Mixing by Time-Dependent Orbits in Spatiotemporal Chaotic Advection

2014 ◽  
Vol 137 (1) ◽  
Author(s):  
Mohammad Karami ◽  
Ebrahim Shirani ◽  
Mojtaba Jarrahi ◽  
Hassan Peerhossaini
Keyword(s):  
Secondary Flow ◽  
Lyapunov Exponents ◽  
Amplitude Ratio ◽  
Flow Patterns ◽  
Chaotic Advection ◽  
Mixing Enhancement ◽  
Flow Pulsation ◽  
Mean Flow ◽  
Homoclinic Connections ◽  
Vorticity Vector

The simultaneous effects of flow pulsation and geometrical perturbation on laminar mixing in curved ducts have been numerically studied by three different metrics: analysis of the secondary flow patterns, Lyapunov exponents and vorticity vector analysis. The mixer that creates the flow pulsation and geometrical perturbations in these simulations is a twisted duct consisting of three bends; the angle between the curvature planes of successive bends is 90 deg. Both steady and pulsating flows are considered. In the steady case, analysis of secondary flow patterns showed that homoclinic connections appear and become prominent at large Reynolds numbers. In the pulsatile flow, homoclinic and heteroclinic connections appear by increasing β, the ratio of the peak oscillatory velocity component of the mean flow velocity. Moreover, sharp variations in the secondary flow structure are observed over an oscillation cycle for high values of β. These variations are reduced and the homoclinic connections disappear at high Womersley numbers. We show that small and moderate values of the Womersley number (6 ≤ α ≤ 10) and high values of velocity amplitude ratio (β ≥ 2) provide a better mixing than that in the steady flow. These results correlate closely with those obtained using two other metrics, analysis of the Lyapunov exponents and vorticity vector. It is shown that the increase in the Lyapunov exponents, and thus mixing enhancement, is due to the formation of homoclinic and heteroclinic connections.

Classification of Pulsating Flow Patterns in Curved Pipes

1996 ◽  
Vol 118 (3) ◽  
pp. 311-317 ◽  
Author(s):  
Shigeru Tada ◽  
Shuzo Oshima ◽  
Ryuichiro Yamane
Keyword(s):  
Secondary Flow ◽  
Amplitude Ratio ◽  
Circular Cross Section ◽  
Volumetric Flow Rate ◽  
Flow Patterns ◽  
Womersley Number ◽  
Dean Number ◽  
Curved Pipes ◽  
Convection Dominated

The fully developed periodic laminar flow of incompressible Newtonian fluids through a pipe of circular cross section, which is coiled in a circle, was simulated numerically. The flow patterns are characterized by three parameters: the Womersley number Wo, the Dean number De, and the amplitude ratio β. The effect of these parameters on the flow was studied in the range 2.19 ≤ Wo ≤ 50.00, 15.07 ≤ De ≤ 265.49 and 0.50 ≤ β ≤ 2.00, with the curvature ratio δ fixed to be 0.05. The way the secondary flow evolved with increasing Womersley number and Dean number is explained. The secondary flow patterns are classified into three main groups: the viscosity-dominated type, the inertia-dominated type, and the convection-dominated type. It was found that when the amplitude ratio of the volumetric flow rate is equal to 1.0, four to six vortices of the secondary flow appear at high Dean numbers, and the Lyne-type flow patterns disappear at β ≥ 0.50.

Mixing Enhancement in a Chaotic Micromixer Using Pulsating Flow

2014 ◽  
Author(s):  
Mohammad Karami ◽  
Mojtaba Jarrahi ◽  
Ebrahim Shirani ◽  
Hassan Peerhossaini
Keyword(s):  
Reynolds Number ◽  
Lyapunov Exponent ◽  
Steady Flow ◽  
Concentration Distribution ◽  
Pulsating Flow ◽  
Chaotic Advection ◽  
Mixing Enhancement ◽  
Mean Flow ◽  
Number Range ◽  
Velocity Amplitude

