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:

image

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.

image

Hot air is less dense than cold air.

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

Light gets bent/distorted

image

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:

image

                  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:

image

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

image

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.

An Engineer, Polarized sunglasses and round airplane windows

When you induce stress on an object and see it through your Polaroid sunglass, then you witness these amazing rainbow patterns.

image

This property of a material where the changes in optical properties of a material is used to determine its stress distribution is known as Photoelasticity.


The simplest way to understand stress distribution in a material is by using a sponge and some wooden planks.

Observe how the lines on the sponge change when one applies the load:

Uniformly Distributed Load

image

                                  PC:  University of Manchester


Concentrated Load

image

When a concentrated load is applied, the lines closer to the loading point become radially distorted but the effect of this distortion dies out as moves away. *

If those lines made sense to you, then the lines that you see through your polarized sunglasses are no different.

image

      Photoelastic visualization of contact stresses on a marble in a C-clamp

image

In addition, the patterns that you observe are directly proportional to load that you apply. You vary the load, you vary the pattern observed.

image

              Source

Why are airplane windows round?

How does knowing the stress concentration help you at all ? When you are an Engineer, knowing the stress concentration tells you the critical stress points in a structure ( or points of probable easiest failure )

image

                     Stress concentration in Square v/s Oval windows

As this Real Engineering video goes on to explain when square windows are used in an aircraft, there is a greater accumulation of stress in the edges than the oval windows.

This increased stress, lead to cracks forming near the sharp edges of the window and causing major havoc, which is why all modern aircraft windows are round.

That being said, it is ironical to note that pilots on aircrafts are not supposed to wear Polaroid sunglasses while flying!

( Check out the previous post to know more)


* Saint- Venant’s principle

Why do pilots use non polarized sunglasses ?

image

Polarized lenses are not recommended for use in the aviation
environment.

While useful for blocking reflected light from horizontal
surfaces such as water or snow,

image

polarization can reduce or eliminate the
visibility of instruments that incorporate anti-glare filters.

image
image
image

Polarized lenses may also interfere with visibility through an aircraft
windscreen by enhancing striations in laminated materials (known as photoelasticity)

image

     Photoelastic visualization of contact stresses on a marble in a C-clamp.

image
image

and mask the
sparkle of light that reflects off shiny surfaces such as another
aircraft’s wing or windscreen, which can reduce the time a pilot has to
react in a “see-and-avoid” traffic situation.

– FAA

*Source:  Polarized v/s non polarized cockpit images

In 2015, a 777-200 made the Newyork-London route in 5 hours,16 minutes where the usual journey time is ~7 hours.

The flight reached ground speeds of up to 1200 km/h (745 mph),riding a powerful jet stream of up to 322 km/h (200 mph) tailwinds and breaking the sonic barrier ( 1224 km/h (761 mph)).

Tail and headwinds

The principle is analogous to those high school problems in relative velocity:

“A man rows a boat in a river. The velocity of the
boat is … Find the stream velocity”

If you are headed downstream i.e in the same direction as the river stream you will reach your destination faster than if you were rowing upstream.

image

Similarly a tailwind is one that blows along the same direction of the aircraft increasing the net speed of the aircraft ,and headwind is one that blows in the opposite direction and slows the craft down.

image

So, does this mean that if you are moving at v kmph and there is a headwind of -v kmph, you would just hover? Hell yeah!

Take a look at this video:

Wind shear

image

A phenomenon known as ‘wind shear’ occurs when the wind speed changes abruptly, which can cause turbulence and rapid increase/decrease in velocity of flight.

This can be really challenging during landing since if the headwind turns tailwind, there is a possibility of the aircraft overshooting the runway due to the increased velocity.

What causes this ?

The aviation industry takes advantage of trade winds and jet streams in order to cut time off the flight and save fuel.

image

Tradewinds are caused by the unequal heating of the atmosphere
at different latitudes and altitudes and by the effects of the Earth’s
rotation (Coriolis effect).

image

                           Trade wind pattern. Credit: Earth Wind Map

Jet streams on the other hand are this narrow current of fast moving
winds in the upper troposphere flowing west to east. And riding one can
definitely make your travel time shorter.

image

                               Jet streams in the northern hemisphere

As a result of jet streams, within North America  the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by more than that amount if it must fly west against it.

How do pilots know about this ?

image

Pilots receive a weather briefing actively during flight. Included in the briefing is the best combination of jetstreams and other wind patterns that the pilot can take advantage of saving time and fuel.

Many airports have runways facing in different directions in order to allow the pilots to use the runway that faces the wind during take off/landing.

Have a great day!

fuckyeahphysica:

The Vapor Cone.

A vapor cone, also known as shock collar or shock egg, is a visible cloud of condensed water which can sometimes form around an object. A vapor cone is typically observed as an aircraft, or object, flying at Transonic speeds. ( slightly slower than the speed of sound) 

The Pressure – Temperature dependence.

As the aircraft approaches the speed of sound, the air pressure around the object drops, and thereby the air temperature drops. If the temperature drops below the dew point, water in the atmosphere condenses to form a cloud in the shape of the shockwave.

Red Bull Stratos and the Vapor Cone.

