Using Depth Average Flow Solver of NASIR Finite Volume Software for Modeling Air Entrainment from Water Surface in Chute Flow

Authors
S. Sabbagh-Yazdi
H. Rezaei-Manizani
Abstract
In this paper, the results of water flow modeling and computation of air distribution in chute spillways are
described. The depth average flow solver of NASIR finite volume software was utilized for modeling of
water flow in chute spillways. In this module, the shallow water equations modified for steep slopes are
solved using cell-centre and cell-vertex schemes of finite volume method that suits unstructured triangular
meshes. Using the computed velocity components and flow depth at each nodal point, the air entertainment
parameters (air inception point, mean air concentration and vertical distribution of air concentration) are
calculated by utilizing some empirical relations. The empirical relations for the air concentration were
adopted by the pervious researchers using the measurements on large scale flow in chute spillways.
In the present work, three modeling strategies were performed: 1) Solution of flow field and then
calculation of air concentration parameter (as a post-processing operation after convergence of the water
flow solution), 2) Coupled solution of the flow equations and air concentration relations considering the
flow bulking and reduction of global friction parameters due to air entrainment and 3) Adding the air
entrainment on density variation effects to the coupled solution of the flow equations and air concentration
relations considering the flow bulking and reduction of global friction parameters due to air entrainment.
In order to assess the quality of the results of the three modeling strategies, the readily available flow
velocity and air concentration measurements on AVIMORE chute spillway were used for comparison
considering the best experimental relations for simulating the entrainment of air into the flow on chute
spillways. In order to provide better understanding of the velocity and air concentration, the vertical
distribution profiles of these parameters were plotted from the multi-layer treatments of depth averaged
computed results.
Comparison of the computed velocity fields and air concentrations with the reported field measurements at
the stations 503 and 505 of AVIEMORE chute spillway presents promising agreements. However, the
accuracy of the 2nd and 3rd modeling strategies (in which the coupled solutions of water flow equations and
air concentration relations are considered and the effect of air content on the reduction of global friction of
the water flow is taken into consideration) was much better than that of the 1st modeling strategy (in which
the air concentration relations are considered as post processing on water flow equation solution for depth
and velocity fields). Furthermore, no significant differences were observed between the results of the 2nd
and 3rd modeling strategies. Hence, it can be concluded that the effect of air concentration on density
variation of the supercritical water flow does not play an important role on the results of the coupled
solutions of water flow equations and air concentration relations.

Keywords

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