How To Calculate Air Velocity

Air velocity is the speed of movement and direction of a volume of gas and it can be calculated using the equation that states Bernoulli's principle. This principle of physics originally applied to the flow of fluids. It establishes the relationship between flow speed and pressure - when speed increases, pressure decreases simultaneously. In keeping with the law of the conservation of energy, pressure can also be interpreted as the fluid's potential energy. The increase in speed that comes with the decrease in pressure demonstrates a transfer of mechanical energy. The total energy of the system being observed does not change.

Despite being originally conceptualized for the movement of fluids, Bernoulli's principle can be applied to the movement of gases. The equation has several forms, each derived from the original to fit the kind of flow being measured and observed. There are generally two types of flow - compressible and incompressible. A compressible flow means that the density of the flowing fluid or gas changes while it moves while incompressible is just the opposite. In the case of air ducts and air piping components of air conditioning systems, the movement of gas is said to be compressible. But in aerodynamics, the equation for incompressible flow can be applied.

1. Figure out the static pressure and density of the flowing gas.

Bernoulli's equation has several variables, the values of which will need to be filled out. The easiest one to determine are static pressure and density as these can be looked up on the standard atmosphere tables prepared by the US National Advisory Committee on Aeronautics (NACA). These tables can be easily searched and downloaded from the internet. Look for the particular given altitude (stated in the problem) and see the corresponding airflow static pressure and density.

2. Figure out the total pressure.
The next value to be determined is total pressure. According to Bernoulli's principle, this is the sum of static pressure and dynamic pressure. For classroom problems, this value is usually given. In the field of aerodynamics, total pressure is measured by a Pitot tube. This is a pressure measurement instrument installed in aircrafts and help pilots determine air velocity.

3. Enter the values and derive air velocity.
Bernoulli's equation for incompressible flows actually contains a variable for gravitational potential. Included in this variable is one for elevation. But in aerodynamics this variable is usually dropped as most aircraft fly at a constant level. So with values of static pressure, density and total pressure established, the only remaining unknown is air velocity. By mathematically deriving a form of the equation where the variable for air velocity is the only one left on one side of the equation, the determined values can be resolved and thus arrive at the quantity for air velocity.

The movement of gases in enclosures such as air ducts and pipes falls under the compressible flow type. Calculating air velocity in such cases is more complex. Since density changes at various points of the enclosure, volume and pressure also change, and consequently so does air velocity. The air velocity of flowing gases in enclosures is thus an average, determined through a statistical form of Bernoulli's principle rather than through its original simple linear equation. Depending on how complex the enclosed system is, there could actually be a huge amount of data that need to be analyzed. Engineers use computer-aided models to resolve such calculations.


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