Pitot Tubes for High-Velocity Gas Flow

An interesting application of the one-dimensional, isentropic flow theory and normal shock theory we have developed is in the calculation of the behavior of pitot-static tubes. ln Sec. 5.8 we showed that for a pitot-static tube the velocity 

This formula gives good results for low-velocity gas flow (less than about 200ft/s), hut for high-velocity gas flows Bernoulli s equation is no longer applicable. For all velocities between zero and sonic velocities, we can assume that the part of the mainstream which is stopped by the impact tube is stopped practically isentropically. If that is correct, then the pressure measured at F, is the reservoir pressure for the flow. Thus, we can use Eq. 8.17, solved for 

In Prob. 8.58 it is shown that as PJP approaches 1.0, this equation approaches Eq. 5.28. Thus, at low velocities the two equations give the same answer. 

Example 8.16. A pitot-static tube immersed in an airstream shows an impact pressure of 151bf/in absolute and a static pressure of lOlbf/in absolute. What is the Mach number of the flow  

From App. A.5 the Mach number is 0.78. If we were interested in the velocity, we also would need to know the absolute temperature.  

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