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.
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.
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
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!
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!
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?)
HI everyone, I’m afraid I’ve slacked with posting recently, but I’ll try and put things up a little more regularly again. I thought a good way to start was with this great Schlieren video from our supersonic tunnel (well beside it). Schlieren imaging uses the fact that as the density of air changes so does it’s refractive properties. This allows us to see these different regions of density. When this useful technique is combined with an 18,000fps high-speed camera you can get fantastic videos like this. The lighter is actually being waved around as quickly as possible but it has been slowed down hugely. The flow you can see from right to left at the top of the picture is actually the hot exhaust from the high-speed camera cooling.
I’m afraid we’ve been tucked away in a wind tunnel all day so today’s post is pretty late and will possibly be of interest to only a handful of people in the world. However, this is a Tollmien-Schlichting wave! The first one measured in our wind tunnel as far as I know.
These waves are an instability (see my week on instabilities) that occur in the very thin boundary layer of fluid (or gas) flowing over a surface. The flow begins off smooth (or Laminar) and instabilities such as the Tollmien-Schlicthing wave which grow exponentially and are one mechanism by which the flow can become turbulent.
Turbulent flow, in general, creates much more drag than laminar flow and so controlling these instabilities on vehicles like aircraft is a leading area of research.
This is the kind of flow viz I absolutely love - found in the most unexpected of places and through the most unexpected of methods!
At first sight this looked like a huge screen showing a film of waves but it actually turned out to be a great way of seeing what the wind was doing. Consisting of thousands of tiny plates free to rotate around one axis, as the wind blew they would change their orientation and thus what they were reflecting. This provides a fantastic view of the wind cascading down the front of the building. It could well be large vortices are being shed off the top of the building forming the wave-like patterns.
A great example you can see yourself. (If you happen to be in Oxford St London anytime soon)
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.
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.
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.
Wake dynamics are important for many classes of fluid machinery. Here we see a cross sectional view showing the wake behind a rotor. The vortex cores can clearly be seen at the top and bottom of the rotor. These are part of a helical structure which can be seen here. If a rotorcraft is descending the wake can play an important role in the aerodynamic load on the rotors. The vortex cores can ‘roll-up’ into one large vortex ring and detach leading to changing aerodynamic load.
After seeing this story on the BBC I thought I’d have a look at mercury. Element Hg and number 80 on the periodic table, its shiny mirror-like surface and upside down meniscus are probably two of its most identifiable traits. But a new one for me is how dense it is - you can float a whole lot more on top of mercury than water, including people.
Although inadvisable due to its poisonous nature, this picture from National Geographic Magazine October 1972 shows a man floating on top of a pool of mercury.
Google mercury and see a whole list of amazing uses and images.
Lasers are not quite so easy to get your hands on but they can be used for pretty much anything. Here a laser sheet and some smoke allows the visualisation of the tip vortices of a large rotating model helicopter blade. These vortices occur due to the difference in pressure between the bottom and top surface of the blade. At the tip this difference causes the flow to ‘curl up’ around the end of the blade.