Abstract
In this study two sources of flow instabilities in the headbox nozzle are presented. Both of them appear close to the nozzle exit and therefore are easily conveyed to the slice jet. This makes their control critical in the view of the jet quality. These instabilities may lead to so-called “small scale faults” in sheet quality, which appear as cockle, fiber orientation streakiness and other small-scale dimensional stability problems. The first source of instability is the nozzle exit itself. The geometry of the nozzle exit is highly asymmetrical due to the slice bar in the upper lip. This results in sudden acceleration and streamline curvature. The results show that remarkable alterations in the structure of the turbulent boundary layer on the lower lip are observed due to the acceleration generated by a slice bar model. However, compared to the boundary layer turbulence, the flow structures evolving in the slice bar shear layer are an order of magnitude stronger. In essence, the slice bar model utilized in the present experiment creates strong streamwise vortices. The other instability is related to the tip of the vane. Recent experimental work has revealed the reason for stable MD-aligned streaks in the mean-flow field created by some vanes at certain speeds. The streaks results from a fluid-structure interaction, in which the flow excites the vane to respond in a characteristic vibration mode. This paper presents the effect of
flow rate, vane tip thickness and vane material on this kind of streaking problem. Also the fundamental nature of the fluid-structure interaction, responsible for enhanced vortex shedding which is the mechanism the streaks are generated, is explained.
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