How to photograph shock waves ?

fyfd:

This week NASA released the first-ever image of shock waves interacting between two supersonic aircraft. It’s a stunning effort, requiring a cutting-edge version of a century-old photographic technique and perfect coordination between three airplanes – the two supersonic Air Force T-38s and the NASA B-200 King Air that captured the image. The T-38s are flying in formation, roughly 30 ft apart, and the interaction of their shock waves is distinctly visible. The otherwise straight lines curve sharply near their intersections. 

Fully capturing this kind of behavior in ground-based tests or in computer simulation is incredibly difficult, and engineers will no doubt be studying and comparing every one of these images with those smaller-scale counterparts. NASA developed this system as part of their ongoing project for commercial supersonic technologies. (Image credit: NASA Armstrong; submitted by multiple readers)

How do these images get captured?

It may not obvious as to how this image was generated because if you have heard about Schlieren imaging what you have in your head is a setup that looks something like:

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But how does Schelerin photography scale up to capturing moving objects in the sky?

Heat Haze

When viewing objects through the exhaust gases emanating from the nozzle of aircrafts, one can observe the image to be distorted.

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Hot air is less dense than cold air.

And this creates a gradient in the refractive index of the air

Light gets bent/distorted

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Method-01 : BOSCO ( Background-Oriented Schlieren using Celestial Objects )

You make the aircraft whose shock-wave that you would like to analyze pass across the sun in the sky.

You place a hydrogen alpha filter on your ground based telescope and observe this:

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                  Notice the ripples that pass through the sunspots

The different air density caused by the aircraft bends the specific wavelength of light from the sun. This allows us to see the density gradient like the case of our heat wave above.

We can now calculate how far each “speckle” on the sun moved, and that gives us the following Schlieren image.

Method-02: Airborne Background Oriented Schlieren Technique

In the previous technique how far each speckle of the sun moved was used for imaging. BUT you can also use any textured background pattern in general.

An aircraft with camera flies above the flight like so:

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The patterned ground now plays the role of the sun. Some versions of textures that are commonly are:

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The difficulty in this method is the Image processing that follows after the images have been taken. 

And one of the main reasons why the image that NASA has released is spectacular because NASA seems to have nailed the underlying processing involved.

Have a great day!

* More on Heat hazes

** More on BOSCO

*** Images from the following paper : Airborne Application of the Background Oriented Schlieren Technique to a Helicopter in Forward Flight

**** This post obviously oversimplifies the technique. A lot of research goes into the processing of these images. But the motive of the post was to give you an idea of the method used to capture the image, the underlying science goes much deeper than this post.

fuckyeahphysica:

The Physics of the ‘Stall’

The proposition that more the angle of attack, more the lift does not hold at all angles.

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At about 14 degrees, something weird happens and the aircraft instead of soaring the skies starts to plummet to the ground.

When this happens it is known as a stall.

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                                                  Source

What causes Lift ?

The main thing to know is that a difference in pressure across the
wing–low pressure over the top and higher pressure below–creates the net
upward force we call lift.

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Upon reaching a certain velocity, the aircraft’s lift is more than its weight and as a result, the aircraft takes off .

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The Concept of a Boundary Layer (BL)

There is a high chance that you might have heard this word even in a casual conversation about wings and that’s because its an important concept in the context of aerodynamics and associated fields.

To understand the physics of a stall, lets consider the interaction of a moving air on a flat plate.

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The nature of airflow over a wing/plate is the result of stickiness or viscosity of air. 

The first layer sticks to the wing/plate not moving at all.

The second layer in frictional contact with the first moves slowly over it.

And the third layer moves somewhat faster than the second

Thus layer by later the flow builds up to the free stream velocity or airspeed. These layers of flow are known as boundary layers.

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What happens to the BL during a stall?

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                                              Source Video

During a stall, these successive tiers of air that form the boundary layer lose their gripping on the surface and break away into turbulence.

( what i mean by turbulence is the chaotic wiggling of the test leads attached to the wing in the animation )

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                                                  Source

It takes a pressure difference between the top and bottom parts of the wing in order to produce lift. But when the flow of air becomes turbulent ( i.e during a stall ), this pressure difference is no longer established.

As a result of which, the lift drastically decreases and the aircraft starts dropping to the ground.

How to get out of a stall ?

