GEM - The Galactic Emission Mapping Project

by George Smoot

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.

For the GEM dish with extensions on the effective dish diameter is about 9.5 meters. If GEM were pointed at the wind (horizon in the direction of wind), it would present an area A = pi R^2 = 71 square meters. Pointed vertically, it presents an area of about 10 square meters to a horizontal wind.

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.