ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 29 Nov 2020 17:53:20 GMT2020-11-29T17:53:20Z50171- On the prolong attachment of leading edge vortex on a flapping winghttps://scholarbank.nus.edu.sg/handle/10635/73715Title: On the prolong attachment of leading edge vortex on a flapping wing
Authors: Lim, T.T.; Teo, C.J.; Lua, K.B.; Yeo, K.S.
Abstract: In this paper, we take a fundamental approach to investigate the effect of spanwise flow on the prolonged attachment of leading edge vortex (LEV) on a flapping wing. By imposing a constant acceleration-constant velocity flow on elliptic wings of various sweep angles and angles of attack, our experimental and numerical results show that while spanwise flow per se has negligible influence on the prolong attachment of the LEV, vortex stretching can significantly delay detachment of the LEV, even for a small spanwise flow. © 2009 World Scientific Publishing Company.
Fri, 30 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/737152009-01-30T00:00:00Z
- Effect of wing-wake interaction on aerodynamic force generation on a 2D flapping winghttps://scholarbank.nus.edu.sg/handle/10635/60087Title: Effect of wing-wake interaction on aerodynamic force generation on a 2D flapping wing
Authors: Lua, K.B.; Lim, T.T.; Yeo, K.S.
Abstract: This paper is motivated by the works of Dickinson et al. (Science 284:1954-1960, 1999) and Sun and Tang (J Exp Biol 205:55-70, 2002) which provided two different perspectives on the influence of wing-wake interaction (or wake capture) on lift generation during flapping motion. Dickinson et al. (Science 284:1954-1960, 1999) hypothesize that wake capture is responsible for the additional lift generated at the early phase of each stroke, while Sun and Tang (J Exp Biol 205:55-70, 2002) believe otherwise. Here, we take a more fundamental approach to study the effect of wing-wake interaction on the aerodynamic force generation by carrying out simultaneous force and flow field measurements on a two-dimensional wing subjected to two different types of motion. In one of the motions, the wing at a fixed angle of attack was made to follow a motion profile described by "acceleration-constant velocity-deceleration". Here, the wing was first linearly accelerated from rest to a predetermined maximum velocity and remains at that speed for set duration before linearly decelerating to a stop. The acceleration and deceleration phase each accounted for only 10% of the stroke, and the stroke covered a total distance of three chord lengths. In another motion, the wing was subjected to the same above-mentioned movement, but in a back and forth manner over twenty strokes. Results show that there are two possible outcomes of wing-wake interaction. The first outcome occurs when the wing encounters a pair of counter-rotating wake vortices on the reverse stroke, and the induced velocity of these vortices impinges directly on the windward side of the wing, resulting in a higher oncoming flow to the wing, which translates into a higher lift. Another outcome is when the wing encounters one vortex on the reverse stroke, and the close proximity of this vortex to the windward surface of the wing, coupled with the vortex suction effect (caused by low pressure region at the center of the vortex), causes the net force on the wing to decrease momentarily. These results suggest that wing-wake interaction does not always lead to lift enhancement, and it can also cause lift reduction. As to which outcome prevails depend very much on the flapping motion and the timing of the reverse stroke. © 2011 Springer-Verlag.
Fri, 01 Jul 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/600872011-07-01T00:00:00Z
- On the aerodynamic characteristics of hovering rigid and flexible hawkmoth-like wingshttps://scholarbank.nus.edu.sg/handle/10635/60980Title: On the aerodynamic characteristics of hovering rigid and flexible hawkmoth-like wings
Authors: Lua, K.B.; Lai, K.C.; Lim, T.T.; Yeo, K.S.
