The Herschel-Bulkley fluid is a generalized model of a non-Newtonian fluid. Generally, the stress of the fluid is related to the strain in a nonlinear way. In the Herschel-Bulkley model, the fluid is characterized by the consistency <math>k</math>, the flow index <math>n</math>, and yield shear stress <math>\tau_0</math>. The consistency is a constant of proportionality, and the flow index is a measure of non-linearity in the stress-strain curve - that is, how shear-thinning or shear-thickening the fluid is. The yield stress is the amount of stress the fluid must experience before it begins to flow.