Multi-stream Flow Research Articles

Visualization of the Anisotropy of the Velocity Dispersion and Characteristics of the Multi-Velocity Continuum in the Regions of Multi-Stream Flows of Gas-Dust Media with Polydisperse Dust.

Collisionless media devoid of intrinsic stresses, for example, a dispersed phase in a multiphase medium, have a much wider variety of space-time structures and features formed in them than collisional media, for example, a carrier, gas, or liquid phase. This is a consequence of the fact that evolution in such media occurs in phase space, i.e., in a space of greater dimensions than the usual coordinate space. As a consequence, the process of the formation of features in collisionless media (clustering or vice versa, a loss of continuity) can occur primarily in the velocity space, which, in contrast to the features in the coordinate space (folds, caustics, or voids), is poorly observed directly. To identify such features, it is necessary to use visualization methods that allow us to consider, in detail, the evolution of the medium in the velocity space. This article is devoted to the development of techniques that allow visualizing the degree of anisotropy of the velocity fields of collisionless interpenetrating media. Simultaneously tracking the behavior of different fractions in such media is important, as their behavior can be significantly different. We propose three different techniques for visualizing the anisotropy of velocity fields using the example of two- and three-continuum dispersed media models. We proposed the construction of spatial distributions of eccentricity fields (scalar fields), or fields of principal directions of the velocity dispersion tensor (tensor fields). In the first case, we used some simple eccentricity functions for dispersion tensors for two fractions simultaneously, which we call surrogate entropy. In the second case, to visualize the anisotropy of the velocity fields of three fractions simultaneously, we used an ordered array (3-vector) of eccentricities for the color representation through decomposition in three basic colors. In the case of a multi-stream flow, we used cluster analysis methods to identify sections of a multi-stream flow (beams) and used glyphs to visualize the entire set of beams (vector-tensor fields).

On Visualization of the Anisotropy of the Dispersion of Velocities of Polydisperse Dust in a Gas-Dust Medium

A large number of publications have been devoted to studying the features of the flow in two-phase flows in a gas-dispersed flow with inertial dust particles (shock waves, jets, turbulence, regular structures such as plasma crystals). In recent years, the study of the behavior of various fractions of impurity particles in polydisperse dust mixtures, expressed in the spatial separation of their distributions, has been of increasing interest. Spatial variations of individual components of the dust mixture make it possible to diagnose the state of the carrier gas phase in those cases when the carrier phase is very rarefied and cannot be observed directly (interstellar gas). This work is a continuation of the authors’ work [14], in which an original algorithm was proposed based on the use of three-dimensional color space resources, which allows visualizing spatial distributions of concentrations of up to three fractions of a polydisperse mixture of dust particles simultaneously, as well as an entropy measure that allows visualizing the degree of polydispersity in a heterogeneous gas-and-dust environment. However, the structural features of the impurity phase (caustics, areas of segregation of dust particles by their size, mass, etc.) are not necessarily spatially combined with its dynamic features (collisionless shock waves, flow turning points, stagnation points, accumulation points). In this paper, we propose a method for visualizing the dynamic features of a polydisperse collisionless mixture of particles in a two-phase gas-dust medium by constructing spatial distribution maps of the particle velocity dispersion anisotropy regions. Strong anisotropy of the velocity dispersion corresponds to the region in which an intense multi-stream flow occurs. Dust is considered as a mixture of several fractions, each of which contains particles of the same type, while the particle sizes in different fractions are different. For each dust fraction, a field is constructed of the spatial distributions of the eccentricities of the velocity dispersion ellipses. To visualize the degrees of anisotropy of the velocity dispersion of two fractions of a polydisperse dust mixture at once, a specially selected entropy measure is proposed. The results of two-dimensional demonstration calculations of a turbulent gas-dust medium and the anisotropy map in the distribution of the velocity dispersion of an impurity dust component are presented.

A membraneless microfluidic fuel cell with continuous multistream flow through cotton threads

A membraneless microfluidic fuel cell with continuous multistream flow through cotton threads

Elastic Turbulence of Aqueous Polymer Solution in Multi-Stream Micro-Channel Flow.

