hi. i am trying to track free surface using VOF model when a focued high density laser beams is used to heat and melt a metal using melting model at the same time. i want to apply a pressure to the free surface and simulate free surface shape,but do not know to do. in other words, i should how to initialize VOF model?

You can use the solidification model of fluent, together with VOF....But you need to model the heat produced by the laser with a heat source within a volume near the free surface.

i use UDF of heat source of laser to model melting and free sueface tracking problem,but udf'definition boundary should i do?

I do not understand your question.