Atmospheric circulation systems have a wide variety of structures and there is no single mechanistic model that describes all their characteristics. However, we can construct simple kinematic models that capture some primary aspects of the flow. For simplicity, we will concentrate on idealized extra-tropical depressions. We will not consider hurricanes and tropical storms in any detail, because the effects of moisture condensation and convection dominate their behaviour.

## Posts Tagged 'Fluid Dynamics'

### Simple Models of Atmospheric Vortices

Published July 29, 2021 Occasional Leave a CommentTags: Fluid Dynamics, Geophysics

### Zhukovsky’s Airfoil

Published February 27, 2020 Occasional Leave a CommentTags: Applied Maths, Fluid Dynamics

A simple transformation with remarkable properties was used by Nikolai Zhukovsky around 1910 to study the flow around aircraft wings. It is defined by

and is usually called the *Joukowsky Map*. We begin with a discussion of the theory of fluid flow in two dimensions. Readers familiar with 2D potential flow may skip to the section *Joukowsky Airfoil*.

### Using Maths to Reduce Aircraft Noise

Published February 6, 2020 Irish Times Leave a CommentTags: Fluid Dynamics, modelling

If you have ever tried to sleep under a flight-path near an airport, you will know how serious the problem of aircraft noise can be. Aircraft noise is amongst the loudest sounds produced by human activities. The noise is over a broad range of frequencies, extending well beyond the range of hearing. The problem of aviation noise has become more severe as aircraft engines have become more powerful [TM180 or search for “thatsmaths” at irishtimes.com].

### Stokes’s 200th Birthday Anniversary

Published August 8, 2019 Irish Times Leave a CommentTags: Fluid Dynamics, History, Physics

Next Tuesday, the 30th of August, is the 200th anniversary of the birth of George Gabriel Stokes. This extended blog post is to mark that occasion. See also an article in The Irish Times.

### Spin-off Effects of the Turning Earth

Published June 6, 2019 Irish Times Leave a CommentTags: Fluid Dynamics, Geophysics, Numerical Weather Prediction

On the rotating Earth, a moving object deviates from a straight line, being deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The deflecting force is named after a nineteenth century French engineer, Gaspard-Gustave de Coriolis [TM164 or search for “thatsmaths” at irishtimes.com].

Coriolis was interested in the dynamics of machines, such as water mills, with rotating elements. He was not concerned with the turning Earth or the oceans and atmosphere surrounding it. But it is these fluid envelopes of the planet that are most profoundly affected by the Coriolis force.

### A Zero-Order Front

Published August 30, 2018 Occasional Leave a CommentTags: Fluid Dynamics, Geophysics, modelling

Sharp gradients known as fronts form in the atmosphere when variations in the wind field bring warm and cold air into close proximity. Much of our interesting weather is associated with the fronts that form in extratropical depressions.

Below, we describe a simple mechanistic model of frontogenesis, the process by which fronts are formed.

### Tides: a Tug-of-War between Earth, Moon and Sun

Published August 2, 2018 Irish Times Leave a CommentTags: Fluid Dynamics, Geophysics

All who set a sail, cast a hook or take a dip have a keen interest in the water level, and the regular ebb and flow of the tides. At most places the tidal variations are **semi-diurnal**, with high and low water twice each day [see TM144, or search for “thatsmaths” at irishtimes.com].

Continue reading ‘Tides: a Tug-of-War between Earth, Moon and Sun’

### Stan Ulam, a mathematician who figured how to initiate fusion

Published May 3, 2018 Irish Times Leave a CommentTags: Algorithms, Fluid Dynamics, Set Theory, Topology

Stanislaw Ulam, born in Poland in 1909, was a key member of the remarkable Lvov School of Mathematics, which flourished in that city between the two world wars. Ulam studied mathematics at the Lvov Polytechnic Institute, getting his PhD in 1933. His original research was in abstract mathematics, but he later became interested in a wide range of applications. He once joked that he was “a pure mathematician who had sunk so low that his latest paper actually contained numbers with decimal points” [TM138 or search for “thatsmaths” at irishtimes.com].

Continue reading ‘Stan Ulam, a mathematician who figured how to initiate fusion’

### Waves Packed in Envelopes

Published April 26, 2018 Occasional Leave a CommentTags: Fluid Dynamics, Geophysics, Wave Motion

In this article we take a look at group velocity and at the extraction of the envelope of a wave packet using the ideas of the Hilbert transform.

### The Heart of Mathematics

Published January 18, 2018 Irish Times Leave a CommentTags: biology, Fluid Dynamics, medicine

At five litres per minute the average human heart pumps nearly 200 megalitres of blood through the body in a lifetime. Heart disease causes 40 percent of deaths in the EU and costs hundreds of billions of Euros every year. Mathematics can help to improve our knowledge of heart disease and our understanding of cardiac malfunction [TM131 or search for “thatsmaths” at irishtimes.com].

