Flow instabilities of Herschel--Bulkley fluids (Q1434717)

The authors deal with numerical simulations that verify the importance of the finite yield stress in Herschel-Bulkley flows. The results confirm the existence of five characteristics flow patterns (``shell'', ``disk'', ``mound'', ``bubble' and ``transition'') that have been observed both experimentally and numerically. By controlling flow parameters, one may be able to a priori fix the jet behaviour which leads to desirable quality and properties of the final parts. Here, the stability maps of the injection process are drawn as a function of flow parameters. It is concluded that the most unstable pattern is that of ``bubble'' and to a lesser degree that of the transition pattern, primarily due to the effects of the yield stress.