Main Channel Flow Research Articles

Investigation of equal and opposing flow junctions

This paper presents an experimental study of a 90° channel junction with equal and opposing flow in the upstream main channel. The aim of the study is to predict the size of the recirculation zones in the downstream branch channel that are formed as the flow makes a 90° turn after colliding and the associated energy loss. Detailed three-dimensional velocity profiles are measured with an acoustic Doppler velocimeter in the upstream main channel and downstream branch channel for different width ratios of the main and branch channels and inlet Froude numbers. The velocity profiles are depth-averaged to determine the maximum width of the recirculation zones in the branch channel. The velocity and flow depth distributions in the main and branch channels near and within junctions are dependent on inlet Froude number and channel width ratio. An analytical model based on mass, momentum and energy principles is developed and used to predict the maximum width of the recirculation zones. The maximum relative error in the predicted maximum width of the separation zone is less than 6·5%.

Determination of apparent and composite friction factors for flooded equatorial natural rivers

This paper presents results, calculated from field measurements taken in several frequently flooded natural rivers, which include D and R relationships, variation of flow resistance with depth of flow, the apparent friction factor, and the composite friction factor for flooded natural rivers. The results obtained have shown the complexity of flow resistance in natural rivers due to the interaction between the main channel and floodplain flow. The interaction has given rise to a pair of apparent shear stresses at the interface region, which can significantly reduce the discharge capacity of the rivers. The apparent shear was quantified in terms of an apparent friction factor, fa , and it was found that the apparent shear stress is many times greater than the averaged boundary shear stress of the rivers. Based on the averaged boundary shear stress and apparent shear stress, the composite (actual) friction factor for the rivers can be estimated accurately (R2 = 0.99) using a statistical method that had been derived.

Experimental Study on Turbulent Flow and Mixing in Counter-Flow Type T-Junction

An experimental study was conducted on turbulent flow and mixing in a counter-flow type T-junction. We measured the velocity and concentration fields simultaneously by combining PIV and PLIF. Special attention was directed to the concentration fluctuation near the channel wall, which might bring about the high-cycle thermal fatigue in case of mixing of hot and cold flows. The velocity ratio of the counter-channel flow to the main-channel flow was changed from 1.0 to 5.0. The fluorescent dye was mixed in the main-channel flow. The dominant structures of the fluctuating velocity and concentration fields that cause the concentration fluctuation near the channel wall were analyzed by POD. It was found that the concentration fluctuation near the channel wall was caused by the superposition of the spanwise wobbling motion of the mixing interface of two flows and the rotational oscillation of the flows.

Continuous separation of cells by balanced dielectrophoretic forces at multiple frequencies

We present a particle-sorting device based on the opposition of dielectrophoretic forces. The forces are generated by an array of electrode chambers located in both sidewalls of a main flow channel. Particles with different dielectric response perceive different force magnitudes and are therefore continuously focused to different streamlines in the flow channel. We relate the particles' dielectric response to their output position in the downstream channel. We demonstrate the performance of the device by separating a mixed yeast cell population into pure fractions of viable and nonviable cells. Finally, we use the device to enrich red blood cells infected with Babesia bovis, a major pathogen in cattle and simultaneously confirm the hypothesis that infection with B. bovis causes significant changes in the dielectric response of red blood cells.

Current Measurements in Rivers by Spaceborne Along-Track InSAR

The global monitoring of river discharges is a technologically challenging problem, with important applications in a variety of disciplines. Due to the limited availability and/or quality of river runoff data from many regions, an increasing use of remote sensing techniques is highly desirable. Altimeter specialists have already demonstrated water level retrievals in rivers from available data. The along-track interferometric synthetic aperture radar [along-track InSAR (ATI)] capabilities of state-of-the-art imaging radars on satellites such as the German TerraSAR-X, which was launched on June 15, 2007, also permit high-resolution line-of-sight surface current measurements. In this paper, we evaluate the potential of current measurements in rivers by spaceborne ATI on the basis of fundamental theoretical considerations, existing spaceborne InSAR data from the shuttle radar topography mission (SRTM), and simulated TerraSAR-X data. We show that an SRTM-derived line-of-sight surface current field in the Elbe river, Germany, agrees well with numerical hydrodynamic model results. The data quality is sufficient to resolve characteristic lateral variations of the currents in the river around a pronounced main flow channel. Assuming that the flow direction is usually aligned with the river bed, even a quasi-2D total surface current field can be derived. Simulations indicate that TerraSAR-X was even better suited for current measurements in rivers. Depending on width, surface roughness, and relative flow direction of a river, current estimates with an accuracy better than 0.1 m/s were possible with an effective spatial resolution of a few hundred meters to kilometers.

