Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D22: Biological Fluid Dynamics: Bird and Insect Wings |
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Chair: Roi Gurka, Coastal Carolina University Room: Georgia World Congress Center B310 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D22.00001: Aerodynamics of owls during flapping flight; comparative study: great horned owl, tawny owl and boobook owl Hadar Ben-Gida, Krishnamoorthy Krishnan, Roi Gurka Owls are known for silent flight during gliding and flapping thanks to their unique wing morphology. However, the current knowledge regarding their aerodynamic capabilities is incomplete. Here, we investigate owl’s aerodynamics; steady and unsteady contributions estimated from near wake flow measurements using long duration time resolved PIV. Three different owl’s species were flown in a wind tunnel in two flight configurations: perch-to-perch and steady flapping flight. Using long-time sampling data, several wingbeat cycles have been analyzed in order to cover both the downstroke and upstroke phases during flight. Drag and lift were obtained using the momentum equation for viscous flows and were found to share a highly unsteady behavior. The owls’ aerodynamics appeared to be different when comparing the two different flight modes over the two wingbeat phases. However, similarity was observed during transition phases. When comparing the flight performance of owls to other birds tested in the same facility, we found that the owls generate significantly more drag and less lift during steady flapping flight. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D22.00002: Model of Bird Alula Generates Arching Vortex on Finite Wings Thomas I Linehan, Kamran Mohseni The inimitable maneuvering capability of birds at high angles of attack is due in part to a miniature collection of feathers located at the bird's wrist termed the alula. The believed aerodynamic benefit of the alula stems from the vortex that it generates when deflected from the wing plane which facilitates the reattachment of flow over the wing. Further understanding of these physics of the alula specifically in the context of finite wings involving complex three-dimensional flow is needed to gauge its use in real world engineering applications. Towards this end, direct force/moment measurements alongside surface oil flow visualizations are conducted in a wind tunnel for which the alula and wing are modeled as rigid flat plates. Results indicate that the high-lift benefit of the alula stems from the formation of a vortex that arches from the leading edge of the wing, at the root of the deflected alula, toward the wing's side edge. The lift enhancement attributed to the alula increases as the alula is placed further from the wing's side edge until a critical distance for which the arching vortex is lost and lift enhancement is subsequently reduced. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D22.00003: Fluid dynamics of the flapping wings of the smallest insects Laura Miller In contrast to larger species, little is known about the flight of the smallest flying insects, such as thrips and fairyflies. These tiny animals range from 300 to 1000 microns in length and fly at Re ranging from about 4 to 60. Previous work with numerical and physical models has shown that the aerodynamics of these diminutive insects is significantly different from that of larger animals, but most of these studies have relied on two-dimensional approximations. We performed a systematic study of the forces and flow structures around a three-dimensional revolving elliptical wing. We used both a dynamically scaled physical model and a three-dimensional computational model. The results showed that dimensionless drag, aerodynamic efficiency, and spanwise flow all decrease with decreasing Re. In addition, both the leading and trailing edge vortices remain attached to the wing over the scales relevant to the smallest flying insects. The numerical portion of the work is then extended to a pair of three-dimensional wings performing a clap and fling. Lift is slightly enhanced, but drag is also significantly increased. Overall, these observations suggest that there are drastic differences in the aerodynamics of flight at the scale of the smallest flying animals. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D22.00004: Dual functions of insect wings: balancing aerodynamics and olfaction Chengyu Li, Haibo Dong, Kai Zhao The ability to track odor plumes to its source (food, mate, etc.) is the key to the survival of many insects. During this odor-guided navigation, flapping wings have been speculated to actively draw odorants to the antennae and enhance olfactory sensitivity. Utilizing an in-house computational fluid dynamics solver, we have quantified the odor plume structures of a fruit fly in forward flight motion and have confirmed that the flapping locomotion induces a strong airflow vortex over its head, thereby enhancing the odor mass flux around its antennae (by ~1.8 times at its peak). Contrary to the common belief that the wing shapes of insects are optimized purely for aerodynamic performance, our results suggest that, because both aerodynamic and olfactory functions are indispensable during odor-guided navigation, the wing shape and size may be a balance between the two functions. Furthermore, we found that the increased odor mass flux is the result of broader spatial sampling range in the vertical direction below the body, but not horizontally. This anisotropic spatial sampling range may also have important implications in understanding the behavior and algorithm of plume tracking in insects. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D22.00005: Optimal wing geometry and kinematics of a hovering rhinoceros beetle for minimum power consumption Sehyeong Oh, Boogeon Lee, Hyungmin Park, Haecheon Choi We investigate the optimal wing geometry and kinematics of a rhinoceros beetle in hovering motion for minimum power consumption. The original wing kinematics of a hovering beetle is measured using high speed cameras. Based on the measured wing kinematics, numerical simulations are conducted using an immersed boundary method. Numerical results indicate that the enhancement of vertical force and reduction of aerodynamic power requirement due to twist of hindwings are less than 3% as compared to their rigid counterparts, and the effect of elytra on the force generation is negligible. Therefore, we consider rigid and flat hindwings for optimization. We develop a predictive aerodynamic model which accurately predicts the force generation and power requirement of the flapping wing. Optimal wing kinematics and geometry are obtained applying this model together with a hybrid of a clustering genetic algorithm and a gradient-based optimizer. We find optimal solutions for the minimizations of aerodynamic and mechanical power consumption, respectively. Optimization results showed that optimal wing kinematics and geometry for mechanical power consumption are closer to those of a rhinoceros beetle than those for aerodynamic power consumption. