James Hilton

Carotid Artery Anatomy and Geometry as Risk Factors for Carotid Atherosclerotic Disease

Background and Purpose— Traditional vascular risk factors do not completely explain the asymmetry, racial, and sex differences in carotid artery disease. Carotid anatomy and geometry may play a role in the pathogenesis of internal carotid artery (ICA) stenosis, but their effects are unknown. We hypothesized that carotid artery anatomy and geometry would be independently associated with ICA stenosis. Method— This is a retrospective study of patients with CT angiography at Monash Medical Centre, 2006 to 2007. Carotid arteries were segmented using semiautomated methods to estimate measures of carotid anatomy and geometry. Measurements of carotid artery geometry were performed according to the recent article by Thomas and colleagues. ICA stenosis was dichotomized as <30% or ≥30% stenosis. Cluster logistic regression was used to examine the associations of anatomy and geometry with stenosis accounting for the paired arteries within subjects, adjusting for age, sex, and vascular risk factors. Results— Mean age of the sample (n=178) was 68.4 years (SD, 14 years). The following were independently associated with ICA stenosis: ICA radius at the bifurcation (OR, 0.20; 95% CI, 0.14–0.29), ICA angle (OR, 1.05 per degree increment; 95% CI, 1.04–1.07), age (OR, 1.05 per year increment; 95% CI, 1.03–1.07), male sex (OR, 1.72; 95% CI, 1.08–2.8), and ever-smoker (OR, 1.85; 95% CI, 1.15–2.96). Conclusions— Carotid anatomy and geometry may enhance the risk of stenosis independent of traditional vascular risk factors and may be of help in very early identification of patients at high risk of developing carotid artery atherosclerosis for aggressive intervention.

Non-universal Voronoi cell shapes in amorphous ellipsoid packs

In particulate systems with short-range interactions, such as granular matter or simple fluids, local structure determines the macroscopic physical properties. We analyse local structure metrics derived from the Voronoi diagram of oblate ellipsoids, for various aspect ratios and global packing fractions φg. We focus on jammed static configurations of frictional ellipsoids, obtained by tomographic imaging and by discrete element method simulations. The rescaled distribution of local packing fractions φl, defined as the ratio of particle volume and its Voronoi cell volume, is found to be independent of the particle aspect ratio, and coincide with results for sphere packs. By contrast, the typical Voronoi cell shape, quantified by the Minkowski tensor anisotropy index β = β02,0, points towards a difference between random packings of spheres and those of oblate ellipsoids. While the average cell shape β of all cells with a given value of is similar in dense and loose jammed sphere packings, the structure of dense and loose ellipsoid packings differs substantially such that this does not hold true.

Assessing sea level-rise risks to coastal floodplains in the Kakadu Region, northern Australia, using a tidally driven hydrodynamic model

The low-lying coastal floodplains of the Kakadu Region in tropical northern Australia encompass World Heritage Kakadu National Park and are highly vulnerable to future sea level-rise (SLR) and extreme weather events, yet there are no modelling tools to assess potential impacts of saltwater inundation (SWI) on freshwater ecosystems and to evaluate future management options. A tidally driven hydrodynamic model was developed to simulate the frequency and extent of SWI in the Kakadu Region for the following four mean SLR scenarios: 0m (present-day, 2013); 0.14m (2030); 0.70m (2070); and 1.1m (2100). Simulations were undertaken at 60-m spatial resolution using October dry-season tides, and a digital elevation model (0.10-m vertical resolution) constructed from LiDAR point cloud data was used to resolve coastal and river-system terrains. Model outputs (maximum extent and frequency of SWI) were used to assess potential loss of freshwater floodplains for each scenario at a park-wide scale and for three case-study areas that differ in tidal influence. Results show little loss by 2030 (–3%), a possible threshold effect by 2070 (–42%) and ameliorating after 2100 (–65%). Although freshwater floodplains further from the coast showed least exposure to simulated SLR, indicating potential refuge areas, all floodplains on Kakadu will be exposed to SWI by 2132 (+117 years).

Effects of spatial and temporal variation in environmental conditions on simulation of wildfire spread

Environmental conditions, such as fuel load and moisture levels, can influence the behaviour of wildfires. These factors are subject to natural small-scale variation which is usually spatially or temporally averaged for modelling fire propagation. The effect of including this variation in propagation models has not previously been fully examined or quantified. We investigate the effects of incorporating three types of variation on the shape and rate of propagation of a fire perimeter: variation in combustion conditions, wind direction and wind speed. We find that increasing the variation of combustion condition decreases the overall rate of propagation. An analytical model, based on the harmonic mean, is presented to explain this behaviour. Variation in wind direction is found to cause the development of rounded flanks due to cumulative chance of outward fluctuations at the sides of the perimeter. Our findings may be used to develop improved models for fire spread prediction. Display Omitted Implementation of a wildfire spread model based on the level set method.Investigation of wildfire propagation under stochastic wind and fuel conditions.Local variation in combustion condition slows the rate of propagation.Local variation in wind direction is found to increase flank spread.A harmonic mean is preferential for spatially varying parameters in spread models.

