Discussion
The velocity of an air stream is 20 m/s. The densities of mercury and air are 13600 kg/m3 and 1.2 kg/m3, respectively. The gravitational acceleration may be taken as 10 m/s2. When a Pitot-static tube is placed in the stream, assuming the flow to be incompressible and frictionless, the difference between the stagnation and static pressure in the flow field (in mm Hg) would approximately be
Answer: B
The pressure difference is about 1.76 mm Hg.
A Pitot-static tube reads the dynamic pressure, the difference between stagnation and static pressure:
\[ \begin{aligned} \Delta p &= \tfrac{1}{2}\rho_{air}V^{2} = \tfrac{1}{2}(1.2)(20)^{2} = 240\ \text{Pa} \\ h &= \frac{\Delta p}{\rho_{Hg}g} = \frac{240}{13600 \times 10} \\ &= 1.76 \times 10^{-3}\ \text{m} = 1.76\ \text{mm} \end{aligned} \]
1760 is the same column expressed in micrometres, a units slip. 0.57 is the reciprocal of 1.76, and \(0.57 \times 10^{-5}\) compounds that with a wrong power of ten.
Post your comment
Please log in or register to post a comment.
Examination Categories
- SSC Exams
- Banking Exams
- Teaching Exams
- Civil Services Exams
- Railways Exams
- Engineering Recruitment Exams
- Defence Exams
- State Government Exams
- Police Exams
- Insurance Exams
- Nursing Exams
- Other Government Exams
- NRA CET
- PG Entrance Exams
- Campus Placement
- MBA Entrance Exams
- CUET and UG Entrance
- Judiciary Exams
- Regulatory Body Exams
- Software Development