Abstract: Insect wings are subjected to fluid, inertia and gravitational forces during flapping flight. Owing to their limited rigidity, they bent under the influence of these forces. Numerical study by Hamamoto et al. (Adv Robot 21(1-2):1-21, 2007) showed that a flexible wing is able to generate almost as much lift as a rigid wing during flapping. In this paper, we take a closer look at the relationship between wing flexibility (or stiffness) and aerodynamic force generation in flapping hovering flight. The experimental study was conducted in two stages. The first stage consisted of detailed force measurement and flow visualization of a rigid hawkmoth-like wing undergoing hovering hawkmoth flapping motion and simple harmonic flapping motion, with the aim of establishing a benchmark database for the second stage, which involved hawkmoth-like wing of different flexibility performing the same flapping motions. Hawkmoth motion was conducted at Re = 7,254 and reduced frequency of 0.26, while simple harmonic flapping motion at Re = 7,800 and 11,700, and reduced frequency of 0.25. Results show that aerodynamic force generation on the rigid wing is governed primarily by the combined effect of wing acceleration and leading edge vortex generated on the upper surface of the wing, while the remnants of the wake vortices generated from the previous stroke play only a minor role. Our results from the flexible wing study, while generally supportive of the finding by Hamamoto et al. (Adv Robot 21(1-2):1-21, 2007), also reveal the existence of a critical stiffness constant, below which lift coefficient deteriorates significantly. This finding suggests that although using flexible wing in micro air vehicle application may be beneficial in term of lightweight, too much flexibility can lead to deterioration in flapping performance in terms of aerodynamic force generation. The results further show that wings with stiffness constant above the critical value can deliver mean lift coefficient almost the same as a rigid wing when executing hawkmoth motion, but lower than the rigid wing when performing a simple harmonic motion. In all cases studied (7,800 ≤ Re ≤ 11,700), the Reynolds number does not alter the force generation significantly. © 2010 Springer-Verlag.
Wed, 01 Dec 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609802010-12-01T00:00:00Z
- A rotating elliptic airfoil in fluid at rest and in a parallel freestreamhttps://scholarbank.nus.edu.sg/handle/10635/84812Title: A rotating elliptic airfoil in fluid at rest and in a parallel freestream
Authors: Lua, K.B.; Lim, T.T.; Yeo, K.S.
Abstract: This paper reports results of DPIV measurements on a two-dimensional elliptic airfoil rotating about its own axis of symmetry in a fluid at rest and in a parallel freestream. In the former case, we examined three rotating speeds (Re c,Ω = 400, 1,000 and 2,000), and in the later case, four rotating speeds (Ro c,Ω = 2.4, 1.2, 0.6 and 0.4), together with two freestream velocities (Re c,u = 200 and 1,000) and two starting configurations of the airfoil (i.e., chord parallel to (α 0 = 0°) or normal (α 0 = 90°) to the freestream). Results show that a rotating airfoil in a stationary fluid produces two distinct types of vortex structures depending on the Reynolds number. The first type occurs at the lowest Reynolds number (Re c,Ω = 400), where vortices shed from the two edges or tips of the airfoil dissipated quickly, resulting in the airfoil rotating in a layer of diffused vorticity. The second type occurs at higher Reynolds numbers (i.e., Re c,Ω = 1,000 and 2,000), where the corresponding vortices rotated together with the airfoil. Due to the vortex suction effect, the torque characteristics are likely to be heavily damped for the first type because of the rapidly subsiding vortex shedding, and more oscillatory for the second type due to persistent presence of tip vortices. In a parallel freestream, increasing the tip-speed ratio (V/U) of the airfoil (i.e., decreasing the Rossby number, Ro c,Ω) transformed the flow topology from periodic vortex shedding at Ro c,Ω = 2.4 to the generation of a "hovering vortex" at Ro c,Ω = 0.6 and 0.4. The presence of the hovering vortex, which has not been reported in literature before, is likely to enhance the lift characteristics of the airfoil. Freestream Reynolds number is found to have minimal effect on the vortex formation and shedding process, although it enhances shear layer instability and produces more small-scale flow structures that affect the dynamics of the hovering vortex. Likewise, initial starting configuration of the airfoil, while affecting the flow transient during the initial phase of rotation, has insignificant effect on the overall flow topology. Unfortunately, technical constraint of our apparatus prevented us from carrying out complimentary force measurements; nevertheless, the results presented herein, which are more extensive than those computed by Lugt and Ohring (1977), will provide useful benchmark data, from which more advanced numerical calculations can be carried out to ascertain the corresponding force characteristics, particularly for those conditions with the presence of hovering vortex. © 2010 Springer-Verlag.
