Counterflow Velocity Research Articles

Model of separation performance of bilinear gradients in scanning format counter-flow gradient electrofocusing techniques.

Counter-flow gradient electrofocusing allows the simultaneous concentration and separation of analytes by generating a gradient in the total velocity of each analyte that is the sum of its electrophoretic velocity and the bulk counter-flow velocity. In the scanning format, the bulk counter-flow velocity is varying with time so that a number of analytes with large differences in electrophoretic mobility can be sequentially focused and passed by a single detection point. Studies have shown that nonlinear (such as a bilinear) velocity gradients along the separation channel can improve both peak capacity and separation resolution simultaneously, which cannot be realized by using a single linear gradient. Developing an effective separation system based on the scanning counter-flow nonlinear gradient electrofocusing technique usually requires extensive experimental and numerical efforts, which can be reduced significantly with the help of analytical models for design optimization and guiding experimental studies. Therefore, this study focuses on developing an analytical model to evaluate the separation performance of scanning counter-flow bilinear gradient electrofocusing methods. In particular, this model allows a bilinear gradient and a scanning rate to be optimized for the desired separation performance. The results based on this model indicate that any bilinear gradient provides a higher separation resolution (up to 100%) compared to the linear case. This model is validated by numerical studies.

Thermal Counterflow in a Periodic Channel with Solid Boundaries

We perform numerical simulations of finite temperature quantum turbulence produced through thermal counterflow in superfluid $^4$He, using the vortex filament model. We investigate the effects of solid boundaries along one of the Cartesian directions, assuming a laminar normal fluid with a Poiseuille velocity profile, whilst varying the temperature and the normal fluid velocity. We analyze the distribution of the quantized vortices, reconnection rates and quantized-vorticity production as a function of the wall-normal direction. We find that the quantized vortex lines tend to concentrate close to the solid boundaries with their position depending only on temperature and not on the counterflow velocity. We offer an explanation of this phenomenon by considering the balance of two competing effects, namely the rate of turbulent diffusion of an isotropic tangle near the boundaries and the rate of quantized-vorticity production at the centre. Moreover, this yields the observed scaling of the position of the peak vortex line density with the mutual friction parameter. Finally we provide evidence that upon the transition from laminar to turbulent normal fluid flow, there is a dramatic increase in the homogeneity of the tangle, which could be used as an indirect measure of the transition to turbulence in the normal fluid component for experiments.

An Experiment Design for Measuring the Velocity Field of a Round Wall Jet in Counter-Flow

In this paper the wall jet, and the problem of jet in counter-flow are introduced briefly and an experimental method for finding the velocity field of a round wall jet in counter-flow is presented. The jet to counter-flow velocity ratio is changed from 1.3 to 25. The measuring technique is two component particle image velocimetry (PIV) by using 4 cameras which provides a large field of view to cover the whole penetration depth of the wall jet at all conditions. The details of the experimental procedure and data extraction are discussed. To validate the accuracy of the test setup, the velocity field of the wall jet in quiescent ambient is obtained in a range of Reynolds number up to 10,000 and is compared with available data in the literatures. The mean velocity field and its main characteristics for the case of jet to counter-flow velocity ratio equal to 17.5 are discussed shortly.

Vortex line density in counterflowing He II with laminar and turbulent normal fluid velocity profiles

Superfluid helium is an intimate mixture of a viscous normal fluid, with continuous vorticity, and an inviscid superfluid, where vorticity is constrained to thin, stable topological defects. One mechanism to generate turbulence in this system is through the application of a heat flux, so-called thermal counterflow. Of particular interest is how turbulence in the superfluid responds to both a laminar and turbulent normal fluid in the presence of walls. We model superfluid vortex lines as reconnecting space curves with fixed circulation, and consider both laminar (Poiseuille) and turbulent normal fluid flows in a channel configuration. Using high resolution numerical simulations we show that turbulence in the normal fluid sustains a notably higher vortex line density than a laminar flow with the same mean flow rate. We examine Vinen's relation, \documentclass[12pt]{minimal}\begin{document}$\sqrt{L}=\gamma v_{ns}$\end{document}L=γvns, between the steady state vortex line density L and the counterflow velocity vns. Our results support the hypothesis that transition to turbulence in the normal fluid is responsible for the TI to TII transition. We also consider the spectral properties of fluctuations of the superfluid vortices, which show a good agreement with previous experimental results.

Mean Concentration Field of a Jet in a Uniform Counter-Flow

An experimental investigation of the scaling factors of mean scalar concentration field of jets issuing into a uniform counter-flow stream is presented. The centerline decay and radial spreading of the mean concentration field were measured by using planar laser induced fluorescence. Jet to counter-flow velocity ratios ranging between 4 to19 were investigated for two different jet diameters. The 5% contour of the mean concentration field of the jet was used to define new scaling factors that generate universal forms for the centerline concentration decay. The jet growth rate in the radial direction was found to be divided into two regions where a linear growth was observed and a region characterized by a power law. Empirical expressions are introduced which predict concentration decay in the established flow region in both the axial and radial directions.

