Browsing by Author "Lua, K. B."

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Effect of clap-and-fling mechanism on force generation in flapping wing micro aerial vehicles
(2019-05-01)
Jadhav, S. S.
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Lua, K. B.
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Tay, W. B.
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機械工程學系
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Department of Mechanical Engineering
The clap-and-fling effect, first observed in a number of insects, serves as a lift-enhancing mechanism for bio-inspired flapping wing micro aerial vehicles (MAV). In our comprehensive literature survey, we observe that the effect manifests differently in insects and contemporary MAVs; insects have active control over the angle of attack and stroke plane of the wing, whereas a number of kinematic parameters of an MAV's flexible wings are determined passively. Although there is consensus that flinging motion significantly enhances aerodynamic lift, the effect of clapping motion is not well-studied. To address this gap, we experimentally quantify the contribution of clapping motion using force measurement and particle image velocimetry. No significant enhancement in lift was observed due to clapping motion, because the momentum jet was too weak. However, the kinematics and flow conditions in our study were notably different from those in the previous studies on insect models. The wings of the MAV are flexible, and deform passively. Hence, the clapping of the trailing edges, and the appearance of a trailing edge momentum jet, was delayed and significantly suppressed. Using force measurement and CFD simulations, it was also found that the lesser the distance between the leading edges of the wings at the end of clap, the higher is the lift due to the subsequent fling.
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.
Numerical study on wide gap Taylor Couette flow with flow transition
(2019-11-01)
Razzak, M. A.
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Khoo, B. C.
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Lua, K. B.
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機械工程學系
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Department of Mechanical Engineering
This study aims to investigate the possible sources of nonaxisymmetric disturbances and their propagation mechanism in Taylor Couette flow for wide gap problems using a direct numerical simulation with a radius ratio of 0.5 and the Reynolds number (Re) ranging from 60 to 650. Here, attention is focused on the viscous layer (VL) thickness in near-wall regions and its spatial distribution along the axial direction to gain an insight into the origin and propagation of nonaxisymmetric disturbances. The results show that an axisymmetric Taylor-vortex flow occurs when Re is between 68 and 425. Above Re = 425, transition from axisymmetric to nonaxisymmetric flow is observed up to Re = 575 before the emergence of wavy-vortex flow. From the variation of VL thickness with Re, the VL does not experience any significant changes in the flow separation region of the inner wall, as well as jet impingement region of both the inner and outer walls. However, a sudden increase in VL thickness in the flow separation region of the outer wall reveals possible sources of nonaxisymmetric disturbances in the flow separation region of the outer wall. These disturbances develop into the periodic secondary flow as the axisymmetric flow transforms into nonaxisymmetric flow, and this leads to the emergence of the azimuthal wave. The periodic secondary flow contributes to a sudden increase in the natural wavelength and rapid reduction in the strength of two counter-rotating Taylor vortices. This in turn leads to a substantial reduction of torque in the transition flow vis-A-vis axisymmetric Taylor-vortex flow. Published under license by AIP Publishing.
Simulation on Vortex Induced Vibration of Circular Cylinder
(2020-01-01)
Chang, T. J.
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Lua, K. B.
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機械工程學系
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Department of Mechanical Engineering
In order to study the vortical flow field and the vibration caused by fluid-structure interaction (FSI), an efficient simulation setup was established. The simulation made use of the built-in Deforming Mesh function, Semi-Implicit Method for Pressure Linked Equations (SIMPLE) Algorithm and SST-kw Turbulence Model of Ansys Fluent to generate the flow field. A User Defined Function (UDF) was written to calculate the displacement of the cylinder based on the simulated flow field information. A range of Reduced Velocity was scanned through in order to find the "locked-in" region, which is the resonance occurring region. The present study focused on the forces acting on the cylinder and the resultant trajectory of different cylinder natural frequencies. The simulation needs further fine tuning to improve the accuracy.
The performance of an acoustic levitator
(2020-01-01)
Hsu, C. H.
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Lin, H. Y.
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Agarwal, A.
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Davis, I.
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Nolan, K. P.
