arXiv subjects

Mao Sun

Publications and source records attributed to Mao Sun.

5 recordsLinked to original sources

arXiv2023

Miniature insect flight

Approximately half of the existing winged-insect species are of very small size (wing length about 0.3-4 mm); they are referred to as miniature insects. Yet until recently, much of what we know about the mechanics of insect flight was derived from studies on relatively large insects, such as hoverflies, honey bees and hawkmoths. Because of their very small size, many miniature insects fly at a Reynolds number (Re) on the order of 10 or less. At such a low Re, the viscous effect of the air is very large: A miniature insect moves through the air as would a bumble bee move through mineral oil. Miniature insects must use new flapping mode and new aerodynamic mechanisms to fly. Over the past decade, much work has been done in the study of the mechanics of flight in miniature insects: novel flapping modes have been discovered and new mechanisms of aerodynamic force generation have been revealed; progress has also been made on the fluid-mechanics related flight problems, such as flight power requirements and flight dynamic stability. This article reviews these developments and discusses potential future directions.

physics.bio-ph↗

arXiv2018

Flapping-pattern change in small and very small insects

Medium and large insects in normal hovering have horizontal, planar up- and downstrokes1-4. The lift of the two half-strokes, generated by the leading-edge vortex, provides the weight-supporting vertical force. But for small insects (wing length R less than about 4 mm and Reynolds number Re very low, about 80 to 10), because of the large effect of air viscosity (as Re becomes very low, moving in air is like in oil), sufficient vertical force could not be produced if using the above wing kinematics. Small insects must use different flapping mode. Here, through analyzing flight data from our recent studies on a relatively-large small insect (fruitfly: R=3 mm, Re=80) and a very small insect (wasp: R=0.5 mm, Re=10), we put forward a hypothesis on how the flapping pattern will change: as insect-size or Re decreasing, a deeper and deeper U-shape upstroke will be used to overcome the viscous effect. And we test this hypothesis by measuring the wing kinematics for species of different sizes to obtain data for Re ranging from 80 to 10 and by computing the aerodynamic forces. The data and computation support our hypothesis: the planar upstroke changes to U-shape upstroke which becomes deeper as size or Re becomes smaller; for relatively-large small insects, the U-shape upstroke produces a larger vertical force than a planar upstroke by having a larger wing velocity, and for very small insects, the deep U-shape upstroke produces a large transient drag that points almost upwards by fast downward acceleration of the wing, providing the required vertical force.

physics.bio-ph↗

arXiv2018

Very small insects use novel wing flapping and drag principle to generate the weight-supporting vertical force

The effect of air viscosity on the flow around an insect wing increases as insect size decreases. For the smallest insects (wing length R below 1 mm), the viscous effect is so large that lift-generation mechanisms used by their larger counterparts become ineffective. How the weight-supporting vertical force is generated is unknown. To elucidate the aerodynamic mechanisms responsible, we measure the wing kinematics of the tiny wasp Encarsia formosa (0.6 mm R) in hovering or very slow ascending flight and compute and analyze the aerodynamic forces. We find that the insects perform two unusual wing-motions. One is rowing: the wings move fast downward and backward, like stroking oars; the other is the previously discovered Weis-Fogh fling. The rowing produces 70 percent of the required vertical force and the Weis-Fogh fling the other 30 percent. The oaring wing mainly produces an approximately up-pointing drag, resulting in the vertical force. Because each oaring produces a starting flow, the drag is unsteady in nature and much greater than that in steady motion at the same velocities and angles of attack. Furthermore, our computation shows that if the tiny wasps employed the usual wing kinematics of the larger insects (flapping back and forth in a horizontal plane), vertical force produced would be only one third of that by the real wing kinematics; i.e. they must use the special wing movements to overcome the problem of large viscous effects encountered by the commonly used flapping kinematics. We for the first time observe very small insects using drag to support their weight and explain how a net vertical force is generated when the drag principle is applied.

physics.bio-ph↗

arXiv2004

A computational study of the aerodynamic forces and power requirements of dragonfly Aeschna juncea hovering

Aerodynamic force generation and mechanical power requirements of a dragonfly (Aeschna juncea) in hovering flight are studied. The method of numerically solving the Navier-Stokes equations in moving overset grids is used. There are two large vertical force peaks in one flapping cycle. One is in the first half of the cycle, which is mainly due to the hindwings in their downstroke; the other is in the second half of the cycle, which is mainly due to the forewings in their downstroke. Hovering with a large stroke plane angle, the dragonfly uses drag as a major source for its weight supporting force (approximately 65% of the total vertical force is contributed by the drag and 35% by the lift of the wings). The vertical force coefficient of a wing is twice as large as the quasi-steady value. The interaction between the fore- and hindwings is not very strong and is detrimental to the vertical force generation. Compared with the case of a single wing in the same motion, the interaction effect reduces the vertical forces on the fore- and hindwings by 14% and 16% of that of the corresponding single wing, respectively. The large vertical force is due to the unsteady flow effects. The mechanism of the unsteady force is that in each downstroke of the hindwing or the forewing, a new vortex ring containing downward momentum is generated, giving an upward force. The body-mass-specific power is 37 W kg-1, which is mainly contributed by the aerodynamic power.

physics.flu-dyn↗

arXiv2004

Aerodynamic Force and Flow Structures of Two Airfoils in Flapping Motions

Aerodynamic force and flow structures of two airfoils in tandem configuration performing flapping motions are studied, using the method of solving the Navier-Stokes equations in moving overset grids. Three typical phase differences between the fore- and aft-airfoil flapping cycles are considered.

physics.flu-dyn↗