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boundary command


boundary x y z 


boundary o p p
boundary os o o
boundary r p rs 


Set the style of boundaries for the global simulation box in each of the x, y, z dimensions. A single letter assigns the same style to both the lower and upper face of the box in that dimension. Two letters assigns the first style to the lower face and the second style to the upper face. The size of the simulation box is set by the create_box command.

The boundary style determines how particles exiting the box are handled.

Style o means an outflow boundary, so that particles freely exit the simulation.

Style p means the box is periodic, so that particles exit one end of the box and re-enter the other end. The p style must be applied to both faces of a dimension.

Style r means a specularly reflecting boundary. Particles that cross this boundary have their velocity reversed so as to re-enter the box. The new velocity is used to advect the particle for the reminder of the timestep following the collision.

Style a means an axi-symmetric boundary, which can only be used for the lower y-dimension boundary in a 2d simulation. The simulation box must also have a value of 0.0 for ylo; see the create_box command. This effectively means that the x-axis is the axis of symmetry. The upper y-dimension boundary cannot be periodic.

Style s means the boundary is treated as a surface which allows the particle-surface interaction to be treated in a variety of ways via the options provided by the surf_collide command. This is effectively the same as when a particle collides with a triangulated surface read in and setup by the read_surf command.

For style s, the boundary face must also be assigned to a surface collision model defined by the surf_collide command. The assignment of the boundary to the model is done via the bound_modify command.


This command must be used before the grid is defined, e.g. by a create_grid command.

For 2d simulations, the z dimension must be periodic.

Related commands:

bound_modify, surf_collide


boundary p p p