**Learning from Nature: Unsteady Flow Physics in Bioinspired Flapping Flight Bioinspired Flapping Flight**

**Learning from Nature: Unsteady Flow Physics in** 

DOI: 10.5772/intechopen.73091

Haibo Dong, Ayodeji T. Bode-Oke and Chengyu Li Additional information is available at the end of the chapter

Haibo Dong, Ayodeji T. Bode-Oke and Chengyu Li

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73091

#### **Abstract**

There are few studies on wing flexibility and the associated aerodynamic performance of insect wings during free flight, which are potential candidates for developing bioinspired microaerial vehicles (MAVs). To this end, this chapter aims at understanding wing deformation and motions of insects through a combined experimental and computational approach. Two sets of techniques are currently being developed to make this integration possible: first, data acquisition through the use of high-speed photogrammetry and accurate data reconstruction to quantify the wing and body motions in free flight with great detail and second, direct numerical simulation (DNS) for force measurements and visualization of vortex structures. Unlike most previous studies that focus on the nearfield vortex formation mechanisms of a single rigid flapping wing, this chapter presents freely flying insects with full-field vortex structures and associated unsteady aerodynamics at low Reynolds numbers. Our chapter is expected to lead to valuable insights into the underlying physics about flow mechanisms of low Reynolds number flight in nature, which will have great significance to flapping-wing MAV design and optimization research in the future.

**Keywords:** insect flight, high-speed photogrammetry, wing kinematics, wing flexibility, unsteady aerodynamics

## **1. Introduction**

In nature, flying is a unique mechanism for generating control and maneuvering forces by flapping the wings. Weis-Fogh and Jensen [1] described flapping flight as a complex physical and biological problem that it is impossible to understand a single part of the process completely. One of the reasons is that the unsteady motion of wings has related flow mechanisms at a Reynolds number (*Re*) of 10 to 10<sup>5</sup> [2]. **Figure 1** illustrates a trend in the relationship between

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

the Reynolds number and the body mass of both natural and man-made flying subjects. At this flow regime, lift producing mechanisms are intrinsically unsteady and vortex-dominated. Spanning over decades, considerable progress in understanding flapping flight has been achieved, and some general unsteady mechanisms have been identified. Examples include clap-and-fling [3–7], leading-edge vortex (LEV) [8–11], rotational lift [8, 12], wake capture [8, 13], wing-wing interactions, and body-wing interactions [14]. By using the rigid wing assumption, many of these mechanisms could explain the fluid phenomena near the wing, which are expressed as the motion of vortices, as well as the instantaneously local and resultant forces.

Flapping flight is a mode of transport widely adopted by natural fliers and has captured the interests of biologists and engineers because of several unique characteristics. From an energetic perspective, the propulsive efficiency of flapping motion can be higher than 85% [15]. Hence, flapping flight can be chosen as an alternative solution for aircraft propulsion to meet the need for high efficiency in energy consumption [16]. From the perspective of maneuverability and controllability, birds and insects have shown remarkable flying capabilities in tight spaces with multiple obstacles. In contrast, conventional aircraft cannot operate in such circumstances. The advantages of flapping flight have led to the development of micro air vehicles (MAVs) which mimic flapping flight. Many aspects of flapping flight, such as wing kinematics, structural response of wing, power consumption, and aerodynamics, are worth exploring. In particular, from a fluid dynamic point of view, the interaction of the flapping appendages with the surrounding air leads to the generation of vortices. Studying these vortex structures is of great importance for two reasons.

First, the vortex formation is related to the aerodynamic force, either indirectly or directly. Exerting forces on fluids can deform the fluids and leave footprints of the propulsion in the wake [17]. Therefore, even without force sensors and directly measuring pressure field, some elementary conclusions on force generation can be drawn based on the observed vortex formation in

**Figure 1.** Characteristics of biological flapping flight and conventional man-made flight based on the Reynolds number and body mass. Picture courtesy of Science & Society Picture Library.

the wake. For example, it has been shown by [18] that the location and orientation of vortex streets behind an oscillating airfoil can indicate if the force acting on the airfoil is drag-indicative or thrust-indicative. When sufficient velocity and vorticity information around a submerged body is given, the wake survey methods can accurately predict force acting on the body [19–21]. Because force exerted by fluids on an immersed body is equal and opposite to the force exerted by the body to the fluids, controlling the vortex shedding in the wake of an airfoil is found to be effective in changing the aerodynamic performance of the airfoil [22].

