A pilot test was run in the LASEN biphase wind tunnel, which
consists of a 12-m long channel with a variable cross-section ranging
from 0.75 to 1 m
.
The first eight meters of floor fetch are
equipped with cooling/heating elements. The flow speed may vary from
0.1 to 25 m/s.
No artificial turbulence was created for the pilot test and the
boundary layer is simply developed along the eight meters of
refrigerated floor.
The ambient air temperature was about 23
C
and the wind tunnel lower surface was chilled to about -10
C.
The similarity rules analyzed above result in the following
scaling factors between model and full scale conditions:
The profiles of speed and temperature obtained in the wind tunnel are
shown in fig. 
. The diagrams
for mean speed and temperature show a favorable comparison with
the theoretical surface layer profiles evaluated assuming a friction
velocity 
of 0.07 m/s and
a roughness length 
of 0.0002 m (0.04 m full-scale).
Only the profile
of turbulence intensity is somewhat lower than the corresponding
theoretical one.
Figure: Wind tunnel profiles of mean speed, turbulence intensity and
temperature compared with a theoretical profile computed assuming
m/s and 
m.
Figure: Schematic of optical measurements in the wind tunnel.
Two measurement methods were attempted to evaluate the seeing
through the boundary layer. The first method consisted in measuring the
temperature spectrum with a micro-thermocouple. Recalling equation
(), 
can be evaluated in principle from any
point of the inertial domain of the temperature spectrum:
However, actual measurement of temperature spectra in this case
presented several problems and no useful results were obtained.
The bandwidth of the micro-thermocouple
used was about 25 Hz, too low for the reduced wind tunnel
scale
.
Furthermore, because of the small signal amplitude it was impossible
to avoid signal pollution from the 50 Hz mains.
The second method consisted in a direct measurement of
the seeing through the boundary layer by means of the
same system used for the 4-cm mirror seeing test, in which the seeing
is evaluated from the image motion of 3-cm diameter laser beam. The
system and the procedure for obtaining seeing values are described in
section above. The frequency range of the
position sensing detector (PSD) which measures the image motion is
1 kHz, and therefore covers amply the inertial range of the wind
tunnel turbulence. The mirror was placed inside a 6-cm
diameter hole located 20 cm downstream of the refrigerated section.
The mirror surface was about 2 cm underneath the tunnel floor.
The optical equipment was located above the wind tunnel as shown in
fig. .
Most test runs were performed at the speed of 2.5 m/s, still
sufficient to generate a turbulent boundary layer, in order to
minimize the disturbance to the optical measurements caused by
vibrations of the wind tunnel.
The main results obtained are summarized in table
below.
Table: Summary of test results.
The baseline test is No. 1, performed
in the empty wind tunnel. Test No. 2 was a repetition, showing that
the measurements are reproduceable.
In test No. 3, which shows a very small amplitude decrease,
there was a a small wood block of size
(l
w
h) 20
6
8 cm located about 40 cm in
front of the light beam. In test No. 4 the block is made of aluminum
and cooled to -10
C: only a tiny amplitude increase is
recorded. Increasing the flow speed (test No. 5), the seeing should
increase but so do also the vibrations, so that the measurement is
hardly significant.
Tests No. 6 and 7 were done when the wind tunnel was reheating,
and show as expected a decrease of amplitude.
In test No. 8 almost no cooling is in effect and
the value measured must be considered as the signal background noise:
it was therefore subtracted quadratically before the evaluation of the
seeing angle.
Some problems were outlined during the tests:
m
, which is marginal with
respect to the effects investigated and a more accurate system should
be used for a further development of this test method.