Textures of Superfluid 3He-B in Applied Flow and Comparison with Hydrostatic Theory

Measurements of the order parameter texture of rotating superfluid 3He-B have been performed as a function of the applied azimuthal counterflow velocity down to temperatures of 0.2Tc. The results are compared to the hydrostatic theory of 3He-B. Good agreement is found at all measured temperatures and rotation velocities when the flow anisotropy contribution to the textural free energy is adjusted. This gives a superfluid energy gap Delta(T) which agrees with that measured by Todoshchenko et al., with Delta(0)=1.97kBTc at 29.0 bar. The B-phase susceptibility, longitudinal resonance frequency, and textural phase transition have been extracted from the measurements as a function of temperature and azimuthal counterflow velocity. Owing to decreasing absorption intensities the present measuring method, based on the line shape analysis of the NMR spectrum, loses its sensitivity with decreasing temperature. However, we find that in practice the measurement of vortex numbers and counterflow velocities is still feasible down to 0.2Tc.

Velocity-Concentration Joint Statistics for the Diffusion Field of Passive Scalar Substance in a Round Jet Discharging into Uniform Counter-Flow

Simultaneous measurements on the velocity and concentration fields of matter in a turbulent round jet injected into a counter stream have been conducted by using a combined system of PIV and PLIF techniques. Experiments were carried out for four cases of jet to counter-flow velocity ratios: Vr = 2.4, 3.4, 5.5 and 7.8. Axial profiles of the concentration statistics along the jet centerline showed that both rms concentration fluctuation and streamwise turbulent flux of mass have two peaks in the near field of the jet and in the mean stagnant region. The radial flux was found to have an tendency to increase in an off-axis area outside the dividing streamline due to a circumferential wandering of the jet about the axis. These features for the axial and radial mass fluxes appeared more obviously as Vr decreased. By investigating the joint PDF between axial velocity and concentration, it was confirmed that alternate streamwise transport of dyed fluid in the jet and ambient fluid in the counter-flow is related with the enhancement in the streamwise turbulent flux of mass in the stagnant region.

Controlled counter-flow motion of magnetic bead chains rolling along microchannels

We demonstrate controlled transport of superparamagnetic beads in the opposite direction of a laminar flow. A permanent magnet assembles 200 nm magnetic particles into about 200 μm long bead chains that are aligned in parallel to the magnetic field lines. Due to a magnetic field gradient, the bead chains are attracted towards the wall of a microfluidic channel. A rotation of the permanent magnet results in a rotation of the bead chains in the opposite direction to the magnet. Due to friction on the surface, the bead chains roll along the channel wall, even in counter-flow direction, up to at a maximum counter-flow velocity of 8 mm s−1. Based on this approach, magnetic beads can be accurately manoeuvred within microfluidic channels. This counter-flow motion can be efficiently be used in Lab-on-a-Chip systems, e.g. for implementing washing steps in DNA purification.

Step width, spacing, and resolution in gradient elution moving boundary electrophoresis. Part 1. Theory and comparison with zone electrophoresis

Gradient elution moving boundary electrophoresis (GEMBE) is a recently described technique for electrophoretic separations in short capillaries or microchannels. With GEMBE, the electrophoretic migration of analytes is opposed by a bulk counterflow of separation buffer. The counterflow velocity is varied over the course of a separation so that analytes with different mobilities enter the separation channel at different times and are detected as moving boundary, step-wise increases in the detector response. The resolution of a GEMBE separation is thus dependent on the rate at which the counterflow velocity is varied, and relatively high-resolution separations can be performed with short microfluidic channels or capillaries. In this paper, a theoretical description of GEMBE is presented that can be used for calculation of expected resolution and for optimization of GEMBE separations. A comparison is made with CZE, a conventional electrophoretic separation technique for which the theoretical understanding is mature. The results indicate that the electric field strength and separation channel length are important parameters for both CZE and GEMBE. However, with GEMBE, the counterflow acceleration is also available as a parameter that can be easily adjusted for optimization of the trade-off between resolution and separation time. This allows for optimization of GEMBE separations by making changes only to the software controlling the apparatus rather than to the hardware of the apparatus itself. Further comparison of the theoretical descriptions of GEMBE and CZE indicates that the time required to achieve a desired resolution is equivalent for the two techniques.

A probabilistic cellular automata model for highway traffic simulation

Abstract This work presents a probabilistic model for the microscopic simulation of traffic roads based on Nagel-Schreckenberg’s model. Each driver’s behavior is described through the combination of a continuous probability function with an anticipatory feature that leads to a counter flow velocity tunning. The simulations developed and described herein give rise to a phase diagram which resembles and enriches the fundamental diagram, in its theoretical as well as for real data.