The hydraulic separator Multidune: Preliminary tests on fluid-dynamic features and plastic separation feasibility

Recycling of plastic materials is a rapidly developing discipline because of environmental awareness, the need to conserve materials and energy, and the growing demand to increase the production economy. The main problem in plastics recovery and recycling is related to the variety of plastic wastes, even if selective collection occurs. Therefore, plastic materials can be recycled either as mixtures or as single types, separating the different typologies by their physical (size, specific mass, etc.) and/or chemical properties. However, separation of plastics in single typologies by traditional processes and devices is difficult due to their typical low variability in properties. This paper presents a new research development for recycling industry: the Multidune separator. This is a device constructed from a sequence of parallel semi-cylindrical tubes of transparent plastic welded together in a plane. The lower half is shifted laterally and then fixed relative to the upper half. Flow is then induced in the lateral direction normal to the axis of the tubes, creating a main flow channel and two recirculation zones. This apparatus creates a differential transport of particles of low specific mass, near to 1g/cm3, allowing their separation. The flow field in the Multidune separator is studied via Particle Tracking Velocimetry (PTV). Eulerian analysis of the data is performed to gather information about the fluid-dynamics features established by different hydraulic heads at the inlet of the Multidune. Preliminary tests on monomaterial samples have been performed, varying several operative parameters to determine the best set of values. Therefore, separation tests have been executed on composite samples, obtaining satisfactory results in terms of plastic separation feasibility.

A microfluidic chaotic mixer using ferrofluid

A novel micromixer which utilizes chaotic mixing induced by ferrofluid actuation is developed. The micromixer is fabricated by a polydimethylsiloxane micromolding technique. The micromixer consists of a T-shaped main mixing channel and two parallel sub-channels that intersect the main channel. Oscillation of a couple of ferrofluid slugs in the sub-channels, induced by external permanent magnet actuation, generates chaotic advection in the main channel flow. To visualize the mixing, red fluorescent particles are supplied to one of the inlets of a T-shaped channel, and mixing flow is observed by a fluorescence microscope and an intensified CCD camera. The mixing experiments are performed with various ferrofluid perturbation frequencies and main stream flow velocities. The optimal mixing conditions determining the upper and lower limits of the most effective Strouhal numbers are found from the experimental results.

A tunable microflow focusing device utilizing controllable moving walls and its applications for formation of micro-droplets in liquids**Preliminary results of the current paper were presented at the 2007 IEEE International Conference of Nano/Micro Engineered and Molecular Systems (IEEE NEMS 2007), Bangkok, Thailand, 16–19 January, 2007.

This study reports a new microfluidic device capable of fine-tuned sample-flow focusing and generation of micro-droplets in liquids by controlling moving wall structures. Two microfluidic components including an ‘active microchannel width controller’ and a ‘micro chopper’ can be used to fine-tune the width of the hydrodynamically pre-focused stream and subsequently generate micro-droplets. In this study, a basic concept of a ‘controllable moving wall’ structure was addressed and applied as the active microchannel width controller and the micro chopper to generate the proposed function. Pneumatic side chambers were placed next to a main flow channel to construct the controllable moving wall structures. The deformation of the controllable moving wall structure can be generated by the pressurized air injected into the pneumatic side chambers. The proposed chip device was fabricated utilizing polymer material such as PDMS (polydimethylsiloxane) to provide the flexibility of the controllable moving wall deformation. The microfluidic chip device with dimensions of 2.5 cm in width and 3.0 cm in length can be fabricated using a simple fabrication process. Experimental data showed that the deformation of the controllable moving wall structure can be adjusted by applying different air pressures, so that the width of the main flow channel can be controlled accordingly. By utilizing the proposed mechanism, the pre-focused dispersed phase stream could be actively focused into a narrower stream, and well-controlled micro-droplets with smaller diameters could be generated. The stream width can be reduced from 30 µm to 9 µm and micro-droplets with a diameter of 76 µm could be generated by utilizing the proposed device. In addition, to generate micro-droplets within smaller diameters, uniform size distribution of the micro-droplets can be obtained. According to the experimental results, development of the microfluidic device could be promising for a variety of applications such as emulsification, nano-medicine and droplet-based microfluidics.

Comparison of modelling approaches used in practical flood extent modelling

Discharge predictions using four codes suitable to modelling flood extent are compared to predictions derived using theoretical and empirical methods.The model codes vary in complexity but represent the types of code used in practical application due to their easy integration with available GIS dataformats. Common to the codes is that turbulent momentum interaction between main channel and over-bank floodplain flow is not explicitly takeninto account. It is shown for different configurations of a hypothetical channel that the codes significantly overestimate discharge capacity as a result.Applying these codes to flood extent modelling therefore requires calibration of roughness values to account for this additional momentum transfer.resulting in roughness factors derived no longer representing physical friction values only, but combined friction and momentum correction factors.As a consequence of calibration being required even in more complex models where turbulence is explicitly modelled, these simpler models that areeasier in application may, however, be equally viable.

