Browsing by Author "Tsai, W. -K."

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EXPERIMENTAL STUDY OF A HYBRID FERROHYDRODYNAMIC INSTABILITY IN MISCIBLE FERROFLUIDS: DROPLET SIZE EFFECTS
(2009-01-01)
Chen, Ching-Yao
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Tsai, W. -K.
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Miranda, Jose A.
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機械工程學系
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Department of Mechanical Engineering
Recently, we have performed a ferrofluid experiment that revealed the existence of a hybrid ferrohydrodynamic situation in which a three-dimensional Collapsing Rosensweig peak and effectively-two-dimensional labyrinthine structures occur simultaneously (C.-Y. Chen et al. Phys. Rev. E., vol. 77 (2008) 056306.) This has been done by immersing a ferrofluid droplet in a thin layer of a miscible nonmagnetic fluid, under the action of a vertically applied magnetic field. We extend the investigation of this new system focusing on the effects produced by ferrofluid droplets of different sizes. It, is found that; larger droplets result in a more promient manifestation of both interfacial instabilities. Moreover, the observations of the labyrinthine fingering patterns are extended into very late stages, where the decaying ferrofluid peak has been completely reimmersed into the surrounding nowmagnetic fluid.
Hybrid ferrohydrodynamic instability: Coexisting peak and labyrinthine patterns
(2008-05-01)
Chen, Ching-Yao
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Tsai, W. -K.
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Miranda, Jose A.
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機械工程學系
;
Department of Mechanical Engineering
We present an experimental study of a different pattern-forming instability occurring when a ferrofluid droplet is immersed in a thin layer of a nonmagnetic fluid, and subjected to a uniform perpendicular magnetic field. The formation of intriguing interfacial structures is observed, and the development of a hybrid-type ferrohydrodynamic instability is verified, where peak and labyrinthine ferrofluid patterns coexist and share a coupled dynamic evolution. Based on our experimental findings we have identified the occurrence of three well defined regimes for the evolution of the miscible Rosensweig peak in which it first grows rapidly, and then gradually decays, to ultimately reimmerse into the surrounding nonmagnetic solvent layer. This unique scenario for the rise and fall of the Rosensweig peak implies the simultaneous emergence of peculiar labyrinthine structures induced by an outward radial flow within the thin nonmagnetic layer. A variety of possible morphologies is revealed (labyrinth, broken tentaclelike fingers, etc.), which result from the interplay between magnetic, diffusive, and convective effects. These free surface flow labyrinthine structures are contrasted to alternative interfacial designs obtained when the experimental system is spatially confined.
Low-complexity hopping DFT design based on a compact recursive structure
(2017-01-05)
Juang, W. -H.
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Lai, S. -C.
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Chen, K. -H.
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Tsai, W. -K.
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Luo, C. -H.
A novel hopping discrete Fourier transform (DFT) algorithm and its architecture design for efficiently computing time-frequency spectra are presented. Since the sliding process is adopted sample by sample, the spectral bin output data rate is the same as the input data rate. Under the conditions of an M-sample complex input sequence (M = 256), and N-point recursive DFT computation (N = 64) for time hop L (L = 4), the proposed method has the following advantages: (i) the computational complexity of Proposed-I requires only four complex additions and four complex multiplications for each frequency bin, after the first spectral component has been finally calculated; (ii) Proposed-II utilises a re-timing scheme to greatly shorten and balance the critical path; (iii) Proposed-II is less computationally complex than Wang et al.\'s method, as the numbers of multiplication and addition operations in the proposed algorithm are 768 and 1024, representing reductions of 50 and 20%, respectively. In addition, the number of coefficients can be reduced by 50% compared with Park et al.\'s method. In the FPGA implementation, the proposed design can be operated at 47.26 MHz. It is thus more suitable for use with real-time analytic applications of time-frequency spectra.
MOTION OF MISCIBLE MAGNETIC FLUIDS IN A VERTICAL CAPILLARY TUBE
(2008-12-01)
Chen, C. -Y.
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Lin, H. -C.
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Tsai, W. -K.
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機械工程學系
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Department of Mechanical Engineering
An experimental study of miscible magnetic fluid motion in a vertical capillary tube is presented. Transporting ferrofluids is dominated by a dimensionless magnetic number Ma, which characterizes the ratio of upward magnetic force to the downward gravity. Two distinct stages of motion, referred to as the sub-critical mode of finite lift and the effective transportation, are identified. These two modes are determined by the values of the sub-critical magnetic number Ma(sub) and critical magnetic number Ma(cr) respectively. For the cases of sub-critical mode (Ma(sub) < Ma < Ma(cr)), the ferrofluids are lifted to quasi-equilibrium heights, which are nearly proportional to the magnetic number Ma. As for the situations of effective transportation (Ma > Ma(cr)), a penetrating finger of ferrofluids is formed similar to the conventional miscible displacements. A dimensionless proportionality of fingertip velocity v, magnetic number Ma and field distribution profile f(z) is obtained as v similar to Ma(1/2) f(z) by scaling arguments. This proportional correlation shows a good agreement with the experiments.