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Abstract

An active mixing mechanism to mix the flow uniformly in a channel using pitching of a square cylinder. The study is to show the behavior of fluid in a channel when a square cylinder is rotating inside the channel at a very low Reynolds number (Re=1) and (Re=100). To understand the behavior of the flow, 2D numerical simulations are conducted for two different cases. In the first case the cylinder is placed very close to the inlet and in the second case it is placed in the middle of the channel. The velocity contours are generated for different cases and flow characteristics of fluid are analyzed.

Introduction

Mixing of fluid in microfluidic devices is done by two methods, active and passive. Generally, active mechanisms are used as it promotes good mixing at a low Reynolds number as compared to passive. The analysis is done for a 2D channel with a square cylinder pitching in it with initial angle of incidence at \(45^o\)C. The flow considered is laminar, incompressible and transient. The walls are stationary with no slip condition.

Model Setup : Geometry

A sketch of mixing channel is shown for first case in Fig. 1, where cylinder is placed at a very small distance from the inlet and in Fig. 2, where it is in the middle of the mixing domain. The fluid is entering at the left side of a 2D channel at a very low Reynolds number. The length of square cylinder is L1 and the initial angle of incidence is \(45^o\)C. The channel length is H5 and the width is V2. H3 denotes the distance at which cylinder is placed from the inlet. The fluid taken into consideration for the study is ‘air’. The cylinder rotates in clockwise direction and thus flow is affected.

Fig. 1 Sketch of Mixing Domain (cylinder close to inlet)


Fig. 2 Sketch of Mixing Domain (cylinder placed in the middle of domain)

Model Setup : Mesh

The meshing is done as shown in Fig. 3. Triangular meshing is adopted in this as the geometry is simple. Edge sizing is done on the walls of the channel as well as on the walls of the cylinder to obtain more accurate results. To generate more fine mesh, inflation is done on the walls of cylinder to adequately capture regions as flow will experience rapid change when it hits the cylinder, shown in Fig. 4.

Fig. 3 Mesh


Fig. 4 Inflation around cylinder walls

Model Setup : Boundary Conditions

Results : Cylinder Close To Inlet

The figures below show the velocity contours for different time in sec. From the contours, it is seen that the results for Re=1 and Re=100 differ just a little bit. Cylinder takes 0.7 sec to complete one rotation.

Fig. 5 Legend View showing different velocities for contours


Fig. 6 Velocity Contours (cylinder placed close to inlet)

Results : Cylinder In The Middle Of Domain

As shown above, similar contours are generated when cylinder is placed in the middle of the domain. Cylinder takes 0.7 sec to complete one rotation.

Fig. 7 Legend view showing different velocities for contours


Fig. 8 Velocity Contours (cylinder placed in middle of domain)

Conclusion

References

[1] Parsa, M. K., & Hormozi, F. (2014). Experimental and CFD modeling of fluid mixing in sinu soidal microchannels with different phase shift between side walls. Journal of Micromechanics and Microengineering, 24(6), 065018.

[2] ANSYS Inc. (2011). ANSYS Fluent 14.0: User’s Guide.

[3] Ortega-Casanova, J. (2016). Enhancing mixing at a very low Reynolds number by a heaving square cylinder. Journal of Fluids and Structures, 65, 1-20.

[4] Kuo, J. N., & Li, Y. S. (2016). Centrifuge-based micromixer with three-dimensional square wave microchannel for blood plasma mixing. Microsystem Technologies, 1-12.