Simulation of the threedimensional behavior of an unsteady large bubble near a structure AIP Conf.The motion of bubbles in a Hele-Shaw cell driven by a surrounding fluid or by gravity has been studied. Assuming that the surrounding fluid wets the solid wall and that the bubble surface is rigid due to the surfactant influence, the translational velocity of an elliptic bubble is estimated. The result indicates that the bubble velocity can decrease by an order of magnitude compared to the prediction of Taylor and Saffman ͓Q. J. Mech. Appl. Math. 12, 265 ͑1959͔͒ due to the surfactant influence. The retarded bubble velocity is apparently in reasonable agreement with the experimental observations of Kopf-Sill and Homsy ͓Phys. Fluids 31, 18 ͑1988͔͒, suggesting that the puzzling observations by them are likely to be due to the surface active contaminants.

The mechanics of a bicomponent two‐layer blown film coextrusion is studied theoretically. As a first step for the modeling of this complex process, we adopt a simple system in which the flow is assumed to be isothermal and the two layers are a Newtonian and an upper‐convected Maxwell fluid (UCM), respectively. The two fluids are chosen to investigate the relative influence of viscous and viscoelastic forces on the flow mechanics of the process. For a given total flow rate, blow‐up ratio, freeze‐line height, and film gage, the radius and the melt thickness profiles of the blown film are determined numerically for various values of the flow rate ratio of the two fluids. When the relaxation time of the UCM layer is small, the flow mechanics including the shape of the bubble (or the radius profile) is not much different from that of a Newtonian single‐layer flow. With increasing relaxation time, the viscoelasticity effect of the UCM layer becomes more and more pronounced and eventually dominates the bubble dynamics even though its layer thickness may be smaller than that of the Newtonian layer.