DISCUSSIONS AND CLOSURES Discussion of “Case Study: Numerical Evaluation of Hydraulic Transients in a Combined Sewer Overflow Tunnel System” by M. Politano, A. J. Odgaard, and W. Klecan October 2007, Vol. 133, No. 10, pp. 1103–1110. DOI: 10.1061/ASCE0733-94292007133:101103 Jose G. Vasconcelos, M.ASCE 1 ; and Steven J. Wright 2 1 Assistant Professor, Dept. of Civil Engineering, Auburn Univ., 208 Har- bert Engrg. Center, Auburn, AL 36849. E-mail: jvasconcelos@ auburn.edu 2 Prof., Dept. of Civil and Environmental Engineering, Univ. of Michigan, 1351 Beal Ave., Ann Arbor, MI 48109. E-mail: sjwright@engin. umich.edu 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 ad- dressed. 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’ experi- ence 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 im- portant for design purposes, may require pipe-filling bore model- ing. 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. Experi- ments 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 flow- regime transition other than Preissmann slot or interface tracking-based models, namely the TPA model of Vasconce- los 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 au- thors mentioned, flow-regime transition models constructed with shock-capturing approaches may develop numerical os- cillations 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 attenu- ate 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 fronts, which in turn resemble gravity currents. These flows have been studied by Benjamin 1968, Baines 1991, and others, and the presented model equations seem unable to represent such flows at the depressurization interfaces. 4. The discussers would like to know how the proposed model distinguishes a gradual from an abrupt flow regime transition interface. Is it based on whether the flow regime transition is coincident with the location of a bore? 5. Fig. 3 of the original paper indicates that the proposed model is able to handle open-channel bores. How does the proposed model test for the formation of such bores within the computational domain? 6. Fig. 9 of the original paper indicates the formation of en- trapped air pockets closer to the Clear Creek station at times 1.98 h, 1.99 h, and 2.00 h. Is there any special treatment for these pockets in the modeling process? 7. How was the value for the acoustic wavespeed of 762 m / s for the West Area CSO Tunnel System estimated? Also, was there any calculation of continuity errors in this tunnel modeling? References Baines, W. D. 1991. “Air cavity as gravity currents on slope.” J. Hy- draul. Eng., 11712, 1600–1615. Benjamin, T. B. 1968. “Gravity currents and related phenomena.” J. Fluid Mech., 312, 209–248. Cardle, J. A., and Song, C. S. S. 1988. “Mathematical modeling of unsteady flow in storm sewers.” Int. J. Eng. Fluid Mech.,14, 495– 518. Song, C. S. S., Cardle, J. A., and Leung, K. S. 1983. “Transient mixed- flow models for storm sewers.” J. Hydraul. Eng., 10911, 1487– 1504. Vasconcelos, J. G. 2005. “Dynamic approach to the description of flow regime transition in stormwater systems.” Ph.D. dissertation, Environ- mental Engineering, The University of Michigan, Ann Arbor, Mich. Vasconcelos, J. G., Wright, S. J., and Roe, P. L. 2006. “Improved simu- lation of flow regime transition in sewers: Two-component pressure approach.” J. Hydraul. Eng., 1326, 553–562. Vasconcelos, J. G., Wright, S. J., and Roe, P. L. 2009. “Numerical oscillations in pipe-filing bore predictions by shock-capturing mod- els.” J. Hydraul. Eng., 1354, 296–305. Closure to “Case Study: Numerical Evaluation of Hydraulic Transients in a Combined Sewer Overflow Tunnel System” by M. Politano, A. J. Odgaard, and W. Klecan October 2007,Vol. 133, No. 10, pp. 1103–1110. DOI: 10.1061/ASCE0733-94292007133:101103 M. Politano 1 ; A. J. Odgaard, M.ASCE 2 ; and W. Klecan 3 1 Assoc. Research Engineer, IIHR—Hydroscience & Engineering, The Univ. of Iowa, 300 South Riverside Dr., Iowa City, IA 52242-1585. E-mail: marcela-politano@uiowa.edu JOURNAL OF HYDRAULIC ENGINEERING © ASCE / JUNE 2010 / 391 J. Hydraul. Eng. 2010.136:391-391. Downloaded from ascelibrary.org by Auburn University on 07/11/14. Copyright ASCE. 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