Modeling of Free-Surface Flows with Air Entrainment.

摘要

This dissertation deals with the computational study of free-surface flows with air entrainment. The aim of the study was to identify a suitable multiphase flow model that is capable of not only simulating the intricate flow physics but is also able to capture the free-surface deformations and predict the air entrainment at a reasonable computational cost. Finite volume based computations were performed using STAR-CCM+ commercial solver. The volume of fluid (VOF) multiphase model was used in the present study. First, a submerged hydraulic jump with an inlet Froude number F1 = 8.2 is simulated to determine the capabilities of the VOF multiphase model in capturing the free-surface deformations and other flow characteristics. The submerged hydraulic jump entrains lesser quantities of air and the free-surface deformations are not as abrupt as the classical hydraulic jump. Hence, this problem was chosen as a benchmark to validate the model. The VOF multiphase model was able to accurately capture the submerged hydraulic jump flow field. Proper orthogonal distribution (POD) analysis of the fluctuating velocity of the submerged hydraulic jump revealed the breakdown of large-scale structures into smaller-scale structures by the interaction of the roller and wall-jet flow, leading to the dissipation of energy.;Subsequently, a classical hydraulic jump with inlet Froude number F 1 = 8.5 was studied using the VOF multiphase model. The mean and unsteady features of the classical hydraulic jump were predicted accurately by the VOF multiphase model. Quadrant decomposition of the Reynolds stresses revealed that the outward and inward interactions were dominant in the classical hydraulic jump flow field. Analysis of the higher-order moments showed that the outward interactions caused a flux of turbulent kinetic energy towards the free surface, leading to interfacial aeration. The VOF multiphase model over-predicted the air concentration in the classical hydraulic jump due to numerical diffusion. Further simulations were performed by including a sharpening factor in the formulation of the VOF multiphase model to contain the numerical diffusion. It was demonstrated that the air concentration showed that the air concentration distributions in a classical hydraulic jump is closely related to the velocity field.