An acoustic levitator uses an array of ultrasonic transducers to generate a standing acoustic pressure field which exerts a radiation force on small particles, allowing the particles to be trapped, relocated, separated or combined. Experiments on levitating particles of different densities and calculations of the acoustic radiation force and moment have been reported in the literature. However, direct inspection on the acoustic pressure field pattern is seldom carried out. This paper reports an investigation on the performance of an existing acoustic levitator design, which uses off-the-shelf components, by comparing the visualized pressure field from Schlieren imaging to analytical simulations. The ability to compare Schlieren imaging results to analytical simulations readily can prove to be a vital tool. Since the simulations provide an ideal pressure field, the imaging of the levitator pressure field can highlight discrepancies between the real and ideal cases. This can be especially useful as a diagnostic tool to identify the cause of a drop in performance of the acoustic levitator in a real world scenario.
Two-dimensional numerical study of isotoxal-star polygonal cylinders in cross-flow
(2019-05-01)
Lee, Y. J.
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Zhou, G.
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Lua, K. B.
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機械工程學系
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Department of Mechanical Engineering
Numerical simulations have been conducted to investigate the flow structures and the force generation of two-dimensional regular and isotoxal-star polygonal cylinders in low Reynolds number (approximate to 150) cross-flow. The isotoxal-star polygons are essentially N-sided regular polygons with the addition of N V-grooves. Generally, transforming the regular polygon into the equivalent isotoxal-star polygon increases the normalized circulation, resulting in an increase in the magnitude of lift fluctuation and mean drag generation; this effect is sensitive to cylinder orientation. The influence of each V-groove on the overall flow structure and force generation of the isotoxal-star polygon is dependent on its placement and orientation, and the V-grooves fall into three main categories. Type I V-grooves are windward facing V-grooves containing the stagnation point; they are generally ineffective at enhancing force generation. Type II V-grooves are those upstream of the separation point but do not contain the stagnation point. They may enhance the magnitude of lift fluctuation if windward oriented, or yield negligible lift enhancement if oriented towards the transverse direction. Type III V-grooves are those downstream of the separation point, and they enhance the magnitude of lift fluctuation if they are oriented towards the vertical direction.
Wing-wake interaction: comparison of 2D and 3D flapping wings in hover flight
(2018-11-01)
Lee, Y. J.
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Lua, K. B.
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機械工程學系
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Department of Mechanical Engineering
The wing-wake interaction of flapping wings while hovering has been investigated, with the focus on the difference in wing-wake interaction between 2D and 3D flapping wings. Numerical simulations are conducted at a Reynolds number of 100, and the flapping configurations are divided into the 2D, quasi-3D and 3D categories. Variations of the aspect ratio and Rossby number allow the flapping configuration to morph gradually between categories. The wing-wake interaction mechanisms are identified and the effect of three-dimensionality on these mechanisms is discussed. Three-dimensionality affects wing-wake interaction through four primary aerodynamic mechanisms, namely, induced jet, downwash/upwash, leading-edge vortex (LEV) shedding due to vortex pairing, and the formation of a closely attached LEV. The first two mechanisms are well-established in the literature. With regard to the LEV shedding mechanism, it is revealed that the interaction between the LEV and the residue vortex from the previous stroke plays an important role in the early vortex shedding of 2D flapping wings. This effect diminishes with increasing three-dimensionality. With regard to the mechanism of the closely attached LEV, the wake encourages the formation of an LEV that is closely attached to the wing's top surface, which is beneficial to lift generation. This closely attached LEV mechanism accounts for most of the lift enhancement that arises from wake effects. Three-dimensionality alters the efficacy of the different aerodynamic mechanisms. Consequently, the dual peak lift coefficient pattern typically seen on 2D flapping wings transforms into the single peak lift coefficient pattern of the 3D flapping wing. It is also demonstrated that the mean lift enhancement due to wing-wake interaction diminishes rapidly when three-dimensionality is introduced. Results suggest that, for wings with parameters close to those of natural flyers, wing- wake interaction yields marginal lift enhancement and a small increase in energy consumption.