Convection-diffusion interaction for oscillatory flow in a tapered tube

Transport of soluble material is analyzed for volume-cycle oscillatory flow in a tapered tube. The equations of motion are solved using a regular perturbation method for small taper angle and order unity amplitude over a range of the Womersley parameter. The transport equation is also solved by a regular perturbation method where uniform end concentrations and no wall flux are assumed. The time-averaged axial transport of solute is calculated for several tapered tubes. There is substantial modification of transport compared to the straight tube case and the results are interpreted with respect to pulmonary gas exchange.     

Light transmission through blood in oscillatory flow

Measurements were made of the intensity of light transmitted through various thicknesses of normal human blood confined between two parallel plane surfaces, one fixed and the other oscillating in its own plane. When the light propagation direction is perpendicular to the direction of shear flow the transmitted intensity contains a steady component and a dominant second harmonic of the oscillation frequency. For a thin layer of blood, the steady intensity is a minimum value when the oscillation amplitude produces unit strain. The second harmonic is very small at small strains, but increases rapidly near unit strain where it is approximately in phase with the strain. For thicker layers, the effects of viscoelastic shear waves reduce the size of the second harmonic and modify its phase. Changes in light transmission are interpreted by relating the optical density of the blood to the total amount of contact between red cells. In oscillatory flow at low strains (less than 1) cell-to-cell contact is reduced by disaggregation of cell groups, and light transmission decreases. Near unit strain, disaggregation becomes complete, cell alignment occurs, and light transmission is minimized. At higher strains cell-to-cell contact is increased by formation of aligned layers of compacted cells separated by parallel plasma layers, and light transmission increases.     

Protein refolding in an oscillatory flow reactor

We demonstrate that an oscillatory flow reactor is a viable reactor for protein refolding via direct dilution. The mixing characteristics of the oscillatory flow reactor are well described and controllable and, importantly, can be scaled-up to process scale without a loss of mixing efficiency. This makes the oscillatory flow reactor an attractive alternative to conventional stirred-tank reactors for process-scale renaturation.     

Gas transport during oscillatory flow in a network of branching tubes

A transport coefficient was measured for a range of oscillatory flow conditions in a branching network of tubes. Measurements were made both across the first generation of a three-generation network and the second generation of a four-generation network. The results for these two series of tests were similar, indicating that there was no significant effect due to the system boundaries. The results are cast in terms of an effective axial diffusion coefficient of the form (Formula: see text) where kappa is the molecular diffusivity, Vt is the local stroke volume (cc); and f is the oscillation frequency (Hz). These results are compared to those obtained by other investigators in branching systems of similar geometry. At low frequency, this result is found to be in approximate agreement with the steady flow result of Scherer, et al. [15]. This expression differs from the oscillatory flow results of Tarbell, et al. [19] for liquids, primarily in terms of the effects of oscillation frequency.     

Pressure excursions during oscillatory flow in a branching network of tubes

The pressure difference across individual branches of a four-generation network of branching tubes was measured with the objective of obtaining general laws to describe the pressure drop in the airways under conditions of oscillatory flow. Fourier decomposition showed that the pressure signals consisted of a dominant component at the excitation frequency (fundamental) and a first harmonic of smaller magnitude. For values of the ratio Re/alpha less than 200, the fundamental mainly represented fluid acceleration, whereas the first harmonic reflected the effects both of viscous dissipation and the change in total cross-sectional area between parent and daughter generations. For values of Re/alpha greater than 200, the magnitude of the fundamental was considerably larger than that due to fluid acceleration alone, suggesting the possibility of onset of turbulence in the branching network. These pressure measurements were applied to a simple model of the dog lung to predict total airway resistance. The results are found to be in substantial agreement with physiological measurements.     

