Mentor Graphics Corporation Mechanical Analysis Division Offers floEFD Meshing Capability

Step-by-step directions guide designers through the use of new technology for engineering fluid dynamics.

Engineering fluid dynamics (floEFD) meshing technology detects fluid regions within a CAD model and automatically meshes them. floEFD uses an octree mesh. To refine the mesh, isotropic splitting is used. This effort results in eight identical smaller cells. A cut-cell treatment is used at solid —fluid boundaries. Using floEFD, the initial mesh, created before the solution and any solution-adaptive refinement, is defined first by constructing a basic Cartesian mesh which is then refined.

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floEFD’s Initial mesh dialog.

Definition of the initial mesh can be fully automated using the dialog box. Or, it can be defined manually if the automatic settings box is checked off. The initial mesh is constructed from a near-uniform Cartesian basic mesh that is subsequently refined according to the following criteria:
• The level of initial mesh slider bar controls the number of cells in the basic mesh.
• The show basic mesh box displays the basic mesh on the model.
• The basic mesh can be stretched to better capture features of the model.

The initial mesh is constructed by refining the basic mesh around the intersection of the solid model, and guided by settings for the minimum gap size and minimum wall thickness. The level of initial mesh performs several functions and refines the basic mesh. In addition, it determines how many times the basic mesh can be split, and sets levels for solid, fluid, and cut cells. Small solid features, local curvature, and narrow channels also have refinement levels together with an associated dimension to denote the smallest size to which cells can be split. The level of initial mesh slider automatically sets values for all these refinement levels to produce the automatic mesh. Once the automatic mesh has been generated, users can check off the automatic setting box and adjust settings manually — providing complete control over the meshing process.

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Basic mesh and resulting default initial mesh for ball valve tutorial case.

The initial mesh settings are applied to the entire computational domain. When specifying a mesh refinement in narrow channels, this is applied to all regions having the same characteristics. However, the initial mesh can be refined locally, with the local region defined by a component (a part or subassembly, as well as a body in multi-body parts), face, edge or vertex, or a defined fluid region.

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Manual refinement of partial cells (left) and fluid cells (right).

Solution adaptive meshing
Solution-adaptive meshing is used for adapting the computational mesh to the solution during calculation. This is useful when capturing flow features such as shocks in high Mach number flows. Cells that do not adequately resolve the gradients of velocity, temperature, pressure, and other parameters on the initial or previously adapted mesh are refined. Cells that are finer than required are coarsened. The octree-based mesh simplifies the procedure. Cells are refined by splitting them into eight smaller cells and coarsened by merging eight identical cells into one.

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Schematic of convergent-divergent channel showing location of shocks.

A validation case that used floEFD considered supersonic flow in a 2D convergent-divergent channel. A uniform supersonic stream of air with a Mach number of 3, a static temperature of 293.2 K, and a static pressure of 1 atm, was specified at the channel inlet between two parallel walls. In the convergent section, the flow decelerated through two oblique shocks. The shape of the convergent section was adjusted to the inlet Mach number so the shape of the shocks matched the geometry. The initial grid was refined near the walls but was not well-suited to capture the above shocks. Adaptive mesh refinements were used to refine the mesh as the solution proceeded. This reduced the overall cells count while concentrating the mesh around the shocks.

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Initial and final (solution adapted) mesh.

The solution adapted mesh accurately captures the sharp shocks as shown in the contour plot of Mach numbers. Mach number values predicted with floEFD along the channel centerline. The reference points are marked by square boxes with numbers are also compared with the theoretical values.

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Contour plot of Mach number and agreement with theory.

Mentor Graphics
www.mentor.com/mechanical