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Alejandro
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Hi,
I have a problem in a simulation in which I use a vof-dpm coupling. I have a bubble that rises through a phase of the vof and reaches the surface, I use a udf that expands the bubble based on the y coordinate of the particle, but at the same time the particle gets a Lot of velocity.

[Martin_Sz]

It sounds like you're encountering an unphysical increase in velocity for your bubble as it reaches the surface in your VOF-DPM coupled simulation. Here are some potential causes and solutions to consider:
* Unbalanced Forces: During bubble expansion, an imbalance between the buoyancy force, drag force, and the force applied by the UDF might be causing the velocity spike. Try reviewing your UDF to ensure it introduces a smooth and balanced force based on the y-coordinate.
* Mesh Resolution: A coarse mesh near the surface could be capturing the bubble expansion inaccurately, leading to numerical instability and velocity spikes. Refine the mesh around the bubble's trajectory to improve accuracy.
* UDF Implementation: Double-check your UDF implementation for any errors that might be causing the force to be applied incorrectly. Consider simplifying the UDF logic to isolate any potential issues.
* Drag Calculation: Ensure your drag force calculation considers the bubble's deformation and velocity accurately. Explore drag models suitable for spherical or ellipsoidal bubbles depending on how your UDF expands the bubble.
* Time Stepping: If the velocity increase is instantaneous, it might be related to the time step. Try reducing the time step to capture the bubble's motion more accurately.
If these suggestions don't resolve the issue, consider consulting Ansys resources or user forums for more specific guidance related to your UDF implementation and CFD software.