Free (open) boundary condition: some experiences with viscous flow simulations |
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Authors: | Evan Mitsoulis Nikolaos A. Malamataris |
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Affiliation: | 1. School of Mining Engineering & Metallurgy, National Technical University of Athens, , Zografou, 157 80 Athens, Greece;2. Department of Mechanical Engineering, TEI of Western Macedonia, , GR‐50100 Kila‐Kozani, Greece;3. Department of Computational and Data Sciences, George Mason University, , 22030 Fairfax, VA, USA |
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Abstract: | The free (or open) boundary condition (FBC, OBC) was proposed by Papanastasiou et al. (A new outflow boundary condition, International Journal for Numerical Methods in Fluids, 1992; 14:587–608) to handle truncated domains with synthetic boundaries where the outflow conditions are unknown. In the present work, implementation of the FBC has been tested in several benchmark problems of viscous flow in fluid mechanics. The FEM is used to provide numerical results for both cases of planar and axisymmetric domains under laminar, isothermal or non‐isothermal, steady‐state conditions, for Newtonian fluids. The effects of inertia, gravity, compressibility, pressure dependence of the viscosity, slip at the wall, and surface tension are all considered individually in the extrudate‐swell benchmark problem for a wide range of the relevant parameters. The present results extend previous ones regarding the applicability of the FBC and show cases where the FBC is inappropriate, namely in the extrudate‐swell problem with gravity or surface‐tension effects. Particular emphasis has been given to the pressure at the outflow, which is the most sensitive quantity of the computations. In all cases where FBC is appropriate, excellent agreement has been found in comparisons with results from very long domains. The formulation for Picard‐type iterations is given in some detail, and the differences with the Newton–Raphson formulation are highlighted regarding some computational aspects. Copyright © 2011 John Wiley & Sons, Ltd. |
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Keywords: | free (open) boundary condition (FBC, OBC) viscous flow Newtonian fluid inertia gravity compressibility pressure dependence of viscosity wall slip surface tension |
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