405,087 research outputs found
The minimal seed of turbulent transition in the boundary layer
No full textThis paper describes a scenario of transition from laminar to turbulent flow in a spatially developing boundary layer over a flat plate. The base flow is the Blasius non-parallel flow solution; it is perturbed by optimal disturbances yielding the largest energy growth over a short time interval. Such perturbations are computed by a nonlinear global optimization approach based on a Lagrange multiplier technique. The results show that nonlinear optimal perturbations are characterized by a localized basic building block, called the minimal seed, defined as the smallest flow structure which maximizes the energy growth over short times. It is formed by vortices inclined in the streamwise direction surrounding a region of intense streamwise disturbance velocity. Such a basic structure appears to be a robust feature of the base flow since it is practically invariant with respect to the initial energy of the perturbation, the target time, the Reynolds number and the dimensions of the computational domain. The minimal seed grows very rapidly in time while spreading, and it triggers nonlinear effects which bring the flow to turbulence in a very efficient manner, through the formation of a turbulence spot. This evolution of the initial optimal disturbance has been studied in detail by direct numerical simulations. Using a perturbative formulation of the Navier–Stokes equations, each linear and nonlinear convective term of the equations has been analysed. The results show the fundamental role of the streamwise inclination of the vortices in the process. The nonlinear coupling of the finite amplitude disturbances is crucial to sustain such streamwise inclination, as well as to generate dislocations within the flow structures, and local inflectional velocity distributions. The analysis provides a picture of the transition process characterized by a sequence of structures appearing successively in the flow, namely, 3 vortices, hairpin vortices and streamwise streaks. Finally, a disturbance regeneration cycle is conceived, initiated by the fast nonlinear amplification of the minimal seed, providing a possible scenario for the continuous regeneration of the same fundamental flow structures at smaller space and time scales
Classification of hyper-variable Corynebacterium glutamicum surface-layer proteins by sequence analyses and atomic force microscopy
No full textHansmeier N, Bartels FW, Ros R, et al. Classification of hyper-variable Corynebacterium glutamicum surface-layer proteins by sequence analyses and atomic force microscopy. J Biotechnol. 2004;112(1-2):177-193.The structural S-layer proteins of 28 different Corynebacterium glutamicum isolates have been analyzed systematically. Treatment of whole C. glutamicum cells with detergents resulted in the isolation of S-layer proteins with different apparent molecular masses, ranging in size from 55 to 66 kDa. The S-layer genes analyzed were characterized by coding regions ranging from 1473 to 1533 nucleotides coding for S-layer proteins with a size of 490-510 amino acids. Using PCR techniques, the corresponding S-layer genes of the 28 C. glutamicum isolates were all cloned and sequenced. The deduced amino acid sequences of the S-layer proteins showed identities between 69 and 98% and could be grouped into five phylogenetic classes. Furthermore, sequence analyses indicated that the S-layer proteins of the analyzed C. glutamicum isolates exhibit a mosaic structure of highly conserved and highly variable regions. Several conserved regions were assumed to play a key role in the formation of the C. glutamicum S-layers. Especially the N-terminal signal peptides and the C-terminal anchor sequences of the S-layer proteins showed a nearly perfect amino acid sequence conservation. Analyses by atomic force microscopy revealed a committed hexagonal structure. Morphological diversity of the C. glutamicum S-layers was observed in a class-specific unit cell dimension (ranging from 15.2 to 17.4 nm), which correlates with the sequence similarity-based classification. It could be demonstrated that differences in the primary structure of the S-layer proteins were reflected by the S-layer morphology. (C) 2004 Elsevier B.V. All rights reserved
The s-layer glycome-adding to the sugar coat of bacteria
No full textThis work was supported by the Austrian Science Fund FWF, projects P19047-B12, P20605-B12, P21954-B20 (to C. Sch¨affer), and P20745-B11 (to P. Messner). Zarschler and Ristl were supported by the Hochschuljubil¨aumsstiftung der Stadt Wien, Projects H-2229-2007 (to K. Zarschler) and H-1897-2008 (to R. Ristl).The amazing repertoire of glycoconjugates present on bacterial cell surfaces includes lipopolysaccharides, capsular polysaccharides, lipooligosaccharides, exopolysaccharides, and glycoproteins. While the former are constituents of Gram-negative cells, we review here the cell surface S-layer glycoproteins of Gram-positive bacteria. S-layer glycoproteins have the unique feature of self-assembling into 2D lattices providing a display matrix for glycans with periodicity at the nanometer scale. Typically, bacterial S-layer glycans are O-glycosidically linked to serine, threonine, or tyrosine residues, and they rely on a much wider variety of constituents, glycosidic linkage types, and structures than their eukaryotic counterparts. As the S-layer glycome of several bacteria is unravelling, a picture of how S-layer glycoproteins are biosynthesized is evolving. X-ray crystallography experiments allowed first insights into the catalysis mechanism of selected enzymes. In the future, it will be exciting to fully exploit the S-layer glycome for glycoengineering purposes and to link it to the bacterial interactome.Peer reviewe
Review article: Linaclotide for the management of irritable bowel syndrome with constipation
No full textBACKGROUND:
Irritable bowel syndrome with constipation (IBS-C) represents a significant burden to patients and healthcare systems due to its prevalence and lack of successful symptomatic resolution with established treatment options. Linaclotide 290 μg has recently been approved by the European Medicines Agency (EMA) for moderate-to-severe IBS-C and by the US Food and Drug Administration for IBS-C (290 μg dose) and for chronic constipation (145 μg dose).