Interaction of peristaltic flow with pulsatile flow in a circular cylindrical tube

The effect of pulsatile flow on peristaltic transport in a circular cylindrical tube is analysed. The flow of a Newtonian viscous incompressible fluid in a flexible circular cylindrical tube on which an axisymmetric travelling sinusoidal wave is imposed, is considered. The initial flow in the tube is induced by an arbitrary periodic pressure gradient. A perturbation solution with amplitude ratio (wave amplitude/tube radius) as a parameter is obtained when the frequency of the travelling wave and that of the imposed pressure gradient are equal. The interaction effects of periodic wall induced flow and periodic pressure imposed flow are visualized through the presence of substantially different components of steady and higher harmonic oscillating flow in the first order flow solution. Numerical results show a strong variation of steady state velocity profiles with boundary wave number and Reynolds number and a strong phase shift behaviour of the flow in the radial direction.     

Structural conformation of spidroin in solution: a synchrotron radiation circular dichroism study

Spider silk is made and spun in a complex process that tightly controls the conversion from soluble protein to insoluble fiber. The mechanical properties of the silk fiber are modulated to suit the needs of the spider by various factors in the animal's spinning process. In the major ampullate (MA) gland, the silk proteins are secreted and stored in the lumen of the ampulla. A particular structural fold and functional activity is determined by the spidroins' amino acid sequences as well as the gland's environment. The transition from this liquid stage to the solid fiber is thought to involve the conversion of a predominantly unordered structure to a structure rich in beta-sheet as well as the extraction of water. Circular dichroism provides a quick and versatile method for examining the secondary structure of silk solutions and studying the effects of various conditions. Here we present the relatively novel technique of synchrotron radiation based circular dichroism as a tool for investigating biomolecular structures. Specifically we analyze, in a series of example studies on structural transitions induced in liquid silk, the type of information accessible from this technique and any artifacts that might arise in studying self-assembling systems.     

Characteristics of glottis-induced turbulence in oscillatory flow: an empirical investigation.

Turbulence inducement from the glottis was scrutinized by employing an idealized model of the larynx and trachea for oscillatory flow conditions. The characterization of turbulence was achieved with the two-component velocity measurements of split-film probe anemometry and with the flow visualization of a smoke-wire technique. The apertures of two different (triangular and circular) shapes were utilized in the airway model to address the distinct effects of the triangular-shaped glottal aperture on the generation, development, and decay of turbulence. One of the salient turbulence characteristics for the triangular aperture case was found to be the relatively high turbulence levels around the center region (2r/D approximately 0) in conjunction with the asymmetric mean axial velocity across the frontal-rear (A-O-P) plane of the trachea at one tracheal diameter (x/D = 1) downstream from the glottis. The detailed turbulence properties such as the Reynolds shear stresses and turbulence intensities for the triangular aperture case differed significantly from those for the circular aperture case within a few tracheal diameters (x/D < 7) downstream from the apertures. The glottis-induced turbulence was incipient during the acceleration phase of inspiration and convected downstream with the traits of decaying turbulence.     

Entry flow in a circular tube of aortic arch dimensions

Steady flow within a uniform circular curved tube formed by two 90-deg elbows was studied as a function of psi, the angle between the planes of curvature of the two elbows. Boundary layer separation was found at two locations. The sites of these separation zones were observed to be essentially independent of psi while the Reynolds number at which separation was first detected was found to decrease as psi increased. The relation between separation and the pathogenesis of atherosclerosis is discussed. Secondary flow pattern was found to depend on psi and in some instances on Reynolds number as well.     

