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04cavitation - Cavitation Notes p 0 =...

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Cavitation Notes ref: PNA pages 181-183 handout p 0 = uniform_stream_total_pressure p 1 = pressure_at_arbitrary_point V 0 = uniform_stream_velocity V 1 = velocity_at_arbitrary_point 1 2 q = ⋅ρ⋅ V 0 = dynamic_or_stagnation_or_ram_pressure 2 1 2 p 0 + ⋅ρ⋅ V 0 = constant Bernoulli 2 1 2 1 2 p 0 := constant ⋅ρ⋅ V 0 p 1 := constant ⋅ρ⋅ V 1 2 2 for propeller immersion, measured at radius r, minimum p 0 is obtained from ... p 0 = p a + ρ⋅ g h ρ⋅ g r p a = atmosphere h = shaft_centerline_immersion ρ⋅ g r accounts for minimum when r vertical up V 0 estimated as (VA^2+( ω *r)^2) 0.5 if p 1 => p v = vapor pressure, cavitation occurs p a + ρ⋅ g h ρ⋅ g r p v define: σ L = local_cavitation_number = and if pressure REDUCTION / q ρ 2 2 2 >= σ L cavitation occurs 2 V A + ω ⋅ r early criteria (Barnaby) suggested limiting average thrust per unit area to certain values (76.7 kN/m 2 = 10.8 psi) for tip immersion of 11 in increasing by 0.35 psi (unit conversions don't match up) kN 76.7 = 11.124 psi earlier PNA (1967) stated Barnaby suggested 11.25 psi 2 m can calculate pressure distributions around blade so can calculate local cavitation situation
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