Decagon Devices DS-2 Gebruikershandleiding - Pagina 7
Blader online of download pdf Gebruikershandleiding voor {categorie_naam} Decagon Devices DS-2. Decagon Devices DS-2 20 pagina's. Sonic anemometer
Pagina afdrukken2 THEORY
2
Theory
The theory behind the sonic anemometer comes from Campbell and
Unsworth (1979).
on air temperature T (K), vapor pressure e(kP a) and atmospheric
pressure, p(kP a), as seen in equation 1.
For a given sound path length, d (m), the number of wavelengths, n
in still air is determined with equation 2.
Here ν is the frequency of the sound (Hz). When the air is moving,
the sound speed is the sum of the wind speed and the speed of sound
in still air. The anemometer transmits a sound pulse in a forward
direction, and a similar pulse in the reverse direction, and computes
the difference in n between the two. If the vector magnitude of the
wind in the direction of the sound is u(m/s), then:
for sound traveling with and against the wind. Subtracting the first
from the second equation we obtain:
Even at the maximum wind speeds for the anemometer, u
2
about 1% of c
, so for a good approximation write.
2
Cambell, G.S. and M.H. Unsworth (1979) An inexpensive sonic anemometer
for eddy correlation. J. Appl. Meteor. 18:1072-1077.
2
The speed c(m/s) of sound in still air depends
c = 20.067 T 1 +
n =
n
∆n
+
n
∆n =
∆n = ∆n + ∆n
u =
0.32e
p
νd
c
νd
=
c + u
νd
c
u
2νdu
=
+
2
2
c
u
2
c
∆n
2νd
4
DS-2
(1)
(2)
(3)
(4)
2
is only
(5)