The Roller node adds a roller (sliding wall) constraint as the boundary condition; that is, the porous matrix displacement is zero in the direction perpendicular (normal) to the boundary, but the porous matrix is free to move in the tangential direction. This boundary condition also sets an impervious (sound-hard) boundary for the fluid pressure. The condition is found under the
More Constraints submenu.
) and select Advanced Physics Options in the Show More Options dialog box. 