T. Boulard et al. Agricultural and Forest Meteorology 100 2000 169–181 171
2.3. Turbulent kinetic energy k and dissipation rate ε
The microscale of turbulence λ is a measure of the dimension of eddies mainly responsible for the
dissipation of turbulent energy into heat.
λ = ¯
u
2
σ
2
2π R
∞
f
2
Ef df
0.5
7 If k, the total turbulent kinetic energy, is calculated as
k = 1
2 σ
2 u
+ σ
2 v
+ σ
2 w
8 where σ
u
, σ
v
, σ
w
, respectively, are the standard devi- ations of the velocity fluctuations in the x, y, z direc-
tions, the turbulence energy dissipation rate is: ε = k
32
λ
−1
9
3. Experimental setup
3.1. Site and tunnel description The measurements were carried out in Avignon
44
◦
N latitude, 5
◦
E longitude and 24 m altitude, in an empty 22 × 8 m
2
tunnel equipped with discontinu- ous openings obtained by separating the plastic sheets
every 4 m on either side of the tunnel with pieces of wood. Schematic views of the tunnel are shown in
Fig. 1. Assuming a weak influence of the gable ends
Fig. 1. Schematic plan of the experimental plastic tunnel. u, v, w
are three components of the air velocity measured by the sonic anemometer in the two sections.
and symmetric air flows along the ydirection with respect to each opening, we have explored the tun-
nel’s turbulent flow characteristics in two transverse sections:
a middle section I, situated 2 m westward from section II mid-way between two consecutive vent
openings; and a ‘vent’ section II, situated in the middle of the vent
openings, with a northern air inlet situated wind- ward and a southern air outlet leeward.
These parallel cross-sections were in the plane of the wind direction.
In order to have a continuously evaporating soil sur- face, the tunnel was abundantly watered before, and
during, experiments.
3.2. Instrumentation Rapid fluctuations in air velocity and temperature
were measured by means of two three-dimensional 3-D sonic anemometers omnidirectional, R3, re-
search ultrasonic anemometer, Gill RD and air hu- midity by means of two krypton hygrometers Camp-
bell, Utah. These four instruments were used to si- multaneously sense air velocity, temperature and hu-
midity fluctuations at two locations in each section. At each location, the measurement head of the krypton
hygrometer was placed within 0.2 m of the sampling volume of the sonic anemometer in order to minimise
the flow distortion.
With only two sampling positions possible at any one time, a difficulty arises from how to deal with
changing external conditions throughout the time needed to measure the 24 different measurement posi-
tions within each cross section Fig. 2. This problem was overcome:
1. by selecting measurements for a fixed northerly
external wind direction; and by using an external reference wind speed U
and the difference in air temperature T
i
− T
o
and humidity X
i
− X
o
between the centre of the green- house subscript ‘i’ and the external air stream
subscript ‘o’ as scaling parameters. T
i
, X
i,
and T
o
, X
o
were measured 1.5 m above the soil surface inside, and outside, the tunnel while U
o
was measured at a height of 5 m, about 20 m from the centre of the tunnel. A detailed list of the normalisation
172 T. Boulard et al. Agricultural and Forest Meteorology 100 2000 169–181
Fig. 2. Measurement positions in the central section of the tunnel. All dimensions are in metres.
d
, Air temperature, humidity and velocity measurements by 3-D sonic anemometers and krypton hygrometers 1 to 24; ×, surface temperature measurements T
s1
- T
s14
; △, reference inside air temperature and humidity measurements T
i
, X
i
.
formulae is given in Appendix A. Appendix B gives relations allowing the reader to derive actual physical
values from the normalised figures using the average values of the scaling parameters ¯
U
o
, 1T
i o
, 1X
i o
measured during the experiment. Three components of wind velocity, air temperature
and humidity fluctuations were measured at 24 posi- tions in each section together with the thermal bound-
ary conditions at 14 positions along the inside soil surface and plastic cover surface Fig. 2. Tempera-
tures were measured by means of thin thermocouples stuck on the plastic cover or soil surface. The manu-
facturer’s calibration was accepted for u, v, w, air tem- perature and humidity measurements. Sampling fre-
quency was 5 Hz. The time duration of each measure- ment record for characterisation of two locations was
about 10 min.
The outside air temperature T
o
and humidity X
o
, wind speed U
o
, and direction θ , and the inside ref- erence air temperature T
i
and humidity X
i
at the centre of the tunnel Fig. 2 were measured each sec-
ond and averaged over the length of each record. Ana- logue signals from the sonic anemometers and kryp-
ton hygrometers were processed on-line and stored in a portable computer with eddy correlation and rapid
Fourier-transform programme options. Outside, and inside, mean climatic conditions and thermal bound-
ary conditions were averaged on-line and results stored in a potable data logger Campbell CR20: Campbell,
Utah. 3.3. Wind conditions
The tunnel was located at Avignon in a region char- acterised by frequent northerly winds channelled by
the Rhône Valley. This wind the Mistral provides re- markable conditions for wind research, because of its
frequency, constancy of direction north and persis- tence McAneney et al., 1988. Measurements were
made one day 241097 during a strong Mistral with an average wind speed of 3.8 m s
−1
. Table 1 sum- marises prevailing weather conditions during the ex-
periment and illustrates the constancy of wind direc- tion and uniformity of the main scaling parameters U
o
, T
i
− T
o
, X
i
− X
o
.
Table 1 Climatic boundary conditions during the experimental period
Parameters Mean
Standard deviation T
o a
◦
C 14.4
0.3 RH
o b
53.0 5.0
T
i c
◦
C 16.5
0.5 RH
i d
56.1 4.5
U
o e
m s
−1
3.8 0.8
Dir
f ◦
259.0 12.9
a
Exterior air temperatures.
b
Relative humidities of the exterior air.
c
Interior air temperatures.
d
Relative humidities of the interior air.
e
External wind speed.
f
Direction.
T. Boulard et al. Agricultural and Forest Meteorology 100 2000 169–181 173
Fig. 3. Polar graph of the experimental air velocity in section I.
4. Results
