Flow(#2) – Plateau- Rayleigh Instability

When you wake up in the morning and open/close your faucet when you brush your teeth, you might have noticed that it undergoes a transition between a smooth jet to a dripping flow like so :

When the velocity of the fluid exiting the faucet is high, it appears smooth for a longer time before it breaks into droplets:

But when you make the velocity of the fluid exiting the faucet low, it seems to form droplets much earlier than before.

Here’s the water breaking into smaller droplets shot in slow motion:

Notice that just by changing the exit velocity of the water you can control when the droplets form.

You can also control the nature of the droplets that form by changing fluids. Here’s how it looks like if you use water as the fluid (left), pure glycerol(center) and  a polymeric fluid(right).


What is causing a jet of fluid to form droplets?

A simple answer to this is perturbations on the surface of the fluid. What does that mean?

Initially the fluid is just falling under the influence of gravity. And velocity of any freely falling object increases as it falls:

But the surface tension of the fluid holds the molecules of the fluid together as they fall down.

Therefore depending on the initial velocity of fluid, the surface tension of the fluid and the acceleration you get a characteristic shape of the jet as it falls down:

This is what you observe as the fluid exits the faucet.


Just after exiting the faucet, there are tiny perturbations on the surface of this fluid as it falls down. This is apparent when you record the flow at 3000fps:

                                            Source: engineerguy

Those tiny perturbations on the surface of the fluid grow as the fluid falls down i.e the jet becomes unstable.

And as a result the fluid jet breaks down to smaller droplets to reach a more thermodynamically favorable state. This is known as the Plateau-Rayleigh instability.


It takes different fluids different time scales to reach this instability. This depends on the velocity of the fluid, the surface tension and the acceleration it experiences. 

And some viscous fluids like honey are also able to dampen out these perturbations that occur on their surface enabling them to remain as fluid thread for an extended time.

A note on inkjet printers

By externally perturbing the fluid instead of making the fluid do its own thing, you can make droplets of specific sizes and shapes.

This engineerguy video explains how this is used in inkjet printers in grand detail. Do check it out.

We started today by trying to understand why water exiting a faucet behaves the way it does. Hopefully this blog post has gotten you a step closer to realizing that. Have a great day!

Sources and more:

* This is a topic that is home to a lot of research work and interesting fluid dynamics. If you like to explore more take a look into the mathematical treatment of this instability here.


Note on average density and why ships do not sink

Floating on the dead sea

Let’s ask a very generic question: I hand you an object and ask you to predict whether the object would float or sink. How would you go about doing that ? Well, you can measure the mass of the object and the volume of the object and can derive this quantity called Average Density (\rho_{avg} )

\rho_{avg} = m_{object}/V_{object}

It is the average density of the entire object as a whole. If this object is submerged in a fluid of density \rho_f , then we can draw the following force diagram:

If \rho_{avg} > \rho_{f} , we note that this generic object would sink and if \rho_{avg} < \rho_{f} it would float!. Therefore in order to make any object float in water, you need to ensure its average density is less than the density of the fluid its submerged in!

Why does a ship stay afloat in sea?

A ship is full of air! Although it is made from iron which sinks in water but with all the air that it is full of, it’s average density (m_{ship}/V_{ship} ) drops down such that \rho_{avg-ship} < \rho_{sea-water} .

Fun Experiment:

If you drop some raisins in soda, you will notice that they raise up and fall down like so (Try it out!):


This is because air bubbles that form on the top of the raisin decrease its average density to the point that its able to make the raisin raise all the way from the bottom to the top. BUT once it reaches the top all the air bubbles escape into the atmosphere (its average density increases) and the raisin now falls down.

Questions to ponder:

  • Why do people not sink in the dead sea ?
  • How are submarines/divers able to move up and down the ocean ? How would you extend the average density argument in this case.
  • Why do air bubbles in soda always want to raise up ?
  • If the total load that needs to on a ship is 25 tons. What should be the total volume of the ship in order to remain afloat if the density of sea water is 1029 kg/m3,

How do hot air balloons work?

  • This post follows the concepts from the previous post on Buoyancy.

