Browsing by Author "Lim, T. T."

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Enhanced thrust performance of a two dimensional elliptic airfoil at high flapping frequency in a forward flight
(2018-01-01)
Dash, S. M.
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Lua, K. B.
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Lim, T. T.
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Yeo, K. S.
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機械工程學系
This study is motivated by our earlier investigation Lua et al. (2016) which shows that a two-dimensional elliptic airfoil undergoing sinusoidal flapping motion experiences thrust deterioration when the flapping frequency exceeds a certain critical value (or critical Strouhal number, St(cr)). To alleviate this unfavorable thrust generation condition, we propose two novel effective angle of attack profiles, namely smooth trapezoid (STEA) profile and elliptic trapezoid (ETEA) profile. These profiles are designed to ensure that the airfoil still experiences the same effective angle of attack amplitude as the sinusoidal flapping while concurrently reducing the detrimental effect of high rotation rate. The effectiveness of these two proposed profiles is confirmed by our numerical and experimental studies. In particular, our results show that at low to moderate flapping frequency, thrust generation is almost invariant to the type of angle of attack profiles applied, but at high flapping frequency (St > St(cr)), the proposed profiles produce higher thrust than the sinusoidal flapping. The thrust augmentation can be attributed to the suppression of the adverse low pressure region in the vicinity of the airfoil, which is a consequence of the reduced rotation rate. Of the two proposed profiles, the ETEA with its steep acceleration phase produces a higher time-average thrust than the STEA near the stroke reversal. Also, in line with our previous finding, thrust augmentation for both STEA and ETEA is a function of the base length of the trapezoid, with a broader one producing a better thrust performance. (C) 2017 Elsevier Ltd. All rights reserved.
Flow past a rapidly rotating elliptic cylinder
(2018-11-01)
Lu, H.
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Lua, K. B.
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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
Previous studies have shown that the characteristics of flow over a circular cylinder can be significantly altered by introducing rotation and ellipticity. While the effect of the former has been extensively studied; the influence of the latter is less explored. In this paper, we report results of a numerical investigation on the flow past a two-dimensional (2D) elliptic cylinder that is rapidly rotating at a Reynolds number of 200, covering a narrow range of high rotational velocity where low-frequency Mode II shedding occurs for a circular cylinder. Results show that aerodynamic forces and moments acting on a rotating cylinder are highly nonlinear with respect to variation in rotational velocity and cylinder thickness. Various wake topologies are revealed, including steady, quasi-steady, periodic and aperiodic flows. At an intermediate rotation rate, a "hovering vortex" is typically formed on one side of the cylinder, and at high rotation rates, multiple vortices are generated and rotated with the cylinder. These rotating vortices are highly related to a plateau in the mean lift over a certain range of cylinder thickness, as well as a sudden force transition from a mean drag to a considerable mean thrust when the thickness exceeds a certain threshold value. (C) 2018 Elsevier Masson SAS. All rights reserved.
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.