American Society of Civil Engineers


Dealing with Zero Flows in Solving the Nonlinear Equations for Water Distribution Systems


by Sylvan Elhay, (Visiting Research Fellow, School of Computer Science, Univ. of Adelaide, South Australia, 5005.) and Angus R. Simpson, (corresponding author), M.ASCE, (Professor, School of Civil, Environmental and Mining Engineering, Univ. of Adelaide, South Australia, 5005. E-mail: asimpson@civeng.adelaide.edu.au)

Journal of Hydraulic Engineering, Vol. 137, No. 10, October 2011, pp. 1216-1224, (doi:  http://dx.doi.org/10.1061/(ASCE)HY.1943-7900.0000411)

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Document type: Journal Paper
Discussion: by Yuriy Kovalenko; formerly, Project Leader, Sistemas y Procesos, Centro de Tecnología Avanzada, 150 Calz. del Retablo, Fovissste, 76150 Querétaro, Mexico E-mail: kovalenko.yuriy@gmail.com and et al.    (See full record)
Discussion: by Nikolai B. Gorev E-mail: gorev57@mail.ru and et al.    (See full record)
Closure:(See full record)
Abstract: Three issues concerning the iterative solution of the nonlinear equations governing the flows and heads in a water distribution system network are considered. Zero flows cause a computation failure (division by zero) when the Global Gradient Algorithm of Todini and Pilati is used to solve for the steady state of a system in which the head loss is modeled by the Hazen-Williams formula. The proposed regularization technique overcomes this failure as a solution to this first issue. The second issue relates to zero flows in the Darcy-Weisbach formulation. This work explains for the first time why zero flows do not lead to a division by zero where the head loss is modeled by the Darcy-Weisbach formula. In this paper, the authors show how to handle the computation appropriately in the case of laminar flow (the only instance in which zero flows may occur). However, as is shown, a significant loss of accuracy can result if the Jacobian matrix, necessary for the solution process, becomes poorly conditioned, and so it is recommended that the regularization technique be used for the Darcy-Weisbach case also. Only a modest extra computational cost is incurred when the technique is applied. The third issue relates to a new convergence stopping criterion for the iterative process based on the infinity-norm of the vector of nodal head differences between one iteration and the next. This test is recommended because it has a more natural physical interpretation than the relative discharge stopping criterion that is currently used in standard software packages such as EPANET. In addition, it is recommended to check the infinity norms of the residuals once iteration has been stopped. The residuals test ensures that inaccurate solutions are not accepted.


ASCE Subject Headings:
Water distribution systems
Algorithms
Head loss (fluid mechanics)
Water flow

Author Keywords:
Water distribution systems
Solving equations
Newton method
Regularization
Todini and Pilati
Zero flows
Convergence
Hazen-Williams
Darcy-Weisbach