Seeing effects in the wake of a building
Scope of the study:
Impact of a new telescope building on the observing quality at the Observatory
Search for the optimal location of the new unit
By: Marc Sarazin, ESO Garching, May 2000
|Various flow patterns contribute to the wake of a building: heat input from the ground by recirculation inside the cavity zone is the main source of thermal turbulence in the wake.|
|The temperature structure function (Ct2, units 0.001 K^(2) m^(-2/3)) in a vertical plane aligned with the wind. A H=30m high building (type VLT-UT) is at y=0. Four regions can be identified: upwind, top, wake and recovered region. Although the inlet flow is recovered at a very large downwind distance (X=20H), only at the top of the building is a significant increase of Ct2 noticeable due to flow lift-off (this effect is reduced in an open structure which has a smaller apparent height).|
The temperature structure function in a horizontal plane behind a row of four 30m high
buildings (type VLT-UTs) at y=-50,-14,18,51 perpendicular to the wind.
The inlet Ct2 is 6.4, 4.3, 2.8 at 5, 10 and 25m height
respectively. The simulation shows a reduction of the Ct2 in the wake in the first 20m above ground due to
mechanical turbulent mixing. |
The ground thermal input is not included in the model.
|Excess seeing (arcsec) versus wind speed (m/s) when observing through the wake at a distance x=2H of the center of the building (Source: VLT Report 62, Appendix A, Nov. 1990, Ed. M. Sarazin).|
|Ct2 measurements at a distance (x=2H) downwind of the NTT building (H=20m) at La Silla were compared to Ct2 measurements at a distance (x=2H') downwind of 3 nearby non insulated water tanks (H'=H/4) at various heights above ground (2H', 4H', 5H')). The Ct2 in the wake of the 4 times smaller water tanks is found to be 10 times larger than in the wake of the NTT building. The water tanks never reach thermal equilibrium with the ambient air.|
|The loci of zero velocity behind a conical hill (Arya and Gadiyaram, 1985, Atm. Env. 20,4 pp729-740).|