Download 5. System Engineering

like OWL which reach heights of about 100m above the ground level values for these
parameters have to given for different heights.
The mean wind velocity can be modelled by a power law. This is shown together with an
alternative logarithmic model in Figure 5-7 a surface roughness similar to that at Paranal, a
height of the boundary layer of of 270 m, and a velocity of 10 m/s at a height of 10 m.
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Figure 5-7: Relationship between the height above the ground and the mean velocity
The values for the three parameters U (z ) , I and L used in this study for heights from 16 to 130
meters are approximately 11m/s to 14 m/s for the mean wind speed, 0.16 to 0.12 for the
turbulence intensity, and 80 m to 100 m for the integral length.
5.4.1.1.1.2
Power Spectral Density
Wind action is a stochastic phenomenon which is conveniently described by the Power Spectral
Densities (PSD) of the wind speed and the aerodynamic pressure on the surfaces and by
correlation functions for these parameters.The pressure field will strongly depend on the
orientation of the telescope with respect to the wind direction, the zenith distance of the
pointing, and the location on the mirror. One may have the following wind load cases :
•
When the telescope is pointing into the wind the characteristics of the incoming wind are not
significantly modified on the telescope surface.The power spectra and correlations of the
pressure fields should therefore be similar to the ones in the boundary layer.
•
When the telescope is pointing away from the wind or towards the zenith there will be
recirculation zones at the edges of the mirror with different power spectra and correlation
functions.
•
Parts of the mirror will be obstructed by the structure supporting the secondary mirror. In
some telescopes this structure is a truss sructure and the turbulence behind it may have the
characteristics of grid turbulence with an integral length L of the order of the grid size.
System Engineering