Viscous liquid flow in micro-channels is typically laminar because of the low Reynolds number constraint. However, by introducing elasticity into the fluids, the flow behavior could change drastically to become turbulent; this elasticity can be realized by dissolving small quantities of polymer molecules into an aqueous solvent. Our recent investigation has directly visualized the extension and relaxation of these polymer molecules in an aqueous solution. This elastic-driven phenomenon is known as ‘elastic turbulence’. Hitherto, existing studies on elastic flow instability are mostly limited to single-stream flows, and a comprehensive statistical analysis of a multi-stream elastic turbulent micro-channel flow is needed to provide additional physical understanding. Here, we investigate the flow field characteristics of elastic turbulence in a 3-stream contraction-expansion micro-channel flow. By applying statistical analyses and flow visualization tools, we show that the flow field bares many similarities to that of inertia-driven turbulence. More interestingly, we observed regions with two different types of power-law dependence in the velocity power spectra at high frequencies. This is a typical characteristic of two-dimensional turbulence and has hitherto not been reported for elastic turbulent micro-channel flows.

Numerical modeling the dynamics of plasma flows interaction in magnetic field

On the basis of numerical simulation, the processes of whistler, Alfven and collisionless shock waves generation are investigated in the interaction of high-speed plasma flows in a magnetic field. The statement of the problem corresponds to the conditions of laboratory experiments at the KI-1 facility (ILP SBRAS) [1]. The numerical model is based on the kinetic description of the ion plasma component and the gas-dynamic approximation for magnetized electrons (hybrid model) [2]. To solve the Vlasov kinetic equation, the author’s modification of the particle method in the cell (PIC) is used. The structure of the generated perturbations is analyzed as a function of the plasma parameters and a magnetic field. The characteristics of shock waves for the regimes of a multistream flow formation and the mechanisms of ion acceleration in the front of a shock wave are studied.

Solution rheological parameters modulate calcium phosphate mineralization in a microfluidic device

Mineralization of calcium phosphate and other materials in vivo and in natural water sources occurs in solutions that are not stagnant, but are flowing. Flow conditions could influence solution mixing and, therefore, mineralization kinetics or mechanism. This work describes the design and characterization of a multi-stream parallel flow microfluidic device that allows for controlled solution mixing and indirect control of laminar flow by altering the microfluidic device width, shape, length, flow rate, and flow velocity. Measurement of solution mixing was accomplished using the protonation of quinine to produce a fluorescent molecule and the rate of calcium phosphate mineralization was monitored by optical microscopy and analysis with Image J software. Experiments were designed to hold the flow rate constant, allowing the solution velocity to vary and to hold the velocity constant, allowing the flow rate to vary. It was found that small changes in laminar flow conditions do not correlate to mineral growth, but solution velocity and flow rate have a substantial effect on calcium phosphate mineralization. AFM and SEM characterization of the mineral produced shows an amorphous material and varying degrees of mineralization possibly due to variation in supersaturation conditions across the solution mixing area. This microfluidic device and analysis procedure allows for improved study of mineralization and the effect of flow conditions relevant to those seen in biological settings.

Post-collapse perturbation theory in 1D cosmology – beyond shell-crossing

We develop a new perturbation theory (PT) treatment that can describe gravitational dynamics of large-scale structure after shell-crossing in the one-dimensional cosmological case. Starting with cold initial conditions, the motion of matter distribution follows at early stages the single-stream regime, which can, in one dimension, be described exactly by the first-order Lagrangian perturbation, i.e. the Zel'dovich solution. However, the single-stream flow no longer holds after shell-crossing and a proper account of the multi-stream flow is essential for post-collapse dynamics. In this paper, extending previous work by Colombi (2015, MNRAS 446, 2902), we present a perturbative description for the multi-stream flow after shell-crossing in a cosmological setup. In addition, we introduce an adaptive smoothing scheme to deal with the bulk properties of phase-space structures. The filtering scales in this scheme are linked to the next-crossing time in the post-collapse region, estimated from our PT calculations. Our PT treatment combined with adaptive smoothing is illustrated in several cases. Predictions are compared to simulations and we find that post-collapse PT with adaptive smoothing reproduces the power spectrum and phase-space structures remarkably well even at small scales, where Zel'dovich solution substantially deviates from simulations.

Quasi-Optical Terahertz Microfluidic Devices for Chemical Sensing and Imaging.

We first review the development of a frequency domain quasi-optical terahertz (THz) chemical sensing and imaging platform consisting of a quartz-based microfluidic subsystem in our previous work. We then report the application of this platform to sensing and characterizing of several selected liquid chemical samples from 570–630 GHz. THz sensing of chemical mixtures including isopropylalcohol-water (IPA-H2O) mixtures and acetonitrile-water (ACN-H2O) mixtures have been successfully demonstrated and the results have shown completely different hydrogen bond dynamics detected in different mixture systems. In addition, the developed platform has been applied to study molecule diffusion at the interface between adjacent liquids in the multi-stream laminar flow inside the microfluidic subsystem. The reported THz microfluidic platform promises real-time and label-free chemical/biological sensing and imaging with extremely broad bandwidth, high spectral resolution, and high spatial resolution.

