External code coupling and parallelism in the TELEMAC system
Fluvial Hydraulics group, Laboratoire National d'Hydraulique, Direction des Etudes et Recherches
Electricité de France
Maritime Hydraulics group, Laboratoire National d'Hydraulique, Direction des Etudes et Recherches
Electricité de France
The software system TELEMAC, based on Element By Element techniques, is dedicated to the Saint-Venant equations either in 2 or 3 dimensions, but also to Boussinesq equations, Mild Slope equations or Wave Action equations, along with water quality and sedimentological modules in 2 or 3 dimensions. We present here two applications based on the PVM (Parallel Virtual Machine) language. The first one is parallelism with domain-decomposition, the second one is external code coupling between modules of the TELEMAC system, by means of a software called CALCIUM
The parallelisation of 3 modules of the TELEMA2C system has been done in the framework of the PCECOWATER project (Parallel Computing of Environment COastal and lake shallow WATER dynamics) is part of the european ESPRIT project 'Parallel Computing Initiative - II.
All the modules in the TELEMAC system use the same library based on the same EBE techniques, mainly consisting of loops over the elements. They a priori offer straightforward solutions for parallelism, but also raise specific problems, because of the small 'grain size' of this parallelism. The technique of domain decomposition has been chosen, it enables the use of clusters of scalar workstations or of Windows-NT machines. Domain partition is done with the Simon's Recursive Spectral Bisection technique (Simon 1994) or the Recursive Coordinate Bisection Technique. The Element By Element method minimises the exchanges of information at the boundaries between sub-domains, and linear systems stemming from implicit schemes are solved over the whole domain with iterative methods. Some algorithms, the method of characteristics for example, required too much effort to be parallelised and were replaced by other techniques such as distributive schemes. A typical speed-up is a factor 6.2 with 8 processors with TELEMAC-2D (Shallow Water Equations). A factor of 40 with 64 processors has been obtained with TELEMAC-3D (Navier-Stokes Equations with a free surface) on a CRAY T3E. Excellent results are obtained by TOMAWAC (Wave Action Equation) with a speed-up of a factor 6.9 with 8 workstations.
3EXTERNAL CODE COUPLING
Coupling different programs of a hydroinformatic system is a necessity. With external code coupling, programs remain separated and communicate through a specific software, CALCIUM in our case. CALCIUM is based on the message passing language PVM and works on UNIX machines. It monitors the execution of any number of FORTRAN or C programs that may communicate between them by means of simple specific READ and WRITE instructions. Applications in the TELEMAC system are:
- Parameter estimation in Shallow-water equations. In this case 3 programs are run concurrently.
- Suspended sediment transport in reservoirs including bank sliding. Modules TELEMAC-2D and SUBIEF-2D are linked through CALCIUM.
- Bed load transport with a coupling between TELEMAC-2D and the module called SISYPHE.
The application to bedload transport is detailed in the paper. The aim of the study was to evaluate the efficiency of tidal currents to ensure a natural flushing of the sediments accumulated in the intake channel of a nuclear power plant. Hydrodynamical and sedimentological modules are run in parallel and exchange their data at given time steps. Two different machines may be used. In the present study, comparisons have shown that bed evolutions without coupling hydrodynamics and bedload transport are overestimated.
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