This problem is, however, loosely synchronous as we can naturally macroscopically synchronize the calculation after each time step-thus a MIMD implementation where each processor processes its own irregular collection of grains is very natural and efficient. Thus, only very localized communication is necessary. Note that Section 4.5 describes a synchronous formulation of this problem. The results of the grain-bed impact simulations have facilitated treatment of two larger scale problems. Ripple and dune simulations are expected to run efficiently with a spatial decomposition on a hypercube.
Water is an important agent for the transport of sediment.
Unlike wind-blown sand transport, underwater sand transport requires simultaneous simulation of the grains and the fluid because water and sand are similar in density. The fluid velocity and the fluid force on each grain are computed by averaging. Since a typical void between macroscopic grains will be occupied by up to 1000 gas particles, the requisite computational speed and memory capacity can be found only in the hypercube architecture. Communication is expected to be minimal and load balancing can be accomplished for a sufficiently large system. Also, these large-scale landscape evolution simulations are suitable themselves for a MIMD parallel machine.