Research and Develop on Series of LNG Coil-Wound Heat Exchanger

The research and development of LNG coil-wound heat exchanger (LCWHE) were discussed in view of the heat exchange in LNG field in petrochemical industry. The basic designing methods and the multi-stream heat exchange processes were illustrated by the cryogenic and high pressure crossing heat exchange equipments of LCWHE with multi-stream and multiphase flow, including Four-stream LCWHE in first stage, Three-stream LCWHE in second stage, Two-stream LCWHE in third stage, Multi-stream main LCWHE etc. A series of LCWHE with different mixed refrigerant and different applications were described. The winding structure characteristics and the working principle of the spiral pipe bundles were elaborated to give a reference for the scientific design and calculation of LCWHE in cryogenic field. The major researched directions and the critical scientific problems were forecasted.

Research and Develop on Series of Cryogenic Liquid Nitrogen Coil-Wound Heat Exchanger

The research and development situation of liquid nitrogen coil-wound heat exchanger were discussed in view of heat exchange in gas purification field in petrol-chemical industry. The basic designing methods and the multi-stream heat exchange process were illustrated by the cryogenic and high pressure crossing heat exchange equipments of liquid nitrogen coil-wound heat exchanger with multi-stream and multiphase flow, including Three-stream back-cooling heat exchangerin first stage, Four-stream back-cooling heat exchangerin second stage, Five-stream back-cooling heat exchangerin third stage, Multi-stream main back-coolingcoil-wound heat exchanger etc. A series of coil-wound heat exchangers with different mixed fluids and different applications were described. The winding structure characteristics and the work principles of the spiral pipe bundles were elaborated to give references for the scientific design and calculation of coil-wound heat exchanger in cryogenic field. The important research directions and the critical scientific problems were forecasted.

Modulating patterns of two-phase flow with electric fields

This paper describes the use of electro-hydrodynamic actuation to control the transition between three major flow patterns of an aqueous-oil Newtonian flow in a microchannel: droplets, beads-on-a-string (BOAS), and multi-stream laminar flow. We observed interesting transitional flow patterns between droplets and BOAS as the electric field was modulated. The ability to control flow patterns of a two-phase fluid in a microchannel adds to the microfluidic tool box and improves our understanding of this interesting fluid behavior.

High Throughput Assay of Diffusion through Cx43 Gap Junction Channels with a Microfluidic Chip

This paper describes a microfluidic-based assay capable of measuring gap-junction mediated dye diffusion in cultured cells. The technique exploits multistream laminar flow to selectively expose cells to different environments, enabling continuous loading of cells in one compartment while monitoring, in real time, dye diffusion into cells of a neighboring compartment. A simple one-dimensional diffusion model fit to the data extracted the diffusion coefficient of four different dyes, 5-(6)-carboxyfluorescein, 5-chloromethylfluorescein, Oregon green 488 carboxylic acid, and calcein. Different inhibitors were assayed for their ability to reduce dye coupling. The chip can screen multiple inhibitors in parallel in the same cell preparation, demonstrating its potential for high throughput. The technique provides a convenient method to measure gap junction mediated diffusion and a screen for drugs that affect gap junction communication.

Spatially Resolved Measurement of Dissolved Oxygen in Multistream Microfluidic Devices

Dissolved oxygen (DO) is an important parameter with significant effect on cellular development and function. Micron-scale laminar flow and hydrodynamic focusing provide ideal tools for the generation of controlled chemical micro-environments and their application as stimuli to cells. In this paper, we demonstrate the generation and characterization of multistream laminar flow and hydrodynamically focused sample streams with defined dissolved oxygen concentrations on chip. A solid-state oxygen sensor layer was integrated into PDMS-based microchannels and calibrated. Several combinations of sample and buffer streams with concentrations ranging from 0 to 34 mg/l DO were generated and measured for up to three independent parallel flow streams. In addition, diffusion-based stream broadening measured with the sensor was used to determine the coefficient of diffusion of O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the flow medium. The devices have the potential to provide novel insights into cell biology and improve the relevance of in vitro cell assays.

