Turbulent flow over a rough surface with suction or blowing is a common fluid mechanics problem that has many practical applications including pulp screening. The present study is motivated by the optimization of aperture geometry in pulp screens used in the pulp and paper industry to separate unwanted contaminants from pulp fibres. In these devices, a dilute suspension of fibres is forced through fine slots to remove the oversized contaminants. To better understand the complex hydrodynamics at the critical region near the screen surface, a Computational Fluid Dynamics study has been conducted to examine how the geometry of the aperture entry, characterized by contour height and wire width, affect the details of the flow field. The results indicate that the flow is dominated by a separation vortex that covers the aperture. For low contour heights the flow is accelerated upstream of the aperture due to the high vorticity at the aperture entry and as profile height increases the turbulence intensity at the screen cylinder surface increases. Although the wire width has less of a impact on the flow field than contour height, the results show that decreasing wire width increases turbulence intensity at the surface due to a increase in apparent roughness of the cylinder.

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