Theoretical description of the unsteady pressure-flow in an annular and/or planar slit under conditions of longitudinal oscillations of one wall of the slit and/or of pressure gradient pulsations is presented. It has been found that for non-Newtonian liquids some flow enhancement (due to reduction of the mean flow resistance) with the preservation of the same mean pressure gradient as in the steady flow is most frequently observed. Moreover, the flow enhancement is a function of the amplitude and frequency of oscillations and/or pulsations and of the Weissenberg number (or mean pressure gradient). The theoretical results are compared with the existing experimental data for polymer melts and solutions. A good qualitative agrement has been found.
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