Engineering Questions with Answers - Multiple Choice Questions
Aerodynamics – Measurement of Velocity in a Compressible Flow
The definition of stagnation pressure can help us calculate the velocity of sound for an incompressible flow. What else do we need for the calculation?
a) Ambient temperature
b) Total pressure
c) Static velocity
d) Static pressure
Explanation: The velocity of sound can be calculated using the Pitot tube which measures the stagnation, also called total pressure and measuring the static pressure as well. There is no need to find temperature. And static velocity is nothing but zero velocity.
For compressible flow also, we can calculate velocity with the Pitot and static pressure found.
Explanation: For the compressible flow- both supersonic and subsonic, we can calculate the Mach number by using the Pitot tube. It measures the total pressure and by measuring the static pressure we can get the Mach number, not velocity, unlike the incompressible flow.
For subsonic compressible flow, which of these is not required to be measured separately to find the velocity of flow?
a) Stagnation pressure
b) Static pressure
c) Mach number
d) Free-stream of sound
Explanation: Unlike the incompressible flow, for compressible flow, even if it’s subsonic, we need other parameters to calculate speed of flow. With the knowledge of static and stagnation pressure we can calculate Mach number directly, and hence it is not required to be separately measured. We need free-stream velocity of sound to get flow velocity from Mach number.
For supersonic compression flows, there is a bow shock formed at the mouth of the Pitot tube.
Explanation: The supersonic flow has a very high velocity. When it finds an obstruction in the form of a Pitot tube, a bow shock is formed for the supersonic flow over a blunt body. As a result, the stagnation pressure at the mouth of the Pitot tube is the pressure behind the bow shock.
The Rayleigh Pitot tube formula does not relate which of these quantities to the others?
a) Pitot pressure
b) Free-stream static pressure
c) Free-stream Mach number
d) Upstream flow velocity
Explanation: The Rayleigh Pitot tube formula helps to calculate the free- stream Mach number using the measured values of free-stream static pressure and Pitot pressure. It is based on the fact that a bow shock is formed at the mouth of the Pitot tube for a compressible supersonic flow. The Pitot pressure is the stagnation pressure behind the bow shock.