### Energy Cascades in Van Gogh’s Starry Night

Published January 4, 2018 Irish Times Leave a CommentTags: Fluid Dynamics, Geophysics

“*Big whirls have little whirls that feed on their velocity,*

*And little whirls have lesser whirls, and so on to viscosity.*“

We are all familiar with the measurement of speed, the distance travelled in a given time. Allowing for the direction as well as the magnitude of movement, we get velocity, a vector quantity. In the flow of a viscous fluid, such as treacle pouring off a spoon, the velocity is smooth and steady. Such flow is called laminar, and variations of velocity from place to place are small. By contrast, the motion of the atmosphere, a fluid with low viscosity, can be irregular and rapidly fluctuating. We experience this when out and about on a gusty day. Such chaotic fluid flow is called turbulence, and this topic continues to challenge the most brilliant scientists [TM130 or search for “thatsmaths” at irishtimes.com].

Continue reading ‘Energy Cascades in Van Gogh’s Starry Night’

### Euler and the Fountains of Sanssouci

Published September 7, 2017 Irish Times Leave a CommentTags: Applied Maths, Euler, Fluid Dynamics

When Frederick the Great was crowned King of Prussia in 1740 he immediately revived the Berlin Academy of Sciences and invited scholars from throughout Europe to Berlin. The most luminous of these was Leonhard Euler, who arrived at the academy in 1741. Euler was an outstanding genius, brilliant in both mathematics and physics. Yet, a myth persists that he failed spectacularly to solve a problem posed by Frederick. Euler is reputed to have bungled his mathematical analysis. In truth, there was much bungling, but the responsibility lay elsewhere. [TM122 or search for “thatsmaths” at irishtimes.com].

### Inertial Oscillations and Phugoid Flight

Published July 13, 2017 Occasional Leave a CommentTags: Applied Maths, Fluid Dynamics, Geophysics

The English aviation pioneer Frederick Lanchester (1868–1946) introduced many important contributions to aerodynamics. He analysed the motion of an aircraft under various consitions of lift and drag. He introduced the term **“phugoid”** to describe aircraft motion in which the aircraft alternately climbs and descends, varying about straight and level flight. This is one of the basic modes of aircraft dynamics, and is clearly illustrated by the flight of gliders.

### The Water is Rising Fast

Published April 20, 2017 Irish Times Leave a CommentTags: Fluid Dynamics, Geophysics

Seventy percent of the Earth is covered by water and three quarters of the world’s great cities are on the coast. Ever-rising sea levels pose a real threat to more than a billion people living beside the sea. As the climate warms, this is becoming a greater threat every year [TM113 or search for “thatsmaths” at irishtimes.com].

### Thank Heaven for Turbulence

Published October 20, 2016 Irish Times Leave a CommentTags: Applied Maths, Fluid Dynamics

The chaotic flow of water cascading down a mountainside is known as turbulence. It is complex, irregular and unpredictable, but we should count our blessings that it exists. Without turbulence, we would gasp for breath, struggling to absorb oxygen or be asphyxiated by the noxious fumes belching from motorcars, since pollutants would not be dispersed through the atmosphere [TM101, or search for “thatsmaths” at irishtimes.com].

### Modelling Rogue Waves

Published May 5, 2016 Irish Times Leave a CommentTags: Applied Maths, Fluid Dynamics, Wave Motion

There are many eyewitness accounts by mariners of gigantic waves – almost vertical walls of water towering over ocean-going ships – that appear from nowhere and do great damage, sometimes destroying large vessels completely. Oceanographers, who have had no way of explaining these ‘rogue waves’, have in the past been dismissive of these reports [TM090, or search for “thatsmaths” at irishtimes.com].

### Richardson’s Fantastic Forecast Factory

Published January 7, 2016 Irish Times Leave a CommentTags: Applied Maths, Computer Science, Fluid Dynamics

Modern weather forecasts are made by calculating solutions of the mathematical equations that express the fundamental physical principles governing the atmosphere [TM083, or search for “thatsmaths” at irishtimes.com]

The solutions are generated by complex simulation models with millions of lines of code, implemented on powerful computer equipment. The meteorologist uses the computer predictions to produce localised forecasts and guidance for specialised applications.

### The Flight of a Golf Ball

Published December 3, 2015 Irish Times Leave a CommentTags: Applied Maths, Fluid Dynamics

Golf balls fly further today, thanks to new materials and mathematical design. They are a triumph of chemical engineering and aerodynamics. They are also big business, and close to a billion balls are sold every year. [TM081: search for “thatsmaths” at Irish Times ].

The character of fluid flow depends on a dimensionless quantity, the Reynolds number. Named for Belfast-born scientist Osborne Reynolds, it determines whether the flow is laminar (smooth) or turbulent (rough). Normally the drag force increases with speed.

