Clumps

A clump is a rigid collection of pieces that can represent complex particle shapes, such as non-convex particles. Clumps are typically composed of spherical pebbles, also known as multi-sphere particles. PFC clumps made up of balls, rigid blocks, and a mixture of these particles can also be defined.

The surface properties of a clump can be specified independently for each piece. Clumps can translate and rotate (defined by \(\{\mathbf{v},\mathbf{w}\}\), the generalized velocity and angular velocity, or spin of the clump centroid). Clump motion obeys the equations of motion. This requires that mass properties, loading conditions, and velocity conditions exist. Mass properties are mass, centroid position, and inertia tensor (\(\{m,\mathbf{x},\mathbf{I}\}\)). Loading conditions are defined by: the force and moment resulting from interactions with other pieces; gravity (defined by \(\mathbf{g}\), the gravitational acceleration vector); and an externally applied force and moment acting on the clump (defined by \(\mathbf{F_A}\) and \(\mathbf{M_A}\)). Velocity conditions are defined by velocity-fixities \(V_f\) with {three values in 2D; six values in 3D} of the clump.

The preferred clump workflow is to first define a set of clump templates (see clump template create) that represent the desired particles. Once the set of clump templates have been defined, one can insert clumps into the domain in three ways:

  • by replicating clumps one at a time (using clump replicate);

  • by generating non-overlapping sets of clumps (using clump generate);

  • by distributing overlapping clumps to match a specified size distribution (using clump distribute).

For clumps composed of spherical pebbles, clump templates can be defined from:

  1. an existing clump (clump template create from-clump)

  2. a list of the positions and radii (\(\{\mathbf{x}^{(i)},R^{(i)}\},i=1,2,\ldots{,n}\)) of the \(n\) pebbles that make up the clump (clump template create pebbles)

  3. a surface description of the particle (either convex or concave), in the form of a closed geometry of {line segments in 2D; triangular facets in 3D} that is manifold and orientable, that is automatically filled with pebbles to represent the particle surface to a desired fidelity (clump template create bubblepack)

For option 1, clumps must first be created with the clump create command. Such individual clumps can be used directly without clump templates for simple systems.

For clumps composed of balls and/or rigid blocks, only option 1 above is available:

Note

the keyword add-pebble of the ball_clump and rblock_clump commands does not add spherical pebbles to the clump, but balls and rigid blocks, respectively.

The user may either specify the mass properties directly or calculate the mass properties based on the sphere distribution with the clump create calculate command, which uses a voxelization approach that accounts for the sphere overlaps.

The mass properties of the clumps can either be directly specified or automatically computed.

Similar commands exist to apply mass properties on individual clumps not part of a clump template, see keywords for commands clump create, ball clump create and rblock clump create.

The clump attribute and clump property commands are used to specify clump attributes and pebble properties as discussed in “Model Components.” The entire attribute/property lists can be listed with the clump list attribute and clump list property commands along with the values of specific attributes/properties.