arXiv subjects

David Powell

Publications and source records attributed to David Powell.

9 recordsLinked to original sources

arXiv2025

Institutional Research Computing Capabilities in Australia: 2024

Institutional research computing infrastructure plays a vital role in Australia's research ecosystem, complementing and extending national facilities. This paper analyses research computing capabilities across Australian universities and organisations, showing how institutional systems support research excellence through local compute resources, specialised hardware, and cluster solutions. Our study finds that nearly 112,258 CPU cores and 2,241 GPUs serve over 6,000 researchers as essential bridges between desktops and national facilities, enabling workflows from development to large-scale computations. The estimated replacement value of this infrastructure is $144M AUD. Drawing on detailed data from multiple institutions, we identify key patterns in deployment, utilisation, and strategic alignment with research priorities. Institutional resources provide critical support for data-intensive projects, facilitate training and higher-degree student research, enable prototyping and development, and ensure data sovereignty compliance when required. The analysis shows how these facilities leverage national investments while addressing institution-specific needs that national systems cannot meet. We present evidence that strategic investment in institutional capabilities yields significant returns through greater research productivity, enhanced graduate training, and improved outcomes. The study offers insights for organisations planning computing strategies and highlights the importance of maintaining robust institutional resources alongside national facilities.

cs.DC↗

arXiv2025

Analytical design of acoustic metasurface cells incorporating meander-line and Helmholtz resonators

Gradient acoustic metasurfaces have shown strong potential for manipulation of acoustic waves across the audible and ultrasonic frequency ranges. The key challenge in designing acoustic metasurfaces is to create a series of sub-wavelength unit cells that match the desired phase response. The most commonly used geometry is a series of Helmholtz resonators, side-coupled to a narrow channel. Despite the existence of a closed-form solution for 3 side-coupled resonators, most reported designs instead make use of 4 or more resonators, which require opaque, optimization-based design approaches. We show that the limiting factor in designs based on side-coupled resonators is the requirement to use elements with a positive imaginary part of impedance, which implies a Helmholtz resonator operating above its fundamental resonance - contradicting the requirement for sub-wavelength volume. We show that by replacing some of the Helmholtz resonators with meander-line elements, the required impedance values can readily be realized within a sub-wavelength volume. The metasurface design approach is demonstrated for a lens operating at 3 kHz and verified numerically. Furthermore, incorporating meander-line elements leads to improved broadband focusing performance, even when no explicit dispersion engineering is included in the design. As this design includes narrow channels, we include the effects of thermo-viscous losses in our modeling, and confirm that our design still gives superior performance to the reference design using only Helmholtz resonators. Our design is expected to lead to more optimal performance of acoustic metasurface designs, and the ability to make use of a closed-form design formula is expected to facilitate the analysis of fundamental performance bounds and enable more explicit achromatic design processes.

physics.app-ph↗

arXiv2024

A Multimodal Vision Foundation Model for Clinical Dermatology

Diagnosing and treating skin diseases require advanced visual skills across domains and the ability to synthesize information from multiple imaging modalities. While current deep learning models excel at specific tasks like skin cancer diagnosis from dermoscopic images, they struggle to meet the complex, multimodal requirements of clinical practice. Here, we introduce PanDerm, a multimodal dermatology foundation model pretrained through self-supervised learning on over 2 million real-world skin disease images from 11 clinical institutions across 4 imaging modalities. We evaluated PanDerm on 28 diverse benchmarks, including skin cancer screening, risk stratification, differential diagnosis of common and rare skin conditions, lesion segmentation, longitudinal monitoring, and metastasis prediction and prognosis. PanDerm achieved state-of-the-art performance across all evaluated tasks, often outperforming existing models when using only 10% of labeled data. We conducted three reader studies to assess PanDerm's potential clinical utility. PanDerm outperformed clinicians by 10.2% in early-stage melanoma detection through longitudinal analysis, improved clinicians' skin cancer diagnostic accuracy by 11% on dermoscopy images, and enhanced non-dermatologist healthcare providers' differential diagnosis by 16.5% across 128 skin conditions on clinical photographs. These results demonstrate PanDerm's potential to improve patient care across diverse clinical scenarios and serve as a model for developing multimodal foundation models in other medical specialties, potentially accelerating the integration of AI support in healthcare. The code can be found at https://github.com/SiyuanYan1/PanDerm.

