Reynolds Number & Flow Regime Checker

What is Reynolds Number?

The Reynolds number is a dimensionless quantity used in fluid mechanics to predict flow patterns in different fluid flow situations. Named after Osborne Reynolds, who first demonstrated its significance in 1883, it represents the ratio of inertial forces to viscous forces within a fluid.

The Reynolds number is calculated using either of these equivalent formulas:

• Re = ρvL/μ — using fluid density (ρ), velocity (v), characteristic length (L), and dynamic viscosity (μ)
• Re = vL/ν — using velocity (v), characteristic length (L), and kinematic viscosity (ν)

Flow Regimes

Laminar Flow

At low Reynolds numbers, fluid flows in smooth, orderly layers with minimal mixing between layers. This is called laminar flow. The fluid moves in parallel layers, with each layer sliding past adjacent layers with little or no mixing. Laminar flow results in lower friction losses and more predictable behavior.

Turbulent Flow

At high Reynolds numbers, the flow becomes chaotic and unpredictable. This turbulent flow is characterized by eddies, swirls, and rapid variation in pressure and velocity. While turbulent flow has higher friction losses, it provides better mixing and heat transfer.

Transitional Flow

Between laminar and turbulent regimes lies the transitional zone, where the flow alternates between laminar and turbulent behavior. Flow in this regime is unstable and can be triggered into turbulence by small disturbances.

Geometry-Dependent Thresholds

The critical Reynolds numbers for transition depend on the flow geometry:

Practical Applications

Reynolds number is essential in many engineering applications:

• Pipe system design: Determining pressure drop and pump sizing
• Aerodynamics: Predicting drag on vehicles and aircraft
• Heat exchangers: Optimizing heat transfer vs. pumping cost
• Wind tunnel testing: Scaling experiments to match real-world conditions
• Chemical reactors: Designing mixing and reaction vessels

Important Assumptions

This calculator assumes steady, incompressible flow of a Newtonian fluid with constant properties. The thresholds provided are approximate; actual transition can be affected by surface roughness, inlet conditions, vibrations, and other disturbances. For safety-critical applications, always consult with qualified engineers.