Modeling Air Entrainment

Dear SWMM group

I am currently attempting to model a section of a CSO system that includes a run of 36" pipe with a grade brake (going from 20% to 3% slope) suspected of having significant air entrainment (due the hydraulic jump). I am attempting to model this system using PCSWMM. Do you know of any way of quantifying the air entrainment? Are there any other models that may be designed to take air entrainment into account? The flow is estimated to be around 70 CFS.

Your comments and suggestions would be greatly appreciated.
John Abdalkhani

Mr. Abdalkhani,

I don't have specific numbers on air entrainment, but the basic idea is the supercritical water flow will induce air flow and air entrainment through the hydraulic jump. Air entrainment is also dependent on the air supply (e.g. if air can't get into the system, or is restricted, the air entrainment is reduced). I was involved in a model study of a large conduit which inherently flowed slightly supercritical, but the bends induced shock waves, similar to hydraulic jumps. When the wave hit the top of the conduit, flow tended to become subcritical and backed up the conduit. Ultimately, these characteristics led to unstable flow, where flow levels and pressures fluctuated, and reduced the pipe capacity. When the air vents were provided upstream of the jump, the flow largely stabilized and hydraulic capacity increased. Air was entrained through the jump, but there was no sign of bulking. It sounds like in your case the upstream flow would have far higher velocities and thus more air entrainment, but again dependent on the air supply characteristics.

CSU has actually done air entrainment studies for water flowing over weirs and other drops for free surface flow. You may want to contact Chester Watson at CSU or Ted Johnson at CDM Denver.

Bill T.
Taggart Engineering Assc., Inc.