A fluid described by a three-parameterrheologicalmodel. A Herschel-Bulkley fluid can be described mathematically as follows: The Herschel-Bulkley equation is preferred to power law or Bingham relationships because it results in more accurate models of rheological behavior when adequate experimental data are available. The yieldstress is normally taken as the 3 rpm reading, with the n and K values then calculated from the 600 or 300 rpm values or graphically.Reference:Hemphill T, Campos W and Pilehvari A: "Yield-Power Law Model More Accurately Predicts MudRheology," Oil & Gas Journal 91, no. 34 (August 23, 1993): 45-50.