Mon, 01 Nov 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/848122010-11-01T00:00:00Z
- Wake-structure formation of a heaving two-dimensional elliptic airfoilhttps://scholarbank.nus.edu.sg/handle/10635/61687Title: Wake-structure formation of a heaving two-dimensional elliptic airfoil
Authors: Lua, K.B.; Lim, T.T.; Yeo, K.S.; Oo, G.Y.
Abstract: This paper is prompted by a recent numerical study (Lewin, G. C., and Haj-Hariri, H., "Modelling Thrust Generation of a Two-Dimensional Heaving Airfoil in Viscous Flow,"Journal of Fluid Mechanics, Vol. 492, Oct 2003, pp. 339-362) that shows that for a two-dimensional (2-D) elliptic airfoil undergoing prescribed heaving motion in a viscous fluid, both leading-edge vortices and trailing-edge vortices contributed to the formation of the wake structures. However, an earlier dye-visualization study (Lai, J. C. S., and Platzer, M. F., "Jet Characteristics of a Plunging Airfoil," AIAA Journal, Vol. 37, No. 12, 1999, pp. 1529-1537) on a heaving NACA 0012 airfoil appears to show that the wake structures were derived from trailing-edge vortices only. The dissimilarity in the two studies remains unclear because there is no corresponding experimental data on a 2-D heaving elliptic airfoil. In this study, digital particle image velocimetry technique was used to investigate the wake-structure formation of a 2-D elliptic airfoil undergoing simple harmonic heaving motion. For the range of flow conditions investigated here, our results show that the type of wake structures produced is controlled by when and how the leading-edge vortices interact with the trailing-edge vortices. Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Sun, 01 Jul 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/616872007-07-01T00:00:00Z
- Helical-groove and circular-trip effects on side forcehttps://scholarbank.nus.edu.sg/handle/10635/58345Title: Helical-groove and circular-trip effects on side force
Authors: Lua, K.B.; Lim, T.T.; Luo, S.C.; Goh, E.K.R.
Abstract: When a slender body, such as a missile, is pitched at high angle of attack to an oncoming flow, it may experience a large side force due to the asymmetric shedding of the tip vortices. The side force is well known to be highly detrimental to the performance of the flight vehicle. We assess the effectiveness of two control devices, namely, the circular trips and the helical grooves, in alleviating the side force on a tangent ogive nose cylinder. Simultaneous side force and pressure measurements taken in a wind tunnel show that the circular trip is generally more effective in reducing the side force than the helical grooves over a wide range of angle of attack. Detailed findings of their performances are reported.
Fri, 01 Sep 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/583452000-09-01T00:00:00Z
- Flowfield around ogive/elliptic-tip cylinder at high angle of attackhttps://scholarbank.nus.edu.sg/handle/10635/58297Title: Flowfield around ogive/elliptic-tip cylinder at high angle of attack
Authors: Luo, S.C.; Lim, T.T.; Lua, K.B.; Chia, H.T.; Goh, E.K.R.; Ho, Q.W.
Abstract: We present the results of experimental investigations on the flowfield around a conventional sharp-nose ogive cylinder and an elliptic-tip ogive cylinder. The studies include simultaneous side-force and surface pressure measurements in a wind tunnel as well as flow visualization in a water tunnel. The results show that changes in the direction of the side force are related to changes in the asymmetry of the pressure distribution along the body. Of the two tip shapes investigated, it is found that the variation of the side force with the roll angle for the elliptic tip is more predictable than that for the sharp ogive tip. Although the flow visualization study shows that the elliptic-tip cylinder with the major axis transverse to the freestream is more effective in delaying the onset of flow asymmetry to a higher angle of attack, the maximum side forces for the two tip geometries are almost the same.
