Author(s): Jorge Molina; Pablo Ortiz

Linked Author(s):

Keywords: Ir cavity propagation; Rising Taylor Bubbles; Geysering; Nearly incompressible two fluids flow

Abstract: Emergence of air pockets in ducts and subsequent geysering event [3] is a physical phenomenon occurring in sewer systems during severe storms. Presented analytical solution for air/water flows in vertical shafts considers gas expansion effects and free surface position to predict Taylor bubbles dynamics and the impulsion of water above them. On the other hand, numerical solution reproduces dynamics of weakly compressible two-phase flows and the interface motion. This model follows the steps of the conservative level-set and is based on the Non-Oscillatory Finite Element (NFEM) algorithm, integrating the sign preserving flux correction methodology [1] to avoid spurious momentum transfers between phases. Both models are used to examine intrusion of air pockets and geysering phenomena. While theoretical approach yields fast results about main characteristics of the dynamics, numerical model provides details filtered by the analytical procedure, especially above the street level. First, emergence of cavities is scrutinised with numerical model to elucidate the minimum flow velocity that prevents the intrusion of air in inclined conduits with a partially closed gate at its end. In these simulations a conservative and monotonic adaptive mesh refinement strategy is employed nearby interface to avoid the artificial amplification of Kelvin-Helmholtz instabilities [2]. Numerics give rise to simple equations predicting the required flow rate with high accuracy. Second, transmission of air between horizontal and vertical ducts and final geyser are analysed. To properly reproduce this event, weakly compressible assumption is essential to capture air expansion/compression processes, achieving a realistic momentum transfer between phases [2]. Analytical, axisymmetric and full three dimensional models are employed to simulate geysers caused by one and multiple rising bubbles, thus comparing their differences and determining conditions that trigger a severe geysering event. Outputs are precise when contrasting with experiments and real geysers. REFERENCES [1] Molina, J. and Ortiz, P. A conservative flux–corrected continuous FEM for fluid interface dynamics. J. Numer. Methods Fluids (2019) 91:287-310. [2] Molina, J. and Ortiz, P. A continuous finite element solution of fluid interface propagation for emergence of cavities and geysering. Comput. Methods Appl. Mech. Eng. (2020) 359:112746. [3] Vasconcelos, J.G. and Wright, S.J. Geysering generated by large air pockets released through water-filled ventilation shafts. J. Hydraul. Eng. (2011) 137: 543-555.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022747

Year: 2022