## Why do shower curtains encroach your showering space ?

You are all set to kick start your day with a shower; You open the faucet and notice that immediately the shower curtain starts to bulge in to your showering space (irrespective of whether its hot or cold water).

What on earth is happening here?

## Shower- Curtain effect

In 2001, Prof.David Schmidt from the University of Massachusetts won his way to the Ig Nobel prize when he found out that the reason why the curtain bends is Vortices!

Let’s run through the logical theories to explain this effect:

Buoyancy effect :

Hot air raises and cooler air moves down, and this causes the shower deflection. True only for hot air, but doesn’t explain why the curtain deflects even for cold water.

Bernoulli effect:

Pressure in a fluid decreases as its velocity is increased. The fast moving water molecules in the shower causes the pressure inside the shower lower than outside.

Since this pressure is lower than outside, this pressure differential independent of temperature would cause the curtain movement..

When Prof.Schmidt ran his simulation, he found out that the spray of water droplets from the shower formed a vortex with its axis normal to the plane of the curtain.

And the pressure at the center of this vortex was lower than those predicted by either effect separately or combined:

Pressure plot

Velocity vectors of air molecules in the shower. (Notice the circulation of air)

Result : The deflection of the curtain

And now we understand that the major contributor that causes the deflection of the curtain is the vortex. But we are still in the dark when it comes to a theory to explain the Shower-curtain effect (A fun research project if anyone is interested).

The next time you step in to take a shower and furious about the damn curtain encroaching all your showering space, just know that a IgNobel was awarded and many research articles published in trying to answer your frustration.*

Physics is life, Have a good one!

* or get a shower weight / a heavier curtain that would prevents this from happening.

## fuckyeahfluiddynamics:As a child, I loved to ride in the car…

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 ,et.al 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

## fuckyeahfluiddynamics: A wall of lava lamps in a San Francisco…

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 it’s London office, they use 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.

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..

Source

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!

FYP! & FYFD

## “If you place a small droplet atop a vibrating pool, it will…

“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)

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.

## This year’s American Physical Society Division of Fluid Dynamics…

This year’s American Physical Society Division of Fluid Dynamics meeting starts this Sunday. I have a couple events scheduled:

Student Lunch, Monday, November 20, 12:55-13:45 (sold out)

FYFD: Getting started in science communication, Monday, November 20, 16:44-16:57, Four Seasons Ballroom

Yes, the ballroom! If you’ve ever struggled to get into an FYFD talk, you shouldn’t have to this year! Also, dear DFD attendees, if you guys manage to pack the ballroom, I will love you forever.

You’ll also see me out and about at the conference, sporting fresh new FYFD t-shirts. I’ll have selected sticker designs for sale in person, too – \$3 each, buy 4 and get the 5th free.

The best way to keep up with me during the conference is through Twitter, and if you need to contact me, you can get to me there or via email at fyfluids[at]gmail.com.

Hope to see you at APS DFD!

## The meeting of land and sea often creates a rich and colorful…

The meeting of land and sea often creates a rich and colorful environment. This satellite image shows Mexico’s Laguna de Términos, a coastal lagoon off the Gulf of Mexico. A skinny barrier island forms the lagoon’s two connections to the ocean; the eastern side is the usual inlet (right), while the western side forms an outlet. Rivers feed freshwater into the lagoon from the south and southwest. These introduce sediments that cause some of the lighter swirls in the image. Winds and tides also contribute to this turbidity. The sheltered nature of the lagoon allows fresh and salt water to mix gradually, providing harbor for many forms of life. Oyster beds thrive in the river mouths; seagrasses prefer the calmer, saltier waters, and mangrove trees line the shore, slowly desalinating water for themselves as their roots shelter young fish and shrimp. (Image credit: NASA Earth Observatory)

## FYFD now has an online store!Whether you’re into stickers or…

FYFD now has an online store!

Whether you’re into stickers or t-shirts, experimental fluid dynamics or CFD, we’ve got you covered. I’m running a special introductory sale through December 15th – holiday shopping anyone? – so it’s a great time to grab some merch!

See a design you want available on more products? Got a concept for a new design? Let me know!

For those attending the APS DFD meeting here in Denver in a few days, I’ll be sporting some of the new t-shirts there, and I’ll be selling selected sticker designs in person (no shipping costs for you!). More DFD details to come.

In the meantime, everyone go check out the store!