Entrainment and suspension of sand and gravel

The entrainment and suspension of sand and gravel are important for the evolution of rivers, deltas, coastal areas, and submarine fans. The prediction of a vertical profile of suspended sediment concentration typically consists of assessing (1) the concentration near the bed using an entrainment relation and (2) the upward vertical distribution of sediment in the water column. Considerable uncertainty exists in regard to both of these steps, especially the near-bed concentration. Most entrainment relations have been tested against limited grain-size-specific data, and no relations have been evaluated for gravel suspension, which can be important in bedrock and mountain rivers. To address these issues, we compiled a database with suspended sediment data from natural rivers and flume experiments, taking advantage of the increasing availability of high-resolution grain size measurements. We evaluated 12 dimensionless parameters that may determine entrainment and suspension relations and applied multivariate regression analysis. A best-fit two-parameter equation (r2=0.79) shows that near-bed entrainment, evaluated at 10 % of the flow depth, decreases with the ratio of settling velocity to skin-friction shear velocity (wsi/u∗skin), as in previous relations, and increases with Froude number (Fr), possibly due to its role in determining bedload-layer concentrations. We used the Rouse equation to predict concentration upward from the reference level and evaluated the coefficient βi, which accounts for differences in the turbulent diffusivity of sediment from the parabolic eddy viscosity model used in the Rouse derivation. The best-fit relation for βi (r2=0.40) indicates greater relative sediment diffusivities for rivers with greater flow resistance, possibly due to bedform-induced turbulence, and larger wsi/u∗skin; the latter dependence is nonlinear and therefore different from standard Rouse theory. In addition, we used empirical relations for gravel saltation to show that our relation for near-bed concentration also provides good predictions for coarse-grained sediment. The new relations extend the calibrated parameter space over a wider range in sediment sizes and flow conditions compared to previous work and result in 95 % of concentration data throughout the water column predicted within a factor of 9.

A BEAD-SPRING MODEL AND MEAN FIELD THEORY BASED RE-CALCULATION REVEALS UNCERTAINTY OF ROUSE-TYPE DNA DYNAMICS IN DILUTE SOLUTION

Double-stranded DNA (dsDNA) is one of the most used model polymers in studying polymer dynamics. Some recent studies with the experimental data via fluorescence correlation spectroscopy (FCS) proposed that the end-monomer dynamics of dsDNA in dilute solution should be fallen into Rouse-type at intermediate times. This viewpoint is inconsistent with the classical polymer dynamics, therefore arousing controversy. To have a further looking clearly at what else meaning could be revealed from the original data of the FCS measurements, we made a re-calculation by two methods: one is based on Lodge and Wu's model (LWM) modified from the classical bead-spring model, in which parameters used in modeling are needed to be adjusted; and another is a mean field theory (MFT) for semiflexible chain with no parameter fitting needed. In LWM, we find not so weak hydrodynamic interactions (HI) which is not expected in Rouse theory, and the scaling of mean square displacement (MSD) is between Rouse-type and Zimm-type. MFT can reproduce experimental data well at larger time scales, whereas also gives rather different picture in intermediate regime — a dynamical scaling between Rouse-like and Zimm-like rather than Rouse-like scaling is found, indicating there may be sample problems or limitations in the setup for the experiment.

Alpha (Vitrea) Transition in Vulcanized Natural Rubber/Styrene Butadiene Rubber Blends Prepared by Mechanical and Solution Mixing

The preparation method of an elastomeric blend can influence the mechanical properties of the vulcanized compound. In this research elastomeric blends composed by natural rubber and styrene butadiene rubber were mixed using two different methods: by mixing in a roll mill and by dissolution of both elastomers in toluene, mixing of both solutions with the curatives and the evaporation of the solvent. Samples with different Natural Rubber/Styrene Butadiene Rubber relation were prepared by both methods and vulcanized at 433K with a system based on sulphur and accelerator (N-t-butyl-2-benzothiazole sulfenamide) up to the time of optimum cure. The blend composition and the preparation methods have a strong influence in the mechanical dynamic properties. Scanning Electron Microscopy observations indicate that, in the blends prepared by the dissolution method, the samples show better miscibility of the constitutive phases than those prepared by the roll milling method. The temperature dependence of the internal friction was studied for each sample using a subresonant forced pendulum at 1 Hz between 190K and 250K. Depending on the blend composition, one or two glass transition temperatures (Tg) associated to the α-relaxation were measured. In the last case each Tgcorresponds to each elastomeric phase of the compound. The loss tangent data for each compound was analyzed using a mixture law of two phases in the frame of the Rouse theory. The adjustment of the data to the proposed model was very good for both preparation method and the whole composition range of the compounds. Then it was possible to obtain the Tg, the main relaxation time and the activation energy values of each compound and, in some samples, the respective values for each elastomeric phase.

Fraction Scaling and Dynamical Properties of Elastomer Materials

Scaling of the real and the imaginary part of dynamic moduli with frequency, for fully cured elastomer materials as gum and active carbon black filled butyl rubbers, is considered experimentally and theoretically. For gum rubber in different ranges of frequency complete agreement with G''-scaling predicted by the Rouse theory is obtained. Obtained slopes for all G' and G'' of filled rubber are much lower.

Molecular Dynamics Simulation of Single Chain in the Vicinity of Nanoparticle

Molecular simulation of single chain in the vicinity of nanoparticle in comparison with pure system is presented. According to the Rouse theory, chains were considered as a sequence of beads connected together by harmonic springs. The motion of atoms was supported by thermal energy and retarded by the resistance of surrounding. New atom position, in given time, was determined by the Smoluchowski equation, that consists of two terms: first one includes the influence of the inter-atomic collisions, the sterical obstacles and the strong intermolecular interactions in friction coefficient, second one express the energy field aggregated from potentials of all atoms. Sinusoidal shear stress was applied to the chain. The output of the model was energy as a function of time. The energy course was also sinusoidal but shifted according to the deformation. The amplitudes and phase shifts were analyzed for the chains under different conditions .The chains were subjected to the model first as the standalone objects. Then, barrier was defined and chains placed in the vicinity of it. The barrier acted as a volume excluded hindrance. This type of chain molecular dynamics could be used as a stand-alone model or it could be suitable component for complex models, for example network model of polymer nanocomposite.