DPM-Drag Macro: UDF, Drag between Particles and Fluid

DPM-Drag Macro, UDF, Drag between Particles and Fluid CFD Simulation

Description

This project simulates a chamber designed for spraying discrete particles into a continuous fluid medium, using ANSYS Fluent, demonstrating how User-Defined Functions can accurately capture the drag forces acting between particles and the surrounding fluid.

The 3D geometry was designed in Design Modeler and meshed in ANSYS Meshing using an unstructured grid totaling 127,100 cells.

Methodology

This simulation uses the Discrete Phase Model (DPM) to represent particle spraying and its interaction with the surrounding fluid, with a custom implementation of the drag force calculation at the core of the setup. Standard drag laws available in Fluent — spherical, Stokes-Cunningham, non-spherical, and high-Mach-number — provide a baseline, but this project instead implements a custom drag law using the DEFINE_DPM_DRAG macro, defining Reynolds-number-dependent drag force relations directly.

Building and applying this UDF involves writing the custom drag force equation, implementing it through the DEFINE_DPM_DRAG macro, compiling and loading the resulting UDF into ANSYS Fluent, and configuring the DPM model to reference this custom drag law in place of a standard built-in option.

Conclusion

Results include a comparison of penetration length across the different drag models, particle tracking visualization for the UDF-based simulation, and a time-dependent analysis of particle trajectories over a 4-second window. This comparison confirms that the custom DPM-drag UDF produces meaningfully different — and more tailored — particle dispersion behavior relative to the standard built-in drag laws, validating this approach as a flexible way to improve prediction accuracy in spray-based particle-fluid systems.

This technique offers particular value in applications where standard drag laws fall short of capturing specific particle or flow characteristics — relevant to industrial coating processes and pharmaceutical aerosol delivery systems — and provides a foundation for extending into more advanced scenarios, such as multiphase flows with custom particle interactions, UDF-enhanced spray nozzle optimization, or the integration of thermal effects into particle-fluid drag calculations.