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

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.