32 research outputs found
Experiments of Interfacial Roughening in Hele-Shaw Flows with Weak Quenched Disorder
We have studied the kinetic roughening of an oil--air interface in a forced
imbibition experiment in a horizontal Hele--Shaw cell with quenched disorder.
Different disorder configurations, characterized by their persistence length in
the direction of growth, have been explored by varying the average interface
velocity v and the gap spacing b. Through the analysis of the rms width as a
function of time, we have measured a growth exponent beta ~= 0.5 that is almost
independent of the experimental parameters. The analysis of the roughness
exponent alpha through the power spectrum have shown different behaviors at
short (alpha_1) and long (alpha_2) length scales, separated by a crossover
wavenumber q_c. The values of the measured roughness exponents depend on
experimental parameters, but at large velocities we obtain alpha_1 ~= 1.3
independently of the disorder configuration. The dependence of the crossover
wavenumber with the experimental parameters has also been investigated,
measuring q_c ~ v^{0.47} for the shortest persistence length, in agreement with
theoretical predictions.Comment: 20 pages, 22 figure
Truncated Levy Random Walks and Generalized Cauchy Processes
A continuous Markovian model for truncated Levy random walks is proposed. It
generalizes the approach developed previously by Lubashevsky et al. Phys. Rev.
E 79, 011110 (2009); 80, 031148 (2009), Eur. Phys. J. B 78, 207 (2010) allowing
for nonlinear friction in wondering particle motion and saturation of the noise
intensity depending on the particle velocity. Both the effects have own reason
to be considered and individually give rise to truncated Levy random walks as
shown in the paper. The nonlinear Langevin equation governing the particle
motion was solved numerically using an order 1.5 strong stochastic Runge-Kutta
method and the obtained numerical data were employed to calculate the geometric
mean of the particle displacement during a certain time interval and to
construct its distribution function. It is demonstrated that the time
dependence of the geometric mean comprises three fragments following one
another as the time scale increases that can be categorized as the ballistic
regime, the Levy type regime (superballistic, quasiballistic, or superdiffusive
one), and the standard motion of Brownian particles. For the intermediate Levy
type part the distribution of the particle displacement is found to be of the
generalized Cauchy form with cutoff. Besides, the properties of the random
walks at hand are shown to be determined mainly by a certain ratio of the
friction coefficient and the noise intensity rather then their characteristics
individually.Comment: 7 pages, 3 figure
Relação entre heterogeneidade ambiental e distribuição de espécies em uma floresta paludosa no Município de Cristais Paulista, SP, Brasil Floristic composition and relationship between environmental heterogeneity and species distribution in a swamp forest from Cristais Paulista, São Paulo State, Brazil
Neste estudo foi caracterizada a composição florística e fitossociológica de uma floresta paludosa no nordeste do Estado de São Paulo, onde variações no padrão de drenagem ocorrem principalmente no sentido perpendicular ao curso d'água, em função da topografia. Nosso objetivo foi investigar a distribuição das espécies em relação ao substrato e a entrada de luz na comunidade. Foram alocadas 60 parcelas de 10×10 m, distribuídas em seis grupos de 10 parcelas cada. Em cada parcela foram medidos e identificados os indivíduos com PAP > 10 cm, avaliadas as propriedades químicas e granulometria do solo, o desnível topográfico, a drenagem e a abertura do dossel. Foram registradas 88 espécies, das quais 61 foram observadas nas parcelas. Uma análise de correspondência canônica (CCA) indicou correlações entre a distribuição das 29 espécies mais abundantes e drenagem, desnível topográfico e Fe no eixo 1 e abertura do dossel no eixo 2. O coeficiente de Spearman indicou correlações significativas entre 66% das 29 espécies e profundidade do lençol freático ou abertura do dossel. Como exemplo, Calophyllum brasiliense e Xylopia emarginata prevaleceram em solos mal drenados; Siphoneugena densiflora e Virola sebifera em solos bem drenados; Myrcia laruotteana e Xylopia sericea em parcelas com maior abertura de dossel. A heterogeneidade ambiental foi um importante fator na determinação da distribuição e a coexistência de espécies, resultando em um incremento na diversidade local.<br>In the present study, we characterized the floristic composition and tree structure of a swamp forest in northeast of São Paulo State, Brazil, where the drainage pattern changes mainly across the watercourse due to the topography. We aimed to investigate the species distribution in relation to substrate and light entrance in the community. Sixty quadrats (10×10 m) were divided into six groups of 10 plots. In each quadrat, individuals with PBH > 10 cm were measured and identified. We also evaluated soil chemical properties, granulometry, topographic unevenness, drainage, and canopy openness. Eighty-eight species were recorded, from which 61 were detected in the quadrats. Canonical correspondence analysis (CCA) indicated correlations between the distribution of the 29 most abundant species and drainage, topographic unevenness and Fe in the axis 1, and canopy openness in the axis 2. Spearman's rank correlation coefficient indicated significant correlations between 66% from the 29 species and water table depth or canopy openness. As examples, Calophyllum brasiliense and Xylopia emarginata were predominant in poorly-drained soils; Siphoneugena densiflora and Virola sebifera, in the drained ones; and Myrcia laruotteana and Xylopia sericea, in the quadrats presenting higher canopy openness. Environmental heterogeneity showed to be an important factor in determining the species distribution and coexistence, which increases local diversity