The 'Reynolds number' in a pipe flow of water (ν=10⁻⁶ m²/s) at V=2 m/s and D=0.1 m is:
Choose the correct answer
Re = 200 (laminar)
Re = 2×10⁵ = 200,000 — turbulent flow; Re = VD/ν = 2×0.1/10⁻⁶
Re = 2 (creeping flow)
Re = 2×10⁸
Correct Answer
B. Re = 2×10⁵ = 200,000 — turbulent flow; Re = VD/ν = 2×0.1/10⁻⁶
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Re = V×D/ν = 2 × 0.1 / 10⁻⁶ = 200,000 = 2×10⁵. Turbulent flow (Re > 4000 for full turbulence). Laminar: Re < 2000 (critical: Re ≈ 2300). At Re = 2×10⁵ in smooth pipe: use Blasius f = 0.316/Re^0.25 = 0.316/(200000)^0.25 ≈ 0.316/21.1 ≈ 0.015. Friction head loss: hf = 0.015×(L/0.1)×(4/2×9.81) = 0.031×10L/L (per metre).
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