With beta features enabled (as described in Introduction), you can allocate user-defined memory to specific zones, rather than the whole domain.
To improve performance, you can limit the user-defined memory (UDM) to specific zones by enabling Zone-Based Memory Allocation, and selecting the desired zones in the Select UDM Zones dialog box (accessed by clicking within the User-Defined Memory dialog box).
Parameters & Customization → User
Defined Memory 
Edit...
The User-Defined Memory dialog box allows you to allocate memory for user-defined storage variables. See the separate Fluent Customization Manual for details.
User-Defined → User Defined
→ Memory...
Controls
- Number of User-Defined Memory Locations
specifies the number of memory locations to be allocated.
- Number of User-Defined Node Memory Locations
specifies the number of node memory locations to be allocated.
- Zone-Based Memory Allocation
specifies that the user-defined memory will only be allocated to the specified zones, rather than the whole domain.
- Edit...
opens the Select UDM Zones Dialog Box.
Important: When writing data files in the legacy format
(.dat files), you must select the UDM quantities using the
Data File Quantities dialog box in order to postprocess them in
CFD-Post. Alternatively, you have the option of exporting the desired quantities to a
.cdat file. This will ensure that all the UDM quantities are
available for postprocessing in CFD-Post. Refer to the Ansys CFD-Post manual for more
information.
The Select UDM Zones dialog box allows you allocate user-defined memory on specific zones. This dialog box is opened by clicking in the User-Defined Memory Dialog Box.
Controls
- Zones
lists the zones available for user-defined memory allocation.
confirms the selection of the highlighted zone(s) for user-defined memory allocation.