An exact general remeshing scheme applied to physically conservative voxelization

We present an exact general remeshing scheme to compute analytic integrals of polynomial functions over the intersections between convex polyhedral cells of old and new meshes. In physics applications this allows one to ensure global mass, momentum, and energy conservation while applying higher-order polynomial interpolation. We elaborate on applications of our algorithm arising in the analysis of cosmological N-body data, computer graphics, and continuum mechanics problems. We focus on the particular case of remeshing tetrahedral cells onto a Cartesian grid such that the volume integral of the polynomial density function given on the input mesh is guaranteed to equal the corresponding integral over the output mesh. We refer to this as "physically conservative voxelization". At the core of our method is an algorithm for intersecting two convex polyhedra by successively clipping one against the faces of the other. This algorithm is an implementation of the ideas presented abstractly by Sugihara (1994), who suggests using the planar graph representations of convex polyhedra to ensure topological consistency of the output. This makes our implementation robust to geometric degeneracy in the input. We employ a simplicial decomposition to calculate moment integrals up to quadratic order over the resulting intersection domain. We also address practical issues arising in a software implementation, including numerical stability in geometric calculations, management of cancellation errors, and extension to two dimensions. In a comparison to recent work, we show substantial performance gains. We provide a C implementation intended to be a fast, accurate, and robust tool for geometric calculations on polyhedral mesh elements.Comment: Code implementation available at https://github.com/devonmpowell/r3

Voids in cosmological simulations over cosmic time

We study evolution of voids in cosmological simulations using a new method for tracing voids over cosmic time. The method is based on tracking watershed basins (contiguous regions around density minima) of well developed voids at low redshift, on a regular grid of density field. It enables us to construct a robust and continuous mapping between voids at different redshifts, from initial conditions to the present time. We discuss how the new approach eliminates strong spurious effects of numerical origin when voids evolution is traced by matching voids between successive snapshots (by analogy to halo merger trees). We apply the new method to a cosmological simulation of a standard LambdaCDM cosmological model and study evolution of basic properties of typical voids (with effective radii between 6Mpc/h and 20Mpc/h at redshift z=0) such as volumes, shapes, matter density distributions and relative alignments. The final voids at low redshifts appear to retain a significant part of the configuration acquired in initial conditions. Shapes of voids evolve in a collective way which barely modifies the overall distribution of the axial ratios. The evolution appears to have a weak impact on mutual alignments of voids implying that the present state is in large part set up by the primordial density field. We present evolution of dark matter density profiles computed on iso-density surfaces which comply with the actual shapes of voids. Unlike spherical density profiles, this approach enables us to demonstrate development of theoretically predicted bucket-like shape of the final density profiles indicating a wide flat core and a sharp transition to high-density void walls.Comment: 13 pages, 13 figures; accepted for publication in MNRA

Work of breathing in exercise and disease

This thesis is focussed on developing new methods and outcomes to assess respiratory function that require little or no volitional effort on behalf of the participants being tested. Specifically to attempt to detach the behaviour of the patient from the accuracy of the test of respiratory function, resulting in techniques that are simpler and easier to administer and undertake for both assessor and participant. It aims to develop methods that reduce the involvement of the participant during assessment of respiratory function. The human body’s way of controlling respiration has evolved into a sophisticated system that optimises breathing pattern to maintain the most efficient homeostatic action of the respiratory system. Eliciting and assessing this automatic response is the key to removing the action of participation from respiratory functiontesting. The focus must therefore be on developing non-invasive, sub-maximal techniques that allow participants to enter into a steady state of respiration and how this can be assessed. Two techniques were investigated; Respiratory Endurance (as the inspiratory work of breathing) and Tidal Breathing Flow Profile, and these were successfully applied in 99 adult participants (68 healthy controls and 31 COPD patients) and 75 children (48 clinical group and 27 healthy controls) who completed 467 respiratory endurance trials whilst seated and exercising, and 249 relaxed tidal breathing trials. The difficulties with lung function assessment are well established and have been described in this thesis. Much recent emphasis has been put on developing existing devices and protocols rather than developing new techniques and approaching these difficulties from alternative viewpoints. This thesis has described the development of innovative techniques to assess the function of the respiratory systems that aim to overcome the issues associated with maximal testing. It was shown that these techniques are easy to undertake for a range of participants, simple to analyse and are able to reliably differentiate between health and disease, suggesting that they could become a useful adjunct to existing methods of respiratory assessment

Studies into cytauxzoon and helminth infections of bobcats (Lynx rufus) of Northwest Arkansas

The purpose of this study was to determine the prevalence of Cytauxzoon felis and gastrointestinal helminth infections in bobcats (Lynx rufus) of Northwest Arkansas, an area known to have numerous cases of cytaux in domestic cats. Sixty bobcat carcasses were collected from trappers located in Mulberry and Decatur, Arkansas. Blood samples from the hearts were used to isolate Cytauxzoon DNA. Next, a polymerase chain reaction ( PCR) procedure coupled with gel-electrophoresis assay for the 18s region of extracted DNA were used to determine the presence of the protozoan in the bobcats at the time of harvest. Out of the 60 bobcats, 54 (90%) were positive for the protozoan’s infection. These findings indicate a large reservoir of Cytauxzoon for possible infection of domestic cats. Along with the detection of Cytauxzoon felis, intestinal helminths of the bobcats were collected and identified. The isolated helminths included Alaria marcianae, Ancylostoma sp., Molineus barbatus, Taenia sp., Spirometra mansonoides, Mesocestoides lineatus, Aonchotheca putorii, Physaloptera praeputialis and Toxocara cati. All helminths found were previously shown to be common in omnivorous and carnivorous sylvatic as well as domestic mammals of the region. It is amazing that bobcats are able to withstand both parasitic infections concurrently, as they roam the forests of Northwest Arkansas

Dividend persistence and return predictability

Evidence of dividend yield return predictability has been presented so widely and consistently that the result has tended to be generally accepted. This paper shows that return predictability of the dividend yield is a spurious result that is due to dividend persistence and finds that standard dividend behaviour explanatory models are also affected by the spurious regression problem. A simulation procedure is utilized to take account of a spurious correlation that compounds the spurious regression problem when the dependent and independent variables in a time series regression are ratios composed of common component variables. The paper’s results therefore imply that extreme care should be taken when using ratios as predictor or explanatory variables in time series regression. The paper introduces a reformulated Lintner first difference dividend behaviour model that is not subject to spurious regression