#flow visualisation

As this video says, 'What does sound look like?'. This is one of those things that's hard to visualise whether you've never thought to try, or you're a hardened fluid dynamic nerd who's seen hundreds of FlowViz videos. However this video shows it in unbelievable detail and clarity.

This is made possible due to the Schlieren flow viz technique, which allows us to see regions of air with different densities - as is so well explained during the video.

A while ago you'll have seen I posted some FlowViz of a special leading edge in one of our wind tunnels. It is one of the exhibits inspired by nature that was on show at the Royal Society's Summer Science Exhibition. Well it has now gone all the way from theory to practice as McLaren used it on their rear wing this weekend.

What's even better is that it fits in perfectly with this week's look at stall and separation. Yes an F1 wing can stall just like the wings we've seen this week. The only difference is, that instead of losing lift, a car loses downforce. These wavy leading edges are probably aimed at stopping separation and stall more efficiently than by using vortex generators.

As promised, how to avoid stall - well delay it at least. This video shows a fantastic real life example of using vortex generators, as visualised by the cotton tufts. Stall occurs when the flow over the wing separates.

Vortex generators work by mixing the fast moving air outside the boundary layer with the slow moving flow inside. This adds momentum to the Boundary layer and helps it remain attached. Adding these vortex generators post production can help improve an aircrafts performance. 

While searching for yesterday's video I came across this one. It's not an unusual video but does demonstrate something that I still think is quite astounding - the air doesn't always flow straight over the wing but can actually reverse it's direction and flow towards the front of the wing.

This situation is called stall and occurs when the flow separates from the upper surface of the wing. (This can be seen well in yesterdays post at 3m05s & 4m49s onwards). This happens at either large angles of attack - like when a plane takes off - or when the plane's air speed is too slow. At this point the plane loses a large amount of lift which is extremely undesirable as it may not be able to support its own weight. 

In this video the first instance occurs at about 25s. The tufts attached to the wing allow us to see clearly the change in the flow direction. 

The Royal Society is hosting a summer exhibition of science this week. For anyone around London I really recommend taking a look! There are all kinds of exhibits including what it's like to see like a dog and The Higgs Boson and more. More importantly for all you flowviz fans there is an actual wind tunnel on our stand with hands on flow viz! You can also learn a load about what's new in the aero-engineering world and see a live demonstration of the Schlieren technique. Send me a message if you're interested and want to know more!

The Imperial Science Festival has been on yesterday and today and there's a whole load of flow-viz going on! We've got a stand with a hands on Schlieren example. From something incredibly simple we've got some fantastic examples - like this one. Oh, and obviously the obligatory high-speed camera allows us to slow everything down.

The video is of a jet coming out of an aerosol can, and shows brilliantly something close to my heart - Laminar to Turbulent Transition. You can see the instabilities (the small waves) beginning to grow and then eventually breaking down to turbulent flow.