Tag: vortices

Categories
Coffee cup science General Observations slow

Coffee & Contrails (II)

vortices in coffee
Vortices forming behind a tea spoon being dragged through coffee.

Drag a tea spoon through your cup of coffee (or tea). Start by dragging the spoon slowly, then faster. Initially, the coffee flows around the spoon smoothly then, as you speed up, small vortices appear at either side of the spoon. Pull the spoon out of the coffee, and the vortices continue to move together through the cup before bouncing off the sides. Such vortices form whenever there is a speed difference between two layers of fluid (gas or liquid), as there is around the spoon being dragged through coffee. It is this effect that is the second connection between the physics of coffee and contrails.

Of course it is not giant tea spoons in the air but aeroplanes. Behind each aeroplane is a series of vortices trailing behind the wings. These vortices do not (normally) cause the contrails, the reason that they form was discussed in Coffee & Contrails (I). However, the vortices do cause some interesting effects in the contrail that we can, occasionally, see.

wake vortex, contrail, coffee in the sky
In this contrail there is a set of protuberances at regular intervals along the lower edge.

As the plane moves through the air, the speed of the air going over the wing is greater than the speed of air under the wing. As well as leading to vortices forming behind the wing, this speed difference results in an air pressure difference (the air pressure under the wing is greater than the air pressure above the wing). The pressure difference (below and above the wing) pushes the plane upwards, or, perhaps more technically, ‘creates lift’ and enables the plane to fly. If you want a good demonstration of the fact that a higher air speed leads to a lower air pressure, get two pieces of flat A4 paper and hold them in front of you such that you are looking through the small gap between them. Now blow into the gap separating the two sheets; they will join together. The reason that they do this is that the air pressure for fast moving air (as you blow) is less than the air pressure for static air (around the paper) and so the difference in air pressure pushes the two sheets together.

Shadowy contrail
Look carefully for another interesting contrail optical effect. There are two contrails here, an obvious one cutting straight down the photo and a second contrail moving more horizontally across the photograph. The second contrail can be seen more clearly by its shadow.

On a clear day, if the air in the higher atmosphere is relatively humid, you will see lots of persistent contrails. These contrails last for a long time in the skies and can drift with the wind. Occasionally at the edge of such a contrail you will see regular protrusions from the contrail, almost as if waves are forming on the contrail and producing white horses in the sky (see picture above). Initially I had hoped that this was a manifestation of the Kelvin-Helmholtz instability however the actual explanation is still quite fascinating. It seems that these protrusions are the result of the “wake vortices“, the vortices that form behind a plane just as the coffee forms vortices behind your spoon. I find it quite impressive to realise that high in the sky, these contrails are showing us that the atmosphere behaves just as if it were a cup of coffee. A definite case for which a coffee is a telescope for viewing the world.

Please leave any comments in the comments box below. If you think of any other connections between the physics of coffee and contrails please share them either here or on my Facebook page.

Categories
Coffee cup science Coffee review General Observations slow

Rain drops at Notes, Covent Garden

Notes Covent Garden, rain, puddles
No one wanted to sit outside when we visited Notes at Covent Garden

It was a cold and wet afternoon in early January when I finally had the opportunity to try Notes (Covent Garden branch). Inside, there were plenty of places to sit while warming up and drying off enjoying a coffee. Although it seems small from the outside, inside, the branch feels quite open, with the bar immediately in front of you as you come through the door. One of the attractions of Notes to me, was the fact that I knew that they served different single estate brewed coffees. I think I tried a “La Benedicion” coffee, or at least that is what I seem to have scribbled in my notepad. We took a stool-seat at the window to look out at the rain as my coffee arrived in a 0.25L glass jar. It is always nice to try different single estate coffees and generally, if I know that a café serves single estate coffees I will seek them out to try them for the Daily Grind.

