Fig. 6.1 shows a comparison of the amplitude of
of rms
pressure fluctuations 
acting on the upper part of an
8-meter telescope in various enclosure configurations.
The additional turbulence created by the slit is significant only
for azimuth angles < 40
. In principle,
the presence of a 50%
permeability windscreen across the slit can provide conditions
on the telescope in which the overall rms
amplitude
of pressure fluctuations is below the typical free flow situation.
Figure 6.1: Comparison of the amplitude of normalized rms
pressure
fluctuations 
acting on
the telescope top ring in different enclosure configurations.
The advantage of the open air enclosure is more evident if one compares the amplitude of pressure fluctuations with frequency larger than 1 Hz, which is a typical bandwidth of the closed loop control system of the telescope drives (fig. 6.2). The free flow wind contains very little turbulent energy at high frequencies, while a slit produces most of the flow turbulence right in that frequency range: only a 20% permeability wind screen can reduce the pressure fluctuations to the free flow level, by dramatically cutting the mean flow speed.
Figure 6.2: Amplitude of rms
pressure fluctuations
with frequency > 1 Hz, acting on
the telescope top ring in different enclosures.