Good morning Jun,
In terms of meshing for the free surface region I would start course and tighten it incrementally through a convergence study, meaning you must run multiple simulations (this is just the CFD way!). The mesh must be specific to the case so I can't really recommend a suitable refinement value.
Start with a course mesh (very little refinement) just to see where the free surface will end up during simulation as then you will have a rough idea of where you should refine the mesh. Then use a refinement region spanning the total area where the free surface interactions occur, both before stating the simulation and after convergence.
As a good starting point Matt (technical support in the USA) runs through a really good free surface example in this webinar (starting at minute 28:40), which relates closely to your project. This should give you a good idea of the mesh refinement requirements for your simulation.
Regarding:
The videoes on Youtube show that we can just right-click to choose the free surface to see the interface but I cannot find that option. Is there anything goes wrong?
Please provide the link to a video where this process occurs as I haven't seen it before.
Regarding:
The results I got after refining seem very hard to analysis, my computer always gets stuck when I see the results and the CPU usage is much less than 100%. is that because of the big data of the study or anything I can do to solve it?
What are your machine specifications? as yes this could be RAM issue with the large mesh.
I hope this helps a little.
All the best,
David
David Short
Technical Support Specialist, Simulation