K. B. Lua

Wake-Structure Formation of a Heaving Two-Dimensional Elliptic Airfoil

This paper is prompted by a recent numerical study 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 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

A quasi-steady aerodynamic model for flapping flight with improved adaptability.

An improved quasi-steady aerodynamic model for flapping wings in hover has been developed. The purpose of this model is to yield rapid predictions of lift generation and efficiency during the design phase of flapping wing micro air vehicles. While most existing models are tailored for a specific flow condition, the present model is applicable over a wider range of Reynolds number and Rossby number. The effects of wing aspect ratio and taper ratio are also considered. The model was validated by comparing against numerical simulations and experimental measurements. Wings with different geometries undergoing distinct kinematics at varying flow conditions were tested during validation. Generally, model predictions of mean force coefficients were within 10% of numerical simulation results, while the deviations in power coefficients could be up to 15%. The deviation is partly due to the model not taking into consideration the initial shedding of the leading-edge vortex and wing-wake interaction which are difficult to account under quasi-steady assumption. The accuracy of this model is comparable to other models in literature, which had to be specifically designed or tuned to a narrow range of operation. In contrast, the present model has the advantage of being applicable over a wider range of flow conditions without prior tuning or calibration, which makes it a useful tool for preliminary performance evaluations.

Aspect ratio effects on revolving wings with Rossby number consideration.

Numerical simulations have been conducted to investigate the effect of aspect ratio (AR) on the mean lift generation of a revolving flat rectangular wing. The purpose of the study is to address some discrepancies reported in the literature regarding the influence of AR on mean lift coefficient. Here, we consider a range of AR from 1 to 10 and Rossby number (Ro) from 0.58 to 7.57, and our results show that different degrees of coupling between AR and Ro yield different trends of a mean lift coefficient with respect to increasing AR. The choice of reference velocity for the normalisation of mean lift forces also has a significant effect on the perceived AR effect. By isolating the effect of Ro, we found that higher AR produces higher mean lift coefficient until it plateaus at a sufficiently high AR. This finding is consistent with conventional fixed wing aerodynamics. Additionally, our results show that increasing AR reduces the three-dimensional wing tip effect and is beneficial to mean lift generation while higher Ro increases leading-edge vortex instability, which is detrimental to mean lift generation. Therefore, mean lift generation on revolving wings is dictated by the competition between these two factors, which represent two fundamentally independent phenomena.

Side Force on an Ogive Cylinder: Effects of Control Devices

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

Does Kutta lift exist on a vortex ring in a uniform cross flow

Past works [Y. K. Chang and A. D. Vakili, Phys. Fluids 7, 1583 (1995); R. Sau and K. Mahesh, AIAA Paper No. 2007-1316] show that a vortex ring ejected normal to a cross flow tilts and deforms as it propagates downstream, and they attribute this phenomenon to the Kutta lift or Magnus effect. Here, we show through a controlled experiment that there is no physical evidence of the existence of a Kutta lift when a fully developed vortex ring is exposed to a uniform cross flow. The observed phenomenon could be attributed to the modification of vorticity distribution of the vortex core due to the combined effect of the cross flow itself and the entrainment of boundary layer material during the formation of vortex ring.

Aerodynamics of two-dimensional flapping wings in tandem configuration

This paper reports a fundamental investigation on the aerodynamics of two-dimensional flapping wings in tandem configuration in forward flight. Of particular interest are the effects of phase angle (φ) and center-to-center distance (L) between the front wing and the rear wing on the aerodynamic force generation at a Reynolds number of 5000. Both experimental and numerical methods were employed. A force sensor was used to measure the time-history aerodynamic forces experienced by the two wings and digital particle image velocimetry was utilized to obtain the corresponding flow structures. Both the front wing and the rear wing executed the same simple harmonic motions with φ ranging from −180° to 180° and four values of L, i.e., 1.5c, 2c, 3c, and 4c (c is the wing chord length). Results show that at fixed L = 2c, tandem wings perform better than the sum of two single wings that flap independently in terms of thrust for phase angle approximately from −90° to 90°. The maximum thrust on the rear wing occurs du...

Design and Characterization of a Novel T-Shaped Multi-Axis Piezoresistive Force/Moment Sensor

In this paper, a T-shaped piezoresistive multi-axis force sensor fabricated by the semiconductor technology is developed. The sensors design, simulation, piezoresistors arrangement, and characterization are discussed. Fourteen piezoresistors are arranged on silicon beams and used as independent strain gauges. The three components (Fx, Fy, and Fz) of an applied force and two components of a moment (Mx, and My) can be simultaneously resolved from the piezoresistance changes induced by the stresses. The sensor was first characterized using a gravity mass reference test bench. The results show the properties of linearity (0.99), sensitivity (force: 1.5 mN; moment: 0.003 Nmm), and small crosstalk (≈5%) between the dominant force component and other components. The fabricated sensor was also verified against a commercial six degree of freedom load cell, and found to perform reliably with high repeatability, low hysteresis (0.5%), and good dynamic response (4 ms).

Wing–Wake Interaction of Three-Dimensional Flapping Wings

Experimental and numerical studies were conducted to investigate the effects of acceleration and deceleration durations on the wake capture effects on three-dimensional fruit-fly-inspired flapping wings. The contribution from the wake capture to lift and drag coefficients was isolated by comparing the forces generated by a wing in a quiescent fluid where the wake capture effect was absent, with the forces generated in a periodic flowfield where the full wake capture effect was encountered. The results showed that wake capture caused lift and drag coefficients to increase noticeably at the start of each flapping stroke. Also, a reduction in deceleration duration enhanced the wake capture effect, and a reduction in acceleration duration caused an earlier onset of wake capture with no significant effect on the force magnitudes contributed by the wake capture. Moreover, the effect of reducing the acceleration and deceleration durations in the same flapping motion resembled superposition of the two individual ...

Experimental Study of Two-Dimensional Flapping Wings in Tandem Configuration

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.

Water-Treading Motion for Three-Dimensional Flapping Wings in Hover

Flapping-wing micro air vehicles are not confined to undergoing the normal-hovering motion implemented by flying insects. In this study, the water-treading motion, which originates from aquatic pro...