I’ve got some fantastic PIV images coming but for today I’ll just put this filler up. Super hydrophobic powder that stays dry even after it’s been in liquid. Someone points out that it is much like many hot coco mixes that just won’t stir in!
Back in the day they really knew how to make educational videos. I have previously shown a video explaining water waves and, after posting footage from our new FlowViz wind tunnel, I came across this gem.
A simple explanation of camber, flaps, stall, separation and slots for a basic aerofoil. There are a whole heap of these videos on youtube, check them out if you want to find out more!
An absolutely fantastic example of flow viz around a model hill from NASA. The flow speed looks pretty slow which is probably required to be able to use smoke as a seeder for the flow viz. Although the Reynolds numbers must be pretty small when compared to an actual hill these visualisations can help validate numerical (CFD) code.
Great use has been used of colour and UV light allowing the wake behind the hill to be seen in great clarity.
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.
There are ways to stop this happening however as we will see tomorrow!
Over the next few posts I’m going to be showing some PIV - or Particle Image Velocimetry - and giving a little explanation of how it works. PIV is probably the best example of a Flow Viz technique which is not only stunning but also provides an almost unprecedented level of detail. It does come with draw backs but images like the third above are a good reason to go to the effort required for a good set of data.
We’ve done quite a bit of this here and so hopefully there will be a few fantastic images/vids!