Arc Welding CFD Simulation, User-Defined Scalar (UDS), ANSYS Fluent Training

Description

Arc welding is a welding process that uses electricity to generate sufficient heat to melt and fuse metal together. Examining this process closely reveals a self-sustaining cycle: the high voltage between the electrode (cathode) and the base metal ionizes the surrounding air, establishing an electric current. This current substantially raises the local air temperature, which in turn further ionizes the air, allowing current to keep flowing between the electrode and base metal and generating even more heat. This feedback loop creates a stable process that sustains a high-temperature region capable of melting metal.

In this simulation, the cathode is positioned 4 mm above the base metal, with the cathode voltage set to 400 V and the base metal held at 0 V, serving as the electrical ground reference. Air enters through the top boundary at 0.01 m/s and 1000 K, with the cathode wall maintained at 2000 K and the ionized air's electrical diffusion coefficient set to 0.005.

The geometry was built in SpaceClaim and meshed in ANSYS Meshing using a structured grid totaling 335,560 elements.

Methodology

Given the very low velocity and very small length scale involved, the flow was treated as laminar, with the energy equation activated. To simulate the electrical behavior driving this process, a User-Defined Scalar (UDS) was introduced, representing voltage or electrical potential (V) — a scalar quantity with no associated mass flow flux. This scalar was governed by a steady-state transport equation (with no source term), using an electrical potential flux formulation based on the ionized air's electrical diffusion coefficient, k.

Current density was computed directly from this voltage field, and to couple the electrical behavior with the thermal solution, a corresponding source term was added to the energy equation representing the resulting Joule heating. This entire formulation was implemented through a custom User-Defined Function (UDF), compiled and interpreted within Fluent, with User-Defined Memory (UDM) used to store and extract the electrical heat release and current density vectors directly from the UDF for post-processing analysis.

Conclusion

Results include velocity and pressure contours around the cathode, captured on the axial plane. The base metal surface reaches a temperature of approximately 1220 K — sufficient to initiate metal melting — while the maximum temperature within the domain reaches an extreme 7036 K, concentrated near the cathode itself.

The electrical heat generation contour confirms that the highest heat generation occurs directly around the cathode, consistent with the steepest electrical potential gradient occurring in that region. The extracted current vectors further illustrate the current's path, originating at the cathode and flowing toward regions of progressively lower electric potential — confirming that the UDS-UDF coupling correctly reproduces the expected physical behavior of the arc welding process.