1 Overview 2 Details of the Model Construction - Canada France ...

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above which a transition between laminar flow over and around the terrain to turbulent flow would be obtained. A set of visualization tests were performed with the upwind terrain probe B6 set at various elevations above the model while varying the flow speed from 10 to 50 cm/s in increments of 10 cm/s. At the time the experiment was conducted we believed that we saw a transition in the flow pattern from a 10 cm/s to 20 cm/s free flow water speed in the tunnel. Namely, the clear formation of a vortex on the leading edge of the observatory building with a scaled height of 5-6 m. However, examination of the video footage after the fact does not lend evidence for this. The video stills in Figure 13 with the B6 probe set 50 mm above the model terrain indicate that the vortex is present and of similar size at all the flow speeds investigated (side view images). Furthermore, the profile and extent of the wake around the building does not change in a noticeable way when looking at the top-down view. The flow speed of 30 cm/s was used for all but a few of the tests. This speed was selected partly on the basis of remaining well above the velocity of the perceived transition between 10 and 20 cm/s. Other factors that influenced this choice was that operation above 40 cm/s for extended periods of time was discouraged by the UWAL staff and that the flushing time (from some quick undocumented tests with the unvented dome) was found to be short enough to allow efficient progress of the experiments while being long enough to allow accurate measurement of the flushing time. Note that at 30 cm/s the Reynolds number Re associated with an ideal spherical obstruction of diameter L is; Re= ρ VL μ =999.7(kg /m3 )⋅0.3(m⋅s −1 )⋅0.2 (m)/1.307x10 −3 ( N⋅s⋅m −2 @10 o C )=4.6x10 4 where ρ is the density and μ is dynamic viscosity of water at the test temperature (average of ~10 C over the testing period). The dynamic viscosity of air depends primarily on temperature; at 0 C it is 1.35x10 -5 N s m -2 . The air density at 4200 m is 0.80 kg/m 3 and with 32 m diameter dome the wind speed to which the above model Reynold's number corresponds to in the real-world observatory is 2.4 cm/s. For comparison, the median wind speed at the location of CFHT on the summit of Mauna Kea is 15 knots (7.7 m/s); which is over 300 times greater than the scale speed obtained in our water tunnel experiment. 8 Description of the tests performed 8.1 Parameters investigated The basic parameters that could be varied in the tests were as follows (numbers in parentheses indicate the possible number of free parameters involved with each item): 1. Water flow speed (1): Except for some quick investigations, this was kept fixed as the dynamics of the flow would not change significantly unless order of magnitude changes were made to the speed. The flow rate was set to 30 cm/s. 2. Dome configuration (3): Unvented dome, dome with small vents, dome with large vents. 3. Dye probe locations. This was narrowed down to a few configurations for the bulk of the tests. Inside the dome (2): Probe A1 alone, and probes A3 & A5 combined. This was used for determination of dome flushing times. Outside the dome (1-2): Probes B1-B5 fired off in succession to view flow around and into the dome (this was not done for all the dome types), and probes B6 and B7 fired off in succession to view flow around the dome on the upwind side and the reverse flow on the downwind side.
4. Windscreen position (3): Either the windscreen was down, 50% raised or 100% raised. 5. Vent plugs (3-4): The purpose of the vent plugs was to simulate throttling of the flow by completely closing some of the proposed dome vents. We investigated two configurations; one in which the upwind vents were closed and the slit was facing partially or completely upwind, with the windscreen at different positions, another with similar dome orientations but with the vents closed on the downwind side. For each configuration the maximum number of vent plugs were installed in the dome (up to 3 plugs) and a fixed set of experiments was run in each configuration, removing a single vent plug once a set of experiments was complete. At the time of the experiment, the configuration of vent plugs tested represented the most likely real-world operational configuration of the dome. Hypothetically, with the upwind vents blocked, the effect of the wind on the telescope shake would be minimized. On the other hand, having the down-wind vents closed allowed us to investigate the potentially compromising effect on flushing time having these vents closed. The reason we might want to have these vents closed is that there was an indication (prior to the tests, but later borne out by our experiments) that the downwind counter-flow might lead to entrainment of the warm air from the building exhaust into the dome. 6. Dome position (4-13): The number of dome positions investigated for each dome configuration depended on the particular configuration with the vents and windscreen plugs installed. With the exception of the boundary layer investigation test, the dome positions explored were from 90 o (dome slit facing east) through 180 o (dome facing south) to 270 o (dome slit facing west) in increments of 15 o . This was the case for each configuration in which the vent plugs were not installed. The north facing orientations were not explored due to the symmetry of the model and the lack of visibility into the dome with the slit facing north (due to the opaque dome shutter). Note however that the symmetry is not perfect due to the structures on the dome floor (e.g. the mezzanine on the south side of the dome floor). Note that in some configurations with the vent plugs installed, small angles to the ENE were explored. Typically, after 20 or more tests had been performed, the videos were downloaded to a laptop. This minimized the possibility of data loss as well as the loss of sequencing of the video files between the two cameras. Care was taken to erase the camera memory after each download session so that file names on the side-view and bottom-view cameras would match. The laptop was backed up to an external disk every evening once testing for the day was complete. Most of the routine experiments involved first installing the maximum number of plugs (on a given upwind or downwind side) that would be used with a set dome (if applicable). The data taking would proceed by taking video at the defined set of angles for a given configuration (see next section for details), almost always in increments of 15 degrees. The probe configuration would then be changed and a new sequence of video at the relevant angles would be taken. This provided the smoothest data taking work flow. However, a few experiments did not fit this mold and these are described below. 8.2 Boundary layer investigation This was previously described in “Determination of the Operating Tunnel Flow Speed”. 8.3 Flow Visualization Near the Heat Vent Locations The purpose of this experiment was to determine if the fluid in the vicinity of the heat vents to the NW and NE of the observatory could be drawn into the dome from certain orientations. The location of the heat vents was visually estimated referring to the topographical map shown in Figure 14. A hand held
Page 1 and 2: Canada-France-Hawaii Telescope Corp
Page 3 and 4: Figure 3: Vent dimensions in inches
Page 5 and 6: Figure 6: View through the shutter
Page 7 and 8: 4 Dye Probe Locations Dye probe por
Page 9 and 10: gradually narrows down over a lengt
Page 11: 6.3 Video capture Two Panasonic Lum
Page 15 and 16: dye probe was held near each vent l
Page 17 and 18: 10 Lessons learned • A dome shutt
Page 19 and 20: Figure 17: Path structure used to o
Page 21 and 22: 11 University of Washington 36” W
Page 23 and 24: Note: at the expected operating win
Page 25 and 26: ing everything needed. • It takes

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