Developing pulsatile flow in a deployed coronary stent

A major consequence of stent implantation is restenosis that occurs due to neointimal formation. This patho-physiologic process of tissue growth may not be completely eliminated. Recent evidence suggests that there are several factors such as geometry and size of vessel, and stent design that alter hemodynamic parameters, including local wall shear stress distributions, all of which influence the restenosis process. The present three-dimensional analysis of developing pulsatile flow in a deployed coronary stent quantifies hemodynamic parameters and illustrates the changes in local wall shear stress distributions and their impact on restenosis. The present model evaluates the effect of entrance flow, where the stent is placed at the entrance region of a branched coronary artery. Stent geometry showed a complex three-dimensional variation of wall shear stress distributions within the stented region. Higher order of magnitude of wall shear stress of 530 dyn/cm2 is observed on the surface of cross-link intersections at the entrance of the stent. A low positive wall shear stress of 10 dyn/cm2 and a negative wall shear stress of -10 dyn/cm2 are seen at the immediate upstream and downstream regions of strut intersections, respectively. Modified oscillatory shear index is calculated which showed persistent recirculation at the downstream region of each strut intersection. The portions of the vessel where there is low and negative wall shear stress may represent locations of thrombus formation and platelet accumulation. The present results indicate that the immediate downstream regions of strut intersections are areas highly susceptible to restenosis, whereas a high shear stress at the strut intersection may cause platelet activation and free emboli formation.     

Not just a sponge: new functions of circular RNAs discovered

<正>One of the most exciting findings in RNA biology is the discovery of numerous circular RNAs(circ RNA)in mammalian genome.Once being considered as low abundance splicing byproducts,circ RNAs are surprisingly abundant     

Superhelical turns of closed circular DNA in solution and in a gel: evidence for a conformational difference

We have compared the number of superhelical turns, tau, in circular covalently closed plasmid pBR beta G DNA obtained by four different methods, each based on one particular distinguishing principle. Three of the methods allow an unequivocal determination of tau under gel electrophoresis conditions, whilst the fourth enables us to determine its value in solution. We were able to detect a significant difference between the two environments, corresponding to an unwinding of the DNA duplex angle by 0.3 degrees when a sample is transferred from solution to gel. The possible existence of such an effect has been generally overlooked by previous investigators. Our result suggests that the previously reported value for the number of base-pairs per helical turn should be adjusted downwards by about 0.10, so that it applies to conditions in solution.     

Monitoring in situ circular dichroism of the intact vitreous: a new approach

For the first time, an attempt has been made to study the vitreous humor in situ by circular dichroism (CD). The vitreous, an avascular and acellular gel-like tissue, is optically transparent and homogeneous, and, thus, light scattering is minimal. The macromolecular components of this tissue, hyaluronate (HA), collagen and noncollagenous protein (NC-P), appear to exist in the matrix in a nonoriented fashion. As a result, no linear dichroism was observed. A typical CD of the vitreous shows a minimum at 206 nm with a shoulder at 220 nm and one small positive peak at approximately 252 nm. Gaussian analysis resolves this spectrum into four component bands. CD analysis of individual components reveals that NC-P makes the major contribution to the dichroic strength of the vitreous; contributions of HA and collagen, on the other hand, are small. The positive peak arises largely from ascorbic acid in the vitreous. CD measurement of the intact vitreous appears to be a useful technique for assessing the structure and changes of the constituent molecules in the normal and diseased vitreous.     

The effect of a turbulent jet on gas transport during oscillatory flow

Axial mass transport due to the combined effects of flow oscillation and a turbulent jet was studied both experimentally and with a simple theoretical model. The experiments show that the distance over which turbulence enhances transport is greatly increased by flow oscillation, and is particularly sensitive to tidal volume. The jet flow rate and jet configuration are relatively less important. To analyze the results, the region influenced by the jet is divided into two zones: a near field in which the time-mean flow velocities are larger than the turbulent fluctuations, and a far field where the time-mean flow is essentially zero. In the far field, axial mass transport is increased due to the turbulence which decays in strength away from the jet. When oscillatory flow is superimposed upon the steady jet flow, the turbulence in the far field interacts with the flow oscillations to augment the transport of turbulence energy and of mass. This transport enhancement is modeled by introducing an effective axial diffusivity analogous to that used in laminar oscillatory flow.