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GATE 2025 AE – Question 17

Aerodynamics · Viscous flows: Hagen-Poiseuille flow, Couette flow and boundary layers · 1 mark · Multiple choice

For a two-dimensional incompressible flow over a flat plate, the laminar boundary layer thickness at a distance $x$ from the leading edge is $\delta$. If $Re_x$ is the Reynolds number defined based on length scale $x$, $\left(\frac{\delta}{x}\right)$ is proportional to ______.

  1. $Re_x^{-1/2}$
  2. $Re_x^{-1}$
  3. $Re_x^{-3/2}$
  4. $Re_x^{-2}$

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Correct answer: (A) $Re_x^{-1/2}$

Explanation

For the Blasius laminar boundary layer, $\delta \approx \frac{5x}{\sqrt{Re_x}}$, so $\frac{\delta}{x} \propto Re_x^{-1/2}$.