Thu, 01 Oct 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/582971998-10-01T00:00:00Z
- Effects of Probe Interference on Side Force of an Inclined Ogive Cylinderhttps://scholarbank.nus.edu.sg/handle/10635/85084Title: Effects of Probe Interference on Side Force of an Inclined Ogive Cylinder
Authors: Ng, Y.T.; Lim, T.T.; Luo, S.C.; Lua, K.B.
Abstract: The effects of probe interference on the side force have been investigated and quantified for an ogive cylinder at α=45 deg.. The experiment was carried out in an open-loop suction wind tunnel with a rectangular test section measuring 0.6 m (height)×1.0 m (width). The results show that when the probe was in close proximity to the vortices, there was either an attenuation or amplification of the side forces, which caused an error in the side-force coefficient that can reach as high as 20% of the corresponding value without the probe. The reference values for the side-force coefficient C Fy0 were obtained.
Sun, 01 Feb 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/850842004-02-01T00:00:00Z
- Side force on an ogive cylinder: Effects of control deviceshttps://scholarbank.nus.edu.sg/handle/10635/58692Title: Side force on an ogive cylinder: Effects of control devices
Authors: Lee, A.S.; Luo, S.C.; Lim, T.T.; Lua, K.B.; Goh, E.K.R.
Abstract: This study is an extension of our earlier work, which examined the effectiveness of using an elliptic tip to control the side force acting on an ogive cylinder. In that study, only one tip was considered, and thus the effect of tip eccentricity on the side force was not known. In the present study, we examine another elliptic tip of a smaller eccentricity to get an insight into how tip eccentricity affects the local and overall side force distribution. Our measurements show that, although the smaller eccentricity tip has a side force distribution similar to that of the larger eccentricity tip, there are some major differences in their flow characteristics. For example, the larger eccentricity tip is found to reduce the onset angle of attack and delay the disappearance of the side force to a higher angle of attack. Furthermore, when α≈60 deg, only the lower eccentricity tip displays a hysteresis effect in its side force distribution. To the best of our knowledge, this phenomenon has not been observed on an elliptic tip before, even though a similar phenomenon has been observed on a conical body with a rounded tip when the cone was subjected to unsteady bleeding.
Wed, 01 Mar 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/586922000-03-01T00:00:00Z
- A study of wake structures of a heaving two-dimensional elliptic airfoil using DPIV techniquehttps://scholarbank.nus.edu.sg/handle/10635/73103Title: A study of wake structures of a heaving two-dimensional elliptic airfoil using DPIV technique
Authors: Lua, K.B.; Lim, T.T.; Yeo, K.S.; Oo, G.Y.
Abstract: The formation of the wake structures of a two-dimensional (2D) elliptic airfoil executing a simple harmonic heaving motion is investigated using Digital Particle Image Velocimetry (DPIV) technique. This work is motivated partly by the lack of experimental data on a 2D heaving elliptic airfoil and partly by the anomaly between the numerical results of Lewin and Haj-Hariri [Lewin, G. C. and Haj-Hariri, H., "Modelling thrust generation of a two-dimensional heaving airfoil in viscous flow," Journal of Fluid Mechanics, Vol. 492, 2003, pp. 339-362] and dye visualization results of Lai and Platzer [Lai, J. C. S and Platzer, M. F., "Jet Characteristics of a Plunging Airfoil", AIAA Journal, Vol. 37, No. 12, 1999, pp. 1529-1537]. The former study found that both the leading edge vortices (LEV) and the trailing edge vortices (TEV) contributed to the formation of the wake structures of a 2D heaving elliptic airfoil, whereas the latter study showed that only the TEV contributed to the wake structures of a heaving NACA 0012 airfoil. Possible reasons for the anomaly are provided.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/731032007-01-01T00:00:00Z
- Experimental study of two-dimensional flapping wings in tandem configurationhttps://scholarbank.nus.edu.sg/handle/10635/73449Title: Experimental study of two-dimensional flapping wings in tandem configuration
Authors: Zhang, X.H.; Lua, K.B.; Lim, T.T.; Yeo, K.S.