Curvature effects in the dynamic propagation of wildfires

The behaviour and spread of a wildfire are driven by a range of processes including convection, radiation and the transport of burning material. The combination of these processes and their interactions with environmental conditions govern the evolution of a fire’s perimeter, which can include dynamic variation in the shape and the rate of spread of the fire. It is difficult to fully parametrise the complex interactions between these processes in order to predict a fire’s behaviour. We investigate whether the local curvature of a fire perimeter, defined as the interface between burnt and unburnt regions, can be used to model the dynamic evolution of a wildfire’s progression. We find that incorporation of curvature dependence in an empirical fire propagation model provides closer agreement with the observed evolution of field-based experimental fires than without curvature dependence. The local curvature parameter may represent compounded radiation and convective effects near the flame zone of a fire. Our findings provide a means to incorporate these effects in a computationally efficient way and may lead to improved prediction capability for empirical models of rate of spread and other fire behaviour characteristics.

Does the principle of minimum work apply at the carotid bifurcation: a retrospective cohort study

BackgroundThere is recent interest in the role of carotid bifurcation anatomy, geometry and hemodynamic factors in the pathogenesis of carotid artery atherosclerosis. Certain anatomical and geometric configurations at the carotid bifurcation have been linked to disturbed flow. It has been proposed that vascular dimensions are selected to minimize energy required to maintain blood flow, and that this occurs when an exponent of 3 relates the radii of parent and daughter arteries. We evaluate whether the dimensions of bifurcation of the extracranial carotid artery follow this principle of minimum work.MethodsThis study involved subjects who had computed tomographic angiography (CTA) at our institution between 2006 and 2007. Radii of the common, internal and external carotid arteries were determined. The exponent was determined for individual bifurcations using numerical methods and for the sample using nonlinear regression.ResultsMean age for 45 participants was 56.9 ± 16.5 years with 26 males. Prevalence of vascular risk factors was: hypertension-48%, smoking-23%, diabetes-16.7%, hyperlipidemia-51%, ischemic heart disease-18.7%.The value of the exponent ranged from 1.3 to 1.6, depending on estimation methodology.ConclusionsThe principle of minimum work (defined by an exponent of 3) may not apply at the carotid bifurcation. Additional factors may play a role in the relationship between the radii of the parent and daughter vessels.

SPARK – A Bushfire Spread Prediction Tool

Bushfires are complex processes, making it difficult to accurately predict their rate of spread. We present an integrated software system for bushfire spread prediction, SPARK, which was developed with the functionality to model some of these complexities. SPARK uses a level set method governed by a user-defined algebraic spread rate to model fire propagation. The model is run within a modular workflow-based software environment. Implementation of SPARK is demonstrated for two cases: a small-scale experimental fire and a complex bushfire scenario. In the second case, the complexity of environmental non-homogeneity is explored through the inclusion of local variation in fuel and wind. Simulations over multiple runs with this fuel and wind variation are aggregated to produce a probability map of fire spread over a given time period. The model output has potential to be used operationally for real-time fire spread modeling, or by decision makers predicting risk from bushfire events.

Dynamics of charged hemispherical soap bubbles

Raising the potential of a charged hemispherical soap bubble over a critical limit causes deformation of the bubble into a cone and ejection of a charged liquid jet. This is followed by a mode which has not previously been observed in bubbles, in which a long cylindrical liquid film column is created and collapses due to a Rayleigh-Plateau instability creating child bubbles. We show that the formation of the column and subsequent creation of child bubbles is due to a drop in potential caused by the ejection of charge from the system via the jet. Similar dynamics may occur in microscopic charged liquid droplets (electrospray processes), causing the creation of daughter droplets and long liquid spindles.

Computer Modeling of Anterior Circulation Stroke: Proof of Concept in Cerebrovascular Occlusion

Background: Current literature emphasizes the role of the Circle of Willis (CoW) in salvaging ischemic brain tissue but not that of leptomeningeal anastomoses (LA). We developed a computational model of the cerebral circulation to (1) evaluate the roles of the CoW and LA in restoring flow to the superficial compartment of the middle cerebral artery (MCA) territory and (2) estimate the size of the LA required to maintain flow above the critical ischemic threshold (>30% of baseline) under simulated occlusion. Methods: Cerebral vasculature was modeled as a network of junctions connected by cylindrical pipes. The experiments included occlusion of successive distal branches of the intracranial arteries while the diameters of LA were varied. Results: The model showed that the region of reduced flow became progressively smaller as the site of occlusion was moved from the large proximal to the smaller distal arteries. There was no improvement in flow in the MCA territory when the diameters of the inter-territorial LA were varied from 0.0625 to 0.5 mm while keeping the intra-territorial LA constant. By contrast, the diameter of the inter-territorial LA needed to be >1.0 mm in order to provide adequate (>30%) flow to selected arteries in the occluded MCA territory. Conclusion: The CoW and inter-territorial LA together play important supportive roles in intracranial artery occlusion. Computational modeling provides the ability to experimentally investigate the effect of arterial occlusion on CoW and LA function.

A power series formulation for two-dimensional wildfire shapes

Computational simulations of wildfires require a model for the two-dimensional expansion of a fire perimeter. Although many expressions exist for the one-dimensional rate of spread of a fire front, there are currently no agreed mathematical expressions for the two-dimensional outward speed of a fire perimeter. Multiple two-dimensional shapes such as elliptical and oval-shaped perimeters have been observed and reported in the literature, and several studies have attempted to classify these shapes using geometric approximations. Here we show that a two-dimensional outward speed based on a power series results in a perimeter that can match many of these observed shapes. The power series is based on the dot product between the vector normal to the perimeter and a fixed wind vector. The formulation allows the evolution and shape of a fire perimeter to be expressed using a small set of scalar coefficients. The formulation is implemented using the level set method, and computed perimeters are shown to provide a good match to perimeters of small-scale experimental fires. The method could provide a framework for statistical matching of wildfire shapes or be used to improve current wildfire prediction systems.