The Force Accumulator feature stores the normal component of contact force on the wall when a grain hits the wall. Each Force Accumulator feature defines a variable, called the accumulated variable, on the selected boundaries. The accumulated variable is discretized using constant shape functions, so its value is uniform over every mesh element and can be discontinuous between adjacent mesh elements.

The name of the accumulated variable is specified in the Accumulated variable name field in the Accumulator Settings section of the settings window. The default variable name, wf, will be used in the remainder of this section when referring to the accumulated variable. The accumulated variable in a boundary element gets incremented according to

where Fn is the normal contact force. For the theory on contact forces, see Contact Force: Linear Elastic Model, Contact Force: Hertz–MD (Mindlin and Deresiewicz) Model, and Contact Force: Hertz–MD with Adhesion Model. It is possible for the same grain to increment the accumulated variable in many different boundary elements, or even in the same element multiple times.

Similar to the Accumulator (Boundary) feature, the Force Accumulator feature also defines global variables; for details, see the Built-in Global Variables subsection under Accumulator Theory: Boundaries and replace rpb with wf.

Unlike the Accumulator (Boundary) feature, the Settings window for the Force Accumulator feature does not include an Accumulator type list; the Force Accumulator feature is an Accumulator boundary feature of Count type. To calculate the accumulated force on a boundary element divided by the boundary element surface area (equivalent to the Density type of the Accumulator boundary feature), use the expression <scope>.<name>/meshvol, for example gran.wp1.facc1.wf/meshvol, in the results processing. See Accumulator Theory: Boundaries to learn more about the accumulator types Count and Density.