Reynolds number for flow of water at room temperature through 2 cm dia pipe at an average velocity of 5 cm/sec is around

Reynolds number (Re) is a dimensionless quantity used in fluid mechanics to predict whether the flow will be laminar or turbulent.

It's calculated using this formula: Re = (Density * Velocity * Diameter) / Viscosity

For water at room temperature, we can use approximate values:
  Density (ρ) ≈ 1 g/cm³
  Viscosity (μ) ≈ 0.01 g/(cm·s)

Let's plug in the given values from the question:
  Diameter (D) = 2 cm
  Velocity (V) = 5 cm/s

Therefore:
  Re = (1 g/cm³ * 5 cm/s * 2 cm) / (0.01 g/(cm·s))
  Re = 1000

Therefore, the Reynolds number is approximately 1000. This indicates a flow that is likely to be in the transition zone between laminar and turbulent flow.