# Displacement Boundary

Displacements cannot be controlled directly in 3DEC; in fact, they play no part in the calculation process, as explained in Theory and Background. In order to apply a given displacement to a boundary of a deformable block model, it is necessary to fix the boundary and prescribe the boundary’s velocity for a given number of steps (using the block gridpoint apply command). If the desired displacement is $$D$$, a velocity, $$V$$ , is applied for a time increment, $$T$$ (e.g., $$D = VT$$ ), where $$T = \Delta tN$$. $$\Delta t$$ is the timestep and $$N$$ is the number of steps (or cycles). In practice, $$V$$ should be kept small and $$N$$ large, in order to minimize shocks to the system being modeled.

The block gridpoint apply command is used to fix the velocity of gridpoints of deformable blocks in the $$x$$-, $$y$$- or $$z$$-direction using keywords velocity-x, velocity-y, velocity-z or just velocity in which case 3 components of velocity are supplied. Veolcity can also be assigned in the the normal direction (keyword :kwd:velocity-normal) along boundaries not aligned with the $$x$$-, $$y$$- and $$z$$-axes. The velocity of rigid or deformable blocks can be fixed with the block fix command (at the current velocity). Use the block apply command to specify a velocity other than the current value for rigid blocks. The velocity can also be altered with a FISH function.

Time-varying velocity histories can be applied via the block gridpoint apply command with the fish or table keywords for deformable blocks or the block apply command with the fish or table keywords command for rigid blocks. These keywords must appear on the same line as the command to prescribe a velocity history. As discussed in Stress Boundary, velocity boundaries should always be assigned after stress boundaries. Fixed velocity conditions can be removed for deformable blocks with the block gridpoint apply-remove command, and for rigid blocks with the block apply remove` command.