Froude Number Calculator
Determine the flow regime: subcritical (Fr < 1), critical (≈1), or supercritical (Fr > 1).
How to Use This Calculator
Measure velocity
Use average section velocity in m/s.
Select characteristic length
For open channels, use hydraulic depth (area/top width). Depth is a common proxy.
Compute Fr
Interpret the regime to assess wave and jump behavior.
Formula
Fr = v / √(g L)
Where: v = velocity (m/s), L = characteristic length (m), g = 9.80665 m/s².
Example: v = 2 m/s, L = 0.5 m → Fr = 2 / √(9.81×0.5) ≈ 0.905 (subcritical).
About Froude Number
Froude number compares inertial to gravitational forces in free-surface flows. It predicts behavior like hydraulic jumps and wave propagation.
- Fr < 1: waves can travel upstream (subcritical).
- Fr ≈ 1: critical flow; minimal specific energy for given discharge.
- Fr > 1: disturbances swept downstream (supercritical).
Frequently Asked Questions
What length should I use?
For rectangular channels, use hydraulic depth A/T. Depth is acceptable for quick checks.
Does Fr apply to closed pipes?
Fr is for free-surface flows. For pressurized pipes, Reynolds and Mach numbers are more relevant.
Can I use imperial units?
Yes, if consistent. g ≈ 32.174 ft/s²; L in feet; v in ft/s.