Secondary Velocity Components Research Articles

A heterogeneous ultrasound open scanner for the real-time implementation of computationally demanding imaging methods.

Ultrasound open scanners have recently boosted the development and validation of novel imaging techniques. They are usually split into hardware- or software-oriented systems, depending on whether they process the echo data using embedded FPGAs/DSPs or a GPU on a host PC. The goal of this work was to realize a high-performance heterogeneous open scanner capable of leveraging the strengths of both hardware and software-oriented systems. The elaboration power of the 256-channel ultrasound advanced open platform (ULA-OP 256) was further enhanced by embedding a compact co-processing GPU system-on-module (SoM). By carefully avoiding latencies and overheads through low-level optimization work, an efficient PCIe communication interface was established between the GPU and the processing devices onboard the ULA-OP 256. As a proof of concept of the enhanced system, the high frame rate color flow mapping technique was implemented on the GPU SoM and tested. Compared to a previous DSP-based implementation, higher real-time frame rates were achieved together with unprecedented flexibility in setting crucial parameters such as the ensemble length (EL). For example, by setting EL=64 and a continuous-time high-pass filter, the flow was investigated with high temporal and spatial resolution in the femoral vein bifurcation (frame rate = 1.1 kHz) and carotid artery bulb (4.3 kHz), highlighting the flow disturbances due to valve aperture and secondary velocity components, respectively. The results of this work promote the development of other computational-expensive processing algorithms in real-time and may inspire the next generation of ultrasound high-performance heterogeneous scanners.

Presentation and Performance of a Planar Solid Oxide Fuel Cell with Chaotic Gas Channels

Flow condition in gas channels determines the distribution and diffusion of reactants, governing electrochemical reactions in the solid oxide fuel cell (SOFC). It is found that the magnitude and direction of fluid velocity change with the geometric position in the channel, which leads to larger secondary flow velocity components and enhances mass transfer between fluids. Laser Doppler velocimetry experiment is conducted to verify the accuracy of numerical results of fluid flow in chaotic flow channels. Herein, a 3D numerical model with coupled multiphysics fields is built to evaluate fluid flow in channels and SOFC performance. The distribution characteristics of reactants in chaotic flow channel and the characteristics of temperature distribution, current density and power density in SOFCs caused by structures of straight and chaotic flow channels are analyzed and discussed. Compared with conventional straight flow channel SOFCs, the average current density of SOFC with chaotic flow channel is improved and the power density is increased by 9.7%. The reactant homogeneity in the electrode is also improved, which reduces the concentration polarization loss of SOFC.

MHD convective rotating flow of viscoelastic fluid past an infinite vertical oscillating porous plate with Hall effects

AbstractIt is considered the unsteady and incompressible magnetohydrodynamic rotating free convection flow of viscoelastic fluid with simultaneous heat and mass transfer near an infinite vertical oscillating porous plate under the influence of uniform transverse magnetic field and taking Hall current into account. The governing equations of the flow field are then solved by a regular perturbation method for a small elastic parameter. The expressions for the velocity, temperature, and concentration have been derived analytically and also its behavior is computationally discussed with reference to different flow parameters with the help of graphs. The skin friction on the boundary, the heat flux in terms of the Nusselt number, and the rate of mass transfer in terms of the Sherwood number are also obtained and their behavior discussed. The resultant velocity enhances with increasing Hall parameter and rotation parameter. The reversal behavior is observed with increasing viscoelastic parameters. The resultant velocity enhances and experiences retardation in the flow field with increasing radiation parameters, whereas the secondary velocity component increases with increasing rotation parameters. The temperature diminishes as the Prandtl number and/or the frequency of oscillations. The concentration reduces at all points of the flow field with the increase in the Schmidt number.

On multiphase wavy movements of non-Newtonian Jeffery fluid in a rotating channel with MHD and compliant walls: exact solutions

This paper discusses the peristaltic flow of magnetized Jeffrey fluid in a two-phase (liquid and dust) scenario inside a rotating planar channel executing compliant walls through the constraints of lubrication theory. The rotation of the channel considers the orthogonal direction axis. The present investigation is made to examine the impact of the magnetic field on the flow rates of the primary and secondary velocity components due to rotational effects. The fluid is electrically conducted by an inclined magnetic field and the rotating channel is assumed to perform the peristaltic wave stream along the vertical surfaces possessing various amplitudes and velocity profiles for dual phases. Exact solutions have been composed for fluid and particulate phases on Mathematica using the built-in tool DSolve. We examined that the velocity profile for the fluid phase gets smaller values than that of the particulate phase under the same parameters by plotting the graph. It is concluded from the obtained readings that the rotational effects and the magnetic field an inverse impact on the flow speed. This study will be applied to the attributes of rotating pumping devices used in medical equipment.

