# What do Aircraft and Fish have in Common?

What do fish and aircraft have in common? Well, water and air are both fluids. And when fish move their tails and bodies from side to side, they push against the surrounding water and leave behind a mini whirlpool or vortex, which contains information about the drag forces experienced by the fish as it moved along. If you can wind back the events that produced the vortex you can work out how it formed in the first place and therefore how much drag the fish felt. This is what Florian Huhn, from the German Aerospace Centre, has managed to do. And because aeroplanes produce very similar vortices in the air, the same technique can be used to develop improved aircraft designs, as he explains…

Florian – We were looking especially at the swirls, at the vortices that the fish typically create. The water slides really close to the skin of the fish, then the water gets some rotation with it and the result of this rotation put into the water when the fish passes are the vortices. Once we have found these vortices behind the fish, what we do is we use the velocity data from the simulations to move this piece of fluid backward in time.

Tom – By tracking the vortex backwards in time Florian and his time are able to see where the fluid making up the vortex originally came from. Interestingly, they found that water from both sides of the fish flows along its body and merges together at the tail where the vortex is then formed. This not only gives us an insight into how fish swim but can also be applied to many other problems.

Florian – At the tip of the wing – take a typical airplane – and we have a huge vortex but its bad for the pilot because if you land at the airport of course there were other planes before you and they all left their wing-tip vortices in the air somewhere. And you don’t want to hit those with your plane because that really shakes the plane.

Tom – Are they what cause the delay between other planes landing?

Florian – I know that there are other causes away form the runway and all these things, but I know its one limiting factor.

Tom – Understanding how these vortices form, that would give us an idea about how to make them smaller or how to make them go away more quickly and therefore leading to potentially more efficient airports.

Florian – Yeah that would be a good thing if that was possible.

You can listen to the full interview for the Naked Scientists here.