Numerical study of the effect of the channel and electrode geometry on the performance of microfluidic fuel cells

Using COMSOL Multiphysics 3.5, 3D numerical models of different microfluidic fuel cells have been developed in this paper to determine the effect of different modifications which have been implemented in the microfluidic fuel cell since its advent. These modifications include the channel geometry aspect ratio and electrode configuration, the third flow between the anolyte and catholyte in the channel (i.e., multi-stream laminar flow), and multiple periodically placed inlets. To be consistent with the convention, the output power of the device is normalized by the electrode surface area; however, the power density calculations are also performed through normalization by the device volume. It is shown that the latter method is more realistic and providing more information from the design point of view since the ultimate goal in designing the microfluidic fuel cell is to fabricate a compact, yet powerful device. Finally, a novel design of the microfluidic fuel cell with a tapered channel is suggested and compared to the non-tapered geometry through the polarization curves. The steps which have been taken in COMSOL to obtain these polarization curves are clearly and thoroughly explained. The Butler–Volmer equation was implemented to incorporate for the electrochemical reactions at the electrodes. The “Conductive Media DC” module, in COMSOL, is used to model the electric fields within the fuel cell. The concentration distributions of the reactant species are obtained using the “Incompressible Navier–Stokes” and “Convection and Diffusion” modules. Solving these equations together predicts the current density for given cell voltage values. The results demonstrate the cell voltage losses due to activation, ohmic and concentration overpotentials. It is shown that for a fixed value of the cell voltage (say 0.45 V), the fuel cell with multiple periodically placed inlets has the highest fuel utilization (i.e., 62.3%); while the “Simple square” geometry depicts 13.8% fuel utilization at this potential. Thus, the multiple-inlets design is particularly suitable for low-voltage applications which require high current. Also, the results of the tapered geometry proposed in this paper show that tapering the channel enhances the polarization curve comparing to the square cross-section geometry with extended electrodes. In essence, the fuel utilization of the “Extended square” geometry is increased from 15.4% to 57.6% by tapering the channel. This is due to the fact that the mixing region growth rate is restricted in the tapered geometry, and hence the electrodes on the top and bottom walls of the channel can be more extended toward the centre of the channel before the crossover occurs.

The origin of `Great Walls'

A new semi-analytical model that explains the formation and sizes of the `great walls' - the largest structures observed in the universe is suggested. Although the basis of the model is the Zel'dovich approximation it has been used in a new way very different from the previous studies. Instead of traditional approach that evaluates the nonlinear density field it has been utilized for identification of the regions in Lagrangian space that after the mapping to real or redshift space (depending on the kind of structure is studied) end up in the regions where shell-crossing occurs. The set of these regions in Lagrangian space form the progenitor of the structure and afterthe mapping it determines the pattern of the structure in real or redshift space. The particle trajectories have crossed in such regions and the mapping is no longer unique there. The progenitor after mapping makes only one stream in the multi-stream flow regions therefore it does not comprise all the mass. Nevertheless, it approximately retains the shape of the structure. The progenitor of the structure in real space is determined by the linear density field along with two non-Gaussian fields derived from the initial potential. Its shape in Eulerian space is alsoaffected by the displacement field. The progenitor of the structure in redshift space also depends on these fields but in addition it is strongly affected by two anisotropic fields that determine the pattern of great walls as well as their huge sizes. All the fields used in the mappingsare derived from the linear potential smoothed at the current scale of nonlinearity which isRnl = 2.7 h−1 Mpc for the adopted parameters of the ΛCDM universe normalized to σ8 = 0.8. The model predicts the existence of walls with sizes significantly greater than 500 h−1 Mpcthat may be found in sufficiently large redshift surveys.

Generation of complex concentration profiles by partial diffusive mixing in multi-stream laminar flow

This paper proposes novel microfluidic concentration gradient generator (CGG) devices that are capable of constructing complex profiles of chemical concentrations by laterally combining the constituent profiles (e.g., linear and bell-shaped) generated in simple Y- or psi-shaped mixers. While the majority of currently existing CGG devices are based on complete mixing of chemical species, our design harnesses partial diffusive mixing in multi-stream laminar flow, and hence, features simple network structures and enhanced device reliability. An iterative simulation approach that incorporates our previous system-level models of CGG networks is developed to locate best-matched combinations of geometrical and operating parameters (e.g., inlet flow rates and inlet sample concentrations) for the device design. Microfluidic CGG chips are fabricated and experimentally characterized using optimal layout and operating conditions selected by the design process. The experimental results not only serve as a benchmark for model verification but also establish the feasibility of concentration gradient generation based on partial mixing for a variety of microfluidic applications.