Hall and Rotation Effects on Radiating and Reacting MHD Flow past an Accelerated Permeable Plate with Soret and Dufour Effects

This article investigates numerically the effects of Hall current and rotation effects on radiating and chemically reacting unsteady MHD natural convection flow past an accelerated infinite vertical permeable plate in the presence of Soret and Dufour effects. The dimensionless coupled non-linear governing partial differential equations of the problem are solved numerically by employing finite element method. The influence of various physical parameters influencing the flow on the primary velocity, secondary velocity, temperature and the species concentration are displayed graphically whilst the numerical results of the primary skin friction, secondary skin-friction, Nusselt number and the Sherwood number are presented in tabular form. Results reveals that magnetic parameter, radiation parameter and chemical reaction rate tends to depreciate both primary and secondary velocity components whilst Hall, Soret and Dufour effects have reverse trend. Rotation parameter tends to retard fluid flow in the primary flow direction and accelerate fluid flow in the secondary flow direction. Thermal boundary layer thickness decreases with increasing radiation parameter whilst the reverse trend is noticed with increasing Dufour effect. Thermal diffusion effect causes to improve concentration boundary layer thickness whilst chemical reaction rate has reverse impact. These parameters have similar effect on the primary and secondary skin-frictions whilst opposite effect was noticed on the Nusselt and Sherwood numbers. This model problem finds an important in engineering and industrial application such as MHD generators, food processing, heat exchangers devices and internal rotation rate of the sun.
 HIGHLIGHTS
 
 The problem investigate the effects of Hall current and rotation effects on radiating and reacting on unsteady magnetohydrodynamics (MHD) natural convection heat and mass transfer flow over an infinite vertical porous plate embedded in a uniform porous medium taking Soret and Dufour effects into account
 The resulting partial differential equations governing the fluid flow are solved numerically using the finite element method. In order to determine the effects of various pertinent parameters and to investigate the important flow features, the numerical calculations for fluid velocity, temperature and species concentration are computed and shown graphically whereas skin friction, Nusselt number and Sherwood number at the plate are evaluated and depicted in tabular form
 The model problem finds an important in engineering and industrial application
 
 GRAPHICAL ABSTRACT

Numerical study on heat absorbing MHD radiating and reacting flow past an impulsively moving vertical plate with ramped temperature and concentration with Hall current

In this paper, we made elaborate investigation of the Hall current and radiation effects on MHD heat absorbing unsteady natural convection flow of a viscous incompressible electrically conducting fluid past an impulsively moving infinite vertical plate with ramped temperature and concentration in the presence of first order chemical reaction. The constructing coupled PDEs of the model along with initial and boundary conditions are solved numerically by utilizing the robust FEM. The effects of significant parameters on the primary and secondary velocities, temperature and concentration within the boundary layer are examined. The computational assessment of the primary and secondary shear stresses, rate of heat and mass transfer at the plate surface are put in tabular form. The thermal and mass buoyancy effects elevate both primary and secondary velocity components whilst reverse trend takes place when radiation and chemical reaction parameters are increased. Both primary and secondary velocity components decrease with increasing magnetic parameter whilst opposite trend are noticed with increasing Hall parameter. An increase in Prandtl number and radiation parameter results to reduce temperature distribution. The concentration distribution decreases with increase in Schmidt number and chemical reaction parameter. The significant difference between the fluid velocities is noticed due to the ramped and isothermal boundary conditions.

Homotopy and adomian semi-numerical solutions for oscillatory flow of partially ionized dielectric hydrogen gas in a rotating MHD energy generator duct

