wind - air flow

 
measuring principle flow/wind

Flow signifies the movement of liquids or gases. The laws of flowing liquids also apply for flowing gases, provided that the flow velocity remains below the speed of sound i.e. gases can be considered to be practically incompressible. Flow arises among other causes from gravity and pressure differences.

flow field – stationary flow
At every given moment, each particle of a flow has a certain velocity in quantity and direction. The space that the flowing particles fill is known as the flow field. Stream lines are used to identify the direction of velocity of the particles. The tangent at any point along the stream line indicates the direction of flow at this point. The conditions are particularly clear when the paths of the particles correspond to the stream lines. This type of flow is known as steady flow.

inviscid flow
In the absence of vortex formation and above all inner friction, the terms "ideal liquid" and "ideal flow" are used.

laminar flow
Laminar flow occurs when there is inner friction but no vortex formation. Inner friction is a consequence of the force effect between molecules. In contrast to outer friction, which arises between the surfaces of two bodies, inner friction is found only within the flowing medium, between neighbouring liquid strata of different velocities.

Beaufort
Wind speed is usually indicated with the aid of the Beaufort Scale. The following table lists the Beaufort wind strengths together with the associated wind speeds and the metrological identification:
 

Beaufort wind strength	wind velocity in knots	wind velocity in m/s	wind velocity in km/h	definition
0	0-1	0-0.2	0-1	still air
1	1-3	0.3-1.5	1-5	light draught
2	4-6	1.3-3.3	6-12	slight breeze
3	7-10	3.4-5.4	12-19	light breeze
4	11-15	5.5-7.9	20-28	moderate breeze
5	16-21	8.0-10.7	29-38	fresh breeze
6	22-27	10.8-13.8	39-49	strong wind
7	28-33	13.9-17.1	50-61	stiff wind
8	34-40	17.2-20.7	62-74	stormy wind
9	41-47	20.8-24.4	75-88	storm
10	48-55	24.5-28.4	89-102	heavy storm
11	46-63	28.5-32.6	103-117	gale force
12	>64	>32.7	>117	hurricane

measuring methods
 

• heat wire: At a hot film anemometer (HFA) an electrical resistance in the form of a thin, structured metal film („heater“) is put onto a carrier substrate. While operating the heater is achieving the heater temperature. A passing (air) flow cools down the heater until there is balance between the electrically fed heat and the thermally discharged heat quantity.

 

• half-shell anemometer: The classic type in the meteorological wind measuring technology. According to the construction starting characteristics between 0.1 m/s and 1 m/s. Heated and unheated construction types available.

 

• ultrasonic sound: An ultrasonic anemometer consists of ultrasonic sound converters being in opposition to each other. The measuring sections being generated by this are vertical one to each other. The converters work as sound sending transducers as well as sound receiving transducers. Via the controlling electronics the corresponding measuring section and ist measuring direction are addressed. With the beginning of a measuring series a sequency of several single measurements is running into all 4 directions of the measuring sections with maximum velocity. The measuring directions (directions of sound propagation) proceed rotating in clockwise direction, first from south to north then from west to east, from north to south and finally from east to west. By this the average values are generated.