In the previous section we used periodic boundary conditions to minimize the effects of walls in the simulations. This approach is good for general studies of system size effects and bulk properties. But as soon as we want to compare the results of the simulations with experiments of a certain geometry this approach is problematic, because it is not clear how the presence of walls influences the suspension. If we are able to simulate a system of the same size as used in the experiment we use solid walls in all directions perpendicular to gravity to match the conditions of the experiments as close as possible and thus avoid effects from the absence of the walls.
Gravity points in direction.
The boundary conditions in the simulations are periodic parallel to gravity and rigid wall perpendicular to it. They tracked the the trajectories of marked particles in a suspension of unmarked particles with a digital imaging system and calculated the local velocities of the particles in the and directions and the corresponding velocity fluctuations. The particles were tracked in a imaging window of approximately 150 by 300 . The depth of the imaging extended through the whole system. With this experimental setup they found velocity fluctuations and .