cs.CV↗

arXiv2022

Metahouse: noise-insulating chamber based on periodic structures

Noise pollution remains a challenging problem requiring the development of novel systems for noise insulation. Extensive work in the field of acoustic metamaterials has led to occurrence of various ventilated structures which, however, are usually demonstrated for rather narrow regions of the audible spectrum. In this work, we further extend the idea of metamaterial-based systems developing a concept of a metahouse chamber representing a ventilated structure for broadband noise insulation. Broad stop bands originate from strong coupling between pairs of Helmholtz resonators constituting the structure. We demonstrate numerically and experimentally the averaged transmission -18 dB within the spectral range from 1500 to 16500 Hz. The sparseness of the structure together with the possibility to use optically transparent materials suggest that the chamber may be also characterized by partial optical transparency depending on the mutual position of structural elements. The obtained results are promising for development of novel noise-insulating structures advancing urban science.

physics.app-ph↗

arXiv2021

Acoustic radiation force and radiation torque beyond particles: Effects of non-spherical shape and Willis coupling

Acoustophoresis deals with the manipulation of sub-wavelength scatterers in an incident acoustic field. The geometric details of manipulated particles are often neglected by replacing them with equivalent symmetric geometries such as spheres, spheroids, cylinders or disks. It has been demonstrated that geometric asymmetry, represented by Willis coupling terms, can strongly affect the scattering of a small object, hence neglecting these terms may miss important force contributions. In this work, we present a generalized formalism of acoustic radiation force and radiation torque based on the polarizability tensor, where Willis coupling terms are included to account for geometric asymmetry. Following Gorkov's approach, the effects of geometric asymmetry are explicitly formulated as additional terms in the radiation force and torque expressions. By breaking the symmetry of a sphere along one axis using intrusion and protrusion, we characterize the changes in the force and torque in terms of partial components, associated with the direct and Willis Coupling coefficients of the polarizability tensor. We investigate in detail the cases of standing and travelling plane waves, showing how the equilibrium positions and angles are shifted by these additional terms. We show that while the contributions of asymmetry to the force are often negligible for small particles, these terms greatly affect the radiation torque. Our presented theory, providing a way of calculating radiation force and torque directly from polarizability coefficients, shows that in general it is essential to account for shape of objects undergoing acoustophoretic manipulation, and this may have important implications for applications such as the manipulation of biological cells.

physics.app-ph↗

arXiv2021

Scalable Metagrating for Efficient Ultrasonic Focusing

Acoustic focusing plays a pivotal role in a wide variety of applications ranging from medical science to nondestructive testing. Previous works have shown that acoustic metagratings can overcome the inherent efficiency limitations of gradient metasurfaces in beam steering. In this work, we propose a new design principle for acoustic metalenses, based on metagratings, to achieve efficient ultrasonic focusing. We achieve beam focusing by locally controlling the excitation of a single diffraction order with the use of adiabatically varying metagratings over the lens aperture. A set of metagratings is optimized by a semi-analytical approach using a genetic algorithm, enabling efficient anomalous reflection for a wide range of reflection angles. Numerical results reveal that our metalens can effectively focus impinging ultrasonic waves to a focal point of FWHM = 0.364{\lambda}. The focusing performance of the metalens is demonstrated experimentally, validating our proposed approach.

physics.app-ph↗

arXiv2020

Moving beyond the classic difference-in-differences model: A simulation study comparing statistical methods for estimating effectiveness of state-level policies

State-level policy evaluations commonly employ a difference-in-differences (DID) study design; yet within this framework, statistical model specification varies notably across studies. Motivated by applied state-level opioid policy evaluations, this simulation study compares statistical performance of multiple variations of two-way fixed effect models traditionally used for DID under a range of simulation conditions. While most linear models resulted in minimal bias, non-linear models and population-weighted versions of classic linear two-way fixed effect and linear GEE models yielded considerable bias (60 to 160%). Further, root mean square error is minimized by linear AR models when examining crude mortality rates and by negative binomial models when examining raw death counts. In the context of frequentist hypothesis testing, many models yielded high Type I error rates and very low rates of correctly rejecting the null hypothesis (< 10%), raising concerns of spurious conclusions about policy effectiveness. When considering performance across models, the linear autoregressive models were optimal in terms of directional bias, root mean squared error, Type I error, and correct rejection rates. These findings highlight notable limitations of traditional statistical models commonly used for DID designs, designs widely used in opioid policy studies and in state policy evaluations more broadly.

stat.ME↗

arXiv2018

Acoustic meta-atom with maximum Willis coupling

Acoustic metamaterials are structures with exotic acoustic properties, having promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in an acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled. Together with its ease of fabrication, this will facilitate the design of future high efficiency acoustic devices.

physics.class-ph↗

arXiv2009

Structural tunability in metamaterials

We propose a novel approach for efficient tuning of the transmission characteristics of metamaterials through a continuous adjustment of the lattice structure, and confirm it experimentally in the microwave range. The concept is rather general and applicable to various metamaterials as long as the effective medium description is valid. The demonstrated continuous tuning of metamaterial response is highly desirable for a number of emerging applications of metamaterials including sensors, filters, switches, realizable in a wide frequency range.

physics.optics↗