Mesh density setting
In the mesh setting, unform size mesh (base mesh) is made to cover the fluid area at first, then the mesh refinement on specified area will be done.
Base mesh settings
Set target number of base meshes and mesh area at "Volume mesh settings" on "Mesh" page. Push 
to preview base mesh on 3D view. Also you can set bease mesh density with 
and 
buttons.
In order to define fluid area, it is necessary to specify a coordinate in the fluid area (calculation area) in the "Computational domain".
Mesh refinement settings
Next, set mesh refinement settings at the specified area. Specify the area with a cuboid, a sphere etc..., and set the refinement level (how many times the base mesh is divided). Push to preview refinemented area on 3D view.
Push button to add the refinement setting. If you delete the setting, push at upper-rigt corner of the setting (blue rectangle).
The result of mesh refinement is as follows.
The difference between the refinement levels must be 1. So mesh of refinement level "1" is automatically inserted between level "0" and level "2". In this example, the area of refinement level "1" is created for 3 meshes because "Number of meshes between levels" parameter is "3".
Feature line extraction and refinement
In CFD, twisted mesh makes the calculation unstable and the result becomes incorrect. In order to prevent this, the sharp edges are “smoothed” when creating the mesh so that the mesh is not too twisted, but in some cases you may want to keep the sharp edges. This is possible by specifying feature angles to keep feature lines.
Set angle that is used to extract feature line (Feature angle) and refinement level at "Feature line" on "Mesh" page.
