fluid dynamics

Playing with a vortex cannon is a ton of fun, and they are remarkably easy to make. You can knock over cups or card houses, create art, or just try your best Big Bad Wolf impression. Or you can supersize things like one group in the Czech Republic did and build a 3m vortex cannon capable of firing 100m! (Seriously, watch it in action here.) And if you’d like to learn more about how vortex rings form and why they’re useful in nature and engineering, check out my vortex ring video. (Image credit: Laborky Cz, source; via Gizmodo)

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Two French scientists at the Pierre and Marie Curie University investigated the physics of popcorn: what causes it to jump? A leg of starch slams against the ground. And what’s behind that popping sound? The explosive release of water vapor. Here’s the study.

To get a microscopic look at the starchy bubbles that make popcorn so fluffy, check out Skunk Bear’s video.

Video: SoraPhotography/iStockphoto 

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What makes drops of food coloring able to dance, chase, sort themselves, or align with one another? This unexpected behavior is a consequence of food coloring consisting of two mixed liquids: water and propylene glycol. Both have their own surface tension properties and evaporation rates, which ultimately drives the behavior you see in the animations above. Both long-range and short-range interactions are observed. The former are due to vapor from each droplet adsorbing onto the glass around the droplet, thereby changing the local surface tension and causing nearby drops to feel an attractive force. The short-range effects are also surface-tension-driven. Droplets with lower surface tension will naturally try to flow toward areas of higher surface tension, which causes them to “chase” dissimilar adjacent drops. You can learn more about the research in the videos linked below (especially the last two), or you can read about the work in this article or the original research paper. (Image credit: N. Cira et al., source videos 1, 2, 3, 4; GIFs via freshphotons; submitted by entropy-perturbation)

I am almost every time put in a trance whilst spectating an aircraft/jet takeoff . There is always something interesting in the occurrence that makes me go nuts!

And this time around, there are these series of rings that one can see in the exhaust plume of a jet engine when it takes off ( usually when the afterburner is on).

I had no clue about the phenomenon nor did I know how to express it in ‘search engine’ terms to find a match.

But upon discussion with some of friends, I was shown this video of a space shuttle launch that seemed to produce a similar pattern.

Hmm.. Interesting

Shock Diamonds

These set of rings/disks that are formed in the exhaust plume are known as Shock Diamonds or Mach discs ( and by many more names ).

And usually occurs at low altitudes when the pressure of the exhaust plume is lower than the atmospheric pressure.

How does it form ?

Since the atmospheric pressure is higher than the exhaust, it will squeeze it inward. This compresses the exhaust increasing its pressure.

The increased pressure also instills a increased temperature.

As a result, ignites any excess fuel present in the exhaust making it burn. It is this burning that makes the shock diamond glow.

Source

The pressure is now more than the atmospheric pressure, and the exhaust gases start to expand out.

Over time, the process of compression and expansion repeats itself until the exhaust pressure becomes the same as the ambient atmospheric pressure.

In other words, the flow will repeatedly contract and expand while gradually equalizing the pressure difference between the exhaust and the atmosphere.

The same occurs in rocket engines as well.

What if?

What if the atmospheric pressure is less than the exhaust plume ( like at higher altitudes ), would we still see shock diamonds ?

Yup, we would! And here’s a picture of it too ( The Bell X-1 at speeds close to Mach 1 )

The same phenomenon as discussed above occurs except that the cycle starts with the exhaust gases expanding to atmospheric pressure first.

Did you enjoy this post?

There is an extensive explanation of shock diamonds given by shock waves which this post does not cover.

And this beckons the start of Supersonic Fluid Dynamics - a marvelous field of its own. If this captivated you, it is definitely worth a google.

Cheers!

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The Delta Flume, The World’s Largest Man Made Wave.

At 9 million litres, the machine can create waves up to 15 feet high. The other end of the trough is a simulated gradually rising coast, which is used to test full scale flood defenses such as dams and dykes. 

The Delta Flume and other machines like it was inspired by a catastrophic flood in The Netherlands in 1953 which took the lives of nearly 2000 people. From this point the Netherlands began devising more inventive ways of flood defense. 

