POLYFLOW - Accuracy of the generalised shear rate reported along the wall of a cylindrical tube

When displaying POLYFLOW results for the Poiseuille flow of a power-law fluid, it can be found that the post-calculated wall shear rate does not exactly match that analytically predicted.
There is no actual remedy to this; but an explanation can be useful. The generalised shear rate is evaluated via a mean least square technique. It is applied to the components of the velocity gradient evaluated at integration points inside elements. In order to prevent the occurrence of negative nodal values for the generalised shear rate, the calculation is performed at points located inside individual elements, and the result is then reported at the closest mesh node. Hence, instead of the possibly expected analytical value of the wall shear rate, one obtains a value calculated in its vicinity of the wall, and it can slightly differ (being either slightly lower or slightly larger).

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