Gradient Elution Moving Boundary Electrophoresis with Channel Current Detection

Gradient elution moving boundary electrophoresis (GEMBE) is a recently described technique for electrophoretic separations in short (1-3 cm) capillaries or microchannels. With GEMBE, the electrophoretic migration of analytes is opposed by a bulk counterflow of separation buffer through the separation channel. The counterflow velocity is varied over the course of a separation so that analytes with different electrophoretic mobilities enter the separation channel at different times and are detected as moving boundary, stepwise increases in the detector response. The resolution of a GEMBE separation is thus dependent on the rate at which the counterflow velocity is varied (rather than the length of the separation channel), and relatively high resolution separations can be performed with short microfluidic channels or capillaries. In this paper we describe an implementation of the GEMBE technique in which a very short (2.5-3.5 mm) capillary or microchannel is used as both the separation channel and a conductivity detection cell. Because the channel is so short, only a single moving boundary "step" is present in the channel at any given time, and the measured current through the channel can therefore be used to give a signal comparable to what is normally generated by more complicated detector arrangements. A theoretical description of the new technique is given along with simulation and experimental data relevant to the optimization of the method parameters such as channel length, counterflow acceleration, and applied field strength. A key theoretical prediction is that although this technique is expected to be a factor of 10 or 20 slower than conventional capillary zone electrophoresis, separation times of the order 1 s or less can still be achieved, making it applicable for ultrahigh-throughput analyses when implemented in a multiplexed format.

Simultaneous Concentration and Separation of Coumarins Using a Molecular Micelle in Micellar Affinity Gradient Focusing

We report the use of a molecular micelle for the simultaneous separation and concentration of neutral and hydrophobic analytes using micellar affinity gradient focusing (MAGF). The technique, MAGF, combines the favorable features of micellar electrokinetic chromatography and temperature gradient focusing. The focusing of neutral coumarin analytes was accomplished by the use the molecular micelle, poly(sodium undecenyl sulfate) (poly-SUS). Concentration enhancements of 10-25-fold/min were achieved for focusing of the coumarin dyes. The effect of varying the temperature gradient on the resolution of two of the coumarin dyes was also investigated, demonstrating that improved resolution could be achieved by reducing the steepness of the temperature gradient. In addition, with scanning-mode MAGF (in which the peaks are sequentially scanned past a fixed detection point by varying the buffer counterflow velocity), the use of poly-SUS was shown to produce repeatable and quantitative analyte peaks, making quantitative separations possible with the MAGF technique. Finally, it was shown that peak areas could be increased in scanning MAGF by reducing the scan rate so that the sensitivity of the method can be adjusted as needed.

Gradient Elution Moving Boundary Electrophoresis for High-Throughput Multiplexed Microfluidic Devices

A novel method for performing electrophoretic separations is described-gradient elution moving boundary electrophoresis (GEMBE). The technique utilizes the electrophoretic migration of chemical species in combination with variable hydrodynamic bulk counterflow of the solution through a separation capillary or microfluidic channel. Continuous sample introduction is used, eliminating the need for a sample injection mechanism. Only analytes with an electrophoretic velocity greater than the counterflow velocity enter the separation channel. The counterflow velocity is varied over time so that each analyte is brought into the separation column at different times, allowing for high-resolution separations in very short channels. The new variable of bulk flow acceleration affords a new selectivity parameter to electrophoresis analogous to gradient elution compositions in chromatography. Because it does not require extra channels or access ports to form an injection zone and because separations can be performed in very short channels, GEMBE separations can be implemented in much smaller areas on a micro-fluidic chip as compared to conventional capillary electrophoresis. Demonstrations of GEMBE separations of small dye molecules, amino acids, DNA, and immunoassay products are presented. A low-cost, polymeric, eight-channel multiplexed microfluidic device was fabricated to demonstrate the reduced area requirements of GEMBE; the device was less than 1 in.2 in area and required only n + 1 fluidic access ports per n analyses (in this instance, nine ports for eight analyses). Parallel separations of fluorescein and carboxyfluorescein yielded less than 3% relative standard deviation (RSD) in interchannel migration times and less than 5% RSD in both peak and height measurements. The device was also used to generate a calibration curve for a homogeneous insulin immunoassay using each of the eight channels as a calibration point with less than 5% RSD at each point with replicate analyses.

