Water (ρ = 1000 kg/m³, η = 1.0 × 10⁻³ Pa·s) flows at 0.05 m/s through a pipe of diameter 4 cm. Find Re and state the flow regime. What speed would make it turbulent? Step 1: Re = ρvd/η = 1000 × 0.05 × 0.04 / 0.001 = 2000. Step 2: Re ≈ 2000 is right at the laminar–transition boundary — the flow is just barely laminar. Step 3: Turbulence needs Re > ~3000: v > 3000 × 0.001/(1000 × 0.04) = 0.075 m/s. Answer: Re = 2000, marginally laminar; above ~0.075 m/s the flow turns turbulent.