Ratio of Λˉ/Λ\bar{\Lambda}/\Lambda in Semi-inclusive Electroproduction

It is shown that the $\bar{\Lambda}/\Lambda$ cross section ratio in semi-inclusive electroproduction of $\Lambda$ and $\bar{\Lambda}$ hyperons in deep inelastic scattering of charged lepton on a nucleon target, can provide useful information on the quark to $\Lambda$ fragmentation functions. This ratio is calculated explicitly in a quark-diquark model, a pQCD based analysis, and an SU(3) symmetry model, with three different options for the contribution from the unfavored fragmentation functions. The $x$-dependence of this ratio is sensitive to the ratio of unfavored fragmentation functions over favored fragmentation functions, $D_{\bar{u}}^{\Lambda}(z)/D_{u}^{\Lambda}(z)$, whereas the $z$-dependence is sensitive to the flavor structure of the fragmentation functions, i.e., the ratio $D_u^{\Lambda}(z)/D_s^{\Lambda}(z)$. Future measurements by the HERMES Collaboration at DESY can discriminate between various cases.Comment: 11 latex files, 6 figure

Extensional flow of Newtonian and Boger fluids through a flow focusing microdevice

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.In this work we perform a numerical and experimental study on the flow of Newtonian and viscoelastic fluids through a microfluidic device in which hydrodynamic flow focusing is produced using two balanced lateral sheath streams that shape a third inlet stream. The flow focusing device used was conceived to achieve a nearly constant extensional rate along the centerline and is shaped much like a conventional cross-slot except for comprising three inlets and one exit channel. The work undertaken comprises experimental flow visualization as well as 2D and 3D numerical calculations using a finite volume method. The Newtonian fluid used in the experiments was distilled water and the viscoelastic fluid was an aqueous solution containing 125 ppm (w/w) of polyacrylamide (PAA, Mw = 18×10(6) g/mol), to which 1% of salt (NaCl) was added. The addition of salt to a shear-thinning PAA solution resulted in a low viscosity Boger fluid, i.e., a viscoelastic fluid with a nearly constant viscosity. The combination of small length scales characteristic of microfluidics with this type of fluid allows us to observe strong elastic effects in the absence of shear-thinning and inertial effects (or at least when these effects are weak). We report a rich variety of flow features, which depend on the fluid used as well as on the operational conditions. Additionally, we show good qualitative agreement between the experimental observations and the numerical predictions.FCT, FEDER and COMPETE for financial support through projects REEQ/262/EME/2005, REEQ/928/EME/2005 and PTDC/EME-MFE/099109/2008

Particle-Antiparticle Asymmetries of Λ\Lambda Production in Hadron-Nucleon Collisions

The particle-antiparticle asymmetries of $\Lambda$ production in 250 GeV/c $\pi^{\pm}$, $K^{\pm}$, and $p$ --nucleon collisions are studied with two model parametrizations of quark to $\Lambda$ fragmentation functions. It is shown that the available data can be qualitatively explained by the calculated results in both the quark-diquark model and a pQCD based analysis of fragmentation functions. The differences in the two model predictions are significant for $K^{\pm}$ beams, and high precision measurements of the asymmetries with detailed $x_F$ and $P_T$ information can discriminate between different predictions.Comment: 14 LaTex pages, 4 figures, to appear in Phys. Lett.

On an elliptic Kirchhoff-type problem depending on two parameters

In this paper, we consider the Dirichlet problem associated to an elliptic Kirchhoff-type equation depending on two parameters. Under rather general and natural assumptions, we prove that, for certain values of the parameters, the problem has at least three solutions

Viscoelastic secondary flows in serpentine channels

AbstractWe report the results of a detailed numerical investigation of inertialess viscoelastic fluid flow through three-dimensional serpentine (or wavy) channels of varying radius of curvature and aspect ratio using the Oldroyd-B model. The results reveal the existence of a secondary flow which is absent for the equivalent Newtonian fluid flow. The secondary flow arises due to the curvature of the geometry and the streamwise first normal–stress differences generated in the flowing fluid and can be thought of as the viscoelastic equivalent of Dean vortices. The effects of radius of curvature, aspect ratio and solvent-to-total viscosity ratio on the strength of the secondary flow are investigated. The secondary flow strength is shown to be a function of a modified Deborah number over a wide parameter range