Second, flapping flight usually operates in a low *Re* regime, and many of flow phenomena can be readily explained by using vortex dynamics. According to the well-known Biot-Savart law, vortex structures in far field can induce velocity change near a flapping wing without direct interaction. Tijdeman and Seebass [23] present an example of how vorticity in downstream induced velocity around an airfoil and caused the oscillation of lift lag behind the motion of the airfoil. A few but important near-field mechanisms for lift enhancement, that is, leading edge vortex [24], rotational forces, and wake capture [8], have also accounted for the presence of vortex structures and their interactions with flapping wings.

#### **1.1. Insect wing and its motion**

the Reynolds number and the body mass of both natural and man-made flying subjects. At this flow regime, lift producing mechanisms are intrinsically unsteady and vortex-dominated. Spanning over decades, considerable progress in understanding flapping flight has been achieved, and some general unsteady mechanisms have been identified. Examples include clap-and-fling [3–7], leading-edge vortex (LEV) [8–11], rotational lift [8, 12], wake capture [8, 13], wing-wing interactions, and body-wing interactions [14]. By using the rigid wing assumption, many of these mechanisms could explain the fluid phenomena near the wing, which are expressed as the motion of vortices, as well as the instantaneously local and resultant forces. Flapping flight is a mode of transport widely adopted by natural fliers and has captured the interests of biologists and engineers because of several unique characteristics. From an energetic perspective, the propulsive efficiency of flapping motion can be higher than 85% [15]. Hence, flapping flight can be chosen as an alternative solution for aircraft propulsion to meet the need for high efficiency in energy consumption [16]. From the perspective of maneuverability and controllability, birds and insects have shown remarkable flying capabilities in tight spaces with multiple obstacles. In contrast, conventional aircraft cannot operate in such circumstances. The advantages of flapping flight have led to the development of micro air vehicles (MAVs) which mimic flapping flight. Many aspects of flapping flight, such as wing kinematics, structural response of wing, power consumption, and aerodynamics, are worth exploring. In particular, from a fluid dynamic point of view, the interaction of the flapping appendages with the surrounding air leads to the generation of vortices. Studying these vortex

First, the vortex formation is related to the aerodynamic force, either indirectly or directly. Exerting forces on fluids can deform the fluids and leave footprints of the propulsion in the wake [17]. Therefore, even without force sensors and directly measuring pressure field, some elementary conclusions on force generation can be drawn based on the observed vortex formation in

**Figure 1.** Characteristics of biological flapping flight and conventional man-made flight based on the Reynolds number

structures is of great importance for two reasons.

2 Flight Physics - Models, Techniques and Technologies

and body mass. Picture courtesy of Science & Society Picture Library.

Insect wings are thin cuticular structures enforced by veins that spread across the wing in intricate patterns. The leading edge of the wing contains thickened veins that provide structural rigidity. These several radially stretched flexion lines on the wing represent regions of increased flexibility along which the wing can deform and yield variable camber [25]. Using a dragonfly forewing as an example, **Figure 2** shows the leading edge of the wing is enforced by multiple vein structures. Wing mass mostly arises from the wing venation, and the pattern of the wing venation varies among species with the wing to body mass ratio ranging between 0.5–4% in dipterans and hymenopterans and 3–10% in butterflies [26, 27].

The distribution of the wing mass has mechanical importance. Spanwise mass distribution defines the wing's moment of inertia about its hinge point and therefore indicates the power required for flapping the wings. The center of mass (CoM) of the insect wings usually lies at about 30–40% of the wing length from the wing hinge, reducing the wing moment of inertia. The chordwise distribution of the wing mass also is important in easing wing rotation at stroke reversal. The wing's center of mass is located below the longitudinal axis of rotation of the wing. Therefore, the inertial forces due to the wing acceleration help flip the wing at the stroke reversal.