Study of Flow Over Side Weirs Under Supercritical Conditions

The present study focuses on the application of momentum principle to the analysis of spatially varied flow under supercritical conditions. Experimental studies were conducted on rectangular side weirs of different lengths and sill heights fitted in the test section of a rectangular aluminium channel that was built in a tilting flume. Measurements of discharges in the main channel and through the side weir were done separately. A pitot tube with direction finder was used to determine the velocities and angle of spill flow with the side weir. Depths of flow were measured both in longitudinal and transverse directions at regular intervals and their profiles were studied. Experiments were conducted with different test plates and flow conditions in the main channel. Coefficients of discharge were computed using momentum principle for different Froude numbers (between 1.5 and 3). The variation of discharge coefficient of the side weir as a function of Froude number was found to exhibit a non-linear relationship. Discharges over side weirs were computed using the computed coefficients of discharge for different Froude numbers and it was verified with the observed discharges. Coefficients of discharge were also computed using energy principle for different Froude numbers. Chi-square test was done between observed discharges over side weirs and discharges computed using momentum and energy principles, it was found that momentum principle is fitting better. Variation of the ratios of longitudinal components of velocity vector of spill flow to the mean velocity of the main channel flow at upstream end of the side weir with Froude number was found to exhibit a non-linear relationship. Variation of the discharge ratios of spill flow and main channel discharges with Froude number was also studied.

The performance of miniature metallic PEM fuel cells

A prototype of metallic PEM fuel cell with thin stainless steel bipolar plates was tested for their potential applications in portable electronic products. The flow field pattern was grown from the stainless steel plates by the electroforming process. The main flow channel has the dimensions of 300 μm (width) × 300 μm (depth). The dimensions of the micro-features were 100 μm width × 50 μm depth and 50 μm width × 50 μm depth. The material of the electroformed flow field pattern is nickel. A prototype of a single cell with total thickness of 2.6 mm, overall reaction area of 4 cm 2 and bipolar plate area of 16 cm 2 was assembled for this study. In order to improve its corrosion resistance, the bipolar plates were coated with 5 μm thick of multi-layered corrosion resistant material. In this study, indices of cell performance, with and without micro-features, of flow field concentration distribution, water-draining capability were studied by both numerical simulation and experimental measurement. The results show that the micro-features will provide a more uniform gas concentration environment in the bipolar plate flow field. Also, the bipolar plate with micro-features could greatly improve cell-draining capability. The results of cell performance tests show that the cell average power density reaches 195 mW cm −2. The long-term tests verify their performance are very stable.

Channel formation by flow stripping: large‐scale scour features along the Monterey East Channel and their relation to sediment waves

AbstractThe Monterey East system is formed by large‐scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour‐shaped depressions ranging from 3·5 to 4·5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude‐supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude‐supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream‐migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net‐erosional or net‐depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream‐migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.

Sediment Transport in River Models with Overbank Flows

Some laboratory sediment-transport experiments are described in which a compound channel with a mobile-bed composed of uniform sand with a d50 of 0.88mm was subjected to overbank flows. The main river channel was monitored to determine the effect of floodplain roughness on conveyance capacity, bed-form geometry, resistance, bed-load transport, and dune migration rate. The floodplain roughness was varied to simulate a wide range of conditions, commensurate with conditions that can occur in a natural river. For a given discharge, the main river channel bed was found to adjust itself to a quasi-equilibrium condition governed by the lateral momentum transfer between the floodplain and main channel flows and the local alluvial resistance relationship appropriate for the proportion of total flow in the main river channel. The sediment transport rate was found to reflect all these influences. The data are summarized in equation form for comparison with other experimental studies and for checking numerical river simulation models.

Discharge Ratio of Side Outflow to Supercritical Channel Flow

This study presents the effects of width changes of a side breach, Froude numbers, and bottom slopes on discharge ratios of a side outflow to the main channel flow when a flow is supercritical. The results compare the differences between theoretical discharge ratios and experimental ones. The differences increase in one of three conditions: increase of the side breach, increase of the bottom slope, and decrease of the Froude number.