Boger fluid flow through hyperbolic contraction microchannels

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Boger fluids are characterized by their constant viscosity and elasticity and are very useful to study pure elastic flow behavior. In this paper we assess the potential of a microfluidic hyperbolic contraction as a device to measure the relaxation time of low viscosity polymer solutions, which are difficult to characterize in a conventional capillary break-up extensional rheometer. For this purpose we initially characterize the shear and extensional rheology of aqueous solutions of polyacrylamide (PAA) at different concentrations (400, 250, 125 and 50 ppm) with 1% (w/w) of NaCl, which result in low viscosity Boger fluids. Subsequently, flow visualizations of their flow through a microfluidic hyperbolic contraction were carried out in order to quantify the relation between their degree of elasticity and the vortex growth upstream of the microchannel.Fundação para a Ciência e a Tecnologia (FCT), COMPETE and FEDER through projects PTDC/ EQU-FTT/ 71800/2006, PTDC/EQUFTT/ 70727/2006, PTDC/EME-MFE/099109/ 2008, REEQ/928/EME/2005 and REEQ/298/EME/2005

Simulations of extensional flow in microrheometric devices

We present a detailed numerical study of the flow of a Newtonian fluid through microrheometric devices featuring a sudden contraction–expansion. This flow configuration is typically used to generate extensional deformations and high strain rates. The excess pressure drop resulting from the converging and diverging flow is an important dynamic measure to quantify if the device is intended to be used as a microfluidic extensional rheometer. To explore this idea, we examine the effect of the contraction length, aspect ratio and Reynolds number on the flow kinematics and resulting pressure field. Analysis of the computed velocity and pressure fields show that, for typical experimental conditions used in microfluidic devices, the steady flow is highly three-dimensional with open spiraling vortical structures in the stagnant corner regions. The numerical simulations of the local kinematics and global pressure drop are in good agreement with experimental results. The device aspect ratio is shown to have a strong impact on the flow and consequently on the excess pressure drop, which is quantified in terms of the dimensionless Couette and Bagley correction factors. We suggest an approach for calculating the Bagley correction which may be especially appropriate for planar microchannels

Microchannels analogues for the study of viscoelastic fluid flows through porous media

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.This work studies the flow behavior and related pressure losses of viscoelastic polymer solutions in microchannels with two different sequences of contraction/expansion, disposed in a symmetric and an asymmetric arrangement, respectively. These microfluidic devices are proposed as simplified microchannel analogues for the flow of Newtonian and viscoelastic fluids through porous media. The results show that the symmetric configuration mimics the pressure gradient of these polymer solutions through a porous medium at low flow rates (below a critical Deborah number, Decr), while the asymmetric arrangement gives the asymptotic limit at high De values (above Decr) as a consequence of the intrinsic differences in the extensional rate profiles defined by each microgeometry.Fundação para a Ciência e a Tecnologia (FCT), COMPETE and FEDER through projects PTDC/ EQU-FTT/ 71800/ 2006, PTDC/EQUFTT/ 70727/ 2006, PTDC/ EME-MFE/ 99109/ 2008 and REEQ/ 262/ EME/ 2005

A new viscoelastic benchmark flow: Stationary bifurcation in a cross-slot

AbstractIn this work we propose the cross-slot geometry as a candidate for a numerical benchmark flow problem for viscoelastic fluids. Extensive data of quantified accuracy is provided, obtained via Richardson extrapolation to the limit of infinite refinement using results for three different mesh resolutions, for the upper-convected Maxwell, Oldroyd-B and the linear form of the simplified Phan-Thien–Tanner constitutive models. Furthermore, we consider two types of flow geometry having either sharp or rounded corners, the latter with a radius of curvature equal to 5% of the channel’s width. We show that for all models the inertialess steady symmetric flow may undergo a bifurcation to a steady asymmetric configuration, followed by a second transition to time-dependent flow, which is in qualitative agreement with previous experimental observations for low Reynolds number flows. The critical Deborah number for both transitions is quantified and a set of standard parameters is proposed for benchmarking purposes

Influence of channel aspect ratio on the onset of purely-elastic flow instabilities in three-dimensional planar cross-slots

In this work, we perform creeping-flow simulations of upper-convected Maxwell and simplified Phan-Thien-Tanner fluids to study the purely-elastic steady bifurcation and transition to time-dependent flow in three-dimensional planar cross-slots. By analysing the flow in geometries with aspect ratios ranging from the near Hele-Shaw flow like limit, up to the very deep, two-dimensional limit, we are able to characterize the mechanism of the cross-slot bifurcation with significant detail. We conclude that the bifurcation mechanism is similar to a buckling instability, by which fluid is redirected via paths of least resistance, resulting in the emergence of peripheral stagnation points, above and below the central stagnation point. The intake of matter at the centre via the inlet axis is thus reduced, being compensated by fluid flowing through low resistance corridors along the central vertical axis, above and below the central point. Furthermore, we propose and locally compute a modified Pakdel-McKinley criterion, thereby producing a scalar stability field and suggesting emergent peripheral stagnation points also indirectly contribute to the onset of time-dependent flow. (c) 2015 The Authors. Published by Elsevier B.V