# Filling zones¶

Filling zones define geometrical areas where you insert particles initially. If
particles are added via the **Fit into closed surface** type, a solid body has
to be provided describing the closed surface where the particles are inserted as
well as a packing algorithm. This algorithm defines how the particles are inserted
into the geometry and the particle size. The assigned solid body can be either
a solid of the geometry domain or a defined volume geometry primitive.

## Packing algorithm¶

The packing algorithm describes how the particles are inserted into the assigned geometry.

### Regular hexahedral packing¶

The regular hexahedral packing algorithm adds spheres in hexahedral arrangement
to the geometry. The size of the spheres is defined by the **Sphere radius** and
the **Gap distance** determines the distance between two spheres’ surfaces in normal
direction. It is very robust.

### Random dense packing¶

The random dense packing algorithm adds spheres of different sizes to the geometry.
The size of a sphere is defined randomly within a given radius interval that is
determined by the **Relative radius fuzziness** with respect to the
mid interval **Sphere radius**. The spheres are then packed dense, so touching each
other in the initial state, which leads to a higher computational effort as
for the **Regular hexahedral packing**. Please note as well that it can happen
that there occur initial forces due to small overlapping contact between the
spheres which results in unexpected opposite directed acceleration of those
particles.