Flow Continuity Velocity at a Size Change
Continuity V2 = V1 (D1/D2)^2 from Q = A V constant. Water at 6 ft/s reduced 4 in to 2 in jumps to 24 ft/s (past the erosion limit); only to 3 in gives a gentler 10.7 ft/s. Expanding slows the flow and recovers pressure. Incompressible full flow. The code velocity limits govern.
Formula and source
V2 = V1 (D1/D2)^2 (from Q = A V constant, A = pi/4 D^2).
The continuity equation for an incompressible fluid, by name.
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