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CROSSFLOW - Crossflow CFL factor

Structured Grids

 CROSSFLOW [CFL] [FACTOR] val [zone_selector]

The CROSSFLOW keyword sets the cross-flow CFL factor to val in the specified zones.

For most Wind-US calculations, the time step is based on a constant CFL number throughout the flow field. Thus, for non-uniform grids, the solution advances at different rates in different parts of the grid. Because the CFL number is directly proportional to the physical time step and inversely proportional to the local grid spacing, the physical time step is small where the grid spacing is small, in order to maintain a constant CFL number. For example, in boundary layers and shear layers, where the grid is often closely packed for better resolution, the solution advances more slowly than in other parts of the flowfield. In fact, most of the iterations in a viscous solution are spent converging viscous regions.

To speed up convergence in these regions, each coordinate direction's eigenvalues are multiplied by a factor before determining the time step associated with the local spacing and flow conditions. This factor varies from 1.0 when the flow is along the coordinate direction, to the specified value when the flow is perpendicular to the coordinate direction. The far-field CFL number is generally the value specified in the input data file with the CFL# keyword, but near the wall the code effectively increases the time step in the boundary layer by the specified factor. This should increase convergence of the boundary layer.

The default cross-flow CFL factor is 2.0 for three-dimensional flow, and 1.0 (i.e., no time step increase in the boundary layer) for two-dimensional flow and axisymmetric flow.