“Circle” is an area emitter. Emitters define the point of particle creation in 3D space, while the particles are part of the "Fluid" container. Emitters can be scaled, rotated, and translated like objects; an arrow indicates the direction of emission.
Emitter Type
Toggle the emitter's type with this drop-down menu. The following parameters and the viewport representation will be updated according to your selection. Bear in mind that type changes will change the emitter's and fluid's behaviour completely.
Volume
This feature creates a defined volume of particles. The size of this volume is defined by the emission area and the value entered here. All dimensions are given in metres. By activating this option, the emitter’s speed value will be set to 0.0 automatically.
Speed
Here the particles' initial speed is defined in metres per second. Higher values will generate more particles per frame. The number of emitted particles also depends on the domain's “Resolution” settings. A value of 0.0 stops the emission of particles.
Vertical Random
To avoid regular emission, it is possible to randomly displace the particles with these functions. The value's range is between 0 and 1.
Horizontal Random
To avoid regular emission, it is possible to randomly displace the particles with these functions. The value's range is between 0 and 1.
Side Emission
Normally the particles are poured out along the emission axis, but it is possible to emit the particles from the emitter's edge. Then, the circle’s inner area remains empty.
Ring Ratio
You are able to define a ring from where the particles will be emitted. Small values create a larger area, higher settings are responsible for thin rings.