Wind Effect on Structural 1

7/30/2019 Wind Effect on Structural 1

1/14

2. Wind characteristics


2/14

Nature of wind from meteorology

Meteorology: the science of the atmosphere and its phenomena;

the study of weather and climate

Micrometeorology: the study of detailed structure of atmospheric flow near

the ground, ex: the mean wind speed profile, the

atmospheric turbulence, etc. (some studies in

micrometeorology can be directly used by structural engineers)

Climatology: the prediction of wind conditions at a given geographical

location, ex: extreme wind climatology


3/14

Temperature of atmosphere

The earths heat balance

[Source: http://clivebest.com/blog/?p=4432]

Sun6000K

original energy source

short wave

atmosphere istransparent

Earth

long wave: absorbed by atmosphere

Earth 250K

The earth is the direct energy source of wind


4/14

A B

1

2

A B

1

2

A B

heating

1

2

A B

heating

1

2

A B

heating

1

2

A B

heating

1

2

A B

heating

1

2

A B

heating

1

2

A B

heating

1

2

A B

heating

Tube 1 & 2 are closed

tA: raised

tB

: maintained

Fluid in A expands

Tube 1 is closed, tube 2 is opened no flow

Humphreys experiment

Tube 1 is opened: flow A B

& tube 2 is opened: flow B A


5/14

Simple model of atmospheric circulation

AirHeating

Solar / terrestialWind = motion of air with respect to the earths surface,

fundamentally caused by variable solar heating of the earths atmosphere

[Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter10/single_cell.html]

A = column of air above equator

B = column of air above pole

Cold

Hot

Equator

PoleGeneral circulation


6/14

Atmospheric hydrodynamics

The motion of an elementary mass of air is determined by the Newtons second

law:

The forces F include:

(1) The horizontal pressure gradient

(2) The Coriolis force

(3) The centrifugal force

(4) The friction force

aF m


7/14

1. The horizontal pressure gradient:

Vertical: main pressure, decrease with height

Changes in pressure gradient

Horizontal: changes are small wrt vertical

Wind direction: high pressure low pressure (isobar)

2. The Coriolis force (apparent force)

v : the velocity vector of the mass

: the angular velocity vector of the earth

Pressure gradient & Coriolis

vF mc 2


8/14

3. The centrifugal force:

r : radius of curvature

4. The friction force

The surface of the earth retards the

flow of air by the effects of friction

Boundary layer of the atmostphere

r

vmF

2

Centrifugal & Friction

v

r

F

d: boundary layer depth

free atmosphere (frictionless wind)

vgr : gradient wind speed

earth surface


9/14

Horse

latitude

Tricellular model of atmospheric circulation

Considering: earth rotation/friction, seasons, geographic Tricellular meridional circulations


10/14

Secondary circulations

Traveling High and Low

pressure areas,

by heating or cooling of

the lower atmosphere


11/14

Wind directions

Mean pressure systems and representative wind vectors at the surface for (a) January and (b) July.

The red dashed lines represent the equatorial trough (and monsoon trough over Asia).[Source: http://www.goes-r.gov]

The motion of air

relative to the earth

surface is determined,

in unaccelerated flow,

by the balance among

these forces


12/14

Moonsoons & Hurricanes

Tropical cyclones with surface wind speed >120km/h

Diameter is in the order of 100km

Depth is in the order of 10km

Hurricane Gladys as seen by the Apollo crew

[Source: http://www.astronautix.com/flights/apollo7.htm]

Moonsoons: seasonal reversing winds associated

with asymmetric heating of land and sea

(summer: ocean to land, with heavy rains)

[Source: http://www.taringa.net]


13/14

Tall convective clouds,

produced by upward motion of warm, moist air

Local wind

Tornado

[Source: http://commons.wikimedia.org/wiki/File:F5_tornado_Elie_Manitoba_2007.jpg]

Dry, warm, down-slope wind / leeward of a mountain range

Foehn wind

[Source: http://fineartamerica.com/featured/foehn-wind-artwork-gary-hincks.html]

Most powerful wind

A vortex of air: ~300m diameter, 30-100 km/h

Thunderstorm [Source: Wind effects on structures, Simiu, p.29]

Small-scale / negligible influence on general circulation


14/14

The Beaufort scale

Degree Wind speed

(m/s)

Description of wind Wind effects

0 0 0.5 Calm

1 0.5 1.7 Light airs Smoke rises nearly vertically

2 1.7 3.3 Light breeze Wind felt on face

3 3.3 5.2 Gentle breeze Leaves move

4 5.2 7.4 Moderate breeze Small branches move, wind extends flags

5 7.4 9.8 Fresh breeze Large branches also move

6 9.8 12.4 Strong breeze Saplings move

7 12.4 15.2 Light gale Strong branches bend, wind is heard in buildings

8 15.2 18.2 Moderate gale Impeded walking, branches break, tree-trunks sway

9 18.2 21.5 Fresh gale Wind displaces lighter objects, strong branches and

saplings break, damage to chimneys

10 21.5 25.1 Strong gale Trees are uprooted, telegraph poles break

11 25.1 29 Destructive gale Severe damage

12 29 and over Hurricane Disastrous damage

Documents

Wind Effect on Structural 1