Abstract: This paper reports a fundamental investigation on the effect of phase difference (φ) between the flapping motion of forewing and hindwing on the lift and thrust generation of two-dimensional (2D) tandem wings in a forward flight condition at a Reynolds number of 5,000. Force sensor was used to measure time-dependent aerodynamic forces acting on the two wings, and digital particle image velocimetry (DPIV) technique was employed to obtain the associated vorticity field and flow structures. Three cases of phase difference were studied, namely 0° (in-phase), 90° and 180° (anti-phase). The results reveal that: (a) the cycleaveraged lift and thrust coefficient of the forewing are higher than the corresponding values of a single flapping wing for all the three cases; (b) the cycle-averaged lift coefficient of the hindwing is approximately the same as that of the single wing and is relatively independent of the phase difference; (c) the thrust coefficient of the hindwing decreases with increasing phase difference, and except for the case of φ = 180°, they are higher than that of the single wing; (d) the maximum thrust coefficient that occurs on the hindwing during in-phase stroking is 85.4% higher than that of a single wing case. Also, it is found that when the leading edge of the hindwing interacted with the wake structures of the forewing, the thrust increased rapidly. In the absence of such interaction, especially for the case of φ = 180°, the leading edge vortex (LEV) on the hindwing interacted destructively with the wake structures of the forewing, leading to a reduction in force generation compared to that of a single flapping wing. This finding supports previous computational modeling studies that the timing of vortex-vortex interaction plays a crucial role in the overall force generation of the hindwing.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/734492013-01-01T00:00:00Z
- Side force on an ogive cylinder: Effects of freestream turbulencehttps://scholarbank.nus.edu.sg/handle/10635/85625Title: Side force on an ogive cylinder: Effects of freestream turbulence
Authors: Luo, S.C.; Lua, K.B.; Lim, T.T.; Goh, E.K.R.
Abstract: The effects of freestream turbulence on the side forces acting on an ogive cylinder at a high angle of attack were studied. For a fixed angle of attack, an increase in turbulence intensity caused the side force to either decrease or increase, depending on the roll angle position of the surface. A small change in turbulence length scale (Lx/D) greatly influenced the reduction of side forces.
Sat, 01 Dec 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/856252001-12-01T00:00:00Z
- On the aerodynamic effects of pitching phase angle on a two-dimensional hovering winghttps://scholarbank.nus.edu.sg/handle/10635/73707Title: On the aerodynamic effects of pitching phase angle on a two-dimensional hovering wing
Authors: Zhang, X.H.; Lua, K.B.; Lim, T.T.; Yeo, K.S.
Abstract: This paper reports a fundamental investigation of the effects of pitching phase angle on the aerodynamics of a two-dimensional (2D) flapping wing executing simple harmonic motion in hovering mode. Direct force measurement and digital particle image velocimetry (DPIV) were employed to obtain the time-dependent aerodynamic forces acting on the wing and the associated flow structures, respectively. Six pitching phase angles, i.e. 0°, 60°, 90°, 110°, 150° and 180°, were studied. For the present set of wing motions, the delayed pitching (i.e. φ < 90) caused the averaged lift to decrease and the averaged drag to increase, overall reducing the lift to drag ratio greatly. On the other hand, the advanced pitching, in the range of 90° < φ ≤ 110°, caused the lift to increase and drag to decrease, resulting in higher lift to drag ratio. But further increase in the phase angle had the opposite effects, resulting in both the lift and lift to drag ratio peaking at φ ≈ 110°. © 2011 IEEE.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/737072011-01-01T00:00:00Z
- Aerodynamic forces and flow fields of a two-dimensional hovering winghttps://scholarbank.nus.edu.sg/handle/10635/84846Title: Aerodynamic forces and flow fields of a two-dimensional hovering wing
Authors: Lua, K.B.; Lim, T.T.; Yeo, K.S.
