Dear Yun.Lu,
Regarding (1), see my post dated Jan 22, 2018 in the following forum topic: Ansys model tower - #8 by Jason.Jonkman.
Regarding the convergence of the BModes solution, are the results for the lowest modes converging with increasing numbers of elements or not? (How far off are they?) I could foresee the eigensolver having numerical problems if the number of elements is too high. Are you using what is called BModesJJ that is discussed in this forum topic: Run BModes with Hydrodynamic effect.?
Best regards,