Hybrid convolution quadrature methods for modelling time-dependent waves with broadband frequency content

This work proposes two new hybrid convolution quadrature based discretisations of the wave equation for interior domains with broadband Neumann boundary data or source terms. The convolution quadrature method transforms the time domain wave problem into a series of Helmholtz problems with complex-valued wavenumbers, in which the boundary data and solutions are connected to those of the original problem through the Z-transform. The hybrid method terminology refers specifically to the use of different approximations of these Helmholtz problems, depending on the frequency. For lower frequencies we employ the boundary element method, while for more oscillatory problems we develop two alternative high frequency approximations based on plane wave decompositions of the acoustic field on the boundary. In the first approach we apply dynamical energy analysis to numerically approximate the plane wave amplitudes. The phases will then be reconstructed using a novel approach based on matching the boundary element solution to the plane wave ansatz in the frequency region where we switch between the low and high frequency methods. The second high frequency method is based on applying the Neumann-to Dirichlet map for plane waves to the given boundary data

Algorithm and performance of a clinical IMRT beam-angle optimization system

This paper describes the algorithm and examines the performance of an IMRT beam-angle optimization (BAO) system. In this algorithm successive sets of beam angles are selected from a set of predefined directions using a fast simulated annealing (FSA) algorithm. An IMRT beam-profile optimization is performed on each generated set of beams. The IMRT optimization is accelerated by using a fast dose calculation method that utilizes a precomputed dose kernel. A compact kernel is constructed for each of the predefined beams prior to starting the FSA algorithm. The IMRT optimizations during the BAO are then performed using these kernels in a fast dose calculation engine. This technique allows the IMRT optimization to be performed more than two orders of magnitude faster than a similar optimization that uses a convolution dose calculation engine.Comment: Final version that appeared in Phys. Med. Biol. 48 (2003) 3191-3212. Original EPS figures have been converted to PNG files due to size limi

On hybrid convolution quadrature approaches for modelling time-domain wave problems with broadband frequency content

We propose two hybrid convolution quadrature based discretisations of the wave equation on interior domains with broadband Neumann boundary data or source terms. The convolution quadrature method transforms the time-domain wave problem into a series of Helmholtz problems with complex-valued wavenumbers, in which the boundary data and solutions are connected to those of the original problem through the Z-transform. The hybrid method terminology refers specifically to the use of different approximations of these Helmholtz problems, depending on the frequency. For lower frequencies we employ the boundary element method, while for more oscillatory problems we develop two alternative high frequency approximations based on plane wave decompositions of the acoustic field on the boundary. In the first approach we apply dynamical energy analysis to numerically approximate the plane wave amplitudes. The phases will then be reconstructed using a novel approach based on matching the boundary element solution to the plane wave ansatz in the frequency region where we switch between the low and high frequency methods. The second high frequency method is based on applying the Neumann-to-Dirichlet map for plane waves to the given boundary data. Finally, we investigate the effectiveness of both hybrid approaches across a range of numerical experiments

When the tail wags the dog? Digitalisation and corporate reporting

To facilitate digital corporate reporting and enable computers to ‘read’ accounting information, standard-setters must construct a taxonomy to assign contextual ‘metadata’ that codifies disclosures arising from accounting concepts, standards and practices. But digitalisation poses a problem for corporate reporting. Within internationally accepted accounting practice, ‘principles-based’ standards give companies significant discretion in deciding what they disclose and how they report accounts of their activity. How would the principles-based nature of corporate reporting be influenced by the construction of a taxonomy that seeks to specify all accounting disclosures? Drawing on literature examining the constitutive potential of classification and formal representation, we use our case study of the digitalisation project undertaken by the global standard-setter, the IASB, to understand how digitalisation intervenes on standard-setting and reporting practice despite the intentions of standard-setters. Our results detail how standard-setters sought to minimise the impact of digitalisation by modelling the taxonomy only on disclosures explicitly required by accounting standards. We reveal the circumstances that led the IASB to change its taxonomy design by seeking to capture not only what should be reported (as prescribed in extant accounting standards) but also what was being reported (as prescribed in a new classification called ‘Common Practices’). We analyse the process by which international accounting disclosure practices were judged to be ‘common’, and demonstrate how the ‘Common Practices’ classification was perceived by early users of the taxonomy. When interpreting the IASB’s Common Practices (what is) as disclosure standards (what should be), digitalisation generates a self-validating feedback loop that can generate more homogenous corporate reporting and push International Financial Reporting Standards beyond the principles-based approach they were designed to engender. Although standard-setters became increasingly aware of the influence of the digital (machine-readable) ‘tail’ on the traditional reporting (human-readable) ‘dog’, their attempts to take tighter control of the taxonomy development process strengthened user perceptions that the taxonomy and its ‘Common Practices’ represented an authoritative view of what should be reported. Our results reveal the process by which digital reporting both represents and intervenes in accounting, and how digitalisation impacts key accounting debates. As digitalisation attempts to provide a universal codification of reporting disclosures, it valorises comprehensive machine-friendly disclosure rules over principles, which offer standardised comparability over entity-specific communication. Our study also offers a perspective on the relations between information representation and intervention that moves beyond a study of passive, ‘reactive’ conformance to consider how representations can intervene despite the intentions of those generating the representation. In doing so, we reveal the constitutive potential of digital representations in generating ‘non-passive’ conformance

On Component Forces in Physics: A Pragmatic View

Do component forces exist? I argue that the answer lies in the affirmative, on historical and operational grounds

Does Scientific Progress Consist in Increasing Knowledge or Understanding?

Bird argues that scientific progress consists in increasing knowledge. Dellsén objects that increasing knowledge is neither necessary nor sufficient for scientific progress, and argues that scientific progress rather consists in increasing understanding. Dellsén also contends that unlike Bird’s view, his view can account for the scientific practices of using idealizations and of choosing simple theories over complex ones. I argue that Dellsén’s criticisms against Bird’s view fail, and that increasing understanding cannot account for scientific progress, if acceptance, as opposed to belief, is required for scientific understanding

Design and development of a novel upper-limb cycling prosthesis

The rise in popularity of the Paralympics in recent years has created a need for effective, low-cost sports-prosthetic devices for upper-limb amputees. There are various opportunities for lower-limb amputees to participate in cycling; however, there are only few options for those with upper-limb amputations. If the individual previously participated in cycling, a cycling-specific prosthesis could allow these activities to be integrated into rehabilitation methods. This article describes the processes involved with designing, developing and manufacturing such a prosthesis. The fundamental needs of people with upper-limb amputation were assessed and realised in the prototype of a transradial terminal device with two release mechanisms, including a sliding mechanism (for falls and minor collisions) and clamping mechanism (for head-on collisions). The sliding mechanism requires the rider to exert approximately 200 N, while the clamping mechanism requires about 700 N. The force ranges can be customised to match rider requirements. Experiments were conducted in a controlled environment to demonstrate stability of the device during normal cycling. Moreover, a volunteer test-rider was able to successfully activate the release mechanism during a simulated emergency scenario. The development of this prosthesis has the potential to enable traumatic upper-limb amputees to participate in cycling for rehabilitation or recreation