Simulation of Flow in Hydraulic Structures using ADH

Abstract

PURPOSE: The purpose of this Technical Note is to describe a process of modeling flow in hydraulic structures. In particular, application is made with the unstructured flow solver, ADH (ADaptive Hydraulics model). ADH also contains solvers for groundwater and two-dimensional shallow water flows. It is capable of refining or coarsening the grid based on error estimates during flow calculations. Three demonstration applications are presented: (a) CAD-to-Grid Process Demonstration at an Intake, (b) Scalability Demonstration with Outlet Manifold, and (c) Free-surface Capability Demonstration with Supercritical Flow Contraction. INTRODUCTION: Modeling of hydraulic structures presents many computational challenges to design/evaluation engineers. Hydraulic structures often have components containing internal (pressure) flow while other components hold external (free-surface) flow. Therefore, three- dimensional (nonhydrostatic) codes must account for both regions of fixed domain limits and regions that have time-varying domains because of free surface movement. Multiple regions of rapid accelerations also characterize hydraulic structures as water passes from slow reservoir flow to high-velocity conduit flow. This difference translates to significant differences in Reynolds number over various portions of the modeled flow field. Free-surface applications to steep channels such as chutes require a model that produces stable solutions for a large range of Froude numbers. The model must solve subcritical and supercritical flow fields and resolve any transitions between these regimes. These flow fields may contain shocks such as oblique standing waves and hydraulic jumps. The solutions may include a highly contorted free surface, and thus requires a robust free-surface model. Finally, the geometric complexity of most hydraulic structures necessitates the use of CAD (Computer Aided Design/Drafting) modeling which leads to a need for a CAD-to-grid generator interface. Visualization, of course, is also important for the engineer to examine the geometric design and the resulting hydraulic conditions. MODELING PROCEDURE: The process of developing computational models of hydraulic structures begins with a three-dimensional CAD description of the parameter-based geometry. This CAD information is then fed to a grid generator where the domain is discretized into a tetrahedral mesh required by ADH (Berger and Stockstill 1999). The resulting mesh and specified boundary conditions are then passed as input to the flow solver. The resulting solution must next be transferred to a visualization tool to facilitate the evaluation of the hydraulic conditions within the structure. The flow solver is capable of determining the internal pressure and elevation of the free surface along with the three velocity components without assuming a hydrostatic pressure distribution. The ADH code is written to take advantage of the newest multiple-processor machines of the Major Shared Resource Center (MSRC), Engineer Research and Development Center (ERDC) Information Technology Laboratory (ITL).