This study determines the simultaneous effects of spatial disturbance and flow pulsation on micromixing by using three different metrics: concentration distribution, Lyapunov exponent and axial vorticity. Numerical simulations are performed for both steady and pulsating flows through a microchannel made up of C-curved repeating units. Moreover, a straight microchannel is analyzed to compare the effects of chaotic advection and molecular diffusion, the main mechanisms of transverse mixing in the chaotic and straight mixer respectively. Simulations are carried out in the steady flow for the Reynolds number range 1≤Re≤50 and in the pulsating flow for velocity amplitude ratios 1≤β≤2.5, and the ratio of the peak oscillatory velocity component to the mean flow velocity, Strouhal numbers 0.1≤St≤0.5. It was found that chaotic advection improves mixing without significant increase in pressure drop. The analysis of concentration distribution implied that full mixing occurs after Reynolds number 50 in the steady flow. When the flow is pulsatile, small and moderate values of the Strouhal number (0.1≤St≤0.3) and high values of velocity amplitude ratio (β ≥ 2) are favorable conditions for mixing enhancement. Moreover, mixing has an oscillating trend along the microchannel due to the coexistence of regular and chaotic zones in the fluid. These results correlate closely with those obtained using two other metrics, analysis of the Lyapunov exponent and axial vorticity.

Chaotic Heat Transfer in a Laminar Pulsating Flow With Constant Wall Temperature

2014 ◽  
Author(s):  
Mohammad Karami ◽  
Mojtaba Jarrahi ◽  
Zahra Habibi ◽  
Ebrahim Shirani ◽  
Hassan Peerhossaini
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Enhancement ◽  
Secondary Flow ◽  
Wall Temperature ◽  
Pulsating Flow ◽  
Mean Flow ◽  
Transfer Enhancement ◽  
Constant Wall Temperature ◽  
Homoclinic Connections

The correlation between heat transfer enhancement and secondary flow structures in laminar flows through a chaotic heat exchanger is discussed. The geometry consists of three bends; the angle between curvature planes of successive bends is 90°. Numerical simulations are performed for both steady and pulsating flows when the walls are subjected to a constant temperature. The temperature profiles and secondary flow patterns at the exit of bends are compared in order to characterize the flow. Simulations are carried out for the Reynolds numbers range 300≤Re≤800, velocity amplitude ratios (the ratio of the peak oscillatory velocity component to the mean flow velocity) 1≤β≤2.5, and wall temperatures 310 ≤ Tw(K) ≤ 360. The results show that in the steady flow, heat transfer enhancement occurs with increasing Reynolds number and wall temperature. However, heating homogenization becomes almost independent of Reynolds number when homoclinic connections exist in the flow. Moreover, at high values of wall temperature, heat transfer enhancement is greater than mixing improvement due to the presence of homoclinic connections. In the pulsating flow, Nusselt number improves with β, and β≥2 is a sufficient condition for heat transfer enhancement. The formation and development of homoclinic connections are correlated with the heating homogenization.

Flow Pulsation and Geometry Effects on Mixing of Two Miscible Fluids in Microchannels

2014 ◽  
Vol 136 (12) ◽  
Author(s):  
Houssein Ammar ◽  
Ahmed Ould el Moctar ◽  
Bertrand Garnier ◽  
Hassan Peerhossaini
Keyword(s):  
Reynolds Number ◽  
Residence Time ◽  
Pure Water ◽  
Mixing Efficiency ◽  
Mixing Enhancement ◽  
Flow Pulsation ◽  
Mean Flow ◽  
Mixing Index ◽  
Two Fluids ◽  
Passive Mixing

Many microfluidic applications involve chemical reactions. Most often, the flow is predominantly laminar, and without active or passive mixing enhancement the reaction time can be extremely long compared to the residence time. In this work we demonstrate the merits of the combination of flow pulsation and geometrical characteristics in enhancing mixing efficiency in microchannels. Mixing was studied by introducing a mixing index based on the gray level observed in a heterogeneous flow of pure water and water colored by rhodamine B. The effects of the injection geometry at the microchannel inlet and the use of pulsed flows with average Reynolds numbers between 0.8 and 2 were studied experimentally and numerically. It appeared that the mixing index increases with the nondimensional residence time (τ), which is inversely proportional to the Reynolds number. In addition, we show that the mixing efficiency depends strongly on the geometry of the intersection between the two fluids. Better mixing was achieved with sharp corners (arrowhead and T intersections) in all cases investigated. In pulsed flow, the mixing efficiency is shown to depend strongly on the ratio (β) between the peak amplitude and the mean flow rate. Optimal conditions for mixing in the microchannels are summarized as a function of Reynolds number Re, the ratio β, and the geometries.

