An automatic PML for acoustic finite element simulations in convex domains of general shape

International audienceThis article addresses the efficient finite element solution of exterior acoustic problems with truncated computational domains surrounded by perfectly matched layers (PMLs). The PML is a popular nonreflecting technique that combines accuracy, computational efficiency, and geometric flexibility. Unfortunately, the effective implementation of the PML for convex domains of general shape is tricky because of the geometric parameters that are required to define the PML medium. In this work, a comprehensive implementation strategy is proposed. This approach, which we call the automatically matched layer (AML) implementation, is versatile and fully automatic for the end‐user. With the AML approach, the mesh of the layer is extruded, the required geometric parameters are automatically obtained during the extrusion step, and the practical implementation relies on a simple modification of the Jacobian matrix in the elementwise integrals. The AML implementation is validated and compared with other implementation strategies using numerical benchmarks in two and three dimensions, considering computational domains with regular and nonregular boundaries. A three‐dimensional application with a generally shaped domain generated using a convex hull is proposed to illustrate the interest of the AML approach for realistic industrial cases

Study of dosimetry techniques applied to electron beams with high dose rate

Tese de mestrado, Engenharia Física, 2023, Universidade de Lisboa, Faculdade de CiênciasElectron beams of 4-20 MeV are used for Total Skin Electron Irradiation (TSEI) of T-cell lymphomas like the mycosis fungoides type. High dose rate electron (HDRE) beams show effectiveness in achieving local control rates close to 100% and low rates of acute and late toxicity. An Elekta Infinity Agility machine was modeled without manufacturer information and the resulting 6 MeV (TSEI compatible energy) beam was compared with reference experimental beam. The dosimetric study was conducted using Advanced Markus® Type 34045 plane-parallel ionization chamber (IC) and GafChromicTM EBT-XD films. Both water (MP3) and solid water (RW3) were used, and beam evaluation involved reference 10x10 cm2 fields and 100 cm SSD as well as open fields and 240 cm SSD TSEI compatible. Functional performance testing was done for evaluation of beam constancy in HDRE operating mode. Results show a 0.983 Gy per 100 MU in reference conditions and a 9.115 times higher dose using HDRE mode. Gamma analysis passing of 100 % (2 mm DTA / 2 % DD) was obtained, for comparison between MP3 and RW3 HDRE beams in an open field configuration and for an SSD of 100 cm. The working range was within the effective range of EBTXD films. On a 240 cm spaced perpendicular plane, relative dose was verified as symmetrical within a 15 % tolerance. In an anthropomorphic phantom, dose was analyzed using the dual-field Stanford technique and dmax shift was seen to occur, discarding interface artifacts, from 1.40 ± 0.05 cm (SSDref) to 0.7 ± 0.2 cm. From TOPAS MC simulation, gamma analysis has shown that the model agrees completely with reference beam. Limited agreement was seen using SSD of 240 cm. MC model mimics the studied linac using reference conditions and SSD of 240 cm is adequate for TSEI implementation. More studies for confirmation of results and others focusing on non-reference conditions are needed

SIMULATION OF WHISTLE NOISE USING COMPUTATIONAL FLUID DYNAMICS AND ACOUSTIC FINITE ELEMENT SIMULATION

The prediction of sound generated from fluid flow has always been a difficult subject due to the nonlinearities in the governing equations. However, flow noise can now be simulated with the help of modern computation techniques and super computers. The research presented in this thesis uses the computational fluid dynamics (CFD) and the acoustic finite element method (FEM) in order to simulate the whistle noise caused by vortex shedding. The acoustic results were compared to both analytical solutions and experimental results to better understand the effects of turbulence models, fluid compressibility, and wall boundary meshes on the acoustic frequency response. In the case of the whistle, sound power and pressure levels are scaled since 2-D models are used to model 3-D phenomenon. The methodology for scaling the results is detailed

Numerical and Analytical Methods in Electromagnetics

Like all branches of physics and engineering, electromagnetics relies on mathematical methods for modeling, simulation, and design procedures in all of its aspects (radiation, propagation, scattering, imaging, etc.). Originally, rigorous analytical techniques were the only machinery available to produce any useful results. In the 1960s and 1970s, emphasis was placed on asymptotic techniques, which produced approximations of the fields for very high frequencies when closed-form solutions were not feasible. Later, when computers demonstrated explosive progress, numerical techniques were utilized to develop approximate results of controllable accuracy for arbitrary geometries. In this Special Issue, the most recent advances in the aforementioned approaches are presented to illustrate the state-of-the-art mathematical techniques in electromagnetics

Development and Application of Efficient Portal Imaging Solutions

__Abstract__ The central subject of this thesis is to derive clinically applicable methods to measure and improve the reproducibility of treatment delivery in radiotherapy by means of portal imaging. The most important criteria that such methods should meet, apart from being effective, is that (1) they are relatively simple to implement and (2) the additional workload required in daily practice is small. This approach was inspired by the observation that routine application of portal imaging in clinical practice, according to well-defined protocols, remains relatively rare. Below, we first sketch the general aims and practice of radiotherapy. From this brief overview, a number of aspects become apparent that are essential to the work described in the following chapters. In particular, the meaning of systematic and random geometrical errors in radiotherapy is emphasised