Remember that epic jump where Felix Baumgartner, as a part of the Red Bull Stratos project broke the sound barrier ( reached Mach 1.25 ) during his descent? But why weren’t vapor cones seen around Felix’s body? Or were they?

image

Vapor cones are formed only near the ground, where plenty of wet air persists. But when Felix broke the sound barrier, there was no wet air that surrounded him that would enable the formation of Vapor cones.

Have a Good day!

PC: twistedsifter

This is a Bonus post from the series. It‘s purpose is primarily to bring out the essence of pressure-temperature dependence that allows us visualize flow in a F1 car

Vortex Generators

So we now understand about boundary layers, flow separation, stalls, what causes them, how to detect and get out of one  (previous post).

One way to delay stalls is by inserting is a small angled plate  on the wing known as Vortex Generators. They impart a rotational/swirling motion to the flow of air on the surface of a wing.

Why do Vortex generators work?

Basically, creating a vortex over a surface allows you to delay boundary
layer* separation.

The swirling/rotational motion of air prevents the separation of air from the wing earlier on. This increases the lift and/or also reduces drag.

image

Benefits of vortex generators

As you saw in the gif above the stallspeed when using VGs drastically
reduces. This means that you can fly (and land safely) at low speeds without stalling.  

And it has also been shown that VGs reduce the noise generated by air inside the aircraft.

Some more benefits have been listed below (source) for the sake of completion:

● An added safety margin for low-speed flight   

● Improved low-speed handling characteristics

● Improved cross wind handling at low speeds 

● Increased safety margin in the event of an engine failure

● Reduced take-off distance, improving short field performance

Now this idea of forcibly generating vortices to increase lift and/or reduce drag finds application in some crazy places and we shall be looking at that in an upcoming post.

Good day!

How big is a A380 tail?

** We edited the second image in order to provide a perspective

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 

image

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.

image

              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.

image

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:

image

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

fuckyeahphysica:

What Is That Hole in the Tail of an Airplane?

When it comes to aviation, you just cannot take anything for granted. Everything serves a purpose, even the hole in the tail of the airplane.

APU ( Auxillary Power Unit )

The primary purpose of an APU on an aircraft is to provide power to start the main engines.

You gotta get them started and provide sufficient air compression for self sustaining operation, right ?

image

For smaller jet engines, one can accomplish this by electric motors.

But when the engine is a colossal giant, you need something much more than a quotidian electric motor to get it to start running.

image

What is it ?

The APU is essentially an air turbine motor, i.e a turbine that is used to produce power by using the air as a fluid

It is attached to the rear end of the aircraft. The hole in the rear end is used to direct the exhaust out of the aircraft i.e APU exhaust.

image

The APU is started by a battery or other means. But once the APU is running, it provides power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft’s main engines.

What other purpose does the APU serve?

APUs are also used to run accessories while the engines are shut
down. This allows the cabin to be comfortable while the passengers are
boarding before the aircraft’s engines are started.

image

                                             Source

Electrical power is
used to run systems for preflight checks. Some APUs are also connected
to a hydraulic pump, allowing crews to operate hydraulic equipment (such
as flight controls or flaps)
prior to engine start.

image

This function can also be used, on some
aircraft, as a backup in flight in case of engine or hydraulic failure.[

GPU ( Ground Power Unit )

image

Many a times you can also find the aircraft connected by some wires. This is the Ground power unit
and supplies the aircraft with electricity while the generators or the auxiliary power unit (APU) are not running.

Some
airports reduce the use of APUs due to noise and pollution, and ground
power is used when possible.


What we behold as merely a small hole on the tail has so much depth to it .Pretty cool eh?

Have a good day!

image

What Is That Hole in the Tail of an Airplane?

When it comes to aviation, you just cannot take anything for granted. Everything serves a purpose, even the hole in the tail of the airplane.

APU ( Auxillary Power Unit )

The primary purpose of an APU on an aircraft is to provide power to start the main engines.

You gotta get them started and provide sufficient air compression for self sustaining operation, right ?

image

For smaller jet engines, one can accomplish this by electric motors.

But when the engine is a colossal giant, you need something much more than a quotidian electric motor to get it to start running.

image

What is it ?

The APU is essentially an air turbine motor, i.e a turbine that is used to produce power by using the air as a fluid

It is attached to the rear end of the aircraft. The hole in the rear end is used to direct the exhaust out of the aircraft i.e APU exhaust.

image

The APU is started by a battery or other means. But once the APU is running, it provides power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft’s main engines.

What other purpose does the APU serve?

APUs are also used to run accessories while the engines are shut
down. This allows the cabin to be comfortable while the passengers are
boarding before the aircraft’s engines are started.

image

                                             Source

Electrical power is
used to run systems for preflight checks. Some APUs are also connected
to a hydraulic pump, allowing crews to operate hydraulic equipment (such
as flight controls or flaps)
prior to engine start.

image

This function can also be used, on some
aircraft, as a backup in flight in case of engine or hydraulic failure.[

GPU ( Ground Power Unit )

image

Many a times you can also find the aircraft connected by some wires. This is the Ground power unit
and supplies the aircraft with electricity while the generators or the auxiliary power unit (APU) are not running.

Some
airports reduce the use of APUs due to noise and pollution, and ground
power is used when possible.


What we behold as merely a small hole on the tail has so much depth to it .Pretty cool eh?

Have a good day!

image