Pulsating Flow for Mixing Intensification in a Twisted Curved Pipe

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
B. Timité ◽  
M. Jarrahi ◽  
C. Castelain ◽  
H. Peerhossaini
Keyword(s):  
Secondary Flow ◽  
Pipe Flow ◽  
Numerical Study ◽  
Pulsating Flow ◽  
Experimental Results ◽  
Chaotic Advection ◽  
Mean Deviation ◽  
Mixing Enhancement ◽  
Curved Pipe ◽  
Deceleration Phase

This work concerns the manipulation of a twisted curved-pipe flow for mixing enhancement. Previous works have shown that geometrical perturbations to a curved-pipe flow can increase mixing and heat transfer by chaotic advection. In this work the flow entering the twisted pipe undergoes a pulsatile motion. The flow is studied experimentally and numerically. The numerical study is carried out by a computational fluid dynamics (CFD) code (FLUENT 6) in which a pulsatile velocity field is imposed as an inlet condition. The experimental setup involves principally a “Scotch-yoke” pulsatile generator and a twisted curved pipe. Laser Doppler velocimetry measurements have shown that the Scotch-yoke generator produces pure sinusoidal instantaneous mean velocities with a mean deviation of 3%. Visualizations by laser-induced fluorescence and velocity measurements, coupled with the numerical results, have permitted analysis of the evolution of the swirling secondary flow structures that develop along the bends during the pulsation phase. These measurements were made for a range of steady Reynolds number (300≤Rest≤1200), frequency parameter (1≤α=r0⋅(ω/υ)1/2<20), and two velocity component ratios (β=Umax,osc/Ust). We observe satisfactory agreement between the numerical and experimental results. For high β, the secondary flow structure is modified by a Lyne instability and a siphon effect during the deceleration phase. The intensity of the secondary flow decreases as the parameter α increases during the acceleration phase. During the deceleration phase, under the effect of reverse flow, the secondary flow intensity increases with the appearance of Lyne flow. Experimental results also show that pulsating flow through a twisted curved pipe increases mixing over the steady twisted curved pipe. This mixing enhancement increases with β.

Pulsating Flow for Mixing Intensification in a Twisted Curved Pipe

2007 ◽  
Author(s):  
Brahim Timite ◽  
Cathy Castelain ◽  
Hassan Peerhossaini
Keyword(s):  
Secondary Flow ◽  
Pipe Flow ◽  
Numerical Study ◽  
Pulsating Flow ◽  
Experimental Results ◽  
Chaotic Advection ◽  
Mean Deviation ◽  
Mixing Enhancement ◽  
Curved Pipe ◽  
Deceleration Phase

This work concerns the manipulation of a twisted curved pipe flow for mixing enhancement. Previous work [1,2,3] has shown that geometrical perturbations to a curved pipe flow can increase mixing and heat transfer by chaotic advection. In this work the flow entering the twisted pipe undergoes a pulsatile motion. The flow was studied experimentally and numerically. The numerical study is carried out by CFD code (Fluent 6) in which a pulsated velocity field is imposed as an inlet condition. The experimental setup involves principally a “Scotch-yoke” pulsatile generator and a twisted curved pipe. Laser Doppler velocimetry (LDV) measurements have shown that the Scotch-yoke generator produces pure sinusoidal instantaneous mean velocities with a mean deviation of 3%. Visualizations by laser-induced fluorescence (LIF) and velocity measurements, coupled with the numerical results, have permitted analysis of the evolution of the swirling secondary flow structures that develop along the bends during the pulsation phase. These measurements were made for a range of steady Reynolds number (300 ≤ Rest ≤ 1200), frequency parameter (1 ≤ α = r0.(ω/υ)1/2 &lt; 20), and two velocity components ratios (β = Umax,osc/Ust). We observe satisfactory agreement between the numerical and experimental results. For high β, the secondary flow structure is modified by a Lyne instability and a siphon effect during the deceleration phase. The intensity of the secondary flow decreases as the parameter α increases during the acceleration phase. During the deceleration phase, under the effect of reverse flow, the secondary flow intensity increases with the appearance of Lyne flow. Experimental results also show that pulsating flow through a twisted curved pipe increases mixing over the steady twisted curved pipe. This mixing enhancement increases with β.

