Browsing by Author "Lua, Kim Boon"

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Numerical study of Taylor-Couette flow with longitudinal corrugated surface
(2020-05-01)
Razzak, Md Abdur
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Cheong, Khoo Boo
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Lua, Kim Boon
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機械工程學系
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Department of Mechanical Engineering
This study investigates the Taylor-Couette flow (TCF) with a longitudinal corrugated surface on a stationary outer cylinder and a rotating smooth inner cylinder using large eddy simulation for three values of amplitude to wavelength ratios (A(*)) (0.1875, 0.2149, and 0.25) to explore the influence of the corrugated surface on the flow structures and the variation of torque for a wider range of Reynolds numbers (Re) (60-650). From the results, four flow regimes are observed. At Re = 60, initially, a pair of secondary vortices appears at the inner wall of the minimum gap region and it evolves to a pair of axisymmetric stationary wall induced vortices (ASSWIVs) in the maximum gap region. As Re increases to 80, 85, and 103 for the three values of A(*) (0.1875, 0.2149, and 0.25), respectively, another pair of axisymmetric stationary secondary vortices is seen at the minimum gap region of the inner wall. A further increase in Re (Re > 125, 130, and 138 for the three values of A(*), respectively) results in the appearance of axisymmetric periodic secondary axial flow. Increasing Re further (Re > 225, 240, and 260 for A(*) = 0.25, 0.2149, and 0.1875, respectively) leads to the emergence of non-axisymmetric and non-periodic secondary axial flow (NANPSAF) with an azimuthal wave. Generally, the torque in TCF with the corrugated surface is found to be lower than TCF with a smooth surface except for the occurrence of the ASSWIV flow regime and weak axial secondary flow in the NANPSAF regime.
Rotating elliptic cylinders in a uniform cross flow
(2018-04-01)
Lua, Kim Boon
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Lu, Hao
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Lim, T. T.
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機械工程學系
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Department of Mechanical Engineering
Numerical simulation of flow past a two-dimensional rotating cylinder of different thickness ratios at Re = 200 has been carried out to investigate how incremental changes in the cross-section of a cylinder from a circle to a thin ellipse affect their lift and drag characteristics as well as flow structures. In contrast to past studies, the present work considers velocity ratios up to 2.5, where vortex shedding of a circular cylinder completely ceases. Results show that a decreasing thickness generally leads to lower mean lift magnitude, but a non-monotonic change in mean drag with a local minimum at a thickness ratio of approximately 0.375. Interestingly, at velocity ratio 2.5, such minimum drag changes sign and turns into a positive thrust; this event occurs at a much smaller velocity ratio than the case of a circular cylinder. Detailed analysis on vorticity contours and surface pressure distributions reveals that the thrust generation is a result of the competition between a prevailing negative pressure below the cylinder due to rotational motion, and a suction effect from a "hovering vortex" formed above the cylinder. Additionally, as the thickness reduces, the wake transits from a von-Karman type to a separated flow dictated by the two edges of the elliptic cylinder. And the crests and troughs in the transient aerodynamic forces become highly related to the instantaneous geometric angle of attack, which can be attributed to a projection effect. (C) 2017 Elsevier Ltd. All rights reserved.
Vortex-induced vibration wind energy harvesting by piezoelectric MEMS device in formation
(2019-12-31)
Lee, Yin Jen
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Qi, Yi
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Zhou, Guangya
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Lua, Kim Boon
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機械工程學系
A silicon chip integrated microelectromechanical (MEMS) wind energy harvester, based on the vortex-induced vibration (VIV) concept, has been designed, fabricated, and tested as a proof-of-concept demonstration. The harvester comprises of a cylindrical oscillator attached to a piezoelectric MEMS device. Wind tunnel experiments are conducted to measure the power output of the energy harvester. Additionally, the energy harvester is placed within a formation of up to 25 cylinders to test whether the vortex interactions of multiple cylinders in formation can enhance the power output. Experiments show power output in the nanowatt range, and the energy harvester within a formation of cylinders yield noticeably higher power output compared to the energy harvester in isolation. A more detailed investigation conducted using computational fluid dynamics simulations indicates that vortices shed from upstream cylinders introduce large periodic transverse velocity component on the incoming flow encountered by the downstream cylinders, hence increasing VIV response. For the first time, the use of formation effect to enhance the wind energy harvesting at microscale has been demonstrated. This proof-of-concept demonstrates a potential means of powering small off-grid sensors in a cost-effective manner due to the easy integration of the energy harvester and sensor on the same silicon chip.