‘Lift and wakes of flying snakes’
by Anush Krishnan, John Socha, Pavlos Vlachos and L. Barba
of Boston Uni and Virginia Tech
Poetic, catchy and attention grabbing title, but I love it for the poetry. If the abstract was a short stanza I would be truly impressed. This was the first paper listed in my daily arxiv summary on Friday and it put a smile on my face. While I know a bit about vortices, more about those of the quantum variety than the classical, I’m not familiar with the research area of the flight of snakes. It has been interesting to learn that there was a bit of an industry in modelling how snakes fly, a question that appears motivated in part by bio-inspired design.
I’d love to see a flying snake in action in real life. There are apparently five species of flying snakes that reside primarily in southeast asia, India, Sri-Lanka and China. They are known by their scientific genus Chrysopelea and are named Chrysopelea ornata, Chrysopelea paradise, Chrysopelea taprobanica, Chrysopelea pelias and Chrysopelea rhodopleuron. They are quite a beautifully patterned reptile and while venomous, a bitey encounter with one of these fellows won’t kill you. However, I’d still rather keep my distance!
As the title implies, Anush Krishnan and collaborators look in detail at the wake of a cross-section of snake in flight and simulate the dynamical evolution of vortices formed by a cross-section of snake, investigating how these influence the lift of the snake. This was simulated solving the incompressible Navier-Stokes equation in two dimensions, which you can have a shot at yourself if you like, as the code is open source and available for the interested and curious. If I find enough free time after tidying up the twenty-odd unfinished projects I have lying around, I’ll have a play. If the words incompressible Navier-Stokes sounds like a foreign language to you, be content with knowing that they are modelling the flow of a classical fluid, such as water or air, and in doing so are assuming there is no sound or shock waves arising in the flight of the snake, or if there are, the effects on what the authors care about are negligible. Intriguingly, the cross section of a snake in flight is not a circle, it’s actually more of a triangular shape with a curved instead of a sharp edge. Somehow the snake flattens its body, therefore elongating it by expanding its ribcage to achieve this figure, which must be optimal for flight. As this description leaves a lot to the imagination, here is a picture from my artistically challenged perspective…
In the paper they give some links to figshare where you can find the exact profile, and also one where you can see a nice movie of vortices being generated by the in-flight snake at various angles (see here).
As a snake glides, they manoeuvre their body into an s-shape. There are some nice videos on you-tube that show you these snakes can clearly also change their trajectory during flight, but most of the important steering seems to depend on how they launch themselves for take-off. While watching I wondered how many bites the phd students received while researching. I guess this varies as a function of experience.
The main result of this nicely written paper is a theoretical agreement with previously experiments that show an angle of attack of 35 degrees gives a significant advancement in lift. The paper is worth checking out if only to look at the lovely pictures of vortices and vortex dynamics. For some parameters it seems they even see what looks like a von-karman vortex street. The illustration of vortex nucleation from the snake cross-section during flight is great, and interesting even from a purely fluid-dynamics perspective. All in all, I enjoyed delving into the poetic, `Lift and wakes of flying snakes’.