Leading edge vortex
The use of leading edge vortices (LEVs) seem to be a universal mechanism in biological systems, used to boost the force production in everything from falling samaras to swimming fishes and flying animals. LEVs are vortices in close proximity to a wing resulting in a low pressure region augmenting the force production. Our research has resulted in the first studies of LEVs in freely flying birds, bats and insects, and a question that arises is: Are LEVs in flying animal all behaving the same?
The simple answer is that LEVs do seem to differ between groups, although the detail of our knowledge about the development of LEVs differs between insects and vertebrates, mainly due to experimental issues making the study of LEVs in birds and bats more challenging.
A LEV that develops on a translating wing, as a result of an increase in angle of attack (angle between the wing and the oncoming air), grows in size and strength until it eventually sheds from the wing with a resulting loss of lift. In animals, the LEVs that develop during a downstroke stays attached to the wing and do not shed until the end of the downstroke. In insects and bats the LEV increase in strength from the base of the wing towards the tip, while in bird (i.e. flycatcher) the LEV gets stronger from the base to mid wing and then gets weaker again towards the tip. In our freely flying insects, as well as in some models, the LEV bursts (becomes turbulent) at some point along the span, a behavior we have not observed in vertebrates.
One of the questions that scientists have tried to answer for a long time is how LEVs are controlled. Apart from factors associated with the flapping motion of the wing (i.e. rotational acceleration) factors relating to the morphology of the wings are being investigated. Wing morphology as well as the ability to control the morphology differs markedly between the mayor groups of flying animals and is therefore of particular interest.
Bat wings consist of thin very compliant layers of skin, tendons and flexible skeletal bones that stretch the skin allowing for passive control. In addition, bending of the digits and muscles within the wing membrane allows for active control of the wing shape. One parameter known to affect the LEV development is camber, and bats increase their camber when flying slowly which may be a way of controlling the LEV.
In bird wings the surface is constructed from multiple small elements, the feathers, which are partly overlapping and linked together. In the flycatcher we see that the angle of attack, which is a factor that affects the development of the LEV, decreases from the base towards the tip of the wing, opposite to what is expected based on the flapping motion of the wings. It is thus likely that the birds control the LEV by adjusting the angle of attack, which is achieved by dynamic twisting of the wing and aeroelastic bending of the primary feathers.
Insect wings are essentially rather stiff, but chordwise flexible, flat plates with relatively little active control of the wing shape. Although the insect wings show some camber during the downstroke, it is far less than what is found in bats and in general we may expect less active control in insects. This may explain the bursting LEVs we find in freely flying insects. On the other hand, the high wingbeat frequencies found in insects may result in a stronger stabilizing effect due to wing rotational acceleration compared to birds and bats.
The data available regarding the development of LEVs in different animal groups is still quite rudimentary and further studies within AFL will shed light on how different animals control the development of the flow.