The FMM-BEM Method for the 3D Particulate Stokes Flow

This work introduces new functions based on the spherical harmonics and the solid harmonics which have been used to construct a multipole development for the 3D Stokes problem in order to reduce the operations costs in the BEM method. We show that the major properties of those functions are inherited from the solid harmonics. The contribution of this paper is the introduction of new formulas that serve to calculate the multipole moments and the transfer functions that are necessary for the schemes of order O(NlogN). Moreover, new translation formulas are introduced to obtain an O(N) scheme. The error truncation of the resulting scheme is discussed. In comparison to the BEM that attains a limit storage at O(104), we present here a method based on FMM-BEM that attains a storage at a limit of O(106). The implementation of the method achieves a high accuracy level at a reasonable cost

Why and how does collective red blood cells motion occur in the blood microcirculation?

International audienceThe behaviour of red blood cells (RBCs), modelled as vesicles, in Poiseuille flow, mimicking the microvasculature, is studied with numerical simulations in two dimensions. RBCs moving in the centre of the Poiseuille flow (as in blood capillaries) are shown to attract each other and form clusters only due to hydrodynamic interactions, provided that their distance at a given time is below a certain critical value. This distance depends on physical parameters, such as the flow strength. Our simulations reveal that clusters are unstable above a threshold value in the number of forming RBCs, beyond which one or few cells escape the pack by a self-regulating mechanism that select the marginally stable size. This size selection depends on the flow strength as well as on the RBC swelling ratio. The results are interpreted via the analysis of the perturbation of the flow field induced by the vesicles and the interplay with bending and tension forces. This sheds a novel light on the process of collective motion of RBCs observed in vivo

Queimadura solar em jovens: estudo de base populacional no Sul do Brasil Sunburn in young people: population-based study in Southern Brazil

OBJETIVO: Analisar a prevalência e fatores de risco para queimadura solar em jovens com idade entre 10 e 29 anos. MÉTODOS: Estudo transversal de base populacional com amostragem em múltiplos estágios, realizado com residentes da zona urbana de Pelotas, Rio Grande do Sul, entre os meses de outubro e dezembro de 2005. Para a coleta de dados foram feitas entrevistas com 1.604 indivíduos, utilizando questionário padronizado e pré-codificado com informações sobre a família e outro questionário aos indivíduos com idade entre dez e 29 anos para avaliação da ocorrência de episódios de queimadura solar. Queimadura solar foi definida como ardência na pele após exposição ao sol. Para as comparações entre proporções, utilizou-se teste do qui-quadrado com correção de Yates. Na análise multivariada utilizou-se a regressão de Poisson com controle para efeito de delineamento e ajuste robusto da variância. RESULTADOS: Das pessoas com idade entre 10 e 29 anos, 1.412 relataram exposição ao sol no último verão. As perdas e recusas somaram 5,5%. Queimadura solar no último ano foi relatada por 48,7% dos entrevistados. As variáveis associadas à ocorrência de queimadura segundo a análise multivariada foram: cor da pele branca (RP=1,41; IC 95%: 1,12;1,79), maior sensibilidade da pele quando exposta ao sol (RP=1,84; IC 95%: 1,64;2,06), idade entre 15 e 19 anos (RP=1,30; IC 95%: 1,12;1,50), pertencer ao quartil de maior renda (RP=1,20; IC 95%: 1,01;1,42) e fazer uso irregular de fotoprotetor (RP=1,23; IC 95%: 1,08;1,42). CONCLUSÕES: A prevalência de queimadura solar na população estudada foi alta, principalmente entre jovens, de pele branca, com maior sensibilidade da pele, de maior renda e que faziam uso irregular de fotoprotetor. A exposição solar em horários seguros e com métodos de proteção adequados deve ser estimulada.<br>OBJECTIVE: To assess the prevalence and risk factors for sunburn in young people. METHODS: Population-based cross-sectional study using a multiple-stage sampling carried out with people living in the urban area of Pelotas, Southern Brazil, between October and December 2005. Data was collected from interviews with 1.604 subjects using a standardized pre-coded questionnaire about their family and another questionnaire applied to those aged between ten and 29 years for assessing the occurrence of sunburn episodes. Sunburn was defined as skin burning after sun exposure. Chi-square test with Yates' correction was used to compare proportions and Poisson regression with design effect control and robust adjustment of variance was applied in the multivariate analysis. RESULTS: Of those aged between 10 and 29 years, 1,412 reported sun exposure in the last summer. Losses and refusals were 5.5%. A total of 48.7% of the interviewees reported sunburn in the last year. The following variables were associated with sunburn in the multivariate analysis: white skin (PR=1.41; 95% CI: 1.12;1.79); higher skin sensitivity to sun exposure (PR=1.84; 95% CI: 1.64;2.06); age between 15 and 19 years (PR=1.30; 95% CI: 1.12;1.50); belonging to the higher quartile of income (PR=1.20; 95% CI: 1.01;1.42); and irregular use of sunscreens (PR=1.23; 95% CI: 1.08;1.42). CONCLUSIONS: The prevalence of sunburn in the population studied was high mainly among white young people with higher skin sensitivity, higher income and who used sunscreens irregularly. Sun exposure during safe times and with adequate protection should be promoted