Predicting equilibrium states with Reynolds stress closures in channel flow and homogeneous shear flow

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. A direct theoretical connection is made between homogeneous shear flow in equilibrium and the log-layer of fully-developed turbulent channel flow. It is shown that if a second-order closure model is calibrated to yield good equilibrium values for homogeneous shear flow it will also yield good results for the log-layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second-order closure models introduce an ad hoc wall reflection term in order to mask deficient predictions for the log-layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling

The prion strain phenomenon: molecular basis and unprecedented features.

Prions are unconventional infectious agents responsible for transmissible spongiform encephalopathies. Compelling evidences indicate that prions are composed exclusively by a misfolded form of the prion protein (PrP(Sc)) that replicates in the absence of nucleic acids. One of the most challenging problems for the prion hypothesis is the existence of different strains of the infectious agent. Prion strains have been characterized in most of the species. Biochemical characteristics of PrP(Sc) used to identify each strain include glycosylation profile, electrophoretic mobility, protease resistance, and sedimentation. In vivo, prion strains can be differentiated by the clinical signs, incubation period after inoculation and the lesion profiles in the brain of affected animals. Sources of prion strain diversity are the inherent conformational flexibility of the prion protein, the presence of PrP polymorphisms and inter-species transmissibility. The existence of the strain phenomenon is not only a scientific challenge, but it also represents a serious risk for public health. The dynamic nature and inter-relations between strains and the potential for the generation of a large number of new prion strains is the perfect recipe for the emergence of extremely dangerous new infectious agents

Molecular characterization of cDNA encoding resistance gene-like sequences in Buchloe dactyloides

Current knowledge of resistance (R) genes and their use for genetic improvement in buffalograss (Buchloe dactyloides [Nutt.] Engelm.) lag behind most crop plants. This study was conducted to clone and characterize cDNA encoding R gene-like (RGL) sequences in buffalograss. This report is the first to clone and-characterize of buffalograss RGLs. Degenerate primers designed from the conserved motifs of known R genes were used to amplify RGLs and fragments of expected size were isolated and cloned. Sequence analysis of cDNA clones and analysis of putative translation products revealed that most encoded amino acid sequences shared the similar conserved motifs found in the cloned plant disease resistance genes RPS2, MLA6, L6, RPM1, and Xa1. These results indicated diversity of the R gene candidate sequences in buffalograss. Analysis of 5' rapid amplification of cDNA ends (RACE), applied to investigate upstream of RGLs, indicated that regulatory sequences such as TATA box were conserved among the RGLs identified. The cloned RGL in this study will further enhance our knowledge on organization, function, and evolution of R gene family in buffalograss. With the sequences of the primers and sizes of the markers provided, these RGL markers are readily available for use in a genomics-assisted selection in buffalograss

Finite element analysis of a fluid-structure interaction in flexible pipe line

This paper describes the basic theory and computing method for transient flow of liquid in flexible pipe such as rubber tubing and arterial system. A mathematical model taking into account tube wall axial and radial motion (in which the dynamic fluid pressure causes circumferential and axial motion of the tube wall) is presented. The tube wall is assumed to be elastic material and the compressibility of the liquid is neglected. Circumferential and axial strain-stress relationships for the tube are considered. The obtained mathematical system is constituted of four non-linear hyperbolic partial differential equations describing the wave  propagation in both pipe wall and liquid flow. The fluid-structure interaction is found to be governed by Poisson’s ratio. In this steady finite element method based on Galerkin formulation is applied. Numerical results show a good similarity with those of the literature obtained by the characteristics method.Key words : Fluid-structure interaction, flexible pipe, rubber, finite element method

Transverse instability of gravity–capillary solitary waves on deep water in the presence of constant vorticity

International audienc

Transverse instability of gravity–capillary solitary waves on deep water in the presence of constant vorticity

International audienc

A nonlinear Schr\"odinger equation for gravity-capillary water waves on arbitrary depth with constant vorticity: Part I

A nonlinear Schr\"odinger equation for the envelope of two-dimensional gravity-capillary waves propagating at the free surface of a vertically sheared current of constant vorticity is derived. In this paper we extend to gravity-capillary wave trains the results of \citet{thomas2012pof} and complete the stability analysis and stability diagram of \citet{Djordjevic1977} in the presence of vorticity. Vorticity effect on the modulational instability of weakly nonlinear gravity-capillary wave packets is investigated. It is shown that the vorticity modifies significantly the modulational instability of gravity-capillary wave trains, namely the growth rate and instability bandwidth. It is found that the rate of growth of modulational instability of short gravity waves influenced by surface tension behaves like pure gravity waves: (i) in infinite depth, the growth rate is reduced in the presence of positive vorticity and amplified in the presence of negative vorticity, (ii) in finite depth, it is reduced when the vorticity is positive and amplified and finally reduced when the vorticity is negative. The combined effect of vorticity and surface tension is to increase the rate of growth of modulational instability of short gravity waves influenced by surface tension, namely when the vorticity is negative. The rate of growth of modulational instability of capillary waves is amplified by negative vorticity and attenuated by positive vorticity. Stability diagrams are plotted and it is shown that they are significantly modified by the introduction of the vorticity