A dimensionless number used in momentum transfer in general and throttling calculations in particular. Proportional to (excess of local static head over vapor pressure head) / (velocity head). A lower Cavitation number (especially Cavitation Numbers < 1) imply a high degree of cavitation.
sigma = (P-Pvap) / (0.5 rho v2)
P: pressure, Pvap: vapor pressure of fluid, rho: density, v: velocity
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