ABSTRACT
The explicit and the semi-implicit methods have been here applied to analyze the central part of the Venice lagoon, considered for simplicity as a closed domain, with no interaction with the northern and southern parts. In both cases the boundary conditions adopted consist in a null normal velocity along the whole boundary and a sinusoidal tidal wave (elevation) with a period of 12 hours at the two in/outlets (Bocca di Malamocco and Bocca di Lido). The presence of three major canals (Malamocco, Malamocco-Marghera and Fasolo) has required the rather highly non-uniform mesh subdivision shown in figure 2, comprising 890 nodes and 1581 elements. Explicit solution. As already mentioned, the time step is in this case severely limited by the stability conditions. to a value of about S sees., which would have led to a quite expensive analysis, if performed on the whole domain. Therefore, the overlapping domain decomposition technique, as introduced in [3),[4) and adopted in [51 has been applied. The particular element distribution has provided a natural decomposition in two subdomains: nc, covering the more finely subdivided area including the canals, between the outer boundary and the r e curve (see figure 3) and n i• comprising the inner coarser mesh up to the r e curve, with the overlapping region bounded by the ri and the r e curves. With this decomposition. it has been possible to perform each time iteration in parallel for the two subregions, with time steps Ate = 5 sees. and A ti = 10 sees .• respectively, accounting for the complete mesh only during the· process of data fitting in the overlapping region. Semi-implicit solution. In this case, as it is well known, the limitation on the time step only depends on the bottom friction.
