Reply to Discussion by M. Pfister on “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators”Appears in the Canadian Journal of Civil Engineering, 36(5): 826-838.

Abstract

In our study, we compared the results of computational fluid dynamics (CFD) using FLUENT software with a small-scale physical model including considerable scale effects and some prototype data only in terms of the air entrainment rates. The verifications of the CFD analyses were based on Richardson extrapolation and other methods in the study. It is noted that the study should be considered in the above framework. In FLUENT, three different Euler–Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. Spillway aerator flows is a free-surface flow and a multiphase flow at the same time. For free-surface flow and large bubble flow, the best multiphase model is VOF, but the VOF model is not appropriate for air–water mixture and small bubble flow. Since there are small bubbles and air–water mixture in spillway flows downstream of an aerator, we suppose that the ASM model is more suitable for modeling of aeration. FLUENT also recommends using the Algebraic Slip Mixture (ASM) model for bubble flows as aeration. It is stated in the tutorial of FLUENT 5.3 (FLUENT 2000) that ASM model can be used at all values of volume fraction. However, it is true that it restricts to relatively small bubbles and low volume fraction when primary phase density is much smaller than the secondary phase density for gas–liquid flows. Among available multiphase models, we think that the ASM model is the most suitable for both free surface and air–water mixture flows since flow aerated by an aerator includes small bubbles. Low gas (air) fraction was only an assumption in our study. Turbulence model