AIM:
To summarise data leading to the approval of linaclotide for IBS-C, with focus on EMA-pre-specified outcome measures.
METHODS:
Literature search of a peer-review database (PubMed) and review of congress abstracts on linaclotide preclinical and clinical trial data in IBS-C.
RESULTS:
Preclinical studies suggest that the guanylate cyclase C agonist (GCCA) linaclotide acts through elevation of cyclic guanosine monophosphate (cGMP) levels, leading to accelerated gastrointestinal (GI) transit through increased fluid secretion and reduced visceral hypersensitivity. Clinical trial data demonstrate that linaclotide improves abdominal symptoms (pain, bloating) and bowel symptoms (constipation) compared with placebo in patients with IBS-C. The most frequent side effect, diarrhoea, results from the therapeutic action of linaclotide. Linaclotide acts locally in the GI tract with minimal systemic exposure, resulting in low oral bioavailability and thus a low risk of relevant systemic adverse effects.
CONCLUSION:
Linaclotide, a first-in-class GCCA, is a promising new drug with a novel, dual mechanism of action that, unlike more well-established agents, can relieve the abdominal pain, bloating and constipation associated with IBS-C and has a low propensity for systemic side effects
On the use of the boundary layer integral equations for the prediction of skin friction and heat transfer
No full textThe usefulness of the energy equation integrated over the thickness of the
boundary layer, in predicing heat transfer rates to smooth body surfaces
in investigated. It is found that on assuming very simple closure
relations, similar to those often used with the momentum equation, highly
accurate predictions are made. It is shown further that the usefulness
of these predictions extend into areas where the momentum equation-skin
friction predictions, which have proved so popular, break down such as
regions of reverse flow and shock/boundary layer interactions. The
technique is has been tested in laminar transitional and turbulent flows
with both experiment and other more complex theories.
The technique is extended to three-dimensional laminar flows with the
inclusion of a crossflow model
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Inner plasma structure of the low-latitude reconnection layer
No full textWe report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer
Investigation of Lagrangian coherent structures in a wake-induced boundary layer transition
No full textThe evolution of coherent structures in a flat plate boundary layer transition induced by the cylinder wake is investigated using the particle image velocimetry (PIV) technique. The finite-time Lyapunov exponent (FTLE), which characterizes the amount of stretching about the flow trajectory, is used to extract the Lagrangian coherent structures. It is revealed that secondary vortex is induced by the cylinder wake vortices in the near wall region,which would evolve into hairpin vortex as it convects downstream. The subsequent evolvement of the hairpin vortex, characterized by the regeneration of offspring hairpin vortex upstream of it, leads to the appearance of the hairpin packet and the boundary layer finally reaches a turbulent state
Experimental characterisation of large scale structures in a high Reynolds number turbulent boundary layer
No full textA very large field of view (4δ x 1δ) with a good spatial resolution owing to the use of four 2k x 2k pixel cameras was conducted in a flat plate boundary layer at two Reynolds numbers (Reθ ≈7,500 and 20,000). Comparing the flow statistics with previously obtained hot-wire data under similar flow conditions show good agreement. The goal of this experiment is to detect and characterise the large scale motions which develop in the log region of a high Reynolds number turbulent boundary layer
Mechanics of inhomogeneous turbulence and interfacial layers
No full textThe mechanics of inhomogeneous turbulence in and adjacent to interfacial layers bounding turbulent and non-turbulent regions are analysed. Different mechanisms are identified according to the straining by the turbulent eddies in relation to the strength of the mean shear adjacent to, or across, the interfacial layer. How the turbulence is initiated and the topology of the region of turbulence are also significant factors. Specifically the cases of a layer of turbulence bounded on one, or two, sides by a uniform and/or shearing flow, and a circular region of a rotating turbulent vortex are considered and discussed.
The entrainment processes at fluctuating interfaces occur both at the outer edges of turbulent shear layers, with and without free-stream turbulence (e.g. jets, wakes and boundary layers), at internal boundaries such as those at the outside of the non-turbulent core of swirling flows (e.g. the ‘eye-wall’ of a hurricane) or at the top of the viscous sublayer and roughness elements in turbulent boundary layers. Conditionally sampled data enables these concepts to be tested. These concepts lead to physically based estimates for critical modelling parameters such as eddy viscosity near interfaces, entrainment rates, maximum velocity and displacement heights
An intrusion layer in stationary incompressible fluids: Part 1: Periodic waves
No full textWaves on a neutrally buoyant intrusion layer moving into otherwise stationary fluid are studied. There are two interfacial free surfaces, above and below the moving layer, and a train of waves is present. A small amplitude linearized theory shows that there are two different flow types, in which the two interfaces are either in phase or else move oppositely. The former flow type occurs at high phase speed and the latter is a low-speed solution. Nonlinear solutions are computed for large amplitude waves, using a spectral type numerical method. They extend the results of the linearized analysis, and reveal the presence of limiting flow types in some circumstances
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