scholarly journals Flat vs. curved rigid-lid LES computations of an open-channel confluence

2019 ◽  
Vol 21 (2) ◽  
pp. 318-334 ◽  
Author(s):  
Pedro Xavier Ramos ◽  
Laurent Schindfessel ◽  
João Pedro Pêgo ◽  
Tom De Mulder
Keyword(s):  
Free Surface ◽  
Secondary Flow ◽  
Open Channel ◽  
Pressure Field ◽  
Large Eddy Simulations ◽  
Numerical Treatment ◽  
Mean Flow ◽  
Large Eddy ◽  
Flow Case ◽  
Channel Confluence

Abstract This paper describes the application of four Large Eddy Simulations (LES) to an open-channel confluence flow, making use of a frictionless rigid-lid to treat the free-surface. Three simulations are conducted with a flat rigid-lid, at different elevations. A fourth simulation is carried out with a curved rigid-lid which is a closer approximation to the real free-surface of the flow. The curved rigid-lid is obtained from the time-averaged pressure field on the flat rigid-lid from one of the initial three simulations. The aim is to investigate the limitations of the free-surface treatment by means of a rigid-lid in the simulation of an asymmetric confluence, showing the differences that both approaches produce in terms of mean flow, secondary flow and turbulence. After validation with experimental data, the predictions are used to understand the differences between adopting a flat and a curved rigid-lid onto the confluence hydrodynamics. For the present flow case, although it was characterized by a moderately low downstream Froude number (Fr ≈ 0.37), it was found that an oversimplification of the numerical treatment of the free-surface leads to a decreased accuracy of the predictions of the secondary flow and turbulent kinetic energy.

scholarly journals Observation and analysis of long-periodic modes in a confluence with dominant tributary inflow

2018 ◽  
Vol 40 ◽  
pp. 05043
Author(s):  
Laurent Schindfessel ◽  
Tom De Mulder ◽  
Mia Loccufier
Keyword(s):  
Numerical Simulations ◽  
Secondary Flow ◽  
Significant Influence ◽  
Laboratory Experiments ◽  
Flow Patterns ◽  
Modal Decomposition ◽  
Periodic Oscillations

Confluences with dominant tributary inflow are found to exhibit long-periodic alternations of the flow patterns. They are shown to exist both in laboratory experiments and in numerical simulations. By means of a modal decomposition, insight is given into these long-periodic oscillations. The origin of these oscillations is investigated and their significant influence on the secondary flow patterns in the downstream channel is revealed.

Characteristics of the mean flow patterns and structure of turbulence in spiral gas streams

AIChE Journal ◽  
1960 ◽  
Vol 6 (4) ◽  
pp. 648-655 ◽  
Author(s):  
W. R. Schowalter ◽  
H. F. Johnstone
Keyword(s):  
Flow Patterns ◽  
Mean Flow ◽  
Gas Streams ◽  
The Mean

Investigation on the Radial Heat-Transfer of Pumping Secondary Flow in Semiclosed Rotating Disk Cavity

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Guohu Luo ◽  
Zhenqiang Yao
Keyword(s):  
Heat Transfer ◽  
Secondary Flow ◽  
Outer Region ◽  
Rotating Disk ◽  
Mean Flow ◽  
Coolant Pump ◽  
Radial Temperature ◽  
Type Flow ◽  
Radial Heat ◽  
Inner Region

Abstract This study investigates the mean flow and radial heat-transfer behaviors in semiclosed rotating disk cavity within the canned reactor coolant pump. The flow in the semiclosed cavity contains the Stewartson type flow at inner region and the Batchelor type flow at outer region. The heat is radially transported from the outer rim of the semiclosed disk cavity to discharge-hole through the nondirect discharge (ND) portion of the superimposed flow from inlet. The effects of rotating Reynolds numbers, cavity aspect ratio and radial location of discharge-hole on the discharge ratio, pumping mass flow rate, local wall shear stress and radial heat-transfer coefficient are examined in the semiclosed rotating cavity flow, respectively. Based on the radial heat transfer behaviors of pumping secondary flow, an equivalent thermal network is proposed and validated by experiments, which can effectively predict the radial temperature distribution from the discharge hole to periphery with the viscous-heating and nonisothermal effects.

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