The reflection of the Notes sign board in a cup of tea
The reflections in a cup of tea

Watching the rain form puddles outside, my thoughts were turned to the reflections bouncing off the water in the puddle. It struck me that the appearance of puddles depends on the water molecules behaving both as individual molecules and as molecules within a group. The rain creates ripples in the puddle which can only occur because each molecule is (weakly) attracted to the other water molecules in the puddle, forming a surface tension effect. A ripple is a necessarily collective ‘action’. On the other hand, the reflection of the lights from the street is the response of each individual water molecule to the incoming light. The reflected image is made from the response of many individual molecules. Reflection is more of an individual molecule thing.

Warning sign, train, turbulence
Such turbulence should be familiar to anyone who has stirred a cup of coffee.

I continued thinking about this when I got home where it occurred to me that there was another connection between rain and coffee. It is often said that “rain helps clear the air”, or something similar. Yet this is not quite true. If you have a coffee in front of you at this instant, take a moment to drag a spoon through it. Note the vortices that form behind the spoon. Such vortices form around any object moving through a fluid. In the case of the coffee it is the spoon through the water. For the rain, as the rain drop falls through the air it creates tiny vortices of air behind it. Just as with the coffee spoon, the size of these vortices depend on the speed and size of the falling drop. These vortices pull and trap the atmospheric dust bringing it down to earth more quickly than rain alone could do. The air is cleaned more by this ‘vacuum cleaner’ action than by the ‘wet mop’ of the rain itself.

I’m sure that there are many other coffee-rain connections that you can make if you sit in a café as I did on a rainy day. Let me know your thoughts on this or indeed, on anything that you notice and think interesting while sitting in a café. There is so much to notice if we just put down the phone or close the laptop while enjoying our brew.

Edited to add: Sadly, this article was posted just as Notes Covent Garden was closing down. Notes still has branches at Trafalgar Square and in Moorgate and is opening new branches in Kings Cross and Canary Wharf in February I believe. Hopefully they will all serve single estate brewed coffee and have good window seats from which to observe the rain when it falls.

Categories
Coffee cup science Coffee review Home experiments

Red Door, Greenwich

Red Door Greenwich, Red Door
Interior of Red Door cafe, Greenwich

Red Door in Greenwich is a great escape from the bustle of the busy streets surrounding it. Although it was crowded when we visited, it was still possible to find a table and have a conversation without too much background noise. I had heard good things about Red Door and wasn’t disappointed. Good coffee (from Monmouth), nice cake and warm surroundings. Definitely a place to go to when in Greenwich. The music that was playing was coming from a record player in the corner. A proper turn-table playing vinyl records. Suddenly, there were so many possibilities for stories for a Daily Grind article. There was the fact that records are analogue based (as opposed to the digital CDs), or perhaps I could write about the physics of a valve amplifier and how it relates to the evaporation of water from coffee (some of the physics is very similar). However what I started to get obsessed with is: what would happen if you put a coffee on a record player?

Now, I am an experimentalist and I do have a record player at home but before I could say “what would happen if…” my plans for experimentation with the record player were blasted out of the water. So I had to make a model record player out of a rotating spice rack. This probably worked better as I could control the speed of rotation, though it did make taking photographs tricky.

record player, turntable
The record player at Red Door

So, what would happen if we put a coffee at the centre of a turntable? The movement of fluids in cups and on record players is extraordinarily complex and is indeed very far from my ‘area of expertise’. However, we can start to understand what might be happening in the cup by making some approximations. Our first approximation is that the coffee in the mug rotates as a ‘rigid body’, meaning that it rotates as a whole. As the coffee cup rotates about its central axis on the “record player” the coffee inside the cup will (eventually) also rotate at the same angular velocity (speed of rotation). The fact that there is a rotation means that there is a force acting on the particles in the coffee liquid. This force produces an acceleration that increases with increasing distance from the axis of rotation. Each coffee particle is of course also subject to the vertical action of gravity. The combined acceleration means that each particle is simultaneously being pulled downwards and inwards. As the acceleration due to rotation increases with increasing radius, the horizontal acceleration becomes increasingly dominant away from the centre of the cup. This leads to the familiar curved surface (a dip at the centre of the mug) that we see with rotating fluids.

vortices, turbulence, coffee cup physics, coffee cup science
This polystyrene cup was rotated about its axis before being stopped. The water inside continues to rotate causing turbulent layers at the edges. These have been visualised with a small amount of blue ink.

