Flow Continuity Velocity at a Size Change
The velocity a flow picks up when the pipe changes size.
Example
You enter
- Upstream velocity (ft/s) 6
- Upstream diameter (in) 4
- Downstream diameter (in) 2
You get
- Downstream velocity 24
Checked against published worked examples from: US DOE.
Details, formula, and sources
ScopeContinuity 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 or tableV2 = V1 (D1/D2)^2 (from Q = A V constant, A = pi/4 D^2).
Source and editionThe continuity equation for an incompressible fluid, by name.
Free accessThe continuity equation Q = A V is a standard fluid-mechanics relation. The code velocity limits and the engineer of record govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesV1_fps, D1_in, D2_in, V2_fps
- Continuity V2 = V1 (D1/D2)^2 from Q = A V constantfluid mechanics
- Incompressible full flow full pipe, incompressible; a gas/two-phase flow does not obey thisfluid mechanics
- No transition loss does not compute the pressure change or the fitting loss at the size changescope of this tile