**Figure 2.** Sketch of the main wing veins (dashed lines) on the left forewing.

The geometrical wing shape is also of great importance in the generation of the aerodynamic force. Wing total area directly affects the magnitude of the aerodynamic force. The wing loading, defined as the ratio of the body mass over the wing area, is an indicator of flight performance. Wing area tends to increase linearly with the body dimension, whereas body mass is a function of volume increasing with the cubed body size. Therefore, insects with larger bodies usually have higher wing loading. In addition to the area, the aspect ratio, defined as the ratio of the wingspan squared to the wing area, is used to describe the wing shape. The aspect ratio of the insect wings varies in a wide range from 2 for some butterflies, to 10 for some Odonata.

Insects modulate their wing kinematics to change the aerodynamic force magnitude and direction. The body motion also affects the net movement of the wing relative to the air and therefore influences the aerodynamics of flight. In maneuvering flights, for instance, the rotation of the body can cause significant asymmetry in the trajectory of the bilateral wings. This effect is more pronounced in low-flapping-frequency insects where the rotation of the body within one wing beat is significant.

#### **1.2. Unsteady aerodynamic of flapping wing**

To achieve efficient flights in a low Reynolds number regime, insects operate their wings with a combination of translational and rotational motion in a stroke plane. The dominant unsteady flow feature that is responsible for the aerodynamic force generation is the vortex formation close to the leading edge of flapping wings. This vortical structure is produced by a laminar flow separation and produces a region of low pressure on the wing toward the leading edge. Ellington et al. [24] first illustrated a direct evidence of the existence of this leading-edge vortex (LEV) by visualizing the flow around a three-dimensional robotic wing at Reynolds number around 10<sup>3</sup> . This unique LEV is similar to the vortical structure produced during dynamic stall observed for conventional airfoils undergoing a rapid pitch motion. However, unlike the vortex produced during a dynamic stall, the LEV is not shed even after traveling many chord lengths of distance. As the flapping wing translates in its stroke plane, a spanwise velocity gradient interacts with the LEV. This causes the axial flow to spiral toward the wing tip direction. The axial flow transported momentum out of the vortex, keeping the LEV attached and stable. The LEV began to detach at the section close to the wing tip and shed into the wake. The vortex system generated by the flapping wing induces downwash in its surrounding fluid and forms a coherent momentum jet to maintain sustained flight.

#### **1.3. Role of wing flexibility**

Flying insects typically have flexible wings to adapt to the flow environment. **Figure 3** illustrates wing deformation of different species of insects with various wing geometry and aspect ratios under various flying modes. Due to the lack of internal musculature extending into the aerodynamic surface of the wing, insects have little active control over wing deformations. Therefore, the most surface morphology of insect wings is a product of the passive mechanical properties, while flapping wings interact with the inertial and aerodynamic forces. It is widely

**Figure 3.** (a) Dragonfly in turning flight, (b) Cicada in forward flight, (c) Butterfly in takeoff flight; all showing large-scale wing deformation.

thought wing deformation would potentially provide new aerodynamic mechanisms of aerodynamic force productions over completely rigid wings in flying.

By applying either a two-dimensional foil or a highly idealized three-dimensional wing model [28–30], recent studies on the dynamic deformations during flapping flight mainly focused on the negative camber resulting from the aerodynamic and inertial forces. The development of high-speed photogrammetry has made the detailed measurements of wing deformation during high-frequency flapping motion possible. The study of deformable wing kinematics of locust [31] used a large number of marker points, and approximately 100 per wing shows that both forewings and hindwings were positively cambered on the downstroke through an "umbrella effect" whereby the trailing edge tension compressed the wing fan corrugated, reducing the projected area by 30% and releasing the tension in the trailing edge. The highfidelity 3D dragonfly wing surface reconstruction performed by Koehler et al. [32] showed that insect wings could present up to 15% positive chordwise camber. Many fliers in nature have flexible wings which deform as the wings interact with the air around them. It has been opined that wing flexibility may provide new aerodynamic mechanisms of aerodynamic force production over completely rigid wings in flying [33–38].

