This controls how much energy particles lose when they collide with each other, and so how much they bounce apart rather than pack together. It is set by two controls on the DEM Inputs tabsheet: the Particle – Particle Co-eff of Restitution value, and the Apply to particle contacts checkbox directly beneath it that switches it on.
Particle-to-particle contact is modelled as a spring: two particles that collide push each other apart with a force set by the particle stiffness Kn. A spring on its own gives back everything it absorbs, so with the checkbox unticked particles rebound off each other perfectly elastically – no matter what restitution is set to.
Ticking Apply to particle contacts adds damping to that spring, sized so a colliding pair separates at the fraction of its approach speed you asked for. That fraction is the coefficient of restitution: 1.0 means the pair rebounds at the full speed it arrived at (nothing lost), and 0 means it does not rebound at all.
Internally the restitution is converted to a damping ratio β – 0 meaning no damping and 1 meaning the most that can be applied before a contact stops rebounding at all:
| Restitution e | Damping ratio β | Damping | Effect on particle separation |
|---|---|---|---|
| 1.0 | 0.0000 | None | No energy lost in a collision at all. Particles rebound fully and spread apart the most. Identical to leaving the checkbox unticked. |
| 0.8 | 0.0709 | Slight | Collisions are still close to elastic. Particles bounce off each other readily and the stream stays open and well separated. |
| 0.5 | 0.2155 | Moderate | About half the approach speed is lost per collision. Noticeably less rebound; material starts to hold together where it is dense. |
| 0.2 | 0.4559 | Strong | Most of the approach energy is absorbed. Particles largely stop bouncing off each other and stay in contact, packing closer together. |
| 0.1 | 0.5912 | Very strong | Collisions are nearly dead. Material packs densely and settles quickly, with very little separation between particles. |
At e = 0.8 (β ≈ 0.07) collisions are nearly elastic, so particles rebound off each other and spread apart. At e = 0.1 (β ≈ 0.59) each collision absorbs most of the approach energy, so particles stop bouncing, stay in contact and pack together. Lower restitution → deader collisions → denser, slower material.
Damping only acts while particles are actually touching each other, so it changes very little in the open, free-falling part of a stream where particles rarely meet. Look for its effect where material is dense and particles are genuinely in sustained contact – a pile, a choked section, or the landing area after a discharge pulley.
Comparing two otherwise identical runs of the supplied Demo 03 Shuttle Head 19 deg transfer project, measured over the discharge region, shows the pattern clearly:
| Measured in the discharge region | e = 0.1 (very strong) | e = 0.8 (slight) |
|---|---|---|
| Particles in contact with a neighbour | 87–90% | 84–86% |
| Mean particle speed | 2.71–2.84 m/s | 2.79–2.88 m/s |
The lower restitution keeps more particles in contact and moving more slowly – denser, more settled material. Averaged over the whole model the two runs differ by well under 1%, because most particles at any moment are in free flight and unaffected; the difference is concentrated where the contacts actually are.
Both the checkbox and the restitution value take effect immediately, including for particles already in the chute, so you can change them during a running calculation and watch the result – see Changing Inputs During a Calculation.