flow visualisation

Posted Aug 6th: I always like to show what can be done with a normal camera like this perfectly timed shot.

It’s all in the timing! This perfectly timed photo shows the amazing shape water can take on (In this case when water drops on someones head) under the influence of gravity and surface tension. Surface tension tries to hold the sheet of water together while gravity is pulling the water downwards. As this is an image you could create with just a camera it is definitely one for ‘science at home’ although you might want to ask the volunteer first.

(Source: BitFlash)

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Shock waves!  Got to play with plasticine for this one. The shock wave is the black line that moves in fro the left and then bounces around the place like crazy.

Looks like 2 years of art school came in handy for something eh?

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Although I don’t only show things you can try at home, I try to provide a large proportion of posts that you can - and here is another easy experiment to try at home.

First take a cup of water and cool it in the fridge or freezer. Second, take another cup of water and add a small amount of food colouring to it. Heat this second cup (be careful) in a microwave or a saucepan. Once this is done carefully pour the heated, coloured liquid onto the colder water. Hopefully two layers will form due to the different densities of the liquids. As the cool water warms up and the coloured liquid cools down the densities change until eventually the coloured water is denser than the layer below.

This situation triggers the Rayleigh-Taylor instability and will hopefully form similar fantastic images to those above.

Photo credit: Physics central

Posted Sept 5th: Trees in a mug. A fantastic image of the fractal patterns forming between two liquids of different densities. It was formed from adding to icing sugar to food colouring. This looks like a classic example of the Saffman-Taylor instability, which occurs when a less dense fluid is moving towards a more dense one, although I’m not sure the respective densities of icing sugar and food colouring.

Source: Reddit - howabootthat Worth checking out the comments for some cool versions

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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!

Fluid dynamics is important in the world of sport. In general laminar flow over a body produces much less drag than turbulent flow does. So it would seem wise to try and remove turbulent flow. However, drag is also produced when the flow separates from a body, quite a lot of drag. In many cases turbulent flow is better at staying attached and not separating thus actually reducing drag. This is the reason golf balls have dimples on.

It is useful to visualise the flow over athletes to see if the flow separates. Adding various forms of roughness can cause the flow to become turbulent and thus reducing separation, and drag. Here roughness elements have been added to a speed skater and the flow is visualised with smoke.

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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.

Source: NASAAgov

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Lasers and impacting droplets can’t fail to impress. PIV is a flow viz technique in which small ‘seeder’ particles are released into a flow and laser light and cameras are used to map the positions of these particles. From these measurements flow quantities like velocity can be calculated.

In this example the interplay between a liquid surface and the air around it is seen as a drop collides with the liquid surface. It looks like the seeding machine is just off the left of screen and producing some pretty good vortices.

Source: IntellectualVentures

After last week’s comparison between an aerofoil and a tuna fish and yesterday’s post from FYFD I couldn’t help but put this one up. Designers increasingly are taking inspiration from nature and the comparison in this image between a B-2 bomber and a Hawk show a distinctive similarity. Although I believe the B-2 took this shape more for stealth reasons than aerodynamic it is amazing how similar a shape is reached in the end.

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We’ve had a few problems in London this week with fog. Stopped trains and cancelled flights galore, but at least there are opportunities for some great photos like these. What one can’t help but notice though, is the distinct wave-like structures we see in the clouds. Once again this demonstrates a familiar phenomenon but perhaps somewhere you wouldn’t expect to see it. Just like in water, waves of cloud can be formed by the wind blowing over the Earth’s surface creating gravity waves.

So Wednesday’s post was really only a warm up for today’s! This is a fantastic super-super slow mo of a shock wave in our supersonic wind tunnel. Shock waves are very thin regions where the flow properties rapidly change. The oscillation of these shock waves can cause damage or fatigue to aeroplanes or other supersonic vehicles, so the study of this oscillation can be very important.

Here the flow is from left to right and the dark, vertical, black line is the normal shock while the slightly fainter lines are oblique shocks coming from the contraction. The actual video was 8 minutes long but represented only 0.5 seconds in real time. That’s slowed down by almost 1000 times! As with Wednesday’s post the shock waves are visualized using Schlieren photography.

(The movement seems mesmerising, perhaps good for a screensaver?)

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After seeing photos like this and this I wanted to see what we could do with a relatively basic camera and a swimming pool. These shots capture the fluid flow around the ball just as it bursts through the surface into the air. Surface tension causes the water to adhere to the ball and pulls up a trail of very turbulent water as the ball exits the water. It almost looks like there is a laminar to turbulent transition of the water over the ball’s surface.

Design it like a Tuna fish! Scientists and engineers are increasingly taking ideas from nature to use in everyday society. The picture above shows a comparison of the profile of a Tuna fish with that of two airplane wing cross sections. It can be seen that the tuna fish shape is similar to that of the top wing which is especially designed to maintain laminar flow for as long as possible (which creates less drag than turbulent flow). Although perhaps a stretch to say this is the reason for the shape of the tuna, it is comparisons like these which are helping us produce ever more efficient, and inventive, designs.

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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!