Dynamic Pressure Measurements

Dynamic pressure measurements are performed using a pitot-static tube. The pitot-static tube is composed of two concentric tubes. At the front end of the tube the outer tube is sealed and only the inner tube is open. This point, called stagnation port, is a blunt obstacle to airflow and therefore the drag coefficient is unity. The pressure exerted to this port consists of the dynamic pressure of the flow and the static ambient pressure (Figure 2).

Figure 3: Pitot-static tube (modified after Brock and Richardson 2001)

At a distance from the stagnation port sufficient to eliminate dynamic flow effects the pitot-static tube has a couple of small holes penetrating only the outer tube. These are called static ports and they are usually distributed equally spaced around the tube. The pressure exerted to the static ports is only the ambient atmospheric pressure (Fig. 1). The rear end bears two connections, one transferring the pressure applied to the outer tube (pstatic) and a second for the inner tube (pstag).

According to Bernoulli’s Equation the pressures can be described using p = ambient pressure, air density and V = air velocity.

After the transformation the equation reads as follows:

The variable can be substituted with p/RT, where R is the gas constant for dry air = 287 J kg-1K-1 and T is the air temperature in K, leading to the final equation:

Since R is the gas constant for dry air, humidity will have an effect, but less than 1%. Although the pitot-tube has to be oriented directly into the airflow a typical tube will tolerate up to 20° of misalignment (BROCK and RICHARDSON 2001).