Friday, September 21, 2012

Tutorial: 2D Adiabatic Compression - Remeshing and Spring Smoothing

This tutorial illustrates the setup and solution of a basic deforming mesh in FLUENT 6.2 using the remeshing and spring-based smoothing approaches.

The dynamic mesh model in FLUENT can be used to model flows where the shape of the domain changes with time due to motion on the domain boundaries. The motion can be either a prescribed motion (e.g., you can specify the linear and angular velocities about the center of gravity of a solid body with time), or an unprescribed motion where the subsequent motion is determined by a user-defined function (UDF). The update of the volume mesh is handled automatically by FLUENT at each time step based on the new positions of the boundaries. To use the dynamic mesh model, you need to provide a starting volume mesh and the description of the motion of any moving zone in the model.

In this tutorial, you will use the spring-based smoothing and remeshing mesh motion methods to update the volume mesh in the deforming region. For zones with a triangular or tetrahedral mesh, spring-based smoothing can be used to adjust the interior node locations based on known displacements at the boundary nodes. The spring-based smoothing method updates the volume mesh without changing the mesh connectivity.

When the boundary displacement is large compared to the local cell sizes, the cell quality may deteriorate or the cells may become degenerate. This leads to convergence problems when the solution is updated to the next time step. To circumvent this problem, FLUENT agglomerates poor-quality cells (cells that are too large, too small, or are excessively stretched) and locally remeshes the agglomeration.
Problem Description
The problem to be considered is shown schematically in Figure . A simplifed 2D geometry consisting of a box is used. The bottom wall of the box represents the piston which moves upward from the bottom dead center position (BDC), slowly compressing the fuid adiabatically. After reaching the top dead center (TDC), the piston moves back downward to the initial position, to complete a cycle.

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