Measurement of Airspeed

Bernoulli equation readily allows one to determine the flow speed once the static and stagnation pressures are known. Rewriting Eqn.3.73 ( Stagnation Pressure)we have

$$V~=~\sqrt{{2~(p_0~-~p)} \over \rho}$$ (3.118)

It is therefore a matter of measuring the static and stagnation pressures at a given location.

Static Pressure is conveniently measured by drilling a hole in the wall or the pipe, called the Pressure Tap (Fig. 3.34). A manometer or a pressure gauge is connected to the tap. During flow static pressure is communicated to the measuring device. Alternately one could use a Static Pressure probe shown in Fig. 3.35. This has holes which communicate the pressure to a measuring device.

Measurement of stagnation pressure requires that the flow be brought to rest. A glass tube or a hypodermic needle aligned with the flow and facing upstream as shown in Fig. 3.36 will do the job. Alternately, what is called a Pitot Tube shown in Fig.3.37, with a hole facing upstream of the flow may be employed. The method shown in Fig. 3.38 suggests itself.

To Manometer or a gauge Figure 3.34: Pressure Tap to measure Static Pressure

To Manometer or a gauge Figure 3.35: Static Pressure Probe

 

Figure 3.36: Stagnation Tube

Figure 3.37: Pitot Tube

But for an accurate determination of flow speed, static and stagnation pressures are to be measured simultaneously . This is made possible by a Pitot-Staic tube shown in Fig. 3.39. This combines the staic pressure probe and the pitot tube. The "staic holes" and the "stagnation hole" are as near to each other as possible.

Figure 3.38: Pitot tube used with a static pressure tap

Figure 3.39: Pitot-static tube

 

 

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney