To create particles that drift in a particular direction, use the Velocity over Lifetime module’s Linear X, Y and Z curves.
To create effects with particles that spin around a center position, use the use the Velocity over Lifetime module’s Orbital velocity values. Additionally, you can propel particles towards or away from a center position using the Radial velocity values. You can define a custom center of rotation for each particle by using the Offset value.
You can also use the Velocity over Lifetime module to adjust the speed of the particles in the Particle SystemA component that simulates fluid entities such as liquids, clouds and flames by generating and animating large numbers of small 2D images in the scene. More info
See in Glossary without affecting their direction, by leaving all the above values at zero and only modifying the Speed Modifier value.