Site for children


J. Perelman
"Entertaining physics". Book 2.
Chapter 6. Properties of liquids and gases

WAVES AND VORTICES

Many of the everyday physical phenomena can not be explained on the basis of the elementary laws of physics. Even this frequently observed phenomenon, as the sea on a windy day, does not give a comprehensive explanation within the school course of physics. And what caused the waves, moving in calm water from the nose coming of the steamboat? Why worry flags in windy weather? Why the sand on the beach is located in waves? Why swirls the smoke coming out of the smokestack?

To explain these and other similar phenomena, it is necessary to know the features of the so-called vortex motion of liquids and gases. Will try to be here to talk a little bit about the vortex phenomena and to note their key features, as in school textbooks of the vortices is barely mentioned.

Imagine the fluid flowing in the pipe. If all fluid particles are moved along the pipe in parallel lines, then we have a simple view of the fluid - calm, or, as physicists say, "laminar" flow.


Calm ("laminar") fluid flow in the pipe.

However, this is not the most frequent case. On the contrary, it is much more fluid flow in pipes, turbulent, from the walls of the pipe going into its axis vortices. This is whirling or turbulent motion. So flowing, for example, the water in the pipes of the water supply network (if you do not have in mind a thin tube where the flow is laminar). Eddy current occurs whenever the rate of flow of this fluid in the pipe (diameter) reaches a certain value, the so-called critical speed


Eddy (turbulent) fluid flow in the pipe.

To explain these and other similar phenomena, it is necessary to know the features of the so-called vortex motion of liquids and gases. Will try to be here to talk a little bit about the vortex phenomena and to note their key features, as in school textbooks of the vortices is barely mentioned.

Imagine the fluid flowing in the pipe. If all fluid particles are moved along the pipe in parallel lines, then we have a simple view of the fluid - calm, or, as physicists say, "laminar" flow. However, this is not the most frequent case. On the contrary, it is much more fluid flow in pipes, turbulent, from the walls of the pipe going into its axis vortices. This is whirling or turbulent motion. So flowing, for example, the water in the pipes of the water supply network (if you do not have in mind a thin tube where the flow is laminar). Eddy current occurs whenever the rate of flow of this fluid in the pipe (diameter) reaches a certain value, the so-called critical speed*.

The vortex current in the pipe fluid can make visible to the eye, if transparent liquid flowing in a glass tube, to introduce a bit of light powder, for example Likopodija. Then clearly differ vortices coming from the walls of the tube to its axis.

This feature swirling currents used in engineering when the device freezers and coolers. The fluid flowing in turbulent tube with cooled walls, much faster results all of its particles are in contact with cold walls than when driving without vortices; we must remember that by themselves are fluid - poor conductors of heat and in the absence of mixing of cooled or warmed very slowly. Lively heat and material exchange blood washed her fabrics are also possible only because of the flow in the blood vessels is not laminar and eddy.

Said pipes applies equally to open channels and riverbeds: in channels and rivers turbulent water flows. When accurately measuring the flow rate of the river tool detects pulsation, especially near the bottom: ripple show constantly changing the direction of flow, i.e., vortices Particles river water move not only along the river channel, as usually imagine, but also from the coast to the middle. Because of this incorrect assertion, though in the depth of the river water has a year-round one and the same temperature, it was +4°C: as a result of mixing, the temperature of flowing water near the bottom of the river (but not the lake) is the same as on the surface. The vortices formed at the bottom of the river, carried away with them a light sand and produce sand waves. The same can be seen on the sandy beach, washed by the incoming wave:


The formation of sand waves on the sea shore by the action of water vortices.

If the flow of water near the bottom was calm, the sand on the bottom would have a smooth surface.

So near the surface of the body, washed with water, formed vortices. About their existence tells us, for example, serpentine winding rope, strung along by the flow of water (when one end of a rope attached, and the other is free). What's going on here? The plot of the rope, near which started the whirlwind, he is fond of them; but the next moment the plot moves already another swirl in the opposite direction produces a serpentine crimps.


Wave motion ropes in flowing water due to the formation of vortices.

From liquids turn to the gases from the water to the air.

Who hath not seen, as the air vortices carry away from earth dust, straw, etc.? It is the manifestation of the vortex flow of air along the surface of the earth. And when the air flows along the water's surface, in places the formation of vortices, spective lower here, air pressure, water rises a hump - induced excitement. The same reason causes the sand waves in the desert and on the slopes of the dunes


The wavy surface of the sand in the desert.

It is easy to understand now, why is worried flag in the wind: it's the same as with a rope in flowing water. Solid vinyl vane does not save when the wind is constant direction, and, in obedience to the vortices varies all the time. The same vortex of origin and the smoke coming out of the smokestack; flue gases flow through the pipe to the vortex motion, which continues for some time by inertia outside of the pipe.


Flying the flag in the wind...


Puffs of smoke coming out of the smokestack.

The great importance of turbulent air movement for aviation. The plane's wings attached to this form, at which place the rarefaction of the air under the wing is filled with substance wing and vortex action on the wing, on the contrary, increases. In the end, the wing is built from the bottom, and the top sticks. Similar phenomena take place and when soaring birds with outstretched wings.


What forces affected wing aircraft.
The distribution of pressure (+) and exhaustion (-) air wing based experiments.
In the end, all the effort, support and sucking, wing fond up.
(The solid lines show the pressure distribution; the dashed line is the same, with a sharp increase flying speed)

How does the wind blew over the roof? The vortices create over the roof of the rarefaction of the air; in an effort to equalize the pressure, the air from under the roof, immersed up, pushing at her. The result is that, unfortunately, we have often to observe: lightweight, loosely attached to the roof blown away by the wind. Large window glass for the same reason, when the wind forced inside (and not break the pressure outside). However, these phenomena are easier explained by the reduction of pressure in the moving air.

When two streams of air of different temperature and moisture flow along one another, each vortices arise. Different forms of clouds in large measure due to this cause.

We see what a vast range of phenomena associated with eddy currents.

* Critical speed for some fluid is directly proportional to the viscosity of the liquid and inversely proportional to its density and diameter of the pipe through which the fluid flows. (Details in the book C. L. Kirpicheva "Conversations on mechanics", the conversation seventh.).

Entertaining physics J. Perelman

 




System Orphus

SUPPORT THE SITE!

Did you like our site and you would like to support it? It's very simple: tell your friends about us!
Read MORE

  © 2014 All children