Stalls can cause problems only when the pilot is not aware that the aircraft is stalling. ( Unlikely but has caused accidents in yester times )

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                                                 Source

As the airplane loses altitude, its nose dips down and airspeed picks up quickly. This restores the lift and the pilot would be able to regain control and bring the aero-plane into level flight.

How are stalls detected ?

On light aircraft there is a reed (much like used on a musical wind
instrument) mounted on one wing root, which is angled such that at the
Angle of Attack which would cause a stall, the reed “plays” which can be
heard in the cockpit.

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Here is a view of where this system is mounted on a Cessna

On some aircrafts, it is a similar principal, however instead of a
reed, it uses a fin which at critical AoA pushes a micro-switch which
activates a buzzer/horn inside the cockpit.

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Here is the assembly on a Beech 18


Large commercial aircraft typically rely on either Angle of Attack (AoA) Vanes or Differential Pitot Tubes  to supply input to flight computers for the purpose of calculating AoA.

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                                                     Source


Review:

A lot of important stuff regarding aerodynamics in this post. Here’s a summary of the post:

Boundary Layer concept  — >  Why do aircrafts stall ? — >  How to get out of one  — > How are stalls detected ?

That’s all folks!

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Hope you enjoyed today’s post and learnt something new.

Have a good one !

This post covers the fundamental principles from which the subsequent posts queued up for this weekend are derived from. Stay tuned.. It is gonna be wild ride.

On Taj Mahal and Lift in airplanes

This is an interesting story that is probably popular among those in the aerospace community on how flaps help provide lift.

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During the World War II, a C-87 cargo plane (pic above) was all set to take off from Agra airport, India. The pilot had specifically asked for a small load of fuel for takeoff.

(because the C-87 did not climb well when heavily loaded  )

But the ground crew accidentally filled it to its full capacity and  forgot to tell the pilot about it.

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The pilot realized this only halfway through the runway and was already committed for take off.

With a three ton overload on the plane, the plane was heading for a fatal crash with one of the towers of the Taj Mahal which was being repaired at that time and was swarming with workmen.

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The pilot gave full throttle but it still refused to rise up.

And in a desperate attempt, he lowered the flaps fully and instantaneously the plane ballooned upwards.

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Surely, it lost some of its forward speed due to the increased drag. But it comfortably cleared the famous tomb, averting an impending disaster. So yeah, flaps on an airplane are no joke.

Have a great one!

* Why does lowering flaps increase lift?

** Physics of stall

*** Stories are great at linking words with experience. And this aids a lot in the learning process.

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From heavy bombers to fighter aircrafts  during the early 20th century, the radial engine was one of the most popular type of engine used.

The big advantage of radials was their large frontal area, which meant they could be air cooled, meaning
less maintenance, failures, and of course a lower cost of initial
purchase and maintenance. 

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But as more complex liquid based cooling systems  developed during the Word War II and the fact that its large frontal area also increased drag decreasing its efficiency, , the radial engine lost its zenith.

Nevertheless. its graceful mechanism of operation will never go out of style and will continue to captivate the minds of many who will retain its heritage.

Other cool stuff

The sound of a Radial Engine

Why are Inline Engines more commonly used than Radial Engines?

A radial engine mounted on a Volkswagen

Have a good one!

PC: Duk

The Blissful Month at FYP!

Here are some of the highlights of the month at FYP. Hope you guys enjoyed reading it as much as i enjoyed drafting each of these posts.:

A vortex portal to another universe – Wing tip vortices.

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Meteor strikes Thailand twice in 3 months.

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The fabric of Space and Time.

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Physics of snapping fingers.

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Why is ( -1 ) x ( – 1 ) = ( +1 )?

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The Magnetic Thinking Putty

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The Marangoni Effect – An affair with Surface Tension.

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This is what happens when Two Black Holes Collide- Gravitational Lensing

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Comets have two tails.

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Suck, Squeeze, Bang and Blow – That’s how a jet engine works!

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Thank you all for your support and for those kind words that you send me. Really appreciate it. Thanks a lot!

The Ultimate goal of FYP is to remove abstractions from the world that we live in and appreciate the physics that dwells everywhere around us. For sometimes it takes only a single bad physics teacher to make you hate the subject forever. But it doesn’t have to be that way!

You are always welcome to ask me anything/ suggest topics / submit / just hangout on Tumblr and also via email ( 153armstrong@gmail.com), whichever is convenient to you 🙂

Love you all and Have a good day!

PC: US coast guard, planetarysystem