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I would like to know the physical foundation of the force fields in blender.

For instance: what are "Wind", "force strength", "flow"?

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The "practical" definition of these parameters is here:

Force Fields in the manual

I don't think there is much documentation about their workings under the hood.

Anyway, each Force type implements one particular type of force in Newton physics.

Forces, the $\mathbf{F}$ in the second law of motion $\mathbf{F}=m\cdot\mathbf{a}$, are the entities that cause an acceleration $\mathbf{a}$ on a object of mass $m$. Different kind of forces can be described as a function that depends on several features of the object being acted on, or of the space in which the force acts:

$$\mathbf{F} = \mathbf{F}(\mathbf{r}, \mathbf{u}, m, q; \mathbf{r_0}, \eta, \ldots)$$

($\mathbf{r}, \mathbf{u}$ = position and velocity vectors of the test object, $m, q$ = mass and charge of the test object, $\mathbf{r_0}$ = position of the effector object, $\eta$ = viscosity of the medium, ...).

Each force can or cannot depend on each of these variables, and it may depend on them in different ways.

  • Wind is an acceleration field with a total translational symmetry, if the falloff is 0 and there is no min/max distance, or a 2D translational symmetry otherwise. It doesn't depend on anything except its own force parameters and, if falloff is enabled, on $\mathbf{r_0}$

  • Force and Charge are (typically spherically symmetric) central force fields (they are aligned along the $\mathbf{r}-\mathbf{r_0}$ direction), the latter depending on $q$

  • Harmonic is a Hookean field. Same variables as Force, but a different equation.

  • Drag is a force depending on the object's velocity $\mathbf{u}$ according to Stoke's or Rayleigh's law

and so on.

Within each type of force, there are options that control the overall strength (Force Strength, a parameter that basically appears the $\mathbf{F}$ functions above as a multiplying factor); allow to tweak the dependence of the strength on the distance from the emitter (Falloff, ...); convert part of the dependence on $\mathbf{r}$ to dependence on $\mathbf{u}$ (Flow); ...

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  • $\begingroup$ Thank you the complete answer. I am investigating how realistic blender deals with the mechanical laws. The description you give is clear and good to use. An example of how I approached the wind force can be downloaded via the following link.dropbox.com/s/4pzekj2ymgf4rc8/Wind.blend?dl=0 $\endgroup$ – Jean Nijhuis Dec 13 '18 at 10:57

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