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GATE 2026 ME – Question 25

Fluid Mechanics · One-dimensional compressible flow, nozzles · 1 mark · Multiple choice

An ideal gas passes isothermally through a long horizontal uniform cross-section pipe under steady flow. Consider that the pressure gradient in the pipe is sufficient for a finite change in the density of the gas. If the flow of the gas is purely pressure driven and subsonic throughout, then the average flow velocity

  1. increases along the flow
  2. decreases along the flow
  3. does not change throughout the pipe
  4. increases in the hydrodynamic entrance region and then decreases thereafter

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Show answer and explanation

Correct answer: (A) increases along the flow

Explanation

The pressure falls along the pipe because of friction. For an isothermal ideal gas the density is proportional to pressure, so the density falls too. The mass flow rate is constant and the area is constant, so $\rho V$ is constant, and the velocity must increase along the flow.