One of the most surprising things about air that may not be intuitive is that it is a fluid and like any other fluids exerts a pressure on objects.

Standing on earth with layers of air above us, we are constantly being ‘weighed down’ by a pressure of ~1atm at all times.

All objects in air are also assisted by a buoyant force that is caused from the pressure difference between the top and bottom surfaces.

Let’s now consider the hot-air balloon in particular. A force diagram is probably the best way to start:

Envelope is the the actual fabric which holds the air inside the balloon

Therefore we see that in order for the hot air balloon to float, we need to have the buoyant force compensate for the net weight from the balloon, the load on it and the air inside it:

F_B > w_{envelope} + w_{load} + w_{hot-air}

Say you built a modest hot-air balloon that just barely managed to get off the ground. How would you make it go higher ?

You don’t want to play around with w_{load} because you don’t want to throw out any of your passengers or your supplies. You can’t really play around with w_{envelope} without re-making the balloon again.

BUT you can reduce the weight of the hot air inside the balloon ( w_{hot-air} = m_{hot-air} g = (\rho_{hot-air} V_{hot-air}) g ) ! How ?

Recall the ideal gas law P = \rho R T . Assuming that the pressure and the volume of the hot-air balloon does not change we note that:

\rho_{hot-air} \propto 1/T_{hot-air}

meaning that if we want to reduce the density of air, we just need to crank up the temperature.

And with a spectacular burner onboard our hot air balloon, we can easily increase the temperature of the air inside the balloon!

That’s pretty much how hot-air balloons work! You increase the temperature of the air inside the balloon to go up, decrease the temperature to go down and skillfully adjust the temperature to hover.

Questions to ponder:

  • When you increase the temperature of air, its density decreases. What do you think happens to all the molecules that were previously inside ? Do they exit the balloon ?
  • Different shaped hot-air balloons is a common sight. Do you think that the Buoyant force changes for each shape? (Review the formula for Buoyant force discussed in the previous post)
  • How do you think Helium balloons work ?

Origins of the Buoyant force

Check out the video description on YouTube on the details of how buoyancy is related to each clip in the video

Useful Preliminaries :

\rho = \frac{m}{V}

w = mg = \rho Vg

p = \frac{F}{A}

p = p_0 + \rho g h

Pressure depends on depth:

In a tank filled with a fluid, the bottom most part of the tank experiences a higher pressure than the top most part of the tank.

This is simply because if you are at the bottom, there are more layers of the fluid above your head “weighing you down” than compared to the top where you have fewer layers.

Fact: Marina Trench is 1000 times the pressure at the sea level.

As a result of the pressure depending on depth, any object placed in a fluid experiences a pressure difference between its top and bottom surfaces.

The top is at a lower pressure and the bottom at a higher pressure. Therefore, there is an upward force called the ‘Buoyant Force’ that acts on the object when you submerge it in a fluid.

Anything that you submerge underwater will feel lighter than it actually is because the Buoyant force acts upward on the objects an help you counteract the effect of gravity.

F_{net} = mg - F_B

(i) If mg>F_B , object sinks

(ii) If mg<F_B , object floats

(i) If mg=F_B , object remains stationary. (It is neutrally buoyant)

Note on the Buoyant force:

  • Air is also a fluid and also offers a Buoyant force on any object.
  • The Buoyant force is a property of the fluid and has nothing to do with the nature of the object that you submerge. A 1 m^3 of Iron, Styrofoam, Lead, etc all would feel the same Buoyant force.

An equation to represent Buoyant force:

So far, all of this has been qualitative. Let’s try to obtain an expression for the Buoyant force that we can work with.

Consider an object submerged in a fluid with density \rho_f as follows:

F_B = (P_{high} - P_{low}) A

F_B = \left[ p_0 + \rho_f g (h+d) - (p_0 + \rho_f g h ) \right] A

where \rho_f -> Density of the fluid.

F_B = \rho_f  g (d A )

F_B = \rho_f g V_{object}

Notice that the Buoyant force only depends on the volume occupied by the object and not it’s density and as a result all objects with the same volume irrespective of its density experience the same Buoyant force!

