Simulation of crack coalescence mechanism underneath single and double disc cutters by higher order displacement discontinuity method

The present research is focused on the numerical crack coalescence analysis of the micro-cracks and cracks produced during the cutting action of TBM disc cutters. The linear elastic fracture mechanics(LEFM) concepts and the maximum tangential stress criterion are used to investigate the micro crack propagation and its direction underneath the excavating discs. A higher order displacement discontinuity method with quadratic displacement discontinuity elements is used to estimate the stress intensity factors near the crack tips. Rock cutting mechanisms under single and double type discs are simulated by the proposed numerical method.The main purposes of the present modeling are to simulate the chip formation process of indented rocks by single and double discs.The effects of specific disc parameters(except speed) on the thrust force Ft, the rolling force Fr, and the specific energy ES are investigated. It has been shown that the specific energy(energy required to cut through a unit volume of rock) of the double disc is less than that of the single disc. Crack propagation in rocks under disc cutters is numerically modeled and the optimum ratio of disc spacing S to penetration depth Pd(i.e. S/Pd ratio) of about 10 is obtained, which is in good agreement with the theoretical and experimental results cited in the literature.

Nonlinear Simulations of Coalescence Instability Using a Flux Difference Splitting Method

A flux difference splitting numerical scheme based on the finite volume method is applied to study ideal/resistive magnetohydrodynamics. The ideal/resistive MHD equations are cast as a set of hyperbolic conservation laws, and we develop a numerical capability to solve the weak solutions of these hyperbolic conservation laws by combining a multi-state Harten-Lax-Van Leer approximate Riemann solver with the hyperbolic divergence cleaning technique, high order shock-capturing reconstruction schemes, and a third order total variance diminishing Runge-Kutta time evolving scheme. The developed simulation code is applied to study the long time nonlinear evolution of the coalescence instability. It is verified that small structures in the instability oscillate with time and then merge into medium structures in a coherent manner. The medium structures then evolve and merge into large structures, and The physics of this interesting nonlinear dynamics this trend continues through all scale-lengths is numerically analyzed.

Investigating species boundaries in Colletotrichum

Colletotrichum is one of the most important plant pathogenic genera that is responsible for numerous diseases which can have a profound impact on the agricultural sector.Species delineation is difficult due to a lack of distinctive phenotypic vari-ation.Therefore,in this study three different genomic approaches based on phylogenetic,evolutionary and coalescent-based methods are applied to establish robust species boundaries.The reliability of five different DNA barcodes was also assessed to provide further insights into species delineation.The ITS region can resolve the placement of taxa up to the species complex level.The GAPDH and TUB2 markers are determined to be the most informative for most complexes.However,no single marker could discriminate between species in all complexes,therefore different molecular approaches based on multi-locus datasets are recommended.This is the first study to provide an estimated divergence time for all species complexes in Colle-totrichum.The estimated divergence time for species complexes ranged between 4.8 to 32.2 MYA.Based on the high level of congruent results obtained from the different molecular approaches,a new species complex,the Colletotrichum agaves complex is introduced.This complex consists of five taxa which are characterised by the presence of straight or slightly curved conidia with obtuse apices.This study shows that coalescent approaches and multi-locus phylogeny are crucial to establish species boundaries in Colletotrichum.The taxonomic placement of three singleton taxa Colletotrichum axonopodi,C.cariniferi and C.parallelophorum is revised.We accept 248 species and provide recommendations regarding species boundaries in the graminicola-caudatum complex.