Closure to “Numerical Oscillations in Pipe-Filling Bore Predictions by Shock-Capturing Models” by J. G. Vasconcelos, S. J. Wright, and P. L. Roe

Abstract

The discussers would like to congratulate the authors for a very interesting paper, in which the gradual flow regime transition i.e., in the absence of a pipe-filling bore front —an outstanding issue in interface tracking models for flow regime transition—was addressed. The proposed model is in this regard an improvement on the family of models presented by Song et al. 1983 and Cardle and Song 1988 , among others. Based on the discussers’ experience in modeling tunnel systems, gradual-flow regime transitions should probably be more common than pipe-filling bores, even though some extreme inflow conditions, which could be very important for design purposes, may require pipe-filling bore modeling. The discussers would like to make a few comments and raise some issues: 1. Geysering events are not necessarily characterized solely by water jets, as investigated by Vasconcelos 2005 . Experiments and field evidence indicate that such phenomena could also be triggered by air pocket expulsion through water-filled vertical shafts in stormwater systems. Careful consideration of the air pocket entrapment is required when dealing with flow-regime transition events. 2. There is already available an alternative to model flowregime transition other than Preissmann slot or interface tracking-based models, namely the TPA model of Vasconcelos et al. 2006 . The TPA model overcomes the limitations of Preissmann-slot models of modeling subatmospheric flows, while retaining a single set of equations to simulate both free-surface and pressurized flows. Indeed as the authors mentioned, flow-regime transition models constructed with shock-capturing approaches may develop numerical oscillations in the vicinity of pipe-filling bores. However, Vasconcelos et al. 2006 propose an approach to attenuate such oscillations. More recently, Vasconcelos et al. 2009 address this issue, and other techniques to attenuate these numerical oscillations are proposed. 3. The discussers doubt that the proposed model would be able to handle a depressurization scenario in which pressurized flows are succeeded by free-surface flows. In such cases, the flow regime interface would be characterized by air intrusion