FlowViz - Fluid Dynamics

PhD student trying to bring fluid dynamics to the masses. Ask me anything - if I don't know it, I'll look it up.

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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.

Something I find amazing is one of the simpler examples in this video - the speaker. Those waves spreading out are what our ears hear as sound, simply changing the frequency of the waves changes the note we here.

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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.

F1 teams use fluorescent paint like the picture HERE to see if their wings are stalling.

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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. 

These devices aren't just used on planes but also on Formula 1 cars to manage the flow around the car.

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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!

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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!

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For those that can't make it to the Royal Society Summer Exhibition of Science here's a video to make up for it! This is what you can see in the wind tunnel on our stand. Despite being an old fashioned method of flowviz, it still gives great insight into the flow over a wing and it's wake.

There is, a Karman Vortex street behind the wing at zero incidence and small angles of attack, clear separation as the incidence is increased and a large turbulent wake.

If you want to ask us any questions on this, like how the flow viz works, or anything else aero/fluid related we've got a Twitter Q&A session tomorrow from midday. Tweet to @aeflowcontrol with the hashtag #asksummerscience.

If you want to see some more of what we're doing check out the links:  http://sse.royalsociety.org/2014/smart-wing-design/

http://www.smartwingdesign.org/

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Royal Society Exhibition

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!

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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.

Normally I'd say this is an example of the Plateau-Rayleigh Instability but there could be a bit more to it according to this paper.

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Following on from yesterday's look at vortices behind the bullet-shaped body used in drag reduction experiments, here is another view of the flow velocity behind the same body. (Of course done using PIV). These are the post-processed results where the blues and blacks are slow moving fluid and the oranges and yellows are fast moving fluid.

In the top case the flow is unaltered and the speaker is left turned off. You can see the recirculating flow and the unsteadiness of the wake.

In the second case the speaker has been turned on and the frequency tuned to minimise drag on the body. You can clearly see the reduction in the amount of recirculating flow and the unsteadiness of flow reduced.

If nothing else once again PIV provides some amazing pictures of the actual fluid flow.

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Second of today's PIV videos

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One of the best things about PIV (apart from amazing pictures) is the ability to measure the entire flow-field at once. Most methods of taking measurements in fluids only measure the flow properties at one point, and often in one direction. Although PIV originally struggled with time dependent measurements, this has largely been overcome and fantastic videos like today's are possible.

This is a time-resolved PIV series behind the test model shown in yesterday's post. There is a large speaker at the back of the body with a small slit around the outer edge of the cylinder. A synthetic jet is created through this slit, and the vortices that form behind the body interact with the separating, fluctuating wake to increase the base pressure. The first video comes from the PIV images and the second come after these images have been post processed and show the vorticity in the wake.

(I am not an expert on this work but for the interested parties more can be found here and this paper: Control of Entrainment in an Axisymmetric Bluff-Body Wake. In preparation for Physical Review Letters, Jan 2014.) 

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One of the overarching areas of research here, which has been going on for some years, has been the attempt to reduce drag on bluff bodies such as lorries. This work started with backward facing steps and moved onto the axisymmetric body in the first photo today. The general idea is to force the flow at the back of the bluff body using a speaker or actuator to "Virtually streamline" the body.

Lots of PIV work has been done on this body, which provides some cool pictures like the second today. This is a shot at the back of the cylinder while PIV was being used. This is one of the classic images from PIV with a darkened room and green laser.

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Today, a great example of the non-linear nature of the Navier-Stokes equations - with a little PIV of course! This video is of the flow caused by a synthetic jet. A synthetic jet is one which has zero net mass flux - i.e. one which does not "pump out" fluid from a source or reservoir. These jets are generally created by something pulsing backwards and forwards - here a speaker.

The bright line on the bottom left is the back of an object (something like a truck) and the small gap at the top is an orifice behind which is a speaker oscillating back and forth. What is amazing is that you can see a constant stream of fluid being ejected towards the right side of the screen, as well as fluid being entrained in from the top and bottom of the image. If this were a linear process all that could happen is the liquid move back and forth with the speaker and no jet would be formed.

Here the PIV has the flow still and the droplets suspended in the air.

This work is part of the flow control group here trying to reduce drag on bluff bodies.

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So after a brief description last week of PIV let's take a look at some images! Today's pics are some stills from one of the PIV experiments carried out in the last few years here. These stills are not really the output we aim for when doing PIV - that is still to come - but they are probably the most recognisable (and coolest). I like to think of these images as PIV for humans, or PIV by sight.

 True PIV requires cameras and post-processing, but the process of seeding the tunnel with oil droplets and using a laser to visualise them allows us to see what is going on just by inspection (assuming the flow is slow enough). These images were taken during PIV behind fractal grids, a set up that allows many different scales of turbulence to be investigated. You can see these different scales within the images.

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So, the basic set-up of PIV? The principle is pretty simple - we disperse seeder particles, like oil droplets, into the flow, upstream of the region we want to look at and we track how far they move in a given time with a camera. It is really important to ensure the camera can see the particles as they move so we shine a high-powered laser onto them. The camera is mounted orthogonally to the plane of the laser sheet. The change in location with time of each particle gives us their velocity in two dimensions.

This set up is shown pretty well in the photo above: The laser sheet can be clearly seen going across the wind tunnel (yes that square rickety thing is a wind tunnel) and the camera is at the top of the picture looking down at the sheet.

There is more to it than that of course but as a starter that's not too far off. I'll explain some of the finer points as we get into the fantastic results

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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!

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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!