Challenges on Three-Dimensional Simulations of Free Surface Flow

Unsteady free-surface flow in open channels with or without in-channel structures are very complex. It is a difficult and challenging subject for civil engineering students to fully understand and comprehend. In this study three-dimensional numerical simulations were performed for several case studies of open channel flows and used to demonstrate several aspects of educational challenges of computational hydraulics. FLOW-3D, developed by the Flow Science, Inc., a three-dimensional computational fluid dynamics (CFD) software was utilized as "Virtual Lab" to study these complex flow phenomena. FLOW-3D utilizes a true volume of fluid (TruVOF) method to compute free surface motion and the fractional area/volume obstacle representation (FAVOR) technique to model complex geometric regions. In this study, flow simulations were presented on three case studies in various channel geometry and channel boundary conditions, and the cases include: the flow simulations of flow characteristics in the experimental channels having weirs with different heights in the middle of simulation domain; the flow simulations under complex channel bottom geometry from supercritical flow to subcritical flow with hydraulic jump due to tailwater effects in a laboratory channel; and the flow simulations of flow characteristics in a long river reach with a broad-crested weir located at the middle of the reach. The simulations demonstrate to use basic hydraulic principles to set appropriate boundary conditions, and conduct model comparison and calibration against results from theoretical equations, other numerical models and laboratory measurements. The detailed analyses revealed flow features of the unsteady free-surface flow in open channels under various boundary conditions. The impacts on water surface profile and flow velocities due to in-channel hydraulic structures were discussed. The study demonstrates challenges of computational hydraulics in how students and researchers critically analyze three-dimensional simulation results using basic one- and two-dimensional hydraulic theories and principles.