Hydrogen-based MHD power generators offer significant advantages over conventional designs. The optimization of these energy devices benefits from both laboratory scale testing and computational simulation. Motivated by this, in the current work, a mathematical model is developed for MHD pumping of partially ionized hydrogen in a rotating duct with oscillatory, Maxwell displacement and magnetic induction effects under an inclined static magnetic field. Perfectly electrically conducting duct walls are assumed. The non-dimensional conservation equations are solved using the power-series based Homotopy Analysis Method (HAM) with an appropriate embedding parameter. Detailed graphical visualization of the impact of emerging parameters on the non-dimensional primary and secondary velocity components (u,v) and magnetic induction components (bx,by) across the duct is presented. Average squared residual errors for all key variables εu,εv,εbx,εby with associated CPU times at various orders of the HAM iteration are also included. Validation with an Adomian Decomposition Method (ADM) is also conducted, and excellent agreement is obtained (tabulated). The computations have shown that with increasing inverse Ekman number strong damping is observed in the primary flow whereas the secondary flow is accelerated, in particular in the core region of the duct. With elevation in Maxwell displacement effect (for the case of a 45° inclined magnetic field i.e. θ=π/4 ) there is a strong decrease in primary magnetic induction at the lower wall of the duct and elevation in magnitudes at the upper duct wall; however, in the core region no tangible modification is computed. The opposite trend is observed for the secondary magnetic induction. With increasing magnetic Prandtl number (i.e. ratio of magnetic Reynolds number to ordinary Reynolds number) in the presence of strong Maxwell displacement current, strong magnetic field and high inverse Ekman number, the primary velocity is accelerated in both the left and right half space of the duct with a dip in magnitude at the centreline. However, the secondary velocity exhibits a much lower enhancement in both zones with only weak acceleration near the duct walls. Both velocity components achieve symmetrical distributions about the duct centreline. A significant depletion in primary magnetic induction is computed near the lower duct wall with enhancement near the upper duct wall; the contrary behaviour is exhibited by the secondary induced magnetic field. Applications of the study arise in hybrid rotating hydrogen based MHD energy generators and furthermore the computations provide a good basis for generalization to 3-dimensional flows with commercial multi-physical fluid dynamic codes e.g. ADINA-F, COMSOL, ANSYS FLUENT-Maxwell wherein further phenomena may be explored including Alfven wave effects and dielectric losses.

Magnetic Rotating Flow of a Hybrid Nano-Materials Ag-MoS2 and Go-MoS2 in C2H6O2-H2O Hybrid Base Fluid over an Extending Surface Involving Activation Energy: FE Simulation

Numeric simulations are performed for a comparative study of magnetohydrodynamic (MHD) rotational flow of hybrid nanofluids (MoS2-Ag/ethyleneglycol-water (50–50%) and MoS2-Go/ethyleneglycol-water (50–50%)) over a horizontally elongated plane sheet. The principal objective is concerned with the enhancement of thermal transportation. The three-dimensional formulation governing the conservation of mass, momentum, energy, and concentration is transmuted into two-dimensional partial differentiation by employing similarity transforms. The resulting set of equations (PDEs) is then solved by variational finite element procedure coded in Matlab script. An intensive computational run is carried out for suitable ranges of the particular quantities of influence. The primary velocity component decreases monotonically and the magnitude of secondary velocity component diminishes significantly when magnetic parameter, rotational parameter, and unsteadiness parameter are incremented. Both the primary and secondary velocities are smaller in values for the hybrid phase Ag-MoS2 than that of hybrid phase Go-MoS2 but the nanoparticle concentration and temperature are higher for hybrid phase Ag-MoS2. The increased values of parameters for thermophoresis, Brownian motion, shape factor, and volume fraction of ϕ2 made significant improvement in the temperature of the two phases of nano liquids. Results are also computed for the coefficients of skin friction(x, y-directions), Nusselt number, and Sherwood number. The present findings manifest reasonable comparison to their existing counterparts. Some of the practical engineering applications of the present analysis may be found in high-temperature nanomaterial processing technology, crystal growing, extrusion processes, manufacturing and rolling of polymer sheets, academic research, lubrication processes, and polymer industry.

Analysis and control of shear flow over a rotating plate in the presence of magnetic field

AbstractShear flow of an incompressible viscous electrically conducting fluid past a uniformly rotating infinite impermeable flat plate in the presence of transverse magnetic field is investigated. The flow is assumed to be governed by Navier‐Stokes equations augmented with an applied magnetic field. Solutions of the corresponding equations are analyzed. It is noted that due to shear flow over a rotating plate, the problem has no axisymmetric solutions. But it is interesting to note that this problem has no unique solution for the secondary velocity components. It is remarkable to note that Navier‐Stokes equations for this problem admit a one parameter family of solutions. Inside the boundary layer, the momentum transfer of the outer shear flow is dominated by the pressure gradient. The secondary velocity component along x‐axis is a linear combination of velocity due to shear and translation. However, it is noted that the contribution from the shear dominates. It is observed that the primary and secondary velocity components can be controlled by the applied magnetic field. It is also found that the torque exerted by the fluid on the plate is influenced by the applied magnetic field but independent of the shear in the external flow.

