[acoustics] Jet aspect-ratio model & overblow threshold (flue height : TSH gap)

[tonykoop]

Story points: 3

Scope

The core mechanic that sets how easily a NAF overblows: the aspect ratio of the air jet (flue height = jet thickness) to the True-Sound-Hole gap (jet travel length) crossing the splitting edge.

Why

The "friend overblew one flute but not the other" difference is a jet-stability problem. A thin, fast jet (shallow flue) reaching the edge quickly (short TSH gap) spikes into the octave with a tiny velocity increase; more jet travel/give absorbs higher velocities.

Implementation plan

• Add docs/acoustics/jet-aspect-ratio.md:
  • Define jet length W (flue exit → splitting edge) and jet thickness (flue height); stability depends on W/thickness and the phase-lock between jet transit time and the bore's acoustic cycle.
  • Short W + thin jet → low overblow-threshold velocity (twitchy). Longer W and/or thicker jet → higher threshold (forgiving).
  • Laminar vs turbulent: too-short/rough flue → turbulent, weak fundamental; perfectly laminar but too thin → high Reynolds sensitivity, jumps octave on a small breath increase.

Acceptance

• Jet aspect-ratio model + overblow-threshold relationship documented.
• Laminar/turbulent stability notes included.
• Cites source.

Links

• Epic Epic: NAF fipple playability & overblow-tolerance tuning #13. Sets up the playability-levers and geometry-spec stories. Shared physics with tonykoop/fujara Epic: NAF fipple playability & overblow-tolerance tuning #13.
• Source: NAF Playability.md, Flute Acoustics.md (2026-07-02).