Theory
Curvature sizing. The fineness scales the background cell by per step, equation (1). Every size derived from it moves with it: the flat-wall cell , the finest wall cell and the levels of the ports. Where the wall cell is set by the levels, the number of cells on a surface goes as , so one step doubles those cells, equation (2). On curved walls the size is set by the curvature threshold of Cells across a pipe diameter (about cells across each pipe), which does not depend on ; there a fineness step only shifts the size to the next power of two of the new . A model made mostly of pipes therefore grows by much less than a factor 2 per step (see Source).
Uniform sizing. The wall cell is fixed by the cells-across rule, , and the background cell is times larger, equation (3). Raising adds one refinement level between the two: the wall cells stay the same, the background becomes twice as coarse, and the bulk of the fluid gets fewer cells. A lower fineness therefore means MORE cells in this mode. Feature edges are refined one level finer than the surfaces (level ), and the fixed floor of the cell limit maxGlobalCells grows with (0.3, 0.7, 1.5, 3 and 6 million cells).
An older case file with no cells-across value sizes the background from the diagonal of the model's bounding box instead, for ; there a higher fineness is finer. The Mesh step always saves a cells-across value.
A mesh independence study (Run step, queue) in curvature mode varies the fineness (2, 3, 4) because it scales every level-driven size by the same ratio; in uniform mode it varies the cells across instead (11, 16, 22) and refuses a fineness study, because the wall cell, which resolves the flow, does not change with there. A refinement study for the grid convergence index has to refine the mesh systematically (Celik et al., 2008).
Equations
| where | meaning | unit |
|---|---|---|
| Background cell size | m | |
| Hydraulic diameter of the fluid volume, | m | |
| Fineness | - | |
| Number of cells whose size is set by the wall levels | - |
| where | meaning | unit |
|---|---|---|
| Wall cell size (uniform sizing) | m | |
| Smallest hydraulic diameter of the inlets and outlets | m | |
| Cells across that diameter | - |
Default and rule
- Default
- 3
- Rule
3 ("normal") in both modes: with curvature sizing it gives ; with uniform sizing two refinement levels between background and wall.
- Source
The divisor that fineness 3 stands for () comes from a mesher setup of case V1 (a 2.0 m multi-chamber muffler with a perforated pipe and three absorptive packings) whose wall sizes matched a reference mesh made with another mesher (22.5 mm background cell); it has not been varied.
Measured effect of one step on V1 (curvature sizing, 6 layers typed, serial meshing): fineness 2 gave 1,304,352 cells in 19 min, fineness 3 gave 1,432,415 cells in 28 min. That is 10 % more cells, not twice, because the walls of that model are mostly pipes sized by the curvature threshold. The mesh-quality advice of the Mesh step nevertheless proposes "fineness +1, about twice the cells" for skewness and inverted-cell failures. The study levels are those of the program's mesh independence study.
Valid range 1 to 5 (whole numbers)
Values
1 to 5, whole numbers (checked when the setting is saved).
Uniform sizing
A higher fineness gives a COARSER background, not a finer wall. To refine the walls in this mode, raise Cells across a pipe diameter.
Curvature sizing on pipe-dominated models
The wall cells of curved walls follow the cells-across threshold, so a fineness step mainly refines flat walls, ports and the bulk. A mesh independence study by fineness has not yet been run on a real model in this mode.