# GEM - The Galactic Emission Mapping Project

## GEM Wind Loading Estimates

### 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.

### Loading on the Dish

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.

### Loading on the Dish

For the GEM dish with extensions on the effective dish diameter is about 10 meters. A screen/shield located and fixed on the ground much be significantly larger than this. The results of calculations such as this have indicated even with the smallest possible radius the wind loading force can be quite large. To minimize this loading we do a number of things: We generally put up a screen very close to the extent of the azimuth scan circle for GEM. We usually use wire mesh screens rather than solid screens. One must be concerned with the corrosion of the screen and the electrical continuity between elements of the mesh. For higher frequencies, this requires care much smaller holes and we occassionally add a rim of foil or solid metal. Tests are done of the effectiveness of the shields. The fence/shild serves two purposes: screening the sidelobes against stray-light from the ground and protecting stay animals and people from accidently entering the instrument scan area.

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.