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 Open Journal of Fluid Dynamics (OJFD) , 2014, DOI: 10.4236/ojfd.2014.45037 Abstract: Chaotic mixing in a curved-square channel flow is studied experimentally and numerically. Two walls of the channel (inner and top walls) rotate around the center of curvature and a pressure gradient is imposed in the direction toward the exit of the channel. This flow is a kind of Taylor-Dean flows. There are two parameters dominating the flow, the Dean number De (∝ the pressure gradient or the Reynolds number) and the Taylor number Tr (∝ the angular velocity of the wall rotation). In the present paper, we analyze the physical mechanism of chaotic mixing in the Taylor-Dean flow by comparing experimental and numerical results. We produced a micromixer model of the curved channel several centimeters long with square cross section of a few millimeters side. The secondary flow was measured using laser induced fluorescence (LIF) method to examine secondary flow characteristics. We also performed three-dimensional numerical simulations for the exactly same configuration as the experimental system to study the mechanism of chaotic mixing. It is found that good mixing performance is achieved for the case of De ≤ 0.1Tr, and that mixing efficiency changes according to the difference in inflow conditions. The flow is studied both experimentally and numerically, and both results agree with each other very well.
 Open Journal of Fluid Dynamics (OJFD) , 2015, DOI: 10.4236/ojfd.2015.53027 Abstract: Chaotic mixing in three different types of curved-rectangular channels flow has been studied experimentally and numerically. Two walls of the channel (inner and top walls) rotate around the center of curvature and a pressure gradient are imposed in the direction toward the exit of the channel. This flow is a kind of Taylor-Dean flow. There are two parameters dominating the flow, the Dean number De (∝ the pressure gradient or the Reynolds number) and the Taylor number Tr (∝ the angular velocity of the wall rotation). In this paper, we analyze the physical mechanism of chaotic mixing in the Taylor-Dean flow by comparing experimental results and numerical ones. We produced three micromixer models of the curved channel, several centimeters long, with rectangular cross-section of a few millimeters side. The secondary flow is measured using laser induced fluorescence (LIF) method to examine secondary flow characteristics. Also we performed three-dimensional numerical simulations with the open source CFD solver, OpenFOAM, for the same configuration as the experimental system to study the mechanism of chaotic mixing. It is found that good mixing performance is obtained in the case of De ≤ 0.1 Tr, and it becomes more remarkable when the aspect ratio tends to large. And it is found that the mixing efficiency changes according to the aspect ratio and inflow condition.
 Micromachines , 2013, DOI: 10.3390/mi4010080 Abstract: We present a continuous-flow active micromixer based on channel-wall deflection in a polydimethylsiloxane (PDMS) chip for volume flows in the range up to 2？μL s ？1 which is intended as a novel unit operation for the microfluidic Braille pin actuated platform. The chip design comprises a main microchannel connected to a series of side channels with dead ends aligned on the Braille pins. Computer-controlled deflection of the side-channel walls induces chaotic advection in the main-channel, which substantially accelerates mixing in low-Reynolds number flow. Sufficient mixing (mixing index MI below 0.1) of volume flows up to 0.5 μL s ？1 could be achieved within residence times ~500 ms in the micromixer. As an application, continuous dilution of a yeast cell sample by a ratio down to 1:10 was successfully demonstrated. The mixer is intended to serve as a component of bio-analytical devices or as a unit operation in the microfluidic Braille pin actuated platform.
 Sensors , 2009, DOI: 10.3390/s90705379 Abstract: In this study a biophysical passive micromixer with channel anamorphosis in a space of 370 mm, which is shorter than traditional passive micromixers, could be created by mimicing features of vascular flow networks and executed with Reynolds numbers ranging from 1 to 90. Split and recombination (SAR) was the main mixing method for enhancing the convection effect and promoting the mixing performance in the biophysical channel. The 2D numerical results reveal that good mixing efficiency of the mixer was possible, with εmixing = 0.876 at Reynolds number ration Rer = 0.85. Generally speaking, increasing the Reynolds number will enhance the mixing. In addition, the sidewall effect will influence the mixing performance and an optimal mixing performance with εmixing = 0.803 will occur at an aspect ratio of AR = 2. These findings will be useful for enhancing mixing performance for passive micromixers.
