The wind loading force on GEM and ground screen/fences is surprisingly large and must be considered carefully, especially for mountain sites where winds are often high.
A simple estimate of wind loading can be done from dimenisional analysis. That is the loading force due to wind on a structure is the rate of momentum delivered by the wind to the structure per unit time. This can be characterized by the simple formula:
Force = Cd * density * velocity^2 * Area
where Cd is the drag coefficient which accounts for fluid flow past the structure as opposed to full momentum transfer. Cd is expected to be roughly 0.8 for our set up.
At sea level air has a density of roughly 1 kilogram per cubic meter.
Putting in this information yields:
Force pointing Horizon = 43750 Newtons * (velocity / 100 km/sec)^2 = 4375 Kg weight * (velocity / 100 km/sec)^2
Force pointing Vertical = 620 Kg weight * (velocity / 100 km/sec)^2
When expecting high winds, GEM is stored pointing vertically and secured with steel cables to tie points which are well secured to the ground. These are typically at a 30 degree angle away and spaced regularly around the dish.
The area of a miminum-size solid screen would typically be about 25 square meters. The fence can quickly get a much larger area. For the shield described in astrophysics note 478 and used in Bishop the effective area of the fence to wind was of order 70 square meters (note that it was made of wire mesh) and would have the same loading as the horizontally pointing dish. The minimum size fence would have loading about 2.5 times that of the vertically pointing dish.
Return to the Smoot Group page for a complete description of Dr. Smoot's group's research activities.