What is the length of a knot in a polymer?

We give statistical definitions of the length, l, of a loose prime knot tied into a long, fluctuating ring macromolecule. Monte Carlo results for the equilibrium, good solvent regime show that ~ N^t, where N is the ring length and t ~ 0.75 is independent of the knot topology. In the collapsed regime below the theta temperature, length determinations based on the entropic competition of different knots within the same ring show delocalization (t~1).Comment: 9 pages, 5 Postscript figure

Energy balance in lubricated drag-reduced turbulent channel flow

We use direct numerical simulation (DNS) to study drag reduction in a lubricated channel, a flow instance in which a thin layer of lubricating fluid is injected in the near-wall region so as to favour the transportation of a primary fluid. In the present configuration, the two fluids have equal density but different viscosity, so that a viscosity ratio can be defined. To cover a meaningful range of possible situations, we consider five different in the range. All DNS are run using the constant power input (CPI) approach, which prescribes that the flow rate is adjusted according to the actual pressure gradient so as to keep constant the power injected into the flow. The CPI approach has been purposely extended here for the first time to the case of multiphase flows. A phase-field method is used to describe the dynamics of the liquid-liquid interface. We unambiguously show that a significant drag reduction (DR) can be achieved for. Reportedly, the observed DR is a non-monotonic function of and, in the present case, is maximum for (flow-rate increase). Upon a detailed analysis of the energy budgets, we are able to show the existence of two different DR mechanisms. For and, DR is purely due to the effect of the surface tension-a localized elasticity element that separates the two fluids-which, decoupling the wall-normal momentum transfer mechanisms between the primary and the lubricating layer, suppresses turbulence in the lubricating layer (laminarization) and reduces the overall drag. For <[CDATA[\u3bb, turbulence can be sustained in the lubricating layer, because of the increased local Reynolds number. In this case, DR is simply due to the smaller viscosity of the lubricating layer that acts to decrease directly the corresponding wall friction. Finally, we show evidence that an upper bound for exists, for which DR cannot be observed: for, we report a slight drag enhancement, thereby indicating that the turbulence suppression observed in the lubricating layer cannot completely balance the increased friction due to the larger viscosity

Produtos iconográficos e cartográficos gerados pela Embrapa Monitoramento por Satélite.

Reune os principais conceitos e informações amplamente utilizados na área de cartografia e geoprocessamento, visando homogeneizar o padrão de apresentação dos produtos da Unidade, segundo as normas correntes, sugerindo conceitos, rótulos e módulos a serem aplicados em mapas e cartas temáticas.bitstream/item/105448/1/890.pd

Propagation of capillary waves in two-layer oil-water turbulent flow

We study the dynamics of capillary waves at the interface of a two-layer stratified turbulent channel flow. We use a combined pseudo-spectral/phase field method to solve for the turbulent flow in the two liquid layers and to track the dynamics of the liquid-liquid interface. The two liquid layers have same thickness and same density, but different viscosity. We vary the viscosity of the upper layer (two different values) to mimic a stratified oil-water flow. This allows us to study the interplay between inertial, viscous and surface tension forces in the absence of gravity. In the present set-up, waves are naturally forced by turbulence over a broad range of scales, from the larger scales, whose size is of order of the system scale, down to the smaller dissipative scales. After an initial transient, we observe the emergence of a stationary capillary wave regime, which we study by means of temporal and spatial spectra. The computed frequency and wavenumber power spectra of wave elevation are in line with previous experimental findings and can be explained in the frame of the weak wave turbulence theory. Finally, we show that the dispersion relation, which gives the frequency as a function of the wavenumber , is in good agreement with the well-established theoretical prediction,

Heat transfer in drop-laden turbulence

Heat transfer by large deformable drops in a turbulent flow is a complex and rich-in-physics system, in which drop deformation, breakage and coalescence influence the transport of heat. We study this problem by coupling direct numerical simulation (DNS) of turbulence with a phase-field method for the interface description. Simulations are run at fixed-shear Reynolds and Weber numbers. To evaluate the influence of microscopic flow properties, like momentum/thermal diffusivity, on macroscopic flow properties, like mean temperature or heat transfer rates, we consider four different values of the Prandtl number, which is the momentum to thermal diffusivity ratio:, and. The drop volume fraction is for all cases. Drops are initially warmer than the turbulent carrier fluid and release heat at different rates depending on the value of, but also on their size and on their own dynamics (topology, breakage, drop-drop interaction). Computing the time behaviour of the drops and carrier fluid average temperatures, we clearly show that an increase of slows down the heat transfer process. We explain our results by a simplified phenomenological model: we show that the time behaviour of the drop average temperature is self-similar, and a universal behaviour can be found upon rescaling by. Accordingly, the heat transfer coefficient (respectively its dimensionless counterpart, the Nusselt number) scales as (respectively) at the beginning of the simulation, and tends to (respectively) at later times. These different scalings can be explained via the boundary layer theory and are consistent with previous theoretical/numerical predictions

