Reynolds Number Calculator Formula

Understand the math behind the reynolds number calculator. Each variable explained with a worked example.

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Formulas Used

Reynolds Number

re = velocity * diameter / kinematic_visc

Laminar Threshold (Re < 2300)

regime = 2300

Variables

VariableDescriptionDefault
velocityFlow Velocity (V)(m/s)2
diameterPipe Diameter (D)(m)0.1
kinematic_viscKinematic Viscosity (nu)(m^2/s)0.000001

How It Works

Reynolds Number

The Reynolds number is a dimensionless quantity that predicts whether fluid flow will be laminar or turbulent.

Formula

Re = V D / nu

where V is the mean velocity, D is the pipe inner diameter, and nu is the kinematic viscosity. For pipe flow: Re < 2300 is laminar, 2300 < Re < 4000 is transitional, and Re > 4000 is turbulent.

Worked Example

Water at 20 deg C (nu = 1e-6 m^2/s) flowing at 2 m/s through a 100 mm pipe.

velocity = 2diameter = 0.1kinematic_visc = 0.000001
  1. 01Re = 2 x 0.1 / 1e-6 = 0.2 / 1e-6 = 200,000
  2. 02Since 200,000 >> 4000, flow is fully turbulent.

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