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Simulation of Flow inHydraulic Structures using ADH

by R. L. Stockstill and R. C. Berger

PURPOSE: The purpose of this Technical Note is to describe a process of modeling flow inhydraulic structures. In particular, application is made with the unstructured flow solver, ADH(ADaptive Hydraulics model). ADH also contains solvers for groundwater and two-dimensionalshallow water flows. It is capable of refining or coarsening the grid based on error estimatesduring flow calculations. Three demonstration applications are presented: (a) CAD-to-GridProcess 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 challengesto 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 andregions that have time-varying domains because of free surface movement. Multiple regions ofrapid accelerations also characterize hydraulic structures as water passes from slow reservoirflow to high-velocity conduit flow. This difference translates to significant differences inReynolds number over various portions of the modeled flow field. Free-surface applications tosteep channels such as chutes require a model that produces stable solutions for a large range ofFroude numbers. The model must solve subcritical and supercritical flow fields and resolve anytransitions between these regimes. These flow fields may contain shocks such as obliquestanding 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 mosthydraulic structures necessitates the use of CAD (Computer Aided Design/Drafting) modelingwhich leads to a need for a CAD-to-grid generator interface. Visualization, of course, is alsoimportant for the engineer to examine the geometric design and the resulting hydraulicconditions.

MODELING PROCEDURE: The process of developing computational models of hydraulicstructures 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 atetrahedral mesh required by ADH (Berger and Stockstill 1999). The resulting mesh andspecified boundary conditions are then passed as input to the flow solver. The resulting solutionmust next be transferred to a visualization tool to facilitate the evaluation of the hydraulicconditions within the structure. The flow solver is capable of determining the internal pressureand elevation of the free surface along with the three velocity components without assuming ahydrostatic pressure distribution. The ADH code is written to take advantage of the newestmultiple-processor machines of the Major Shared Resource Center (MSRC), Engineer Researchand Development Center (ERDC) Information Technology Laboratory (ITL).

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4. TITLE AND SUBTITLE Simulation of Flow in Hydraulic Structures using ADH

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Army Engineer Research and Development Center,Coastal andHydraulics Laboratory,3909 Halls Ferry Road,Vicksburg,MS,39180

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APPLICATIONS: Application of ADH has been made to hydraulic structures varying fromhigh-velocity channels to navigation components. This technical note presents three applicationsas demonstrations of the modeling process, and it documents the model’s features andcapabilities. In each case the flow solution was run for steady-state conditions.

CAD-to-Grid Process Applied to an Intake: The first application is a model of theMarmet Navigation Lock intake structure. The multiported intake model demonstrates therequired CAD-to-Grid process for complicated geometries. The intake modeled about 215 m ofthe upper approach, the upper miter gates and recesses, the intake ports, and filling systemculverts as shown in the CAD drawing provided in Figure 1. The tetrahedral mesh, shown inFigure 2, had 20,194 nodes and 87,953 elements. Figure 3 shows the details of the discretizationnear the intake ports. Flow in the approach channel is slow and deep. The flow accelerates as itenters the intake ports. Further accelerations occur as the port flows combine into a singleculvert on each half of the structure. These accelerations are shown in the velocity and pressurecontours along a plane located in the center of the culvert beginning at the port faces (Figures 4and 5, respectively).

Scalability Demonstrated with Outlet Manifold: Next, the parallel performance of theADH code is shown by application of the flow model to a multiported navigation lock outlet.The outlet model is an application requiring high resolution to describe the port shapes as shownon the surface mesh of Figure 6. The tetrahedral mesh had 48,000 nodes and 244,000 elements.Initial runs were made to obtain computational timing information for documentation of ADH’sscalability. The model was run on a Department of Defense (DoD) High-PerformanceComputing Center’s Cray T3E. The multiple processor speedups are illustrated by the timingspresented in Figure 7. Significant speedups were obtained when up to 64 processors were used.Additional processors did not speed up the computation time in this application due to the largerrelative communication time required between processors. This is a relatively small test case.Parallel performance will be even better for larger problems. The flow solution is shown on thevelocity contours plotted on a horizontal plane through the center of the structure (Figure 8).

Free-surface Capability Demonstrated with Supercritical Flow Contraction:Finally, free-surface capabilities are demonstrated by application to a supercritical flowcontraction having an approach Froude number of 4. This application demonstrates ADH’s free-surface modeling capabilities in flow fields having multiple shocks. The free-surface solution ischallenging due to the presence of oblique standing waves within and downstream of thetransition. The flow solution is presented on the steady-state finite element mesh shown onFigure 9. The top of the mesh maps the oblique standing waves generated within the contraction.

SUMMARY AND CONCLUSIONS: The ADH flow solver will provide a rapid andeconomical method of predicting the hydrodynamic conditions in and around hydraulicstructures. The parallel code allows reasonable computational time requirements for extremelylarge problems.

RECOMMENDATIONS FOR FUTURE RESEARCH: Accurate modeling of hydraulicstructures requires accounting for turbulence effects. Shear stresses generated at fixed

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boundaries play an important role in the composition of flow in conduits and channels. Futureefforts should include the development of a turbulence model within ADH.

ADDITIONAL INFORMATION: Funding for this effort was provided by the U.S. Departmentof Defense High Performance Computing Modernization Office through the Common High-performance Software Support Initiative (CHSSI). Additional information may be obtained fromDr. R. L. Stockstill, Coastal and Hydraulics Laboratory, U.S. Army Engineer Research andDevelopment Center (ERDC) at 601-634-4251 or e-mailRichard.L.Stockstill@erdc.usace.army.mil; or Dr. R. C. Berger, Coastal and HydraulicsLaboratory, U.S. Army Engineer Research and Development Center (ERDC) at 601-634-2570 ore-mail Charlie.R.Berger@erdc.usace.army.mil.

REFERENCE:

Berger, R. C., and Stockstill, R. L. (1999). “A finite element system for flows,” Proceedings of the 1999ASCE Water Resources Engineering Conference, “Water Resources into the New Millenium, PastAccomplishments and New Challenges,” Seattle, WA.

Figure 1. Three-dimensional CAD model of lock intake

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Figure 2. Surface mesh of lock intake

Figure 3. Surface model at the port faces

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Figure 4. Velocity contours on a plane through center of intake and culverts

Figure 5. Pressure contours on a plane through center of intake and culverts

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Figure 6. Surface mesh inside multiported outlet

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Figure 7. Computational timings for lock outlet manifold model

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Figure 8. Velocity contours on a plane through center of outlet manifold

Figure 9. Computational mesh resulting from supercritical flow in a contraction