– If you find that your simulation is not physically correct, you can try to use higher values for Subframes, expecially when big velocities are involved.

– Higher value of Subframes means smaller value for the delta time used in the ODE solver, that is to say more accuracy in calculating every step.

– In this example you can see how Subframes was used to obtain a more realistic simulation (particles with high velocity in a box).

– Using this menu you can choose the integration method (an integration method is a numerical procedure for solving the equations of motion).

– In all these examples the only force applied is the gravitational one.

– **Euler** is the less accurate, the result is physically not correct. See how a bouncing particle increases its energy over time (due to the small errors in calculating the aproximated position step by step).

– **Verlet** is more accurate than Euler and energy does not increases. In this example there is no damping force applied, the loss of energy is due to a ‘numerical dissipation’.

– **Midpoint** (default) will be the best choice in most cases.

– In the next example the “loss of energy” (for numerical dissipation) is minimal when choosing Midpoint. Remember: there are not other forces but gravity! In absence of air friction (or other types of forces) the particle should reach its initial position every time!

– **RK4** is the slowest but also the most accurate one, you can try it where Midpoint is found not to be accurate enough.

– The amount of simulation time (in seconds) that passes during each frame.

– When Subframes is set to 0, Timestep is equal to the delta time (calculation frequency) used in the ODE Solver (see ODE Solver on Wikipedia).

– Default value is 0.04 as at 25 fps one frame duration is 0.04 seconds (1 frame / 25fps).

– In this example not only the the 1st flow of particles runs ‘slowler’ than the 2nd one, but also it is shorter as the same number of particles is emitted in half time.

– In these examples the green objects (named in the Outliner as “Umbrellas.green”) and the blue ones (nemed in the Outliner as Umbrellas.blue) are all set as colliders.

– The Collection selected is the only one that interacts with particles.

– In the Outliner you can see the two collection:

– Enabling this option, particles die when they hit a Collider Object (the small grey board in this example).

– Damp: Reduces particle velocity (deceleration, friction, dampening).

– In this example the two particles are emitted with the same upward velocity. The bigger the Damp value, the more the particle reduces its velocity.

– Drag: A force that reduces the velocity of the particle in relation to it’s speed and size (useful in order to simulate Air-Drag or Water-Drag).

– Drag=0 : all particles fall at the same velocity.

– Drag>0 : small particles fall faster than the bigger ones.