In 1851 George Gabriel Stokes defined how drag forces effect spherical objects in a viscous fluid in the formula: Fd = 6pi * u * R * v.
An object in a viscous fluid like water always experiences a downward force – because of its weight and gravity – as well as an upward force. The upward force equals the weight of water which was displaced by the object. Furthermore, a floating object will collide with other particles on its way to the water surface. So, the downward forces must be added by drag forces as a consequence of these collisions. Just like that, a settling object, will experience additional upward drag forces. The object has to face further drag forces - caused by the viscosity and the flow velocity of the fluid relatively to the object.
A fluid with a high viscosity like honey makes it much more difficult for a particle to settle down. At the same time, objects within a fluid with a high flow velocity will be dragged into the flow direction. Both the viscosity and flow velocity are expressed in Strokes law. - R is the radius of the object in the water. Therefore, D in the formula U = W + D can be replaced by Fd for drag forces.
As Stokes law is only applicable for spherical objects the formula can be added by a form factor for different object geometries. Strokes law plays a very important role in the water and waste water industry e.g. to determine the size of settling basins and for the calculation of lamella clarifiers.