Analytical solution for solute transport resulting from instantaneous injection in streams with transient storage

An analytical solution is presented for solute transport in rivers including the effects of transient storage. The traditional advection–dispersion equation for transport in the main channel is linked to a first order mass exchange term between the main channel and the transient storage zones. This system of equations is solved analytically for the case of an instantaneous injection of a tracer mass in a river with constant and uniform flow. The solution enables to estimate the temporal and spatial evolution of the tracer concentration downstream of the injection point. The correctness of the solution is verified by comparison with the numerical model OTIS (USGS). The solution is programmed in MATLAB and linked to a non-linear least squares optimisation algorithm to obtain an effective and reliable method to estimate solute transport parameters from observed breakthrough curves. The procedure is successfully applied to the Chillán River, Chile, where five tracer experiments were conducted. The observed concentration profiles vs. time at the different measuring locations are well reproduced by the model. In all cases the exchange of solute between the main flow channel and transient storage zones is markedly present.

Electroforming of metallic bipolar plates with micro-featured flow field

In addition to mechanical properties, uniform fuel dispersion, efficient removal of water and high electric conductivity are also important functions of a bipolar plate. The capillary effect of micro-featured flow field may attract water from the carbonic diffusion layer and promote more evenly dispersion of fuels into the diffusion layer. Thus, it may improve the performance of proton exchange membrane (PEM) fuel cells. In this research, the Lithography Galvanik Abformung (LIGA) manufacturing processes with electroforming technology are investigated for the production of micro-featured flow field of the metallic bipolar plates. The micro-features are designed in conjunction with the existing flow channel to form an integrated flow field system. Instead of silicon wafer, a 4 in. wafer size SS304 stainless steel plate is used as the substrate. The LIGA processes of photo masking, spin coating, exposure and development are employed to create electric conducting die with flow field pattern. Electroforming of this metallic plate coated with flow field patterned photo resist will result in the main flow channel on the SS304 plate. The same processes were conducted for the second iteration to form micro-features. Thus, metallic bipolar plates with micro-features are produced using the electroforming technology. A single cell with total cell area of 16 cm 2 and reaction area of 4 cm 2 was produced. It has micro-features of 100 and 200 μm width and of 50 μm depth. The dimensions of the main flow channel were 300 μm in width and 200 μm in depth. Single cell tests were conducted to evaluate its performance. The cell performance of the single cell with SS304 metallic bipolar plates exceeds similar size single cell with silicon or glass fiber substrates. The electroforming is a promising technology for metallic bipolar plates with micro-features and micro-fuel cell.

On-chip high-speed sorting of micron-sized particles for high-throughput analysis

A new design of particle sorting chip is presented. The device employs a dielectrophoretic gate that deflects particles into one of two microfluidic channels at high speed. The device operates by focussing particles into the central streamline of the main flow channel using dielectrophoretic focussing. At the sorting junction (T- or Y-junction) two sets of electrodes produce a small dielectrophoretic force that pushes the particle into one or other of the outlet channels, where they are carried under the pressure-driven fluid flow to the outlet. For a 40 microm wide and high channel, it is shown that 6 microm diameter particles can be deflected at a rate of 300/s. The principle of a fully automated sorting device is demonstrated by separating fluorescent from non-fluorescent latex beads.

Optimizing water intake angle by flow separation analysis

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C221 T形合流管内の高・低温流混合における乱流熱輸送(OS-6 熱流動現象の計測およびデータ解析技術II)

Turbulent thermal mixing in rectangular channels with a T-junction was investigated experimentally, which simulated the mixing region in the automobile HVAC unit. The Reynolds number and temperature of the main-channel flow were 2.5×10^4 and 12℃, respectively, and the flow velocity in the branch (60℃) was set at the same value as that of the main channel. The simultaneous measurements of velocity and temperature were conducted using LDV and cold-wire thermometer to make clear the distribution of the turbulent heat flux with a special attention to its spanwise variation. The mechanism of the turbulent thermal mixing of hot and cold flows is discussed based on the distributions of the turbulent heat flux.

D133 気泡微細化を伴うサブクール流動沸騰 : 噴流の噴き付けを付加した長い伝熱面の高熱流束除熱(OS-15 電子機器・デバイスの熱問題と現状技術IV)

Subcooled flow boiling was performed in a horizontal rectangular channel with auxiliary channel and multi-needle nozzle. The heating surface is 10cm in length and 1cm in width and placed at the bottom of the main channel. Subcooled jet was issued into the heating surface from the auxiliary channel assembled overhead of the main channel through the multi-needle nozzle. The maximum heat flux was 5MW/m^2 at 40K of liquid subcooling, 0.5m/s of main-channel flow and 4.6m/s of jet velocity for distilled water. No coalesced bubbles were observed in the downstream section of heating surface and the pressure fluctuations were very low.