Hello everyone!
To consider the effect of water sloshing at the free surface of the liquid in the Abaqus. 
I used acoustic element for liquid and at free surface I used membrane element connect with spring(node to ground) element to be able to observe sloshing phenomenon.
With this setting, the spring stiffness(k) is defined as 
k = Fluid density * acceleration of gravity * A.
My question is:
A here is the free surface area of the liquid 
or A = (Liquid's free surface area / Number of nodes on that surface).
Hope someone can help me answer my question!
Thank you.

Abaqus ​​​​​​​