#### **1.4. Quantification of wing flexibility**

The geometrical wing shape is also of great importance in the generation of the aerodynamic force. Wing total area directly affects the magnitude of the aerodynamic force. The wing loading, defined as the ratio of the body mass over the wing area, is an indicator of flight performance. Wing area tends to increase linearly with the body dimension, whereas body mass is a function of volume increasing with the cubed body size. Therefore, insects with larger bodies usually have higher wing loading. In addition to the area, the aspect ratio, defined as the ratio of the wingspan squared to the wing area, is used to describe the wing shape. The aspect ratio of the insect wings varies in a wide range from 2 for some butterflies, to 10 for

Insects modulate their wing kinematics to change the aerodynamic force magnitude and direction. The body motion also affects the net movement of the wing relative to the air and therefore influences the aerodynamics of flight. In maneuvering flights, for instance, the rotation of the body can cause significant asymmetry in the trajectory of the bilateral wings. This effect is more pronounced in low-flapping-frequency insects where the rotation of the body

To achieve efficient flights in a low Reynolds number regime, insects operate their wings with a combination of translational and rotational motion in a stroke plane. The dominant unsteady flow feature that is responsible for the aerodynamic force generation is the vortex formation close to the leading edge of flapping wings. This vortical structure is produced by a laminar flow separation and produces a region of low pressure on the wing toward the leading edge. Ellington et al. [24] first illustrated a direct evidence of the existence of this leading-edge vortex (LEV) by visualizing the flow around a three-dimensional robotic

produced during dynamic stall observed for conventional airfoils undergoing a rapid pitch motion. However, unlike the vortex produced during a dynamic stall, the LEV is not shed even after traveling many chord lengths of distance. As the flapping wing translates in its stroke plane, a spanwise velocity gradient interacts with the LEV. This causes the axial flow to spiral toward the wing tip direction. The axial flow transported momentum out of the vortex, keeping the LEV attached and stable. The LEV began to detach at the section close to the wing tip and shed into the wake. The vortex system generated by the flapping wing induces downwash in its surrounding fluid and forms a coherent momentum jet to maintain

Flying insects typically have flexible wings to adapt to the flow environment. **Figure 3** illustrates wing deformation of different species of insects with various wing geometry and aspect ratios under various flying modes. Due to the lack of internal musculature extending into the aerodynamic surface of the wing, insects have little active control over wing deformations. Therefore, the most surface morphology of insect wings is a product of the passive mechanical properties, while flapping wings interact with the inertial and aerodynamic forces. It is widely

. This unique LEV is similar to the vortical structure

some Odonata.

sustained flight.

**1.3. Role of wing flexibility**

within one wing beat is significant.

4 Flight Physics - Models, Techniques and Technologies

wing at Reynolds number around 10<sup>3</sup>

**1.2. Unsteady aerodynamic of flapping wing**

The ability to capture the flight trajectory and flapping locomotion of flying insects is essential for studying flapping flight and quantifying the associated unsteady aerodynamics. Because most fliers flap too fast for the human eye to capture every detail, photogrammetry has been used to study birds [39], bats [40], and insects [41, 42].

Several previous studies have investigated the mesosurface morphological details of the wings of tethered [43, 44] and free-flying [45, 46] insects. However, these studies focused primarily on static wings. Laser scanning was used to measure the surface roughness of severed insect wings [47]. Dragonfly forewing and hindwing structures have been studied using a micro-CT scanner [48]. Corrugation in insect wings, for example, locust wings, has also been investigated [44]. However, to study corrugation, for a tethered locust, a large number of marker points (approximately 100 per hindwing) are used. Tracking these large numbers of marker points in free-flight studies is undesirable. Despite these quantitative visualizations of insect wing morphology, few works have been done on detailed measurements of 3D wing morphology during free flight. The small wing size, fast flapping motion, and unpredictability of insect movement complicate the tracking of the details of wing kinematics and deformation.
