Such characterizations have been proposed, for example, by Huang and Kokini (59) and Bagley (60) on wheat flour doughs and by Janssen et al. (24) and Kokelaar et al. (29) on flour and gluten doughs.
Because these methods are restricted to deformation rates much lower than those encountered in forming processes (mixing, sheeting), other techniques have been proposed, approximating a rheological point of view but closer to real processing conditions. Most is known about converging flows (61, 62). In convergent flow (for example, at the entry of a capillary), both shear and elongational deformations are present. Cogswell (63) was the first one to propose expressing the entrance-pressure drop as the sum of two parts, one due to the shear, the other to the elongation:
Apsheai is computed using the lubrication approximation (which means neglecting elongation terms):
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