Cavitation number (sigma)

The cavitation number $\sigma = (p_0 - p_v) / (\tfrac12 \rho V^2)$ is the dimensionless margin between local static pressure $p_0$ and vapor pressure $p_v$, normalized by dynamic pressure. Cavitation begins when $\sigma$ falls to the section’s inception value, so a low $\sigma$ (deep loading, high speed, shallow immersion) is dangerous. It sets the operating envelope tested in a cavitation tunnel and read against the Burrill chart or the cavitation bucket.