object drop in a fluid boundary conditions
 

Hi,
I am simulating an object drop in a fluid using immersed boundary method.
Consider the object placed in a fluid domain from where it moves (initial velocity zero) under gravity. I want to know what boundary conditions to apply to the fluid domain. The fluid is initially static, when a stationary object drops through it under gravity;

What type of fluid(s) is involved in this?

Water is the fluid.

Single phase water or air/water multiphase (ie free surface)?