26.9.2 Patching Values in Selected Cells

Once you have initialized (or calculated) the entire flow field, you may patch different values for particular variables into different cells. If you have multiple fluid zones, for example, you may want to patch a different temperature in each one. You can also choose to patch a custom field function (defined using the Custom Field Function Calculator dialog box) instead of a constant value. If you are patching velocities, you can indicate whether the specified values are absolute velocities or velocities relative to the cell zone's velocity. All patching operations are performed with the Patch dialog box (Figure  26.9.2).

Solution Initialization Patch...

Figure 26.9.2: The Patch Dialog Box

1.   Select the variable to be patched in the Variable list.

2.   In the Zones to Patch and/or Registers to Patch lists, choose the zone(s) and/or register(s) for which you want to patch a value for the selected variable.

When shell conduction is enabled, the names of the Zones to Patch will appear as shell:wall-name. The wall-name is the name of the wall on which a shell conduction zone has been created.

3.   If you wish to patch a constant value, simply enter that value in the Value field. If you want to patch a previously-defined field function, enable the Use Field Function option and select the appropriate function in the Field Function list.

4.   If you selected a velocity in the Variable list, and your problem involves moving reference frames or sliding meshes , indicate whether the patched velocities are absolute velocities or velocities relative to the motion of each cell zone by selecting Absolute or Relative to Cell Zone under Reference Frame. (If no zone motion occurs in the problem, the two options are equivalent.) The default reference frame for velocity patching in ANSYS FLUENT is relative. If the solution in most of your domain is rotating, using the relative option may be better than using the absolute option.

5.   Click the Patch button to update the flow-field data. (Note that patching will have no effect on the iteration or time-step count.)

Using Registers

The ability to patch values in cell registers gives you the flexibility to patch different values within a single cell zone. For example, you may want to patch a certain value for temperature only in fluid cells with a particular range of concentrations for one species. You can create a cell register (basically a list of cells) using the functions that are used to mark cells for adaption. These functions allow you to mark cells based on physical location, cell volume, gradient or isovalue of a particular variable, and other parameters. See Chapter  27 for information about marking cells for adaption. Section  27.11.1 provides information about manipulating different registers to create new ones. Once you have created a register, you can patch values in it as described above.

Using Field Functions

By defining your own field function using the Custom Field Function Calculator dialog box, you can patch a non-constant value in selected cells. For example, you may want to patch varying species mass fractions throughout a fluid region. To use this feature, simply create the function as described in Section  31.5, and then perform the function-patching operation in the Patch dialog box, as described above.

Using Patching Later in the Solution Process

Since patching affects only the variables for which you choose to change the value, leaving the rest of the flow field intact, you can use it later in the solution process without losing calculated data. (Initialization, on the other hand, resets all data to the initial values.) For example, you might want to start a combustion calculation from a cold-flow solution. You can simply read in (or calculate) the cold-flow data, patch a high temperature in the appropriate cells, and continue the calculation.

Patching can also be useful when you are solving a problem using a step-by-step technique, as described in Section  26.18.2.