Mosquitoes flap their long, thin wings four times faster than similarly sized insects. Imaging and computational analysis of mosquito flight illuminates some aerodynamic mechanisms not seen before in animal flight. See Letter p.92
Ever wondered why you hear a distinctive, high-pitched whine right before a mosquito bites at dusk? This is because mosquitoes flap their wings at a rate of nearly 800 beats per second, 4 times faster than many insects of a similar size1. Several other aspects of their flight also make them unique. For example, their long, slender wings move through an arc (called the stroke-amplitude arc) of only about 40° between the wing's most anterior and posterior positions. Yet, fruit flies have a stroke amplitude of about 150° (ref. 2), and even honeybees, known for their small-amplitude wing beat, have a stroke amplitude more than double that of mosquitoes3. On page 92, Bomphrey et al.4 present a closer look at mosquito flight, using state-of-the-art approaches to model air movement around the wing.
How does a wing generate lift? In what is known as an aerofoil, as air moves over a wing, the flow across the wing's upper surface is accelerated more than the air flow under its lower surface because of the shape of the wing and what is termed the angle of attack: the angle above the horizontal at which the wing encounters air passing over its surface. If air moves across the surface of a wing from left to right, the velocity difference between the upper and lower surfaces creates a net clockwise circular movement of air. This creates an upward lift that is proportional to the strength of the circulation.
A more intuitive explanation of this phenomenon might be to consider how lift is generated when you stick your hand out of the window of a moving car. If you hold your hand horizontally, there is little upward or downward force. However, as you rotate your hand upwards to increase the angle of attack, you will feel a greater upward force in addition to a force pushing your hand backwards. Not only is your palm now pushing air downwards, but also the top of your hand is bending the air into a downward jet. The overall result is that, on average, the air is directed downwards by both surfaces of your hand. Because a downward momentum is imparted to the air, an equal and opposite force acts upwards on your hand by Newton's third law, providing a lifting force.
Insects have ways of enhancing the lift provided by an aerofoil. Experimental, computational and theoretical research over the past two decades has revealed the complexity of lift generation in insects ranging from the fruit fly to the hawkmoth5,6,7. The basic flight aerodynamics for most insects are surprisingly similar. The wing beat includes a long-distance main sweeping movement known as translation, during which most of the lift is generated.
One mechanism that enhances lift during translation is the presence of a stable leading-edge vortex (LEV), an area of rotating air flow around the top of the wing (known as the leading edge), and the formation and separation of a trailing-edge vortex (TEV), another area of rotating air flow around the bottom of the wing (the trailing edge)8. The wing motion separates the air flows at the top and bottom edges of the wing, creating sheets of concentrated vorticity (rotating air). Negative-pressure regions form in these rotating regions, and the vortex sheets roll up into vortices near the leading and trailing edges. The stable, close 'attachment' (association) of the LEV to the leading edge of the wing maintains a negative-pressure region on the wing's upper surface, enhancing lift. This creates a pressure difference between the upper and lower surfaces of the wing that pushes it upwards.
Another mechanism of lift enhancement during the translational phase is through wake capture. As an insect hovers and its wings flap back and forth, it moves through the wake generated during the previous half stroke. By doing so, it encounters greater air-flow speeds than if it were to move through still air. This additional airflow strengthens the LEV and enhances circulation and lift.
By contrast, the short-amplitude wing strokes of mosquitoes are unable to take full advantage of these types of translational lift-enhancement mechanism, given the relatively shorter duration of the translational phase in mosquitoes. It had been speculated that mosquitoes instead rely on rotational mechanisms that occur as the wing changes direction between the forward, downward stroke and the reverse, upward stroke. However, numerous technical challenges had made rigorous study of this idea intractable until now.
To analyse mosquito flight aerodynamics, Bomphrey and colleagues used 8 high-speed video cameras that recorded images at 10,000 frames per second, 3D numerical simulations of this challenging fluid–structure interaction problem, and the technique of particle-image velocimetry to reveal the air-flow patterns around mosquito wings (see Supplementary Video 1 for the paper4). Similarly to the observations made of most insect, bird and bat flight9, the authors found that stable, attached LEVs enhance lift in mosquitoes (Fig. 1).
However, the authors identified two additional lift-generating mechanisms in mosquitoes that have not been described in any other animal: rotational drag and the strengthening of the TEV through wake capture. Rotational drag had been postulated as a mechanism for generating upward force10, but had not been observed in any flight system. TEV strengthening through wake capture had not previously been observed or proposed as a lift-enhancing mechanism. Both mechanisms emerge from the aerodynamics that occurs as the wing rotates through the air when it pivots between the forward and backward stroke.
In the case of rotational drag, as the wing begins to rotate about the leading edge at the end of a forward translational stroke, this pushes the air downwards and generates negative pressure on the wing's upper surface that enhances lift. If the wing were to continue to rotate by pivoting on the leading edge, once the wing became perpendicular to the ground it would start to generate negative lift because the wing would then push the air upwards. However, mosquitoes use a clever trick: rather than having a fixed pivot for wing rotation around the leading edge, the axis of rotation moves, and halfway through the rotation, the centre of rotation migrates from the leading edge of the wing to the trailing edge. This means that, towards the end of rotation, the wing pushes down rather than up on the air. It is not clear how mosquitoes accomplish this feat. This phenomenon has not been described in other insects, although it is possible that some might use the trick.
In TEV enhancement through wake capture, the mosquito wing's trailing edge encounters strong air flow from its wake at the end of each translational sweep. The movement of the air relative to the wing separates the flow at the trailing edge and induces the formation of the TEV. This TEV has a strong negative-pressure region in its centre that enhances lift during stroke reversal. This aerodynamic mechanism emerges from the dynamics of the short-amplitude wing beat of mosquitoes and is not seen in insects that use longer translational phases.
The discovery of these mosquito aerodynamic mechanisms raises some questions. Do other animals use rotational drag and strengthening of the TEV through wake capture? When is it beneficial to use higher-frequency, low-amplitude wing strokes? What are the consequences of these types of aerodynamics for the evolution of wing design, behavioural ecology and the energetics of insect flight? Could these aerodynamic mechanisms provide insights into how the design of miniature drones might be improved? Mosquito-flight investigations are certainly on their way to generating plenty of future research buzz.
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Applied Sciences (2018)