Louise Wright

Prediction of acoustic radiation from axisymmetric surfaces with arbitrary boundary conditions using the boundary element method on a distributed computing system

This paper presents a computational technique using the boundary element method for prediction of radiated acoustic waves from axisymmetric surfaces with nonaxisymmetric boundary conditions. The aim is to predict the far-field behavior of underwater acoustic transducers based on their measured behavior in the near-field. The technique is valid for all wavenumbers and uses a volume integral method to calculate the singular integrals required by the boundary element formulation. The technique has been implemented on a distributed computing system to take advantage of its parallel nature, which has led to significant reductions in the time required to generate results. Measurement data generated by a pair of free-flooding underwater acoustic transducers encapsulated in a polyurethane polymer have been used to validate the technique against experiment. The dimensions of the outer surface of the transducers (including the polymer coating) were an outer diameter of 98 mm with an 18 mm wall thickness and a length of 92 mm. The transducers were mounted coaxially, giving an overall length of 185 mm. The cylinders had resonance frequencies at 13.9 and 27.5 kHz, and the data were gathered at these frequencies.

A multi-thermogram-based Bayesian model for the determination of the thermal diffusivity of a material

The determination of thermal diffusivity is at the heart of modern materials characterisation. The evaluation of the associated uncertainty is difficult because the determination is performed in an indirect way, in the sense that the thermal diffusivity cannot be measured directly. The well-known GUM uncertainty framework does not provide a reliable evaluation of measurement uncertainty for such inverse problems, because in that framework the underlying measurement model is supposed to be a direct relationship between the measurand (the quantity intended to be measured) and the input quantities on which the measurand depends. This paper is concerned with the development of a Bayesian approach to evaluate the measurement uncertainty associated with thermal diffusivity. A Bayesian model is first developed for a single thermogram and is then extended to the case of several thermograms obtained under repeatability and reproducibility conditions. This multi-thermogram based model is able to take into consideration a large set of influencing quantities that occur during the measurements and yields a more reliable uncertainty evaluation than the one obtained from a single thermogram. Different aspects of the Bayesian model are discussed, including the sensitivity to the choice of the prior distribution, the Metropolis–Hastings algorithm used for the inference and the convergence of the Markov chains.

Parameter Estimation from Laser Flash Experiment Data

Optimisation techniques are commonly used for parameter estimation in a wide variety of applications. The application described here is a laser flash thermal diffusivity experiment on a layered sample where the thermal properties of some of the layers are unknown. The aim is to estimate the unknown properties by minimising, in a least squares sense, the difference between model predictions and measured data. Two optimisation techniques have been applied to the problem. Results suggest that the classical nonlinear least-squares optimiser is more efficient than particle swarm optimisation (PSO) for this type of problem. Results have also highlighted the importance of defining a suitable objective function and choosing appropriate model parameters.

Ultrasonic Characterization of Thermal Distribution in Vicinity for a Cylindrical Thermal Lesion in a Biological Tissue

The study considers an ultrasonic characterization on the thermal distribution in vicinity for a cylindrical thermal lesion formed in a biological tissue. The cylindrical heat source is made of a standard nichrome wire with the diameter of 1 mm. The wire was inserted inside a pork muscle housed in a cuboidal container made of perspex. The heat is conducted radially outwards from the wire to the surrounding tissue. Thermal distribution near the heated wire was predicted by numerically solving a bioheat transfer function using FemLab (Comsol, Inc.). As the wire temperature was raised from the environmental temperature 20 °C to more than 80 °C in steps of 5 °C, ultrasonic B-scan images were acquired at each temperature. We assessed the feasibility of detecting the lesion boundary using changes in echogenicity, changes in centroid frequency due to attenuation, tissue moving characteristics resulting from changes in the speed of sound, and elastograms. These observations will be of use in improving ultrasonic monitoring and guiding in HIFU surgery and thermo-therapeutic process in general.

Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables

Submarine fiber optic earthquake detection Seismic networks detect earthquakes and are common on continents, where they are easy to install. However, most of Earths surface is under the oceans, where placing seismometers is difficult. Marra et al. now find that ordinary submarine telecommunication cables can be used to detect earthquakes. Small strain changes associated with the passage of seismic waves were detected with laser light sent through in-use fiber optic cables by ultrastable lasers. This strategy could turn intercontinental fiber optic cables into ocean-bottom strain sensors, dramatically improving our ability to record earthquakes. Science, this issue p. 486 Ultrastable lasers can be used to detect earthquakes in land-based and submarine fiber optic cables. Detecting ocean-floor seismic activity is crucial for our understanding of the interior structure and dynamic behavior of Earth. However, 70% of the planet’s surface is covered by water, and seismometer coverage is limited to a handful of permanent ocean bottom stations. We show that existing telecommunication optical fiber cables can detect seismic events when combined with state-of-the-art frequency metrology techniques by using the fiber itself as the sensing element. We detected earthquakes over terrestrial and submarine links with lengths ranging from 75 to 535 kilometers and a geographical distance from the earthquake’s epicenter ranging from 25 to 18,500 kilometers. Implementing a global seismic network for real-time detection of underwater earthquakes requires applying the proposed technique to the existing extensive submarine optical fiber network.

Measuring the relative concentration of particle populations using differential centrifugal sedimentation

The factors that affect the accuracy and precision of differential centrifugal sedimentation (DCS) for the analysis of nanoparticle concentration are described. Particles are separated by their sedimentation rate and detected using light absorption. In principle, the relative concentration of particles in different populations can be found, but the uncertainty in such measurements is unclear. We show that the most appropriate measurement of particle concentration using this technique is the mass concentration, rather than the number concentration. The relative mass concentration of two discrete populations can be measured with reasonable precision, usually without resorting to complicated data analysis. We provide practical approaches to find the relative mass concentrations for two cases: spherical particles of different materials and agglomerated particles of the same material. For spherical particles made of different materials, naive analysis of the results can provide relative mass concentrations that are many orders of magnitude in error. Correction factors can be calculated that reduce the error to less than 50%. In the case of agglomerated particles we show that errors of less than 20% are possible and demonstrate, in the case of gold particles, that a combination of UV-visible spectroscopy and DCS enable practical values of mass and number based particle concentrations to be obtained.