🌊 Reynolds Number Calculator
Compute Reynolds number for pipe, flat plate, annulus, and rectangular duct geometries. Classify flow regime (laminar/transition/turbulent), compute hydraulic diameter, entry length, and critical velocity.
📝 Configuration
Re = ρ·V·D_h / μ
D_h = D (pipe), L (plate)
D_h = D_o−D_i (annulus)
D_h = 4WH/[2(W+H)] (duct)
Laminar: Re < 2300, Turbulent: Re > 4000
📊 Results
Configure inputs and click Compute to view results.
📘 Methodology
Reynolds Number
Re = ρVD_h/μ is the ratio of inertial to viscous forces. It determines the flow regime: laminar (Re < 2300 for internal flow, Re < 5×10⁵ for external), transition, or turbulent.
Hydraulic Diameter
For non-circular cross-sections, the hydraulic diameter D_h = 4A/P (4× cross-sectional area / wetted perimeter) is used. For an annulus D_h = D_o − D_i, for a rectangular duct D_h = 2WH/(W+H).
Entry Length
The hydrodynamic entry length is the distance for the velocity profile to fully develop. Laminar: L_e ≈ 0.05·Re·D. Turbulent: L_e ≈ 4.4·Re^(1/6)·D (shorter relative to pipe diameter).