GATE 2020 CH – Question 27

Fluid Mechanics and Mechanical Operations · Turbulent flow, boundary layer and flow past immersed bodies · 1 mark · Multiple choice

A constant-property incompressible fluid flows parallel to a smooth thin wide flat plate at constant free-stream speed. Reynolds number at 2 m from the leading edge is 8000. At 1 m, the boundary-layer flow is

Laminar
Turbulent
Transitioning from laminar to turbulent
Inviscid

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Show answer and explanation

Correct answer: (A) Laminar

Explanation

Local Reynolds number is $Re_x=U_\infty x/\nu$. Halving x from 2 m to 1 m halves it from 8000 to 4000. This is far below the usual smooth-flat-plate transition Reynolds number of order $5\times10^5$, so the boundary layer remains laminar, A. Pipe-flow transition criteria do not apply to this external-flow geometry.

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