# POLYFLOW - evaluation of a typical wall shear rate

 Considering a flow rate Q, and some geometric dimensions (width L, half height H, radius R), what is a typical wall shear rate g-dot?The formulae below provide typical values; they assume that the geometry is smooth and that the viscosity is constant. For shear-thinning materials, the actual wall shear rates can be some 50% larger.2D planar flow (between two plates):- the flow rate Q is given per transverse unit of length [thus e.g. cm^3/s/cm]- the channel has a height 2H- the typical wall shear rate is given by g-dot = 3.Q/(2.H^2)2D axisymmetric flow (through a circular tube):- the flow rate concerns the whole circular cross section- the cross section has a radius R- the typical wall shear rate is given by g-dot = 4.Q/(pi.R^3)2D axisymmetric flow (through an annular tube):- the flow rate concerns the whole annular cross section- the cross section has an average radius R and an opening 2e- the typical wall shear rate is given by g-dot = 3.Q/(4.pi.R.e^2) or Q/(4.R.e^2)3D flow:- the flow rate concerns the actual cross section- the cross section has a length L and a width 2H (with H < L)- the typical wall shear rate is given by g-dot = 3.Q/(2.L.H^2)- for an annular cross section, the actual geometry can be unrolled.

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