1. n. [Drilling Fluids]

The velocity gradient measured across the diameter of a fluid-flow channel, be it a pipe, annulus or other shape. Shear rate is the rate of change of velocity at which one layer of fluid passes over an adjacent layer. As an example, consider that a fluid is placed between two parallel plates that are 1.0 cm apart, the upper plate moving at a velocity of 1.0 cm/sec and the lower plate fixed. The fluid layer at the lower plate is not moving and the layer nearest the top plate is moving at 1.0 cm/sec. Halfway between the plate, a layer is moving at 0.5 cm/sec. The velocity gradient is the rate of change of velocity with distance from the plates. This simple case shows the uniform velocity gradient with shear rate (v1 - v2)/h = shear rate = (cm/sec)/(cm/1) = 1/sec. Hence, shear rate units are reciprocal seconds.

Synonyms: rate of shear

See related terms: apparent viscosity,  attapulgite,  Bingham plastic model,  Brookfield viscometer,  centipoise,  dynamic filtration,  gel strength,  Herschel–Bulkley fluid,  high-pressure, high-temperature viscometer,  Newtonian fluid,  non-Newtonian fluid,  plastic fluid,  plastic viscosity,  power-law fluid,  pseudoplastic,  rheological property,  rheology,  shear stress,  viscosity,  yield point