SUMMARYThe analysis and improvement of an immersed boundary method (IBM) for simulating turbulent ows over complex geometries are presented. Direct forcing is employed. It consists in interpolating boundary conditions from the solid body to the Cartesian mesh on which the computation is performed. Lagrange and least squares high-order interpolations are considered. The direct forcing IBM is implemented in an incompressible ÿnite volume Navier-Stokes solver for direct numerical simulations (DNS) and large eddy simulations (LES) on staggered grids. An algorithm to identify the body and construct the interpolation schemes for arbitrarily complex geometries consisting of triangular elements is presented. A matrix stability analysis of both interpolation schemes demonstrates the superiority of least squares interpolation over Lagrange interpolation in terms of stability. Preservation of time and space accuracy of the original solver is proven with the laminar two-dimensional Taylor-Couette ow. Finally, practicability of the method for simulating complex ows is demonstrated with the computation of the fully turbulent three-dimensional ow in an air-conditioning exhaust pipe.
Mechanical and chemical processes used in the extraction of flax fibres for the production of technical flax fabrics and other flax products have a significant effect on their biochemical composition, structure and properties. In this work, we investigated the effect of different chemical extraction treatments on the biochemical composition and physical chemical properties of flax fabrics and their influence on the microstructure and mechanical properties of thermo-compressed flax fabrics reinforced epoxy composites. A unidirectional (UD) flax tow woven fabric with minimal processing was chosen in order to retain as much of the original flax cell wall structure as possible. The flax fabric was treated by various aqueous and organic solvents with increasing solvation capacity, so as to gradually extract cell wall components from the fibres. The treated flax fibre fabrics were characterised in terms of biochemical composition, wettability and dimensional characteristics. The influence of chemical extraction treatments and the role of cell wall components on the microstructural and mechanical properties of UD flax/epoxy biocomposites were investigated and discussed by means of Scanning Electron Microscopy (SEM), image analysis, Differential Scanning Calorimetry (DSC) and transverse tensile tests. Our results demonstrate that noncellulosic cell wall components of flax fibres play a key role in the dispersion of flax yarns within the epoxy matrix, and in the mechanical behaviour of biocomposites.
In several industrial sectors, structural composite materials with good impact resistance are required to design parts submitted to crashes or falling objects. This work analyses the impact behaviour of short hemp fibres reinforced biocomposites through mechanical measurements, high speed imaging and finite element modelling. A drop-weight impact machine was instrumented with a high speed camera to measure the propagation of macro-cracks and correlate it to the force-displacement dynamic response at several impact energy levels. PP-hemp composites exhibit higher absorbed energies (up to 40%) than PP-glass composites due to higher strain at break. The video tracking analysis highlights that for a given cumulated crack length, PP-hemp composite absorbs much more energy, related to differences in failure mechanisms. The developed finite element model is in good agreement with the experimental measurements and the fracture growth pattern, thus constituting a useful tool to predict the impact response of biocomposite parts.
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