Energy Spectra of Developed Turbulence in Helium Superfluids

We suggest a “minimal model” for the 3D turbulent energy spectra in superfluids, based on their two-fluid description. We start from the Navier–Stokes equation for the normal fluid and from the coarse-grained hydrodynamic equation for the superfluid component (obtained from the Euler equation for the superfluid velocity after averaging over the vortex lines) and introduce a mutual friction coupling term, proportional to the counterflow velocity, the average superfluid vorticity and to the temperature dependent parameter q = α/(1 + α′), where α and α′ denote the dimensionless parameters characterizing the mutual friction between quantized vortices and the normal component of the liquid. We then derive the energy balance equations, taking into account the cross-velocity correlations. We obtain all asymptotical solutions for normal and superfluid energy spectra for limiting cases of small/big normal to superfluid density ratio and coupling. We discuss the applicability of our model to superfluid He II and to 3He-B.

Superfluid turbulence in rotating containers: Phenomenological description of the influence of the wall

In this paper a previous equation for the evolution of vortex line density L in counterflow superfluid turbulence in rotating containers is generalized, in order to take into account the influence of the walls. This model incorporates the effects of counterflow velocity V and of angular velocity {omega} of the container, and introduces corrective terms depending on {delta}/d, {delta} being the intervortex spacing, of the order L{sup -1/2}, and d the diameter of the channel. The stability of the solutions for L, for several regimes of averaged counterflow velocity V and angular velocity {omega}, is analyzed. Our mathematical analysis reveals that qualitative consistency allows us to reduce the four coefficients characterizing the dependence on {delta}/d to only one additional independent coefficient, linked to the critical angular velocity {omega}{sub c} needed for the appearance of vortex lines in a rotating superfluid.

Generalization of Vinen’s equation including transition to superfluid turbulence

A phenomenological generalization of the well known Vinen equation for theevolution of vortex line density in superfluid counterflow turbulence is proposed. Thisgeneralization includes nonlinear production terms in the counterflow velocity andcorrections depending on the diameter of the tube. The equation provides a unifiedframework for the various phenomena (stationary states and transitions) presentin counterflow superfluid turbulence: in fact, it is able to describe the laminarregime, the first-order transition from laminar to turbulent TI state, the twoturbulent states, the transition from TI to TII turbulent states, and it yields a slowerdecay of the counterflow turbulence than the classical local description. Finally, acomparison with the experimental results shows that the contribution of the new termsis prevalent in the laminar and in the turbulent TI regime, while in the fullydeveloped turbulent TII regime the equation reduces to the original Vinen equation.

Use of the particle image velocimetry technique to study the propagation of second sound shock in superfluid helium

Transient counterflow velocity fields induced by second sound shock in helium II have been measured using the particle image velocimetry (PIV) technique. The arrival of the shock front, passage of the shock tail, and onset of heat diffusion are clearly visualized as velocity profiles of suspended micron-size particles. Measured particle velocities are compared to calculated normal fluid velocities. Other observations include a critical energy flux for the onset of quantum turbulence and random motion of particles behind the shock. These results indicate that PIV may be a useful tool in the study of second sound and superfluid turbulence.

1403 一様流中の円形対向噴流に及ぼす噴出口形状の影響について

The influence of jet exit conditions on a round jet in a uniform counter-flowing stream has been investigated using LDV and PIV techniques. Three sets of different initial conditions were generated using three nozzle types : a smooth contraction (or contoured) nozzle, a sudden contraction (or orifice) nozzle and a non-contraction (or straight pipe) nozzle. Measurements were conducted for the jet to counter-flow velocity ratios from 1.5 to 6.5. Downstream variations of the axial mean and rms velocities were compared for three types of nozzles. The penetration distance for the straight nozzle was found to increase compared with those for the contoured and orifice nozzles.

1304 円形対向噴流による物質拡散場に対する噴流断面濃度場の PLIF 計測

The concentration field of a passive scalar in the cross plane of a round jet opposing a uniform water flow has been investigated using PLIF techniques. In order to capture images in good focus from a camera oriented obliquely to the object plane, the camera configuration has been designed according to the Scheimpflug condition and a water prism has been introduced to reduce the amount of radial distortions that arise when imaging through a water channel. A random time-variation in the centroid of instantaneous concentration field has been confirmed to exist at the cross plane of x/d=9 for the case of jet to counter-flow velocity ratio of V_r=5.1.

DESIGN AND TESTING OF A COMBINED CONDUCTION-CONVECTION ROTARY PADDY DRYER

ABSTRACT A rotary drum dryer prototype was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase moisture reduction rates. Ambient air forced inside the drum counter-flow to the direction of the cascading grains brought about “dryeration” of the hot grains, resulting in cooler grain output and increased moisture reduction rates. Its partial drying capacity doubled that of the benchmark pre-dryer at 5 rpm drum speed and quadrupled at 7 rpm, requiring only a single-pass operation. Tests using freshly harvested and re-wetted paddy showed that partial drying capacity, final moisture content and moisture reduction rate were all significantly affected by counter-flow air velocity, Its overall thermal efficiency was also 50% higher.