flowviz

youtube

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.

youtube

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!

youtube

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

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.

Source:

James Stack, Jonathan Colby, William Tsai

youtube

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.

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/

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.

Space bubbles!  Once again the lack of gravity in space allows some cool physics to be seen.  Here, a bubble within a drop of water produces an interesting image of Andre Kuipers aboard the International Space Station.  A drop of water acts a little bit like a double convex lens and, depending on the distance of the object from the bubble and the distance of the camera to the bubble, the image will either be inverted or not.  Here the drop acts as one lens, inverting the image a first time, and the bubble within acts as a second inverting the image again.  

(Credit: AP Photo/NASA)