A multi-stream flow technique to obtain isotropic texture in extruded thermotropic liquid crystalline polymer melts

In this article, we present a simple method toward achieving global orientation isotropy in a capillary-extruded thermotropic liquid crystal polymer (TLCP), that is, a random copolyester of 73 mol% hydroxybenzoic acid (HBA) and 27 mol% hydroxynaphthoic acid (HNA), by passing it through porous, flow altering metal foam inserts placed in the pathway of extrusion. TLCP melt flow through foam metal inserts causes a rapid creation of large number of small streams of varying cross-section that subjects all regions of the flowing body to intense shearing leading to high orientation in individual streams. These individual streams of varying velocity and direction interact to form a structure similar to a woven rope. As a direct result of this randomization effect of these small streams, the extruded liquid crystal polymer tends toward isotropic macro properties. The degree of orientation anisotropy is evidenced by scanning electron microscopy (SEM) and micro beam wide angle x-ray diffraction (WAXD) techniques and contrasted in samples extruded with and without the presence metal foam inserts. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers

Conditional velocity statistics in the double scalar mixing layer – A mapping closure approach

In this work we use 3D direct numerical simulations (DNS) to investigate the average velocity conditioned on a conserved scalar in a double scalar mixing layer (DSML). The DSML is a canonical multistream flow designed as a model problem for the extensively studied piloted diffusion flames. The conditional mean velocity appears as an unclosed term in advanced Eulerian models of turbulent non-premixed combustion, like the conditional moment closure and transported probability density function (PDF) methods. Here it accounts for inhomogeneous effects that have been found significant in flames with relatively low Damköhler numbers. Today there are only a few simple models available for the conditional mean velocity and these are discussed with reference to the DNS results. We find that both the linear model of Kutznetzov and the Li and Bilger model are unsuitable for multi stream flows, whereas the gradient diffusion model of Pope shows very close agreement with DNS over the whole range of the DSML. The gradient diffusion model relies on a model for the conserved scalar PDF and here we have used a presumed mapping function PDF, that is known to give an excellent representation of the DNS. A new model for the conditional mean velocity is suggested by arguing that the Gaussian reference field represents the velocity field, a statement that is evidenced by a near perfect agreement with DNS. The model still suffers from an inconsistency with the unconditional flux of conserved scalar variance, though, and a strategy for developing fully consistent models is suggested.

Direct numerical simulations of the double scalar mixing layer: Part II: Reactive scalars

The reacting double scalar mixing layer (RDSML) is investigated as a canonical multistream flow and a model problem for simple piloted diffusion flames. In piloted diffusion flames, the reacting fuel and oxidizer streams are initially separated by a central pilot stream at stoichiometric composition. The primary purpose of this pilot is to delay the mixing of the pure streams until a stable flame base can develop. In such multistream systems, the modeling of turbulent scalar mixing is complicated by the multiple feed streams, leading to more complex fine-scale statistics, which remain as yet an unmet modeling challenge compared to the simpler two-feed system. In Part I we described how multimodal mixture fraction probability density functions (PDFs) and conditional scalar dissipation rates can be modeled with a presumed mapping function approach. In this work we present an efficient and robust extension of the modeling to a general multistream reacting flow and compare predictions to three-dimensional direct numerical simulations (DNS) of the RDSML with a single-step reversible chemistry model and varying levels of extinction. With high extinction levels, the interaction with the pilot stream is described. Additionally, state-of-the-art combustion modeling calculations including conditional moment closure (CMC) and stationary laminar flamelet modeling (SLFM) are performed with the newly developed mixing model. Excellent agreement is found between the DNS and modeling predictions, even where the PDF is essentially a triple-delta shape near the flame base, so long as extinction levels are moderate to low. The suggested approach outlined in this paper is strictly valid only for flows that can be described by a single mixture fraction. For these flows the approach should provide engineers with fine-scale models that are of accuracy comparable to those already available for binary mixing, at only marginally higher complexity and cost.

Characterization of microfluidic fuel cell based on multiple laminar flow

This paper discusses a membraneless microfluidic fuel cell using the multi-stream laminar flow to keep the fuel and oxidant streams separated when they are flowing at a low Reynolds number state. The system we used consists of a microfluidic channel with three inlets. The fuel, oxidant, and the electrolyte solution enter the channel from the inlets and continue to flow in parallel without turbulent mixing (diffusive exchange occurs across the interface at a micro scale). During the working process of the fuel cell, controllable output potential and current of the fuel cell can be obtained and studied by changing the flow rate of the stream between the fuel and the oxidant. Results indicate that this novel design can be used in microscopic scale power source systems such as the lab-on-chip control systems because it is easy to integrate with other micro functional systems.