Comparative study of models of impedance boundary conditions in acoustic problems

En este artículo se presentan distintas soluciones para la implementación numérica de condiciones de contorno de impedancia (reactancia local) en problemas acústicos. Para ello se analizan 2 tipos de ecuaciones: las ecuaciones de Euler y la ecuación de ondas, y se estudian diferentes soluciones para los contornos tanto en algoritmos de diferencias finitas en el dominio del tiempo (FDTD) como en algoritmos pseudo-espectrales en el dominio del tiempo (PSTD). El análisis de las distintas propuestas numéricas existentes en la literatura se realiza mediante exhaustivos experimentos numéricos que permiten estudiar el comportamiento absorbente de las distintas condiciones de contorno en función de la frecuencia y del ángulo de las ondas incidentes. Este novedoso estudio comparativo permite al ingenierio acústico escoger el modelo numérico que más se adapte a sus necesidades.In this paper, different implementations of numerical locally reacting boundary conditions are studied for acoustic problems. In this comparative study we analyze two types of equations, the Euler equations and the wave equation. We also analyze both finite-differences time-domain (FDTD) algorithms, and pseudo-spectral time domain (PSTD) numerical schemes. We compare different numerical implementations existing in the literature by means of exhaustive numerical experiments. These numerical experiments allow for the study of the absorbing properties of the different schemes as a function of the frequency and the angle of the incident sound waves. This novel comparative study will help the acoustic engineer in order to choose the proper numerical scheme for his/her simulations.Peer Reviewe

Erratum to “Comparative study of models of impedance boundary conditions in acoustic problems”

Peer Reviewe

Erratum to “Comparative study of models of impedance boundary conditions in acoustic problems”

Peer Reviewe

Comparison of expansion-based explicit time-integration schemes for acoustic wave propagation

We have developed a von Neumann stability and dispersion analysis of two time-integration techniques in the framework of Fourier pseudospectral (PS) discretizations of the second-order wave equation. The first technique is a rapid expansion method (REM) that uses Chebyshev matrix polynomials to approximate the continuous solution operator of the discrete wave equation. The second technique is a Lax-Wendroff method (LWM) that replaces time derivatives in the Taylor expansion of the solution wavefield with their equivalent spatial PS differentiations. In both time-integration schemes, each expansion term J results in an extra application of the spatial differentiation operator; thus, both methods are similar in terms of their implementation and the freedom to arbitrarily increase accuracy by using more expansion terms. Nevertheless, their limiting Courant-Friedrichs-Lewy stability number S and dispersion inaccuracies behave differently as J varies. We establish the S bounds for both methods in cases of practical use, J≤10, and we confirm the results by numerical simulations. For both schemes, we explore the dispersion dependence on modeling parameters J and S on the wavenumber domain, through a new error metric. This norm weights errors by the source spectrum to adequately measure the accuracy differences. Then, we compare the theoretical computational costs of LWM and REM simulations to attain the same accuracy target by using the efficiency metric J/S. In particular, we find optimal (J,S) pairs that ensure a certain accuracy at a minimal computational cost. We also extend our dispersion analysis to heterogeneous media and find the LWM accuracy to be significantly better for representative J values. Moreover, we perform 2D wave simulations on the SEG/EAGE Salt Model, in which larger REM inaccuracies are clearly observed on waveform comparisons in the range J≤3.C. Spa has received funding from the Chilean Agency CONICYT under the project FONDECYT 11140212, whereas O. Rojas and J. de la Puente have received funding fromthe European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no.777778 MATHROCKS. The research leading to these results hasreceived funding from the European Union’s Horizon 2020 research and innovation programme under the ChEESE project, grant agreement No. 823844. We also acknowledge funding from the Spanish Ministry Project Geofísica de Altas Prestaciones TIN2016-80957-P.Peer ReviewedPostprint (author's final draft

Unravelling the geographical palimpsest through fieldwork: discovering a sense of place

Fieldwork enables students to gain a greater understanding of the people and places that they encounter. Urban areas are popular destinations for fieldwork because they present a landscape that is in a continual state of change. Yet, as this article indicates, the past is ever present in the urban landscape as each place can be regarded as a palimpsest, where layers of history, geography, culture and politics co-exist. Through active participation in fieldwork, students have the potential to unravel this palimpsest and discover a deeper sense of that place. The example used here reflects upon urban fieldwork in Barcelona as experienced by a group of undergraduate students who, as they carry out and reflect upon their field work, benefit from gaining a sense of place informed by a range of geographical processes and meanings. The article concludes with a series of suggestions for those embarking on fieldwork and wishing to unravel the geographical palimpsest

Gaining a “sense of place”: students’ affective experiences of place leading to transformative learning on international fieldwork

This paper reveals the extent to which undergraduate students demonstrate transformative learning whilst on international fieldwork in Barcelona, Spain. Groups of students create a series of discrete active learning situations that allow them and their peers to engage more fully with their locale and in turn experience a deeper understanding of “place”. Reflective field trip notebooks are used to capture the experiences of students. Results show that through the use of the affective domain (Krathwohl’s taxonomy) and applying cyclic experiential learning (Kolb) combined with critical reflection (Mezirow), students demonstrate progression and, in some cases, regression along Krathwohl’s taxonomy

Influence of manufacturing practices on quality of pharmaceutical products manufactured in Kenya

Objective: To establish the quality of pharmaceutical products manufactured by the respective industries in Kenya and determine the effect of manufacturing practices on the quality of these products. Design: Cross-sectional study. Setting: Industries examined are in Nairobi, Kenya. Laboratory analysis was carried out using available facilities at Kenya Medical Research Institute and University of Nairobi, Faculty of Pharmacy. Interventions: Structured Questionnaires were administered to examine how the code of good manufacturing practices has been used in the production of each pharmaceutical product by respective companies. Questionnaires designed to evaluate the distribution and carry out limited post-market surveillance study were administered to community pharmacy outlets. Drugs were sampled and analyzed for their quality according to the respective monographs. Main Outcome Measures: The questionnaires administered to the industry included the source of raw materials, quarantine procedure before and after manufacture, manufacturing procedure, quality audit, quality assurance procedure, equipment, and staff. That administered to the pharmacy outlet included availability, affordability and acceptability of locally manufactured pharmaceutical products. Quality analysis of products involved the establishment of the chemical content, dissolution profile, friability, uniformity of weight and identity. For antibiotic suspensions the stability after reconstitution was also determined. Results: There were 15 respondents and two non-respondents from the industry and six out of nine respondents from the pharmacy outlets. The ratio of qualified staff to product range produced seemed to influence product quality. Industries producing several products with only limited number of pharmaceutical staff had more products failing to comply with pharmacopoeia specifications compared to those producing only few products. Nevertheless, all companies are well equipped with quality control equipment, in accordance with type of product manufactured. Private pharmacies stocked few of the locally manufactured products. The reason, they said, was due to low doctor and/or patient acceptance. Compliance with quality specifications as set out in respective monographs was overall 76%. Conclusion: Although the local pharmaceutical industries have adopted good manufacturing practices leading to many good quality products currently in commerce, these manufacturing practices are not comprehensive and measures need to be taken to continue improving them. East African Medical Journal Vol.81(6) 2004: 287-29