Comparing a CFD simulation of a box gutter to the Plumbing Code

I have generated a computer simulation of a working box gutter to hopefully help in understanding how things work, and to explore how such a mathematical model compares with the plumbing Code.

SO WHAT IS CFD?

CFD stands for Computational Fluid Dynamics. It is a way of generating a computer simulation of fluid flow in real time. It consists of dividing the fluid into a mesh of very small particles. Analyzing all forces on each particle, then tracing the resultant movement of each particle through the obstacles in the allotted time frame. Some more info can be found HERE.

ANALYZING THE RESULTS

The above simulation is very pretty, but not very good at measuring any meaningful depths. So lets take a closer look at the mesh.

The important things to note are:-

a) The deepest flow is at the upstream end, because the water surface must always fall in the direction of the flow.

b) The flow over the brink is 0.7*critical depth. This is a hydraulic principle of a free outfall from an open channel.

c) The downpipe does not flow full. The flow is restricted by the entry throat diameter.

WHAT ARE THE CODE RESULTS

The simulation is based on 11 L/s, with a 300mm wide box gutter, and no slope.

From the Plumbing Code AS/NZS 3500.3, Fig I1 gives the gutter depth for no slope as 170, and with 1:200 slope, depth = 145 mm.

From fig I3, for a DP dia of 125, flow = 11 L/s rainwater head details are :-

• depth of water = 112
• Total depth of RWH = 187,
• and length of RWH, at BG depth of 145 (1:200) = 173

From Fig I6 :-

• Critical depth "Loc" = 52 mm
• 0.7 * critical depth = 37

PLOTTING THE RESULTS ON THE MESH

Mesh Side View

The background grid is 100mm spacing, so you can visually check the dimensions, and confirm I am not pulling any legs.

The Code does not give any values for the freeboard, maybe this is because the freeboard may vary with width and flow, as in some other plumbing Codes. However I find that a value of about 60mm seems to work. This gives a water depth of 110 mm.

The total gutter depth, and rain water head dimensions are also plotted to the code sizes.

As you can see, there is remarkable agreement with the Plumbing Code.

So we can all go away with a warm inner glow, knowing that a CFD simulation agrees with the Plumbing Code. (or is it the other way around?)

BTW my FREE PROGRAMS also give the same results.

 Ken Sutherland is a 'retired' (from design work) Hydraulic/civil Engineer who spent too much time analyzing the Plumbing Code and associated formulas. He now offers the benefit of this research and some of the resultant design programs, for free on the internet, for the benefit of the Construction industry, Architects, Engineers, Hydraulic consultants, students, and every other man and his dog.