How to photograph shock waves ?


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.


A note on triple and critical points

The Pressure – Temperature of a pure substance is commonly plotted as follows:


When you take a look at such a graph, there are two points on it that are particularly fascinating. (rest are all boundaries) : The Triple and Critical point.

Most people might be familiar with a triple point. This is where the solid liquid
and gaseous phase are in equilibrium.


 In the above animation, Gas phase is at the Top half of the container, Liquid at the bottom and Solid phase is sandwiched between the two phases.

If one keeps moving past the triple point and along the liquid vapor boundary line on the P-T diagram, one would notice that this boundary line terminates at a distinct temperature and pressure !!!

This point is known as a Critical point.


Beyond this point distinct liquid and gas phases do not exist and the substance is known as a super-critical fluid.

In order to understand what that really means, watch the following didactic demonstration of a super-critical fluid transition: (It is absolutely beautiful)

One of the reasons why the critical point is crucial is that close of the critical point, small changes in pressure or temperature result in large changes in density. And industries love to take advantage of this to fine tune their processing methods.**

Have a good one!

* We have discussed the phase diagrams for only pure substances here. But phase diagrams for mixtures are interesting too. Check this out.

** Decaffeinating coffee + Video demonstration of decaffeinating using Supercritical CO2  


As a child, I loved to ride in the car while it was raining. The raindrops on the window slid around in ways that fascinated and confused me. The idea that the raindrops ran up the window when the car moved made sense if the wind was pushing them, but why didn’t they just fly off instantly? I could not understand why they moved so slowly. I did not know it at the time, but this was my early introduction to boundary layers, the area of flow near a wall. Here, friction is a major force, causing the flow velocity to be zero at the wall and much faster – in this case roughly equal to the car’s speed – just a few millimeters away. This pushes different parts of large droplets unevenly. Notice how the thicker parts of the droplets move faster and more unsteadily than those right on the window. This is because the wind speed felt by the taller parts of the droplet is larger. Gravity and the water’s willingness to stick to the window surface help oppose the push of the wind, but at least with large drops at highway speeds, the wind’s force eventually wins out. (Image credit: A. Davidhazy, source; via Flow Viz)

Amazing.. I tried it at the Freeway the other day and look at the traveling wave pattern that it generates:

Thanks @fuckyeahfluiddynamics​!

Throwback Mondays: Pilot Wave Hydrodynamics

Two months ago, FYPhysics! in collaboration with FYFD on Tumblr dedicated an entire series exploring Pilot wave hydrodynamics. (You can check it out here)

And from one of the posts we learned that when you blow air through a nozzle at a water surface, you can see a circular wave propagating outwards.

BUT when a vibrational excitation is given to this propagating wave, that wave is split into two traveling waves moving in opposite directions – One towards the source ( called time-reversed waves ) and one away from it.

Now this paper by Bacot , takes this to the next level by positioning these nozzles in the shape of an Eiffel tower.

Now when one changes the effective gravity through a vibration, the time-reversed waves refocus at the center giving us back the Eiffel tower structure.


The group was also successful in refocusing a smiley face using the same principles which is equally mind blowing. (You can watch the video here)

Have a great week


A wall of lava lamps in a San Francisco office currently helps keep about 10% of the Internet’s traffic secure. Internet security company Cloudflare uses a video feed of the lava lamps as one of the inputs to the algorithms they use to generate large random numbers for encryption. The concept dates back to a 1996 patent for a product called LavaRand. The idea is that using a chaotic, unpredictable source as a seed for random number generators makes it much harder for an adversary to crack your encryption. 

With lava lamps, a lot of that chaos comes from the fluid dynamics involved – without perfect knowledge of thousands of variables, it would be impossible to simulate the lava lamp wall and get the same outcome as the real one – but there’s also randomness that comes from the measurement. People walking by, shifts in lighting, and random fluctuations of individual pixels all help make the video feed unpredictable. For those interested in the details of how Cloudflare uses their lava lamps, the company explains things for both technical and non-technical readers. You can also check out Tom Scott’s video for a good overview. (Image and video credit: T. Scott; submitted by Jean H.)

Chaotic pendulums


                                      Chaotic double pendulum

In addition to the Lava lamps, in it’s London office Cloudflare uses a
chaotic  pendulum with three arms that twist and turn together to
generate random numbers. 