We can make this even better by realizing that V_{object} = V_{fluid- displaced} because when an object submerges in water it pushes away all the fluid that was already there to occupy it for itself.

How much fluid does it need to displace ? For submerged objects it’s exactly how much volume it needs to accommodate the object in the fluid. Therefore we can rewrite the above formula like so:

F_B = (\rho_f  V_{fluid-displaced}) g

F_B = m_f g (since \rho = m/V )

This is the Archimedes principle. It reads that the Buoyant force experienced by any object is equal to the weight of the fluid displaced.


Things to explore:

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.

Why every spot is not a ‘sweet spot’ ?

Yesterday’s post was about the acoustic sweet spot. 

The crazy thing about the sweet spot is that it is a psycho-acoustic phenomenon and it reveals itself only when the arrival time of the sound is equal to your ears.

To answer why every spot is not a sweet spot, we need to learn about the precedence effect. 

The precedence effect.

It is the psycho-acoustic phenomenon whereby an acoustic signal arriving first at the ears suppresses the ability to hear any other signals.


As you move away from the acoustic sweet spot, the sound from the speaker nearest to us to take precedence ( since it’s arrival time is less ) and suppresses the sound from other speakers.

This kills the realism from the image since now, all that you are hearing is the sound from one speaker ( a consequence of the precedence effect ). 

And no! It’s not the fault of the speakers. It’s a natural survival skill that evolution has granted us with to hear danger and judge it’s proximity.

PC: bnoack.com

The acoustic sweet spot.

The acoustic sweet spot is defined as the listening position equidistant to each of the two front channels as they are from each other, so the arrival time of the sound is equal at your ears.


It was brought in the vogue of the public by the big bang theory where Sheldon cooper tries to find the sweet spot in a theater.


Why is it important?

It is called the ‘sweet spot’ for a real good reason. 

In a motion picture, an image is considered to be ‘good’ if the location of the performers can be clearly located. This is known as stereo imaging and it adds realism to the image. 

The only person who hears this perfectly is the one who is in the sweet spot. ( no wonder Sheldon is obsessed with his spot! )


At this juncture, it is highly recommended that you check out the virtual barber shop to experience the acoustic sweet spot for yourself. 

The virtual barber shop places you in the sweet spot and abuses sound technology to bring you this high-quality audio realism.

We will dive deeper once you are done with your haircut! Have a good day.

Types of Damping

Damping is an influence within or upon an oscillatory system that has the effect of reducing, restricting or preventing its oscillations.

There are 4 types of damping:(in the order of the animations shown)

1. Under Damped System.

The system oscillates (at reduced frequency compared to the undamped case) with the amplitude gradually decreasing to zero.

2. Critically Damped System.

The system returns to equilibrium as quickly as possible without oscillating.

3. Over Damped System.

The system returns to equilibrium without oscillating.

4. Un-Damped System.

The system oscillates at its natural resonant frequency

( Sources: xmdemo, timewarp,wikipedia)

Physics of the ballpoint pen

People often brag about Large Hadron Collider as having one of the most sophisticated Technology in the world. True, but even if you are living in France, it’s still inaccessible! I believe that accessibility is the true trait of technology.

Look around the place that you are sitting in. Do you see a Ball Point pen lying around in the vicinity? Chances are that it is, are really high. Today on FYP, we will unravel the modest physics that governs it.

The physics.


Behold the ball in a ball point pen!.

To write you glide your pen onto the paper right? So what you are doing is rolling the ball that is present on the pen’s tip.

The ink flows continuously under the influence of gravity from the ink reservoir to the ball.

The ball rolls and the ink gets transferred onto the paper.

How does the ink stay inside the pen?

Put a drinking straw into a glass of water (or any liquid) and then put your finger over the top end of the straw so it’s air tight. You can now lift the straw out and the liquid will not fall out of the straw!

Now switch characters and imagine the liquid to be the ink and the straw to be the ink reservoir and voila!


Fun Fact.

Rollerball pen and Ballpoint pens work on the same principle. They differ in the type of ink used. While Ballpoint pens have a thicker oil based ink, the rollerball uses a liquid ink, thus giving its fluidity.


(Sources : http://home.howstuffworks.com/pen3.htm )