Abstract: This paper reports the results of an experimental investigation on a two-dimensional (2-D) wing undergoing symmetric simple harmonic flapping motion. The purpose of this investigation is to study how flapping frequency (or Reynolds number) and angular amplitude affect aerodynamic force generation and the associated flow field during flapping for Reynolds number (Re) ranging from 663 to 2652, and angular amplitudes (α A) of 30°, 45° and 60°. Our results support the findings of earlier studies that fluid inertia and leading edge vortices play dominant roles in the generation of aerodynamic forces. More importantly, time-resolved force coefficients during flapping are found to be more sensitive to changes in α A than in Re. In fact, a subtle change in α A may lead to considerable changes in the lift and drag coefficients, and there appears to be an optimal mean lift coefficient (C1) around α A = 45°, at least for the range of flow parameters considered here. This optimal condition coincides with the development a reverse Karman Vortex street in the wake, which has a higher jet stream than a vortex dipole at α A = 30°and a neutral wake structure at α A = 60°. Although Re has less effect on temporal force coefficients and the associated wake structures, increasing Re tends to equalize mean lift coefficients (and also mean drag coefficients) during downstroke and upstroke, thus suggesting an increasing symmetry in the mean force generation between these strokes. Although the current study deals with a 2-D hovering motion only, the unique force characteristics observed here, particularly their strong dependence on α A, may also occur in a three-dimensional hovering motion, and flying insects may well have taken advantage of these characteristics to help them to stay aloft and maneuver. © 2008 Springer-Verlag.
Mon, 01 Dec 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/848462008-12-01T00:00:00Z
- Role of tip and edge geometry on vortex asymmetryhttps://scholarbank.nus.edu.sg/handle/10635/61253Title: Role of tip and edge geometry on vortex asymmetry
Authors: Lim, T.T.; Lua, K.B.; Luo, S.C.
Abstract: Flow visualization studies were performed on an ogive-cylinder to investigate the flow over a delta wing at high incidence. Sharp tip and sharp edges (STSE), Ogive tip and Sharp edges (OTSE) and ogive tip and round edge (OTRE) model were considered for the study. Charge-coupled device (CCD) video cameras were used to capture the flow patterns and asymmetric lifting of the tip vortices from the wing. Investigation results suggested that both axisymmetric ogive tip and edge geometry were responsible for the phenomenon of vortex asymmetry and overall force distribution.
Thu, 01 Mar 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612532001-03-01T00:00:00Z
- Side force on an ogive cylinder: Effects of surface roughnesshttps://scholarbank.nus.edu.sg/handle/10635/61286Title: Side force on an ogive cylinder: Effects of surface roughness
Authors: Luo, S.C.; Lua, K.B.; Goh, E.K.R.
Abstract: The effects of surface roughness on the side force acting on an ogive cylinder at high incidence were studied. Examination of the circumferential pressure distribution showed revealed that, with the roughened surface at certain roll angles, the boundary layer on both sides of the cylinder appeared turbulent. The results indicted that aluminium powder type of surface roughness used was capable of triggering the laminar turbulent transition in the boundary layers of the ogive cylinders resulting in a side force that fluctuated more with roll angle.
Mon, 01 Jul 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612862002-07-01T00:00:00Z
- Side force on a rough-surface ogive cylinder: Effects of freestream turbulencehttps://scholarbank.nus.edu.sg/handle/10635/61285Title: Side force on a rough-surface ogive cylinder: Effects of freestream turbulence
Authors: Lua, K.B.; Luo, S.C.; Goh, E.K.R.
Abstract: Side force on a rough-surface Ogive cylinder was discussed. Effects of freestream turbulence were also studied. The turbulent floe was also studied. Results showed that an increase in the turbulence intensity causes the side force to decrease.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612852003-01-01T00:00:00Z