Solving the World’s Toughest CFD Problems

# Combined Sewer Overflow Hydraulics

Today’s post is about a simulation for a municipal application. I usually draw on work-related inquiries for these blog posts, and this week a municipal design consultancy called asking about how we handle the combined sewer overflow problem (heads up the answer is very well!). Typically there are two important aspects. One is understanding the straight hydraulics in both a transient and a steady state sense, the other is the characterization of particulate dynamics in the flow stream. In this case, the interest was purely on understanding the hydraulics under various upstream water elevations and pressures, in both free stream and fully pressurized conditions.

The ‘lay of the land’ here is that we have an interceptor sewer carrying the flow under potentially free surface or pressurized conditions. A T-junction allows some of the fluid to make its way to a regulator vault which can then discharge some of the fluid into a separate culvert through a vertical drop-shaft.

To clarify the geometry, I created a rotating animation so that you can see the inflow, the regulator vault and the drop shaft where some of the fluid exits into an underlying culvert from different viewpoints:

I ran two scenarios. In the first scenario we begin with a stagnant low level of fluid in the main sewer, then adjust the inflow boundary condition with time to simulate a rising water level. Initially this only increases the flow rate and fluid elevation in the interceptor sewer. At a given height the water begins to ‘overflow’ into the regulating vault section of the structure. The fluid then exits into the culvert.

In the second scenario, we restart the simulation once the fluid occupies the full volume of the interceptor sewer, and then continue to increase to upstream pressure: the sewer interceptor now operates under pressurized conditions. What is interesting to observe here is that while the sewer interceptor chamber now runs under fully pressurized, confined flow conditions, the regulator vault continues to operate under free surface conditions due to the limiting effect of the regulating slide gate.

Finally, although this wasn’t asked for, it is noteworthy that particle tracking can be included in the analysis. FLOW-3D can assign particle sources of different sizes and densities and fully couple their behaviors with the hydraulics of the flow. We can use particles as tracers of course, but more dynamically sophisticated behaviors are equally straightforward to implement.

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I leave you with a link to an interesting read on a related topic, “Testing a Complex Hydraulic Design of a Sewer Transition with FLOW-3D. Comparison with a Physical Model,” by Daniel Valero, Rafael García-Bartual, Ignacio Andrés and Francisco Valles of the Polytechnic University of Valencia.

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