mesh module 6dof

Syntax

6dof 6dof_keywords 6dof_values

• zero or more 6dof_keyword/value pairs may be appended 6dof keywords = mass (obligatory) or com (obligatory) or moi (obligatory) or vcm or angmom or suspension or forceflags or torqueflags or externalForces

mass value = mass
   mass = mass of the rigid body (mass units)
com values = x, y, z
    x, y, z = coordinates of the center of mass of the body (distance units)
moi values = Ixx, Iyy, Izz, Ixy, Ixz, Iyz
    Ixx, Iyy, Izz, Ixy, Ixz, Iyz = moments of inertia around the given axis
vel values = vx, vy, vz
    vx, vy, vz = velocities of the center of mass of the body (velocity units)
angmom values = ax, ay, az
    ax, ay, az = initial angular momentum around x-, y-, and z-axis
suspension values = k_t kt c_t ct k_r kr c_r cr
    k_t = obligatory keyword
    kt = stiffness for translational spring
    c_t = obligatory keyword
    ct = damper coefficient for translational damper
    k_r = obligatory keyword
    kr = stiffness for rotational spring
    c_r = obligatory keyword
    cr = damper coefficient for rotational damper
forceflags values = f1 f2 f3
    f1,f2,f3 = flags for x,y,z which can be 0 or 1
torqueflags values = t1 t2 t3
    t1,t2,t3 = flags for x,y,z which can be 0 or 1
externalForce values = f1, f2, f3
    f1, f2, f3 = values for externally acting forces
limit_vel values = v
    v = maximum velocity (magnitude) of the body

Examples

fix  cube all mesh/surface/stress/6dof file cube.stl type 1 com 0. 0. 0. vel 0. 0. -0.5 mass 40 moi 0.266666 0.266666 0.266666 0. 0. 0.

Description

This mesh module integrates the motion of the mesh assuming it to be a rigid body with the given com (center of mass) and moi (moments of inertia). Optionally, you can specify an initial velocity and angular momentum of the rigid body. Please note the difference between angular velocity omega and angular momentum (which is I*omega)

Note that if a fix gravity is used, the gravitational force acts on the center of mass of the body

With the optional suspension keyword, an additional suspension force and torque is activated which tries to pull the body back into its original position (which is the com as defined by this command) and its original orientation. The force and torque consist of translational and rotational spring/damping parts and write

f = kt * dx - ct * vel t = kr * dalpha * axis - cr * omega

where vec and omega are the body velocity and rotational velocity. dx represents the translational displacement of the body from its original position. dalpha and axis represent the rotational displacement of the body from its original orientation, where dalpha is the the angle and axis is the the corresponding rotation (unit) axis.

With the optional forceflags and torqueflags keywords, motion integration can be turned off separately for the x-, y-, and z-axis by setting the correspondig entry 0.

If the limit_vel keyword is set, then the maximum velocity magnitude of the body is set to the corresponding value.

Restart, fix_modify, output, run start/stop, minimize info

This mesh module stores a global vector with 10 components for access by various output commands. The first 3 components (10-12) store the position of the center of mass of the body. The next 4 components (13-16) are the quaternion of the rotation of the body and the last three components (17-19) are the change of the angle in x, y and z direction between the current position and the initial position. The position of the data in the output depends on the additional mesh modules that are used (in case of mesh/surface/stress/6dof there are 9 components for mesh/surface/stress, the output for the 6dof module comes afterwards). Please note that if this module is available the total force and torque (in the output of mesh module stress the contributions of the particles, gravity and the suspension model (if activated). Furthermore, this module writes the state of the rigid body (translational and rotational) to binary restart files so that a simulation can continue correctly. This module supports fix_modify with options 6dof/forceflags, 6dof/torqueflags, 6dof/vel, 6dof/angmom, 6dof/omega and 6dof/move_mesh. All options, except 6dof/move_mesh, work as their counterpart in the definition of the module (see Syntax). The option 6dof/move_mesh takes three values as input and shifts the position of the mesh according to these three values. The following

fix_modify my_fix_id 6dof/move_mesh 0. 0. 1.

would shift the mesh in my_fix_id 1 unit in z-direction.

Restrictions

This mesh module requires the usage of mesh module stress and that module needs to preceede the 6dof module.

When using this module, along with scaling or rotate the body, all the 6dof_keyword/value pairs have to represent the state after scaling/rotation. Mesh elements may not be deleted in case due to leaving the simulation box for a fixed boundary. In this case, an error is generated. See boundary command for details. This module can not be used in conjunction with another fix that manipulates mesh geometry, such as a fix move/mesh or the mesh module servo

Related commands

Mesh module stress, fix wall/gran, fix mesh/surface

Default

vel = 0 0 0, angmom = 0 0 0, forceflags = 1 1 1, torqueflags = 1 1 1 , suspension = off