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:

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.

Hot air is less dense than cold air.

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

Light gets bent/distorted

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:

                  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:

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

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.

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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.

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.

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

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.

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).

                           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.

                               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 ?

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!

The Vapor Cone.

fuckyeahphysica:

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?

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

y250 vortex

fuckyeahphysica:

F1 is more than just racing, it is an engineering battle. In this gif you can see the absolute control of wing tip vortices generated from the front wing. This is just an example to show the extreme aerodynamics that these vehicles are engineered for.

Have a great day!

* What are wing tip vortices ?

** Smoke angels and wing tip vortices

One of my friends pointed out to me that this is known as a y250 vortex since the vortex originates 250mm from the car’s centerline and it is on the y axis.

The y250 vortex is actually a single vortex but is formed from 3-4 cascades of blades on the front wing (see pic below).

From the previous posts we understood about the role that vortices played in delaying the onset of boundary layer on a wing by using vortex generators. And we also learned that any wing generates a vortex at the tip.

This is the case with the front wing on a F-1 car as well. Here is a computation model that illustrates this.

                               CFD model of the y250 vortex

To crudely put it, the front wings on a F1 car behaves like an inverted wing and contribute to 25-40% of the down-force produced.

But the most important function of the front wing is to aerodynamically redistribute the airflow to the various parts of the car. This begs the question:

What exactly does the y250 vortex do ?

                                  PC(amazing gif): motor sport tv

This gif illustrates what happens to the y250 vortex after it leaves the front wing. It deflects the air flow from the front wing preventing its interaction with the back tire thereby reducing drag.

It also prevents unwanted air flowing under the floor of the car greatly increasing the downforce produced. And reduced drag and increased downforce in coalition with a lot of other factors yields faster lap times.

But why does the vortex follow the surface ?

Quite frankly the true answer is great design and engineering! But this can be attributed to the Coanda effect. Simply put it is the tendency of a fluid jet to stay attached to a convex surface.

The most popular example of this effect is the spoon in a water faucet

In the case of vortices formed in a F1 car it just follows the surface of the car due to the Coanda effect as well.

Why don’t we see this vortex all the time?

If you have been following F1 you know that you are not the spectacle to this sort of display every race. 

The vortex is a region of low pressure and only under the right track conditions water vapor in the air condenses giving viewers a sneak peek into the magnitude of engineering and fluid dynamics that has gone behind each ecstatic lap.

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Aerodynamics and the vortices generated by the front wing is absolutely vital (and complicated) to the performance of a F1 car.

In this post we have taken a look at only one aspect of the front wing (y250 vortex). We urge you to check out the resources to quench your thirst for more.

Have a great day!

Resources :

* What on earth is a Boundary Layer ?

Red Bull’s Y250 and the Batchelor vortex

Basics of Front wing aerodynamics – (Link1)

Basics of Front wing aerodynamics – (Link2)

F1 air flow explained (gif source + amazing video)

Source: youtube.com

A vortex portal to another universe.

fuckyeahphysica:

This is known as wingtip vortex. It is a ramification of the design of the wing and how it works.

How does an aircraft fly? Think of it like this, due to the design of the wing, larger number of air molecules are hitting the bottom portion of the aircraft than the top.

As a result, a upward force acts on the wing, hence the wing lifts!

This works fine till we get to the wing tips.

In the wingtip, the air from a higher pressure wants to move to the region of lower pressure. And as a result, this forms vortices ( fancy name for the swirling motion of air ) known as Wingtip Vortex. ( because its formed in the wing tips!!! )

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Why do birds fly in a V formation?

Migratory birds take advantage of each other’s wingtip vortices by flying in a V formation so that all but the leader are flying in the upwash from the wing of the bird ahead. ( Look at the image, each one is exactly out of phase in its wing motion ).

This upwash makes it easier for the bird to support its own weight, reducing fatigue on migration flight.

And somehow birds know about this and recalibrate themselves in flight?

Wow! There is so much more to a bird’s flight that that meets the eye. I will take up the same sometime down the line. But, If you are really curious to find out why, read this nature article.

Have a Good Day!

PC: John Benson, boldmethod,mathcareer, natgeo, NASA.

EDIT – Also do check out the Smoke Angels.

Now we now know about stalls, boundary layers, vortex generators and wing tip vortices. In the concluding post of this mini series we will try to apply the knowledge gathered thusfar in the context of a F1 car. Stay tuned…

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.

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!

The Physics of the ‘Stall’

fuckyeahphysica:

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

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.

                                                  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.

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

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.

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.

What happens to the BL during a stall?

                                              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 )

                                                  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 )

                                                 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.

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.

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.

                                                     Source

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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!

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.

Thrust Vectoring

When you are in the combat zone, agility of a fighter jet is of utmost importance. But as an engineer, if you have already fiddled around with the wing structure your next option would be to fiddle around with the direction of the thrust.

Thrust vectoring is primarily used for directional control in rockets and jets. And one achieves this by manipulating the direction of thrust .

This generates the necessary moments (and forces) that enable the directional control of the aircraft. 

An aircraft traditionally has three “degrees of freedom” in aerodynamic
maneuverability; pitch, yaw and roll. **

The number of “dimensions” of
thrust vectoring relates directly to how many degrees of freedom can be
manipulated using only the vectored engine thrust.

Therefore, 2D
vectoring allows control over two degrees of freedom (typically pitch
plus either roll or yaw) while 3D controls all three.

Lockheed Martin F35B

The F-35B short takeoff/vertical landing (STOVL) variant is the world’s first supersonic STOVL stealth aircraft.

It achieves STOVL by swiveling its engine 90 degrees and directing its thrust downward during take off/lvertical landing mode.

In the following gif you can witness the transition from a 90 degree tilted engine towards a forward thrust engine during flying.

Unlike other variants of the Lockheed Martin F-35 the F-35B has no landing hook. And as a result, witnessing its landing is rather pretty special.

But nevertheless, this is one of those posts which addresses a topic that has been a gold mine for research. If this sort of thing fascinated you, there have been a lot of research conducted by NASA do check them out.

Have a great day!

*Rockets – How to turn during flight ?

** Aviation 101 : Pitch Roll and Yaw

F1 is more than just racing, it is an engineering battle. In this gif you can see the absolute control of wing tip vortices generated from the front wing. This is just an example to show the extreme aerodynamics that these vehicles are engineered for.

Have a great day!

* What are wing tip vortices ?

** Smoke angels and wing tip vortices

Source: youtube.com

In mathematics there is a concept known as ‘Conformal Mapping’ which allows you convert a given shape to a completely different one by making a transformation.

In the joukowski transform you take all the points on a circle and apply the following transform:

And the resulting transformed points resemble an aerofoil shape. Pretty cool huh ?

** Conformal mappings are a really cool topic in complex analysis but also equally extensive. If you want to know more about them click here

Source: youtube.com