 Physics , 2010, Abstract: Twisted tapes are used to induce swirling flow and improve mixing. The flow induced by a 180 degree twisted tape with length (pitch) 60 mm and diameter 25.4 mm in a circular pipe was investigated using Laser Doppler Velocimetry (LDV) measurements. Tangential velocity profiles downstream of the twisted tape swirler were measured at multiple locations along the pipe axis, across the horizontal diameter of the pipe. The profiles showed an unexpected transition along the pipe axis from regular swirling flow to an apparent counter-rotation near the pipe axis, and then reverting back to regular swirling flow. Injecting fine air bubbles into the flow showed the existence of two co-rotating helical vortices superimposed over the main swirling flow. The close proximity of the two co-rotating vortices creates the local reversing flow at the pipe centerline. The secondary vortices are analyzed with high speed camera videos and numerical simulations.
 Computer Science , 2008, Abstract: A novel design for vortex modulation of a passive chaotic micromixer, named a circulation-disturbance micromixer (CDM), has been achieved and analyzed experimentally and numerically. The systematic numerical analyses - topological flow characteristics and particle tracking method - have been developed, that enable visualization of detailed mixing patterns. To display the cross section of mixing region of flows in our CDM, the biotin-streptavidin binding is detected through the fluorescence resonance energy transfer (FRET) pair of fluorescent proteins - R-phycoerythrin (RPE) and cross-linked allophycocyanin (clAPC). We expect the diagnosis technique using FRET will be successfully applied to biochemical analysis in microfluidic system.
 Physics , 1998, Abstract: Chaos control in some of the one- and two-dimensional traffic flow dynamical models in the mean field theory is studied.One dimensional model is investigated taking into account the effect of random delay. Two dimensional model takes into account the effects of overpasses, symmetric distribution of cars and blockages of cars moving in the same direction. Chaos synchronization is performed within both replica and nonreplica approaches, and using parameter perturbation method.
 Physics , 2013, DOI: 10.1134/S102833581401011X Abstract: Flows between concentric, counter rotating spherical boundaries are under investigation in the gap with size equal to inner sphere radius. Outer sphere rotational rate is fixed, while inner sphere rotational rate has time periodic variations. The amplitudes and frequencies of these variations are small relative to both spheres averaged rotational rates. With amplitude increase transition from initial periodical flow to chaos is occurred. To determine state of the flow time series of azimuthal velocity, taken with laser Doppler anemometry, were used. We demonstrate appearance of flow states in the form of chaos-chaos and cycle-chaos-chaos intermittency. A procedure is considered which allow quantitatively confirm distinct properties of different patterns of time alternating flow state with intermittency.
 Physics , 2010, Abstract: Adequate knowledge of the nature of river flow process is crucial for proper planning and management of our water resources and environment. This study attempts to detect the salient characteristics of flow dynamics of the Karoon River in Iran. Daily discharge series observed over a period of six years (1999-2004) is analyzed to examine the chaotic and scaling characteristics of the flow dynamics. The presence of chaos is investigated through the correlation dimension and Lyapunov exponent methods, while the Hurst exponent and R\'enyi dimension analyses are performed to explore the scaling characteristics. The low correlation dimension ($2.60 \pm 0.07$) and the positive largest Lyapunov exponent ($0.014 \pm 0.001$) suggest the presence of low-dimensional chaos; they also imply that the flow dynamics are dominantly governed by three variables and can be reliably predicted up to 48 days (i.e. prediction horizon). Results from the Hurst exponent and R\'enyi dimension analyses reveal the multifractal character of the flow dynamics, with persistent and anti-persistent behaviors observed at different time scales.
 Physics , 2007, DOI: 10.1063/1.3392773 Abstract: We experimentally study the turbulent flow between two coaxial and independently rotating cylinders. We determined the scaling of the torque with Reynolds numbers at various angular velocity ratios (Rotation numbers), and the behaviour of the wall shear stress when varying the Rotation number at high Reynolds numbers. We compare the curves with PIV analysis of the mean flow and show the peculiar role of perfect counterrotation for the emergence of organised large scale structures in the mean part of this very turbulent flow that appear in a smooth and continuous way: the transition resembles a supercritical bifurcation of the secondary mean flow.
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