Question

A pilot tube is inserted in an air flow (at STP) to measure the flow speed. The tube is inserted so that it points upstream into the flow and the pressure sensed by the tube is the stagnation pressure. The static pressure is measured at the same location in the flow, using a wall pressure tap. If the pressure difference is $30mm$ of mercury, determine the flow speed.

• A. $80.8m/s$
• B. $40m/s$
• C. $10.23m/s$
• D. None of these

Answer 1

Answer: (A)

$80.8m/s$

The difference in pressure is due to stagnation of air.

Pressure of 30mm of mercury is $P=\frac{30\times {10}^5}{760}=3947pascal$

So $P=\frac{\rho v^2}{2}=3947$ , density of air is $\rho =1.2kg/m^3$

$⟹v=80.8m/s$