conclusions are drawn
Computation of steered nonlinear fields using offset KZK axes
No full textIntroduction/Background: Commercial medical scanners utilise electronic beam steering and the second harmonic signals generated by tissue nonlinear processes to form images at the second harmonic frequency. Furthermore, characteristics of nonlinearly generated harmonics within the beam itself contribute to improvements in lateral resolution and quality of the reconstructed image. Fully understanding these harmonic features offers new possibilities for redesigning or optimising array transducers to attain better imaging performances, as well as in other areas such as contrast agent responses and
microbubble destruction strategies for drug delivery. However, such an understanding is hindered at present by the computational difficulty in accurately predicting all the nonlinear characteristics of steered beams. This paper addresses this issue, presenting a KZK based method for analysing beams
steered at arbitrary angles.Method: The parabolic KZK equation is often used to study nonlinear characteristics of medical ultrasound beams. This equation is traditionally applied in a propagation direction perpendicular to the surface of the transducer, and has been shown to model the pulse propagation well along the central axis of the transducer [1]. However, its accurate angle of application is restricted to within approximately only 16 degrees away from the central transducer axis due to the underlying approximations employed in the derivation of the KZK equation.
This restriction causes a problem for studying steered beams, since typical steering ranges up to the order of 45 degrees away from the transducer centreline. To overcome this problem whilst at the same time exploiting the traditional KZK benefits, we develop an iterative method for displacing the KZK axes away from the central transducer axis to investigate arbitrary beam or field angles of interest. Results/Conclusions: The implemented algorithm is based on a time domain solution of the KZK equation, on a standard 3GHz PC with 2GB RAM producing runtimes in the order of a few hours per investigation angle. The transducer used is a 64 element linear phased array operating at 1.7MHz (height 10.5 mm, width 0.27mm, kerf 0.065mm) with beam steering at 0 and 45 degrees. Validation of the basic KZK algorithm without steering has been given previously at linear pressures (54kPa) against both experimental measurements and Field II and at nonlinear pressures (700kPa) against measurements described previously in [1]. The KZK axes are swept over a range of angles covering the full spectrum 499 of -90 to +90 degrees relative to transducer centreline, in order to investigate the entire emitted field for both steered and unsteered beams. Element pitch is then increased to investigate grating lobes. All results are decomposed into first and second harmonic fields to compare relative frequency domain properties. The overall contribution is a better characterisation of linear and nonlinear field characteristics at high angles than is possible with the traditional KZK approach
Initial results on size discrimination of similar underwater objects using a human hearing model
No full textConnection between X-Waves, Fourier-Bessel series and optimal modelling aperture for circular symmetric arrays
No full textThis paper addresses various unresolved issues raised in publications [1], [2], and [3], in connection with the study and application of limited-diffraction and non-diffracting beams. Nondiffracting beams have the property of a constant radial profile with propagation distance, subject to an infinite aperture source, and the related theories advanced in the context of medical imaging have resulted in the possibility of extremely high fame rates. However, all the fundamental theory assumes an infinite-aperture source being available. In reality this is not possible, and when nondiffracting beams are implemented on finite source apertures they become limited diffraction beams. [1], [2], [3] have studied the use of and numerical differences between nondiffracting theory and limited diffraction implementation of such beams for field computation and tuning. In [1], [2] it was shown that an iterative technique could be applied to extend the effective modelling aperture from the physical limits of the transducer (limited diffraction basis functions) towards an infinite modelling aperture (Bessel beams
/ X-waves), to both tune and compute the emitted field from any given circular symmetric flat array with linear propagation conditions. This technique involved the concept of a modelling aperture spanning the gap between the physical limit of the transducer and infinity, which, when increased iteratively resulted in convergence of the corresponding computed field and source driving function. However, the technique relied on a combination of iterative combinations and 137 numerical field convergence to within a pre-selected limit in order to terminate the computations at an appropriate point.
In this paper, a formal mathematical connection between the limited-aperture (limited diffraction) basis functions and the nondiffracting infinite aperture theory (Bessel beams / X-Waves) is established as a function of the increasing modelling aperture. The result is that a specific optimal modelling aperture may then be specified as a function of frequency spectrum, spatial field extent to be investigated, and pulse repetition frequency. Consequently the previous iterative technique may be replaced by a single one-shot computation to achieve the same result. As a result, the new technique is significantly more efficient than the previous technique and the specific saving in computation depends on the particular transducer considered, but typically computational reductions are in the order of 50%. The global contribution of the paper is twofold : firstly a formal mathematical connection between limited diffraction beams and nondiffracting beams as function of increasing modelling aperture, and secondly the derivation of the optimal modelling aperture required for computation and tuning of circular symmetric fields with minimal computational demands
Risk factors for proliferative sickle retinopathy
No full textThe prevalence, incidence, and risk factors associated with proliferative sickle retinopathy (PSR) were investigated in 786 patients with homozygous sickle cell (SS) disease and 533 patients with sickle cell haemoglobin C (SC) disease. PSR was more common in SC disease, in which there was a significant predominance of males, and it increased with age in both genotypes. In SC disease the risk of developing PSR was highest between 15 and 24 years in males, between 20 and 39 years in females, and in SS disease between 25 and 39 years in both sexes. PSR tended to be bilateral, especially in SC disease. There was no evidence of familial clustering of PSR in SC siblings, and insufficient numbers of SS siblings were available to test for clustering. Haematological risk factors associated with PSR in SS disease were a high haemoglobin in males and a low fetal haemoglobin in both sexes and in SC disease, a high mean cell volume, and a low fetal haemoglobin in female
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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