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Air (density = 1.2 kg/m³, kinematic viscosity = 1.5×10⁻⁵ m²/s) flows over a flat plate with a free-stream velocity of 2 m/s. The wall shear stress at a location 15 mm from the leading edge is τ_w. What is the wall shear stress at a location 30 mm from the leading edge?

1. τ_w / 2
2. √2 τ_w
3. 2 τ_w
4. τ_w / √2

Correct answer: τ_w / √2

Solution

The wall shear stress in a laminar flow over a flat plate decreases with distance from the leading edge, specifically following the relationship that it is inversely proportional to the square root of the distance from the leading edge. Therefore, at 30 mm, the wall shear stress is reduced to τ_w / √2 compared to its value at 15 mm.

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