The Reynolds number is defined as Re = *VL*/*ν* where *V* is the flow speed, *L* the length scale and *ν* the viscosity coefficient. The transition from laminar to turbulent flow occurs at a critical value of Re which depends on details of the system, such as surface roughness.

### The Hodograph

Published April 9, 2015 Occasional Leave a CommentTags: Astronomy, Fluid Dynamics, Geophysics

The Hodograph is a vector diagram showing how velocity changes with position or time. It was made popular by William Rowan Hamilton who, in 1847, gave an account of it in the Proceedings of the Royal Irish Academy. Hodographs are valuable in fluid dynamics, astronomy and meteorology.

### Plateau’s Problem and Double Bubbles

Published January 22, 2015 Occasional Leave a CommentTags: Algorithms, Fluid Dynamics, Physics, Topology

Bubbles floating in the air strive to achieve a spherical form. Large bubbles may oscillate widely about this ideal whereas small bubbles quickly achieve their equilibrium shape. The sphere is optimal: it encloses maximum volume for any surface of a given area. This was stated by Archimedes, but he did not have the mathematical techniques required to prove it. It was only in the late 1800s that a formal proof of optimality was completed by Hermann Schwarz [Schwarz, 1884].

### Falling Bodies [1]: Sky-diving

Published November 13, 2014 Occasional Leave a CommentTags: Fluid Dynamics, Geophysics, Mechanics

Aristotle was clear: heavy bodies fall faster than light ones. He arrived at this conclusion by pure reasoning, without experiment. Today we insist on a physical demonstration before such a conclusion is accepted. Galileo tested Aristotle’s theory: he dropped bodies of different weights simultaneously from the Leaning Tower of Pisa and found that, to a good approximation, they hit the ground at the same time.

### El Niño likely this Winter

Published November 6, 2014 Irish Times Leave a CommentTags: Fluid Dynamics, Geophysics, modelling, Physics

This week’s *That’s Maths* column in *The Irish Times* (**TM056** or search for “thatsmaths” at irishtimes.com) is about El Niño and the ENSO phenomenon.

In 1997-98, abnormally high ocean temperatures off South America caused a collapse of the anchovy fisheries. Anchovies are a vital link in the food-chain and shortages can bring great hardship. Weather extremes associated with the event caused 2000 deaths and 33 million dollars in damage to property. One commentator wrote that the warming event had “more energy than a million Hiroshima bombs”.

### How Big was the Bomb?

Published September 18, 2014 Irish Times 2 CommentsTags: Applied Maths, Fluid Dynamics, Physics, Wave Motion

By a brilliant application of dimensional analysis, G.I.Taylor estimated the explosive energy of the first atomic blast, the Trinity Test (see this week’s *That’s Maths* column in *The Irish Times, *TM053, or search for “thatsmaths” at irishtimes.com).

### White Holes in the Kitchen Sink

Published January 9, 2014 Occasional Leave a CommentTags: Applied Maths, Fluid Dynamics, Mechanics, Physics, Wave Motion

A tidal bore is a wall of water about a metre high travelling rapidly upstream as the tide floods in. It occurs where the tidal range is large and the estuary is funnel-shaped (see previous post on this blog). The nearest river to Ireland where bores can be regularly seen is the Severn, where favourable conditions for these hydraulic jumps occur a few times each year.

But you do not have to leave home to observe hydraulic jumps. Continue reading ‘White Holes in the Kitchen Sink’

### Interesting Bores

Published January 2, 2014 Irish Times Leave a CommentTags: Applied Maths, Fluid Dynamics, Geophysics, Mechanics, Wave Motion

This week’s *That’s Maths* column in the* Irish Times* ( TM036 ) is about bores. But don’t be put off: they are very interesting.

### A Mathematical Dynasty

Published September 26, 2013 Occasional Leave a CommentTags: Analysis, Fluid Dynamics, History

The idea that genius runs in families is supported by many examples in the arts and sciences. One striking case is the family of Johann Sebastian Bach, the most brilliant star in a constellation of talented musicians and composers.

In a similar vein, several generations of the Bernoulli family excelled in science and medicine. More that ten members of this Swiss family, over four generations, had distinguished careers in mathematics.

Recently, I kayaked with two friends on the River Shannon, which flows southward through the centre of Ireland. Starting at Dowra, Co. Cavan, we found it easy paddling until we reached Lough Allen, when the going became very tough. It was an uphill struggle.

*Could we really be going uphill while heading downstream?* That seems bizarre but, in a certain sense, it is possible.

### The Pitch Drop Experiment

Published March 21, 2013 Irish Times Leave a CommentTags: Fluid Dynamics, Physics

Later this year a big black blob of sticky pitch will plummet from a funnel and plop into a beaker. The story is recounted in this week’s *That’s Maths* ( TM017 ) column in the *Irish Times*.

In one of the longest-running physics experiments, the slow-flowing pitch, under a bell-jar in the University of Queensland in Brisbane, will ultimately lose its battle with gravity …

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