Yet we know that this cannot be the full story. If we suddenly stop rotating the mug, the coffee in the mug continues to rotate for a while but does not do so indefinitely; it slows down. We can understand this by refining our approximation that the coffee inside the mug rotates as a rigid body. In fact, the coffee is a viscous liquid and the viscosity means that the layer of coffee immediately adjacent to the mug walls will move at the same speed as those walls: Stationary wall, stationary coffee. The coffee towards the centre of the cup meanwhile continues to rotate for a while. Imagine suddenly stopping the record player so that the mug is now still but the coffee inside continues to spin around the central axis. Stress is being produced between the stationary ‘layers’ of coffee next to the mug wall and neighbouring ‘layers’ of rotating coffee. This stress leads to turbulence. We can make this turbulence visible if, instead of coffee we use a mug of water. Rotate the mug of water as before and then suddenly stop the mug rotating. As with the coffee, the water continues to rotate. Now drop a tiny amount of water soluble ink or food colouring into the very edge of the water (I used a cocktail stick dipped in ink and held against the mug wall so that a small amount dripped into the water). As the water continues to spin, the ink is caught up in the turbulence and the vortices it produces can be seen. These concepts of boundary layers and turbulence are important for many applications including weather systems and car design. We need to understand how liquids (or gases) flow past each other in order to predict the weather and we need to know how they flow past solid objects in order to make cars more aerodynamic. In the coffee however I think that this turbulence is one of those things that is worth just creating and appreciating. A great demonstration of beauty, art and science in a mug of coffee.

Please do share your pictures of these coffee cup vortices if you manage to create them, particularly if you are able to see the effect with cream in coffee. You could either write about your results in the comments section below or email me photographs of your coffee and I will include them on this page. As always, enjoy your coffee.

My thanks to Kate & Edward of Red Door for sending me the photos of Red Door.

Extra photos of vortices in a rotating coffee:

Rotating coffee

An attempt at visualising the vortices using cream in coffee. Not so successful though you can see at least 2 well defined vortices in the top left of the image. Introducing the second liquid right at the edge of the mug seems critical, not so easy with cream as it is with ink!

Categories
Coffee cup science Observations

The destructive power behind a spoon

Have you ever sat waiting for someone in a coffee shop, slightly bored? Resisting the urge to check your email or Twitter on the phone (perhaps the battery is dead), you have been stirring your coffee and playing with the vortices that form behind the spoon. Have you wondered why they form? Or played with detaching a vortex from the spoon and getting it to ‘bounce’ off of the side of the cup?

chimney, coffeecupscience, everydayphysics, coffee cup science, vortex
The spiral around this chimney helps to prevent vortex formation in high winds

Such vortices form behind objects in a flow of liquid when either the speed of the liquid, or the size of the object, reaches a critical value. The research about how and why these vortices form is a huge field. From improvements to plane design, through understanding insect flight and even into how wind instruments such as flutes work, understanding these vortices is a challenging topic. It is also useful to know about the behaviour of these vortices when designing chimneys in order to prevent their collapse.

Chimneys are of course stationary, but when they are in high winds, vortices form around the chimney just like the vortices behind the spoon (rather than the spoon moving through the coffee, the wind moves past the chimney). At relatively low speeds, the wind forms small whirlwinds as we see behind the spoon in the coffee. At higher wind speeds, the vortices forming behind the chimney can start to detach and form a pattern known as a Karman vortex sheet. As each vortex detaches from the chimney it subjects the chimney to a small force. Under some conditions and around some objects, this can result in the rather beautiful sounds of the Aeolian harp. Under more extreme conditions, it can result in the collapse of chimneys. The Ferrybridge C cooling towers collapsed in 1965 in high winds as a result of the turbulence around the cooling towers. To minimise the chances of such vortex sheets forming, chimneys are now designed with a spiral pattern (pictured) around them. Far from being an aesthetic feature, this spiral channels the wind so that vortex sheets cannot form behind the chimney.

Something to think about next time you’re waiting for someone in a cafe.