2022-11 - Downpipe

A vertical 1200mm dia downpipe which conveys stormwater down a 7m high retaining wall is simulated by OpenFOAM.
My colleague came to discuss his design of downpipe a few days ago. To convey 6m3/s stormwater runoff down a retaining wall, he estimated that a 1200mm dia downpipe with very steep gradient had sufficient capacity based on Colebrook-White Equation.
Actually, this is not the first time colleagues come to discuss this question. From his design, he didn't consider control at downpipe inlet which may result in excessive headwater level.
OpenFOAM simulation was carried out under the following conditions:
Dia of downpipe = 1200mm
Downpipe is assumed vertical
Atmospheric pressure at downpipe outlet
Inflow increases linearly from 0m3/s to 6m3/s in 60s and then kept constant
Radius of bend is 1D
A short horizontal piece of 1D long is used to guide flow in channel above retaining wall to the downpipe
It is interesting to note the followings:
1. For low flow rate (Q less than 4 to 5m3/s), control is at pipe inlet and downpipe is not full. Headwater level reaches about 3.5m maximum
2. For higher flow rate, inlet flow hits opposite wall of downpipe and results in back flow to submerge pipe inlet. Resulting full bore flow pulls upstream water into the pipe quickly similar to siphon effect.
From this simple CFD simulation, it demonstrated that excessive water level at crest of retaining wall could be developed due to inlet control, despite capacity of full bore downpipe is estimated sufficient. A simple solution is to add more downpipes assuming inlet control.

[@white2785]

Hi Eddy. Keep posting FLOW -3D videos. You're cool