# Differential drag on a cup anemometer

• cocoon95
In summary, to calculate the differential drag on a specific size cup and with a specific wind velocity, assuming the wind is constant and in the same horizontal plane as the four cups, you will need to know the drag coefficient of an open and closed cup and use the normal drag equation. The drag coefficient for an open cup is about 1.42, while for a closed cup it is only 0.38. If you slice half of the cup off with a vertical cut and blank the end, creating a 1/4 sphere with a flat plate at the end of the cup, the differential drag will still depend on the surface area and the difference in drag coefficient. Shape matters in determining the drag coefficient, so a 1/
cocoon95
If differential drag on a four cup anemometer causes its rotation, how does one calculate the differential drag on a specific size cup and with a specific wind velocity assuming the wind is constant and in the same horizontal plane as the four cups.?

thanks for the link. It got me started in the right direction.

okay, The drag coefficeint is clear. If however I slice half of the cup off with a vertical cut and blank the end, so I really have a 1/4 sphere with a flat plate at the end of the cup, does the differential Cd remain the same for a relative wind perpendicular to the 1/4 cup vs the 1/4 sphere. Or does if change and only depend on the surface area of the 1/4 cup and 1/4 sphere?
Remember that shape matters!

An open cup is around CD of 1.42, closed side is only 0.38.
http://www.windpower.org/en/tour/wtrb/drag.htm , the area is the same - so the force is just the area * the difference in Cd

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Knowing that the CD for an open cup is 1.42 and the closed side 0.38, is the CD for a 1/2 cup the same as a whole cup or does the CD change due to the different shape. If it changes would it be expected to change by a very small amount say < 0.05 or something larger like 0.10.

## What is differential drag on a cup anemometer?

Differential drag on a cup anemometer is a principle used to measure wind speed. It involves the difference in air resistance between the cups of the anemometer as they spin in the wind.

## How does differential drag affect the accuracy of a cup anemometer?

Differential drag can affect the accuracy of a cup anemometer if the cups are not evenly spaced or if they have different weights. This can cause variations in the speed at which the cups spin and therefore affect the measurement of wind speed.

## What factors can affect the differential drag on a cup anemometer?

The factors that can affect differential drag on a cup anemometer include the size, weight, and spacing of the cups, as well as the air density and wind velocity.

## How can the accuracy of a cup anemometer be improved?

To improve the accuracy of a cup anemometer, the cups should be evenly spaced and have the same weight. Additionally, the anemometer should be calibrated regularly and placed in an open and unobstructed location to minimize any external influences on differential drag.

## Is differential drag only used in cup anemometers?

No, differential drag can also be used in other types of anemometers, such as propeller or vane anemometers. However, cup anemometers are the most commonly used type of anemometer that utilizes the principle of differential drag.

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