Modeling the Transition from Gravity to Pressurized Flows in Sewers

  • Li J
  • McCorquodale A
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Abstract

Blow-off of storm/combined manhole covers may be attributed to the severe pressure Ixansients which can occur during the transition from free surface to pressurized flow in a sewer system (Fig.l). Observations fi'om a physical sewer model with a submerged outlet indicate that the pressure transients were composed of an initial low frequency water hammer type pressure surge and a subsequent high frequency pressure fluctuation due to the release of trapped air at the upstream manhole. A mathematical model, which was based on the assumption of rigid water columns and a compressible air bubble, was derived to simulate the pressure trmmients. The frequency of pressure transients predicted by the calibrated mathematical model is in close agreement with that recorded during the laboratory experiments. The attenuation of the pressure fluctuations, however, is underestimated. This may be attributed to the superposition of various air release processes observed during the experiments and the steady-state friction factor used in the mathematical model. This new approach for modelling pressure transients in storm/combined sewe1"s provides an efficient alternative to the traditional Preissman's slot method for mixed flow analysis.

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Authors

  • J Li

  • A McCorquodale

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