A register is the group of cells that are marked for refinement/coarsening but not adapted.
There are two types of registers:
An adaption register is basically a list of identifiers for each cell in the domain. The identifiers designate whether a cell is neutral (not marked), marked for refinement, or marked for coarsening. Invoking the Mark command creates an adaption register. It is called a register because it is used in a manner similar to the way memory registers are used in calculators. For example, one adaption register holds the result of an operation, another register holds the results of a second operation, and these registers can be used to produce a third register.
The adaption function is used to set the appropriate identifier. For example, to refine the cells based on pressure gradient, the solver computes the gradient adaption function for each cell. The cell value is compared with the refining and coarsening threshold values and assigned the appropriate identifier.
Adaption registers can be created using geometric data, physical features of the flow field, and combinations of these information. After they are created, the adaption registers can be listed, displayed, deleted, combined, exchanged, inverted, and changed to mask registers.
Hybrid Adaption Functions
The hybrid adaption functions are created to confine the adaption to a specific region (using masks) and/or create a more accurate error indicator. FLUENT provides a few basic tools to aid in creating hybrid adaption functions.
For example, you can capture the shock wave generated on a wedge in a supersonic flow field by adapting the grid to the gradients of pressure. The pressure gradient near the surface of the wedge, however, is relatively small. You can therefore use the velocity field to resolve the equally important boundary layer near the surface of the wedge.
The relative weight of the two functions in the hybrid function is determined by the values of the refinement and coarsening thresholds you specify for each of the flow field variables.
To refine the shock and boundary layer only near the leading edge of the wedge, create a circle at the leading edge of the wedge using the region adaption function, change this new register to a mask, and combine it with the hybrid gradient function.
The GUI and text interface commands generate adaption registers that designate the cells marked for refinement or coarsening. These registers can be converted to mask registers.
Mark registers maintain only two states: ACTIVE and INACTIVE. If the adaption register is converted to a mask, cells marked for refinement become ACTIVE cells, while those that are unmarked or marked for coarsening become INACTIVE.
This process is illustrated in Figures 26.11.1, 26.11.2, and 26.11.3.
Figure 26.11.1 shows a cloud of cells representing an adaption register (shaded cells are marked cells). Figure 26.11.2 illustrates the active cells associated with a mask register. If the mask is applied to (combined with) the adaption register, the new adaption register formed from the combination has the marked cells shown in Figure 26.11.3.
This example does not differentiate between refinement or coarsening marks because the mask is applied to both types of marks. For more information on combining registers, see Section 26.11.1.