text: In the study of stirred tanks, the Froude number governs the formation of surface vortices. Since the impeller tip velocity is  (circular motion), where  is the impeller frequency (usually in rpm) and  is the impeller radius (in engineering the diameter is much more frequently employed), the Froude number then takes the following form:
  The Froude number finds also a similar application in powder mixers. It will indeed be used to determine in which mixing regime the blender is working. If Fr<1, the particles are just stirred, but if Fr>1, centrifugal forces applied to the powder overcome gravity and the bed of particles becomes fluidized, at least in some part of the blender, promoting mixing