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D22.00006: Neural-Inspired sparse wing sensors for insect flight Thomas Leonard Mohren, Steven L Brunton, Bingni Wen Brunton, Tom L Daniel Controlling high-dimensional systems with fast and robust feedback is a central challenge in the design of many modern engineering systems. This work is inspired by flying insects, which use a few embedded strain-sensitive neurons to achieve rapid and robust flight control despite large gust disturbances and a highly unsteady environment. Even more remarkable is how they achieve this with limited brain capacity and latency in communication. We investigate the sensing of exogenously induced body rotations by biological strain sensors on the wings of insects. Combining a structural wing model and electro-physiological wing neuron recordings, we show that arrays of neural-inspired sensors are able to detect exceedingly small differences in wing deformation that result from body rotations. Furthermore, we used sparse optimization to show that very few sensors in optimized locations on the wing are required to accurately classify rotation. The combination of temporal filtering with nonlinear neural thresholding and the optimized spatial distribution of biological sensors is crucial to achieve fast and robust, yet computationally efficient, sensory feedback. We believe this paradigm holds great promise for engineering design of high dimensional control problems. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D22.00007: Coherent relationship between passive rotation and active stroke of a flapping wing Yang Xiang, Haotian Hang, Zifeng Wen, Hong Liu By referring to the flapping flight of insects, human has realized the flight of micro aerial vehicles (MAVs). To reduce the system mass and decrease the mechanical complexity of MAVs, the wings are generally designed to be passively rotated by means of aerodynamic and wing inertial forces. However, it is an open question how to modulate the passive rotation of the flapping wing during flight and steering maneuvers. Therefore, we designed a passive-rotation flapping apparatus and measured the time varying wing kinematics by using the high-speed camera. Experimental results showed that the passive rotation strongly depends on the kinematics of active stroke of wing. By modulating the stroke amplitude and frequency, one obtained different rotating amplitudes and velocities, as well as the phases between stroke reversal and wing rotation, which is of particular importance for unconventional force generation. Based on the amplitude and frequency of wing stroke, a driving Reynolds number is applied to characterize the active stroke. To characterize passively rotating motion, a rotational Reynolds number is proposed. Furthermore, a coherent relationship between the driving Reynolds number and the rotational Reynolds number was demonstrated. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D22.00008: Estimating lift from unsteady wakes of flapping wings Shizhao Wang, Tianshu Liu, Guowei He The Kutta-Joukowski (KJ) theorem usually leads to puzzling results when it is applied to estimating the lift from the unsteady wakes generated by flapping wings. We investigate this puzzling problem by using three different flapping wing models, where the unsteady wakes are obtained by numerically solving the Navier-Stokes equations at a low Reynolds number. It is found that neither the unsteady nor the time-averaged lift coefficient is correctly predicted when the parameters for the KJ theorem are selected according to the widely accepted models in the literature. We propose a wake-sectional KJ model to predict the time-averaged lift, where the spanwise distance between the streamwise vorticity centriods is computed as a effective span length. Furthermore, we quantitatively identified that the phase difference of unsteady lift is caused by the quasi-steady assumption. We show the phase difference can be corrected by using an added mass lift model. This work is helpful to clarify the errors in estimating the lift from the wakes in animal flight. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D22.00009: Effects of Timing and Magnitude of Wing Stroke-Plane Tilt on the Maneuverability of Flapping Flight Chao Zhou, Long Chen, Jianghao Wu, Bo Cheng Hummingbirds perform a variety of agile maneuvers and one of them being the escape maneuver as they steer away from the threats using only 3-4 wingbeats in less than 150 ms. One kinematic feature that enables this maneuverability is the tilt of wing stroke plane. Here we investigate how timing and magnitude of the stroke-plane tilt affect this maneuverability using a flapping wing model with AR = 3 and Re=1000. The wing stroke plane is initially horizontal and then begins to tilt backward. Under different flapping amplitude, we quantify the effects of timing (relative to the phase of flapping cycle) and magnitude of this stroke-plane tilt on wing aerodynamic forces and moments using experimental, CFD and quasi-steady methods. Results show that backward thrust and pitching moment are maximized when the tilting occur near the end of downstroke and in the middle of upstroke, respectively. Good agreement is obtained among experimental, CFD, and quasi-steady model results, indicating that the effect of unsteady wake is negligible. Comparing the optimal stroke-plane tilting kinematics with those measured in hummingbirds suggests that hummingbirds attempt to maximize the backward acceleration in the beginning of the escape maneuver. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D22.00010: Passive rotation of a flapping wing with an inhomogeneous mass distribution Haotian Hang, Yang Xiang, Hong Liu By flapping wings, insects obtain lift to sustain flight. According to experimental and theoretical investigation, it is found that the wing of insects can be rotated passively owing to the aerodynamic and wing inertial forces. In addition, the wings of insect have various shapes and an inhomogeneous mass distribution, which introduces an additional torque owing to mismatch of rotation axis and the action point of the resultant of gravity force and buoyancy force. In this paper, we experimentally investigated the passive rotation of the flapping wing with an inhomogeneous mass distribution with a Reynolds number around 250 and measured the time varying kinematics by using high-speed camera. Experimental results showed that larger additional torque results in larger stroke angle, which generates larger conventional lift force due to the wing translation. Moreover, increasing additional torque can lead to advanced wing rotation, which is also beneficial to lift generation. Then, we compared the experimental results to the prediction of a quasi-steady numerical model. Our work demonstrates that the passive rotation of flapping wing depends sensitively on the additional torque, which potentially is used as an effective control method for design micro aerial vehicles (MAVs). |
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