E.Y.K. Ng

CFD Modelling of Abdominal Aortic Aneurysm on Hemodynamic Loads Using a Realistic Geometry with CT

The objective of this study is to find a correlation between the abdominal aortic aneurysm (AAA) geometric parameters, wall stress shear (WSS), abdominal flow patterns, intraluminal thrombus (ILT), and AAA arterial wall rupture using computational fluid dynamics (CFD). Real AAA 3D models were created by three-dimensional (3D) reconstruction of in vivo acquired computed tomography (CT) images from 5 patients. Based on 3D AAA models, high quality volume meshes were created using an optimal tetrahedral aspect ratio for the whole domain. In order to quantify the WSS and the recirculation inside the AAA, a 3D CFD using finite elements analysis was used. The CFD computation was performed assuming that the arterial wall is rigid and the blood is considered a homogeneous Newtonian fluid with a density of 1050 kg/m3 and a kinematic viscosity of 4 × 10−3 Pa·s. Parallelization procedures were used in order to increase the performance of the CFD calculations. A relation between AAA geometric parameters (asymmetry index (β), saccular index (γ), deformation diameter ratio (χ), and tortuosity index (ε)) and hemodynamic loads was observed, and it could be used as a potential predictor of AAA arterial wall rupture and potential ILT formation.

Parametric Study of Femtosecond Pulses Laser Hole Drilling of Silicon Wafer

Clark MXR femtosecond (fs) laser micro structuring with pulse duration of 200fs up to a wavelength of 1030nm with a nominal frequency of 1 KHz on silicon wafer was studied in air using the direct focusing technique. The three steps of laser drilling process, namely hole entrance, spatter formation and hole exit, were conducted to investigate the laser parameters such as laser power, focus position, focus lens and number of pulses and their effects on the drilling hole geometry and spatter characteristic. The results show that the minimum entrance diameter is achieved while Zf =-0.2mm. It was easier for the laser to break through the material with longer lens focus length (f=100mm). However, smaller hole was produced when the focus length was shorter. The average diameter is as small as 23µm and aspect ratio is 1:30 at laser power of 200mW.

Theoretical study of pre-formed hole geometries on femtosecond pulse energy distribution in laser drilling.

Maxwells wave equation was solved for fs laser drilling of silicon. The pre-formed hole walls influence on the propagation behavior of subsequent laser pulses was investigated. The laser intensity at hole bottom shows distinct profile as compared with that at hole entrance. The multi-peaks and ring structure of the laser intensity were found at hole bottom. The position of maximum laser intensity (MLI) in relation to the wall taper angle was studied. It was found that the position of the MLI point would be closer to the hole entrance with increasing taper angle. This observation provides valuable information in predicting the position of plasma plume which is a key factor influencing laser drilling process. The elliptical entrance hole shape and zonal structure at the hole bottom reported in the literatures have been reasonably explained using the laser intensity distribution obtained in the present model.