Randomness from radioactive decay

A Geiger counter is a device used for the detection and measurement of all types of radiation: alpha, beta and gamma radiation.

When the counter detects a high energy particle, it creates an electric current. This current is amplified by electronic
circuitry, creating a crackling sound. 


           USB Geiger counter(amazon)

In Cloudflare’s Singapore office, a pellet of uranium encased in a glass
bell jar has its radiation monitored using a Geiger counter.

By monitoring how many counts are generated in a certain time, you can also generate truly random numbers.

Pilot Wave Hydrodynamics: Series Wrap-up

This week FYP in collaboration with FYFD brought to you an exclusive Tumblr series.on Pilot Wave Hydrodynamics. In case you had missed it out, here’s an overview:

1) We started with Chladni & inverse Chladni patterns – Basically normal modes of vibration on a plate with a subtle twist.


2) But what happens if the vibrating medium was water? This lead to a discussion on  Faraday instability;


3) Along the way, we discovered that at the right excitation, droplets placed atop a vibrating bath could be made to walk & bounce.


4) Using many of these bouncing droplets one could form extremely stable complex lattices


5) Then we started to subject the system to quantum experiments such as the single and double-slit experiments,


6) And found that they formed similar diffraction and Interference patterns with the bouncing droplets as well..



7) Maybe it was luck, so we tried quantum tunneling.


8) But even that was reproduced on the macroscopic scale using these bouncing droplets



9) Absolutely fascinated by what we had seen thus far, we then explored Pilot wave theory in its raw essence and its potential as an interpretation for Quantum mechanics


10) Hoping that this fascinated you, we left you with a treasure of useful resources to aid you in your ecstatic adventure. 

We hope you enjoyed this journey that spanned almost two centuries’ worth of scientific discoveries, feel free to share with us your thoughts, comments, and questions on this series.

Have a great weekend!


“If you place a small droplet atop a vibrating pool, it will happily bounce like a kid on a trampoline”. And when lots of these droplets are placed in a lattice, their behavior as a collective is absolutely fascinating.

In this series of gifs, you can see the evolution of complex lattices from simple droplets eventually leading to an instability that drives them apart.

Now a key thing to note is that when you have 7 droplets, you will not obtain a hexagonal lattice configuration per se. Those lattices had to be obtained artificially but can be very stable after they are formed.


Source: Archimedean lattices in the bound states of wave interacting particles

The key point of distinction when one talks about lattice in this vernacular is that in solid state physics, a crystal lattice is the depiction of three-dimensional solid as points.

And one obtains these crystalline solids through crystallization.


In contrast, when we are talking about lattices in pilot wave hydrodynamics, they are formed by the standing waves of the bouncing droplets.

In the upcoming posts, we will take a dive into some quantum mechanical experiments and their pilot wave hydrodynamic counterparts.

In case you had missed out, here are the previous posts on this collaborative series on Pilot wave Hydrodynamics with FYFD : 1) Introduction; 2) Chladni patterns; 3) Faraday instability, 4) Bouncing droplets

Water spirals are absolutely magnificent

This gif from one of the Slow mo guys videos is extremely fascinating. Water spirals have been of considerable research interest in Fluid Dynamics and as it turns out the gif is only a mode of ejection for the water.

Here are the other modes:


These are dependent on the angular velocity of the sphere The sphere is rotating
clockwise with an angular velocity of (a) 84, (b) 137, © 357, and (d)
406 rad/s.

And as one keeps increasing the angular velocity, the stability of the sheet is destroyed and the fluid ejects as small droplets from the equatorial region.

Source: Langley, Kenneth &
Maynes, Daniel & Truscott, Tadd. (2015). Eggs and milk: Spinning
spheres partially immersed in a liquid bath. Physics of Fluids. 27.
032102. 10.1063/1.4913574.

Impact of a laser on a drop

In this amazing work by the Fluids group at University of Twente, a drop’s response to a focused laser pulse has been analyzed in slow motion. The drop reacts to the energy imparted by the laser in many different ways, such as
vaporization or even plasma generation.


Tightly focused laser beam leading to a white plasma glow and a violent ablation from the drop

This extremely violent reaction propels the drop to several m/s before it explodes or breaks up. Now what cool applications can you think for this ? Let us know!


                    PC: xkcd      

Learn more at:

Physics of Fluids Group University of Twente

Laser Umbrella – What if? by xkcd


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

y250 vortex

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.

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)