How to Reduce Drag when Cycling

Cyclists can use up to 90% of their energy overcoming drag, which was the motivation behind the work of Ivaylo Nedyalkov at the University of New Hampshire, who was able to measure the force on each individual cyclist in a train formation to determine the best position to reduce your overall drag.

This video is part of a collaboration between FYFD and the Journal of Fluid Mechanics featuring a series of interviews with researchers from the APS DFD 2017 conference.

Sponsored by FYFD, the Journal of Fluid Mechanics, and the UK Fluids Network. Produced by Tom Crawford and Nicole Sharp with assistance from A.J. Fillo.

Climate Change will increase Turbulence on Flights

We’ve seen many recent extreme weather events – from mudslides in Columbia to flooding in Australia – which scientists say are a consequence of climate change; but it’s not just the weather that is affected. The Earth’s atmosphere is made up of several layers of air which all flow around each other in patterns known as jet streams and an increase in temperature will cause these to speed up. This is bad news for air passengers, including the 1 million people currently airborne at this very instant, because an increase in the speed of the jet streams will cause more turbulence making flying less comfortable and potentially more dangerous. I spoke to atmospheric scientist Paul Williams…

  • Climate change will cause a 59% increase in light turbulence, 94% increase in moderate turbulence, and 140%  increase in severe turbulence.
  • Turbulence is measured on a scale from 1 to 7 where 1 means light turbulence, 3 means moderate, 5 means severe, and 7 means extreme.
  • Light turbulence is a slight strain against the seat belt, moderate turbulence causes unsecured objects to become dislodged and makes walking around difficult, and severe turbulence results in anything that isn’t strapped down being catapulted around the cabin.
  • Turbulence is caused by wind shear – the higher you go up into the atmosphere the windier it gets – and instabilities within these layers of shear generate turbulence.
  • As the atmosphere is heated, the temperature increase causes the jet streams to move faster, creating more wind shear and thus more turbulence.
  • The researchers hope that results such as this will encourage us to think more carefully about our carbon footprint as there are likely many effects of Climate Change that we do not know about.

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

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.

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