Insects make fluid dynamic studies extremely difficult. They're small;they'll rarely fly steadily in a wind tunnel; and they tend to flap their wings extremely quickly. A three quarter milligram fruit fly, for example, can flap its 2mm long wings 250 times per second, making observing the air motion around the wings nearly impossible. To examine the flow, some researchers have painstakingly built scaledup models, but others, like Sun Mao of the Beijing University of Aeronautics and Astronautics, dream of simulating an insect entirely within a computer. The necessary physics has been known for more than 150 years, but only recently have computers been able to solve these equations for something as complex as an insect.
Computational fluid dynamicists like Sun Mao dream of using simulations to ask questions that would be difficult or impossible to answer experimentally. For example, how is a fruit fly different from a bumble bee? Just run the simulation twice and compare the results. How much power do insects use moving their wings, and how much do they use pushing the air around? Just turn off the simulated air, and let the computer do the rest. In his paper with Du Gang in Acta Mechanica Sinica, Sun Mao is beginning to achieve this dream of going where experimentalists dare not tread.
The researchers simulated eight different insects, from 0.7 mg fruit flies up to 1.6 g hawkmoths. Sun and Du were interested in how the different animals produce forces and how they modulate the power used to move their wings. Some insects, they thought, might save power by storing energy elastically in some type of spring and using that energy to accelerate the wings, rather than relying on muscular power alone. To create the virtual insects, they took morphological and kinematic data from previous studies, but they couldn't find much information on each insect's wing angle of attack – the angle between a wing and the oncoming flow. Because this angle strongly affects the lift force supporting the insect, they simulated various angles of attack and took the minimum angle necessary to support each animal.
Sun and Du confirmed that, despite the 1500-fold weight difference, all the insects produce high lift forces using a mechanism called `delayed stall, 'observed experimentally with model insects. Interestingly, the different insects use different amounts of energy to do it. Large insects have a large inertial power, meaning that the difficult issue is accelerating and decelerating their wings. Because the wings' inertia is their primary problem,they could benefit substantially by storing energy elastically. In contrast,small insects have a small inertial power but large aerodynamic power, meaning that they can easily accelerate their wings, but have trouble with air resistance, and therefore wouldn't gain much from elastic energy storage.
These results aren't too surprising, but they would have been extremely difficult to show experimentally. They also make an interesting prediction:that large insects can benefit from elastic energy storage. More importantly,they demonstrate some of the promise of computational fluid dynamic simulations.