• Journal of Mechanical Science and Technology
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• v.14 no.5
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• pp.547-552
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• 2000

The objective of this paper is to present a modified fractional step method of keeping a constant mass flow rate in spatially periodic flows, because original fractional step methods do not precisely keep the mass flow rate constant in time. In the modified method, the mean and fluctuating pseudo-pressure gradients are separately obtained at each time step. This method is successfully applied to channel and pipe flows and shown to be suitable for maintaining a constant mass flow rate in time.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.11-16
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• 2002

In this paper, we present two successful results from active controls of flows over a circular cylinder and a sphere for drag reduction. The Reynolds number range considered for the flow over a circular cylinder is 40-3900 based on the free-stream velocity and cylinder diameter, whereas for the flow over a sphere it is $10^{5}$ based on the free-stream velocity and sphere diameter. The successful active control methods are a distributed (spatially periodic) forcing and a high-frequency (time periodic) forcing. With these control methods, the mean drag and lift fluctuations decrease and vortical structures are significantly modified. For example, the time-periodic forcing at a high frequency (larger than 20 times the vortex shedding frequency) produces $50{\%}$ drag reduction for the flow over a sphere at $Re=10^{5}$. The distributed forcing applied to the flow over a circular cylinder results in a significant drag reduction at all the Reynolds numbers investigated.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.1758-1760
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• 2008

Under typical operating conditions, flows in microfluidic devices are laminar and molecular diffusion across the channels is slow, which makes an efficient mixing in microfluidic devices difficult to achieve. The mechanism to achieve effective mixing in laminar flows is that of repetitive stretching and folding. Essential is to generate spatially periodic flows with crossing cross sectional streamlines. A mapping method is employed to analyze mixing in micromixers, enabling us to investigate the progress of mixing both qualitatively and quantitatively. The progress of mixing is characterized by a measure of mixing, called the discrete intensity of segregation. The mapping method is applied to mixing in such micromixers as the staggered herringbone mixer, the barrier embedded micromixer, and the three-dimensional serpentine channel to demonstrate the capability of the numerical scheme to tackle general mixing problems in microfluidic devices.