Turbulence and Interface Waves in Stratified Oil–Water Channel Flow at Large Viscosity Ratio

We investigate the dynamics of turbulence and interfacial waves in an oil–water channel flow. We consider a stratified configuration, in which a thin layer of oil flows on top of a thick layer of water. The oil–water interface that separates the two layers mutually interacts with the surrounding flow field, and is characterized by the formation and propagation of interfacial waves. We perform direct numerical simulation of the Navier-Stokes equations coupled with a phase field method to describe the interface dynamics. For a given shear Reynolds number, Reτ= 300 , and Weber number, We= 0.5 , we consider three different types of oils, characterized by different viscosities, and thus different oil-to-water viscosity ratios μr= μo/ μw (being μo and μw oil and water viscosities). Starting from a matched viscosity case, μr= 1 , we increase the oil-to-water viscosity ratio up to μr= 100 . By increasing μr , we observe significant changes both in turbulence and in the dynamics of the oil–water interface. In particular, the large viscosity of oil controls the flow regime in the thin oil layer, as well as the turbulence activity in the thick water layer, with direct consequences on the overall channel flow rate, which decreases when the oil viscosity is increased. Correspondingly, we observe remarkable changes in the dynamics of waves that propagate at the oil–water interface. In particular, increasing the viscosity ratio from μr= 1 to μr= 100 , waves change from a two-dimensional, nearly-isotropic pattern, to an almost monochromatic one

Interaction between thermal stratification and turbulence in channel flow

Transport phenomena in high Reynolds number wall-bounded stratified flows are dominated by the interplay between the turbulence structures generated at the wall and the buoyancy-induced large-scale waves populating the channel core. In this study, we want to investigate the flow physics of wall-bounded stratified turbulence at relatively high shear Reynolds number and for mild to moderate stratification level - quantified here by the shear Richardson number varying in the range. By increasing stratification, active turbulence is sustained only in the near-wall region, whereas intermittent turbulence, modulated by the presence of non-turbulent wavy structures (internal gravity waves), is observed at the channel core. In such conditions, the wall-normal transport of momentum and heat is considerably reduced compared with the case of non-stratified turbulence. A careful characterization of the flow-field statistics shows that, despite temperature and wall-normal velocity fluctuations being very large at the channel centre, the mean value of their product - the buoyancy flux - vanishes for. We show that this behaviour is due to the presence of a phase delay between the temperature and the wall-normal velocity signals: when wall-normal velocity fluctuations are large (in magnitude), temperature fluctuations are almost zero, and vice versa. This constitutes a blockage effect to the wall-normal exchange of energy. In addition, we show that the friction factor scales as, and we propose a new scaling for the Nusselt number,. These scaling laws, which seem to be robust over the explored range of parameters, complement and extend previous experimental and numerical data, and are expected to help the development of improved models and parametrizations of stratified flows at large

Armazenamento de sementes de pinhão manso em diferentes embalagens e ambientes.

O presente trabalho teve como objetivo avaliar a qualidade fisiológica de sementes de pinhão manso armazenadas em diferentes embalagens e ambientes. O experimento foi conduzido na Universidade Federal de Viçosa, em Viçosa-MG. As sementes, com teor de água de 8,3%, foram acondicionadas em embalagem de pano e de plástico e armazenadas por 450 dias em condições de laboratório (sem controle de temperatura e U.R.); sala refrigerada (18 a 20 ºC e 55 a 60% de U.R.); câmara fria (10 a 12 ºC e 55 a 60% de U.R.) e câmara fria (5 a 7 ºC e 60 a 65% de U.R.). No início do armazenamento e a cada 90 dias, foram determinados o teor de água, a germinação e o vigor das sementes. Redução na qualidade fisiológica das sementes de pinhão manso ocorreu durante o armazenamento, independentemente das condições de ambiente e embalagem. As sementes podem ser armazenadas por 270 dias em ambiente não controlado, em Viçosa-MG. Para armazenamento por tempo superior a 270 dias, é recomendada a utilização de ambiente refrigerado, com temperatura mais ou menos 18-20 °C, independentemente da embalagem utilizada

Diffusion of a ring polymer in good solution via the Brownian dynamics

Diffusion constants D_{R} and D_{L} of ring and linear polymers of the same molecular weight in a good solvent, respectively, have been evaluated through the Brownian dynamics with hydrodynamic interaction. The ratio $C=D_{R}/D_{L}$, which should be universal in the context of the renormalization group, has been estimated as $C= 1.11 \pm 0.01$ for the large-N limit. It should be consistent with that of synthetic polymers, while it is smaller than that of DNAs such as $C \approx 1.3$. Furthermore, the probability of the ring polymer being a nontrivial knot is found to be very small, while bond crossings may occur at almost all time steps in the present simulation that realizes the good solvent conditions.Comment: 11 pages, 4 figure