Study of the Couple Stress Convective Micropolar Fluid Flow in a Hall MHD Generator System

The steady non-isothermal convective heat transfer in magnetohydrodynamic micropolar fluid flow over a non-linear extending wall is examined. The fluid flow is treated with strong magnetic field. The influence of magnetic field, Hall current, viscous dissipation, and couple stress are mainly focused in this work. The fluid flow problem is solved by analytical method. The impact of developing dimensionless parameters on primary, secondary, and angular velocity components and temperature profile are determined through graphs. The greater magnetic parameter, local Grashof number, Hall parameter, power-law index, Prandtl number, nonlinear wall geometric parameter, and couples stress parameter reduce the primary velocity component while the material parameter, and Eckert number escalates the primary velocity component. The greater local Grashof number, material parameter, power-law index, Prandtl number, and wall geometric parameter reduces the secondary velocity component while the greater magnetic parameter, Hall parameter, Eckert number, and couple stress parameter escalates the secondary velocity component.

Investigations of Soret, Joule and Hall effects on MHD rotating mixed convective flow past an infinite vertical porous plate

We made an elaborate scrutiny on the Soret and Joule effects of MHD mixed convective flow of an incompressible and electrically conducting viscous fluid past an infinite vertical porous plate taking Hall effects into account. Perturbation technique is used to solve the non-dimensional equations. The effects of the various non-dimensional parameters on velocity, temperature and concentration within the boundary layer are examined. Besides that, computational deliberations or discussions are also undertaken on the effects of the pertinent or significant parameters on the skin-friction coefficient and rates of heat and mass transfer in terms of the Nusselt and Sherwood numbers respectively. The concentration distribution increases with increase in Soret effect and decrease with increase in chemical reaction parameter. An increase in Prandtl number results to decrease the temperature distribution. Both the primary and secondary velocity components and temperature increases with increasing heat source parameter. Skin friction coefficient decreases with an increase in permeability parameter, whereas it shows reverse effect for thermal and mass Grashof numbers. Nusselt number increases with an increase in Prandtl number. Sherwood number reduces with increasing Soret number.

Morphological and functional properties of the conducting human airways investigated by in vivo computed tomography and in vitro MRI.

The accurate representation of the human airway anatomy is crucial for understanding and modeling the structure-function relationship in both healthy and diseased lungs. The present knowledge in this area is based on morphometric studies of excised lung casts, partially complemented by in vivo studies in which computed tomography (CT) was used on a small number of subjects. In the present study, we analyzed CT scans of a cohort of healthy subjects and obtained comprehensive morphometric information down to the seventh generation of bronchial branching, including airway diameter, length, branching angle, and rotation angle. Although some of the geometric parameters (such as the child-to-parent branch diameter ratio) are found to be in line with accepted values, for others (such as the branch length-to-diameter ratio) our findings challenge the common assumptions. We also evaluated several metrics of self-similarity, including the fractal dimension of the airway tree. Additionally, we used phase-contrast magnetic resonance imaging (MRI) to obtain the volumetric flow field in the three-dimensional-printed airway model of one of the subjects during steady inhalation. This is used to relate structural and functional parameters and, in particular, to close the power-law relationship between branch flow rate and diameter. The diameter exponent is found to be significantly lower than in the usually assumed Poiseuille regime, which we attribute to the strong secondary (i.e., transverse) velocity component. The strength of the secondary velocity with respect to the axial component exceeds the levels found in idealized airway models and persists within the first seven generations. NEW & NOTEWORTHY We performed a comprehensive computed tomography-based study of the conductive airway morphology in normal human subjects, including branch diameter, length, and mutual angles. We found significant departure from classic homothetic relationships. We also carried out MRI measurements of the three-dimensional inspiratory flow in an anatomy-based model and directly assessed structure-function relationships that have so far been assumed. We found that strong secondary flows (i.e., transverse velocity components) persist through the first seven generations of bronchial branching.

