Medials For Meshing And More

INTRODUCTION The goal of an automated FE modelling system is to accept a general problem definition as input and to return results of prescribed accuracy. A general problem definition will include the geometric model of the component to be analysed as well as all the required attributes such as loading, restraints and material properties. Automatic, adaptive mesh generation is an essential prerequisite for generating analysis results of prescribed accuracy for a given computational domain. However for many problems, the geometric design model is too complex a domain to analyse in a realistic timeframe. The purpose here is to argue that the medial axis transform of a geometric domain is a powerful tool for recognising features which are significant in the derivation of appropriate analysis models from design geometry. THE MEDIAL AXIS TRANSFORM The medial axis of a 2D region is the locus of the centre of an inscribed disc of maximal diameter as it rolls around t

Response of perennial ryegrass (Lolium perenne) to renovation in Australian dairy pastures

This study reports on the effect of oversowing perennial ryegrass (Lolium perenne L.) into a degraded perennial ryegrass and white clover (Trifolium repens L.) pasture to extend its productive life using various intensities of seedbed preparation. Sites in New South Wales (NSW), Western Australia (WA), South Australia (SA) and Tasmania (Tas.) were chosen by a local group of farmers as being degraded and in need of renovation. Control (nil renovation) and medium (mulch and graze, spray with glyphosphate and sow) renovation treatments were common to all sites whereas minimum (mulch and graze, and sow) and full seedbed (graze and spray with glyphosphate and then full seedbed preparation) renovation were imposed only at some sites. Plots varied in area from 0.14 to 0.50 ha, and were renovated then sown in March or April 2000 and subsequently grazed by dairy cows. Pasture utilisation was estimated from pre- and post-grazing pasture mass assessed by a rising plate pasture meter. Utilised herbage mass of the renovated treatments was significantly higher than control plots in period 1 (planting to August) and 2 (first spring) at the NSW site only. There was no difference among treatments in period 3 (first summer) at any site, and only at the WA and NSW sites in period 4 (March to July 2001) was there a response to renovation. As a result, renovation at the NSW site only significantly increased ryegrass utilisation over the whole experimental period. Ryegrass plant density was higher at the NSW, WA (excluding minimum renovation) and Tas. (excluding full renovation) sites 6 months after renovation but this was only sustained for 12 months for the minimum and medium treatments at the NSW and Tas. sites, respectively, presumably due to reduced competition from naturalised C4 summer grasses [kikuyu (Pennisetum clandestinum) and paspalum (Paspalum dilatatum)] in NSW At the NSW, WA and SA sites, the original ryegrass plant density was low (<35 plants/m2) compared with the Tas. site where density was around 185/m2. The response to renovating a degraded perennial ryegrass pasture varied between sites in Australia. Positive responses were generally small and were most consistent where renovation removed competing C4 summer grasses

A Geometry Centred Process in Airframe Design

The requirement for different abstractions of the core geometry is a key challenge in development of future design systems. In particular the fidelity of the analysis models tends to increase as more knowledge of the design is gained, but these higher fidelity models must coexist alongside coarser models and information must pass consistently and reliably between them. For example a detailed 3D solid of a joint may be embedded within a beam-shell model which ultimately obtains its loads from an aerodynamic model of the cross-section. Much of the difficulty for analysis model derivation arises due to the complexity of the master solid geometry. Shapes and configurations need to be identified, and abstracted to a suitable analysis form. An aerodynamic model may require only the exterior profile of the aircraft, but the structural model may require that stiffeners be represented as a line at the neutral axis with associated cross-sectional properties, or the skin is reduced to a 2D surface. The approach presented here is to remove the geometry as the reference source and replace it with a more abstract representation of the aircraft, and thereafter to use a common geometry generation process for the various disciplines. Model creation then becomes the central focus of the design process for all the disciplines being integrated. This then becomes an implicit part of the systems engineering process. The solid geometry, analysis, manufacturing and economic models can then be derived as required from a common design representation ensuring consistency, ease of use and reliability.</p