Pilot test for direct measurement of seeing

A pilot test was run in the EPFL/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.

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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 by the 50 Hz mains.

The second method consisted of 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 small wood block of size (lwh) 2068 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:

• It would have been necessary to decouple the optical system and the mirror from the wind tunnel to eliminate the effect of vibrations.
• The resolution of the PSD used is 3 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.