(CityLab)

What happens when you step on lava? (First off, don’t try this yourself.) Lava is both very dense and very viscous, so, as illustrated in the animation above, it does not give all that much under pressure. If you were to fall on it, you’d land, sink a little bit, and then get burned. It’s also interesting to note that the lava springs back after being indented. Basaltic lava like that found in Hawaii, where this clip originates, does have viscoelastic properties, which might explain the elasticity of the deformed fluid. (Image credit: A. Rivest, source video; via Gizmodo)

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Flow Visualizations

Spread across the world are a legion facilities that allow the creation of artificial oceans ( wave tanks ) which are used for experimenting with surface waves. From testing simple parts for behavior to comprehension of complex flow phenomenon in oceans and seas, wave tanks are freaking awesome!

And how they work is simple:

At one end of the tank an actuator ( those giant hands that are moving in unison ) generates waves; the other end usually has a wave-absorbing surface.

Pretty cool eh?

Sources: Edinburgh University  Jason Truscott dantheman733

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Fluid

Interactive webtoy by David Li is a fun fluid dynamics simulator which lets you play around with various parameters - here is a video of it in action:

Fluid simulation is a GPU implementation of the FLIP method (with various additions). Particle rendering uses spherical ambient occlusion volumes. 

You can find out more and try it out for yourself here

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Step 1: Put a bubble in a droplet in zero g 

Step 2: Shoot other droplets at it 

Step 3: Astronuat Reid Wiseman blows your mind with some ISS fluid dynamics! 

If the force of the droplet hitting the big bloop is less than the surface tension, the droplet will bounce off. If the force is greater, they’ll join up. Amazing!

FLOW-3D トイレフラッシング解析
“いったい、トイレはどのように動作していると思いますか。それらは実際にかなり複雑で­す。ハンドルが押されると水は、便器を一杯にします。便器内の水がトラップ(防臭弁)­の上部より上昇した時、堰きタイプの流動が始まります。流れが十分速い時、泡は、サイ­フォンを作成しているトラップ上部にできます。その時点で、サイフォンは便器の水を抜­いてトイレを流します。

多くの場所で、節水が重要な問題です。低流量のトイレは、家庭と商業利用のために必要­です。しかし、もしトイレが最初の試みで勤めを果たさないならば、節水はできません。­最適な結果を得るための様々なデザインをモデル化するのにFLOW-3Dを使用するこ­とができます。”

One of the many ways to get a plane airborne is to blow fast moving air along the wings, generating lift. The above clipping is the scenario in action during a violent storm.

And surprisingly this method of moving the medium of traverse ( air ) instead of the object itself is the principle of operation of Wind Tunnels.

What is a wind tunnel ?

They are tube shaped facilities where powerful fans move air through the tube. The object is placed ( bolstered ) in a test section and the speeds of the air blown are controlled by fans 

By moving air around an object, the wind tunnel simulates the conditions during operation.

The object can be a smaller-scale model of a vehicle, one piece of a vehicle, a full-size aircraft or spacecraft, or even a common object like a tennis ball.

              NASA Tests Boeing Aircraft Tail in World’s Largest Wind Tunnel  

Usually, the object carries special instruments to measure the forces produced by the air on the object.

Engineers also study how the air moves around the object by injecting smoke or dye into the tunnel and photographing its motion around the object. Improving the flow of air around an object can increase its lift and decrease its drag.

It saves a lot of time and money required for the testing and analysis of designs, and prototypes.


I leave you guys with this clipping from a wind tunnel testing facility at NASA:

The Flappy Plane : This phenomenon is known as Flutter in Aerodynamics. It is an unstable oscillation that can lead to destruction


Have a Great Day!

More about Wind Tunnels: NASA, explainthatstuff

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Gross! But also a complicated physical phenomenon

“There’s a whole range of droplet sizes in this cloud, and the cloud is made of hot and moist air,” says Lydia Bourouiba. “And it’s turbulent, so that means that it has swirls and eddies, and it’s moving very fast.”

The conditions in the surrounding room — like airflow, moisture and temperature — can change how all those swirls and zigzags move, she says.

Bourouiba leads a research group at the Massachusetts Institute of Technology that studies fluid dynamics of disease transmission. If she can understand how a sneeze moves, she says, she can better understand how to prevent microbes from moving from a sick person or contaminated surface to somebody else.

You can watch a video from Science Friday exploring the phenomenon:

MORE HERE!