Finite element computation of transient dissipative double diffusive magneto-convective nanofluid flow from a rotating vertical porous surface in porous media

This article aimed to investigate the transient dissipative magnetohydrodynamic double diffusive free convective boundary layer flow of electrically conducting nanofluids from a stationary or moving vertical porous surface in a rotating high permeability porous medium, considering buoyancy, thermal radiation and first-order chemical reaction. Thermo-diffusion (Soret) and diffuso-thermo (Dufour) effects are also considered. Darcy’s law is employed. The mathematical model is formulated by considering water-based nanofluids containing metallic nano-particles for both stationary and moving plate cases. Three nanofluids are examined, namely copper, aluminium oxide or titanium oxide in water. The transformed non-linear, coupled, dimensionless partial differential equations describing the flow are solved with physically appropriate boundary conditions using Galerkin weighted residual scheme. For prescribed permeability, numerical results are presented graphically for the influence of a number of emerging parameters. Validation of finite element solutions for skin friction and Nusselt number is achieved via comparison with the previously published work as special cases of the present investigation and very good correlation obtained. Increasing rotational parameter is observed to reduce both primary and secondary velocity components. Primary and secondary velocities are consistently elevated with increasing Soret, Dufour, thermal Grashof and solutal Grashof numbers. Increasing Schmidt number, chemical reaction and suction parameter both suppress nano-particle concentration whereas the converse behavior is computed with increasing Soret number. The study is relevant to high-temperature rotating chemical engineering systems exploiting magnetized nanofluids and also electromagnetic nanomaterial manufacturing processes.

EXTERNALLY HEATED PROTOSTELLAR CORES IN THE OPHIUCHUS STAR-FORMING REGION*

ABSTRACT We present APEX 218 GHz observations of molecular emission in a complete sample of embedded protostars in the Ophiuchus star-forming region. To study the physical properties of the cores, we calculate H2CO and c-C3H2 rotational temperatures, both of which are good tracers of the kinetic temperature of the molecular gas. We find that the H2CO temperatures range between 16 K and 124 K, with the highest H2CO temperatures toward the hot corino source IRAS 16293-2422 (69–124 K) and the sources in the ρ Oph A cloud (23–49 K) located close to the luminous Herbig Be star S1, which externally irradiates the ρ Oph A cores. On the other hand, the c-C3H2 rotational temperature is consistently low (7–17 K) in all sources. Our results indicate that the c-C3H2 emission is primarily tracing more shielded parts of the envelope whereas the H2CO emission (at the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino emission dominates. In some sources, a secondary velocity component is also seen, possibly tracing the molecular outflow.

Resonant oscillations of a gas in an open tube in the shock-free wave mode

Nonlinear gas oscillations excited in an open tube by a flat piston at one of the tube ends are studied. The sinusoidal piston oscillations in the shock-free wave mode are created by a vibration exciter near the first eigenfrequency. Expressions for gas pressure oscillations are obtained for a tube with a nonrounded end without a flange and secondary flow velocity components. The influence of the piston displacement amplitude on the pressure range and secondary flow velocity of gas is studied. The theoretical calculations of the gas pressure are compared with experimental data. An estimate for the velocity of particle motion along the tube axis is presented with calculated values of the secondary flow velocity.

A new perspective on the interstellar cloud surrounding the Sun from UV absorption line results

We offer a new, more inclusive, picture of the local interstellar medium, where it is composed of a single, monolithic cloud that surrounds the Sun in all directions. Our study of velocities based on Mg II and Fe II ultraviolet absorption lines indicates that the cloud has an average motion consistent with the velocity vector of gas impacting the heliosphere and does not behave like a rigid body: gas within the cloud is being differentially decelerated in the direction of motion, and the cloud is expanding in directions perpendicular to this flow, much like the squashing of a balloon. The outer boundary of the cloud is in average 10 pc away from us but is highly irregular, being only a few parsecs away in some directions, with possibly a few extensions up to 20 pc. Average H I volume densities vary between 0.03 and 0.1 cm3 over different sight lines. Metals appear to be significantly depleted onto grains, and there is a steady increase in this effect from the rear of the cloud to the apex of motion. There is no evidence that changes in the ionizing radiation influence the apparent abundances. Additional, secondary velocity components are detected in 60% of the sight lines. Almost all of them appear to be interior to the volume holding the gas that we identify with the main cloud. Half of the sight lines exhibit a secondary component moving at about - 7.2 km/s with respect to the main component, which may be the signature of an implosive shock propagating toward the cloud's interior.

