Question

A Pitot-tube is used for measuring

• A. Flow velocity
• B. Flow pressure
• C. Flow rate
• D. Total energy of flow

Hint:

A Pitot-tube measures flow velocity by using the difference between stagnation and static pressure, applying Bernoulli’s equation: \( v = \sqrt{\frac{2 \Delta P}{\rho}} \).

Answer 1

The Correct Option is A

Step 1: Understand the function of a Pitot-tube.
A Pitot-tube is a device used to measure the velocity of a fluid flow, commonly in aerodynamics and fluid mechanics. It consists of a tube with an opening facing the flow (stagnation point) and another opening to measure static pressure, often through a side port or a separate static tube. Step 2: Analyze how a Pitot-tube works.
The Pitot-tube measures the stagnation pressure (total pressure) at the tip, where the flow velocity becomes zero (stagnation point).
It also measures the static pressure of the flow through a side port or a separate static tube.
The difference between the stagnation pressure and static pressure is the dynamic pressure, which is related to the flow velocity via Bernoulli’s equation (for incompressible flow): \[ P_{\text{stagnation}} - P_{\text{static}} = \frac{1}{2} \rho v^2, \] \[ v = \sqrt{\frac{2 (P_{\text{stagnation}} - P_{\text{static}})}{\rho}}, \] where \( \rho \) is the fluid density, and \( v \) is the flow velocity. Thus, the primary purpose of a Pitot-tube is to calculate the flow velocity by measuring the pressure difference. Step 3: Evaluate the options.
(1) Flow velocity: The Pitot-tube directly measures velocity using the pressure difference, as derived above. Correct.
(2) Flow pressure: While the Pitot-tube measures pressures (stagnation and static), its purpose is to use these to find velocity, not just pressure. Incorrect.
(3) Flow rate: Flow rate requires velocity and cross-sectional area (\( Q = vA \)). A Pitot-tube measures velocity, but flow rate calculation requires additional information. Incorrect.
(4) Total energy of flow: The Pitot-tube measures stagnation pressure, which is related to the total energy per unit volume, but its primary use is for velocity, not total energy directly. Incorrect. Step 4: Select the correct answer.
A Pitot-tube is used for measuring flow velocity, matching option (1).