Fanno Flow Calculator
Adiabatic flow with friction in a constant-area duct. Type into any box — including the duct length — and the rest follow, on either branch, at any γ.
| Quantity | Subsonic accelerates to choking | Supersonic decelerates to choking |
|---|
Duct length to choking
4fL*/D
supersonic ceiling
M = 1, choked
What this is computing
NOT WRITTEN YET — Iliya writes this, 300–500 words.
Deliberately loud so it cannot ship by accident. Worth covering:
- Why friction drives both branches towards M = 1 rather than always slowing the flow — the subsonic case accelerating is the fact students refuse to believe, and entropy is the reason.
- What choking actually means here, and what physically happens if the duct is longer than L*: the flow does not go supersonic, it backs up and the mass flow drops.
- Why the supersonic branch has a maximum duct length while the subsonic one does not.
- The friction-factor trap. This page uses 4fL*/D with the Fanning f; with the Darcy–Weisbach f_D = 4f the same number is f_D L*/D. Getting this wrong is a factor-of-four error and it is extremely common.
- Why the reference is sonic rather than inlet conditions, and how to use the table for a duct between two stations by differencing.