Measurements

These wind tunnel tests were performed at the boundary layer wind tunnel of the Danish Maritime Institute as part of the test program with the cylindrical enclosure selected for the VLT (see section 4.1.3). The model included the same complete enclosure with a fully modeled telescope inside it at a scale of 1:64 (see fig. ). The tests were run at a nominal wind speed of about 12 m/s, with several venting configurations of the enclosure as described in section 4.1.3 above and covered the range of wind directions 0, 10, 20, 40, 90 and 180 in combination with telescope zenith angles of 10, 35 and 60. Two types of measurements were performed:

• Measurements of the pressure difference between the upper and lower faces of the primary mirror. Pressure taps were mounted on opposite sides of the primary mirror at four symmetrical locations. The scanivalve setup was such that instantaneous pressure differences were measured. These instantaneous pressures were then expressed as coefficients normalized by the reference external wind speed.

• Measurement of wind velocity close to the upper surface of the mirror, by two single-wire hot-wire probes, respectively vertical and horizontal, located about 10 mm (model scale) above the mirror surface.

  
Figure: Plots of (rms of the pressure coefficients, normalized to the external wind speed) versus the azimuth angle with respect to the wind direction for different telescope altitudes and enclosure configurations (see the table at page ).