A Study of Unsteady Rotating Hydromagnetic Free and Forced Convection in a Channel Subject to Forced Oscillation under an Oblique Magnetic Field

A theoretical analysis is presented for transient, fully-developed magnetohydrodynamic free and forced convection flow of a viscous, incompressible, Newtonian fluid in a rotating horizontal parallel-plate channel subjected to a uniform strength, static, oblique magnetic field acting at an angle  to the positive direction of the axis of rotation. A constant pressure gradient is imposed along the longitudinal axis of the channel. Magnetic Reynolds number is sufficiently small to negate the effects of magnetic induction. The channel plates are electrically non-conducting. The conservation equations are formulated in an (x,y,z) coordinate system and normalized using appropriate transformations. The resulting non-dimensional coupled ordinary differential equations for primary and secondary velocity components and transformed boundary conditions are found to be reciprocal of the Ekman number ( 2 K = 1/Ek), non-dimensional pressure gradient parameter (Px), Hartmann number ( 2

Particle motion in square channel flow of a viscoelastic liquid: Migration vs. secondary flows

The viscoelasticity-induced migration of a sphere in pressure-driven flow in a square-shaped microchannel is investigated under inertialess conditions. The effects of fluid rheology, i.e. of shear thinning and normal stresses, is studied by means of 3D finite element simulations. Two constitutive models are selected, in order to highlight differences due to rheological properties.A strong influence of the suspending fluid rheology on the migration phenomenon is shown, by particle trajectory analysis. When the second normal stress difference is negligible and, as a consequence, no secondary flows appear, the particle migrates towards the channel centerline or the closest corner, depending on its initial position. As shear thinning is increased, the center-attractive region is reduced, and the migration rate is faster. On the other hand, the existence of secondary flows, linked to the existence of a second normal stress difference, alters the migration scenario. The competition between the particle-wall hydrodynamic interactions, promoting the migration mechanism, and the secondary flow velocity components gives rise to further ‘equilibrium’ positions within the channel cross-section. Particles driven towards such positions trace out a spiral trajectory, following the vortex structure of the secondary flows. However, as the particle dimension is increased or the Deborah number is reduced, the cross-streamline migration velocity overcomes the secondary flow velocity. In this case, most of the particles are driven towards the channel centerline, i.e. a strong flow-focusing effect results.

Flow fields, bed shear stresses, and suspended bed sediment dynamics in bifurcations of a large river

Channel bifurcations associated with bars and islands are important nodes in braided rivers and may control flow partitioning and thus affect downstream confluences, as well as the formation and dynamics of bars. However, the morphodynamic processes associated with bar formation are poorly understood, and previous studies have largely concerned laboratory experiments, small natural streams, or numerical analyses with large Froude numbers, high slopes, and low Shields stresses. In these cases, the morphologic changes at bifurcations are relatively rapid, with predominant bed load transport and the suspended load playing a minor role. In this paper, the evolution of the flow structure and suspended bed sediment transport along four expansion‐diffluence units in the Rio Paraná, Argentina, are described. The Rio Paraná is a large multichannel river with a bed composed of medium and fine sands and possesses low Froude numbers and high suspended bed material transport. Primary and secondary flow velocity components were measured with an acoustic Doppler current profiler (ADCP) along the expansion‐diffluence units, and the backscatter signal of the ADCP was calibrated to allow simultaneous measurements of suspended bed sediment concentrations. The interactions between these variables show that the cores of primary flow velocity and suspended bed sediment concentration do not necessarily follow the thalweg at the bifurcation and that inertial effects on the suspended bed sediment may influence the morphodynamics of bar formation. It is suggested that changes in flow stage, as well as the presence of vegetation, may further increase the deposition of suspended bed sediment at the bar head. This study suggests that the ratio of suspended bed material to bed load is an important factor controlling the morphodynamics of bifurcations in large sand bed braided rivers.

Optimization of an electrokinetic mixer for microfluidic applications

This work is concerned with the investigation of the concentration fields in an electrokinetic micromixer and its optimization in order to achieve high mixing rates. The mixing concept is based on the combination of an alternating electrical excitation applied to a pressure-driven base flow in a meandering microchannel geometry. The electrical excitation induces a secondary electrokinetic velocity component, which results in a complex flow field within the meander bends. A mathematical model describing the physicochemical phenomena present within the micromixer is implemented in an in-house finite-element-method code. We first perform simulations comparable to experiments concerned with the investigation of the flow field in the bends. The comparison of the complex flow topology found in simulation and experiment reveals excellent agreement. Hence, the validated model and numerical schemes are employed for a numerical optimization of the micromixer performance. In detail, we optimize the secondary electrokinetic flow by finding the best electrical excitation parameters, i.e., frequency and amplitude, for a given waveform. Two optimized electrical excitations featuring a discrete and a continuous waveform are discussed with respect to characteristic time scales of our mixing problem. The results demonstrate that the micromixer is able to achieve high mixing degrees very rapidly.