Dear Hamda,

I made a series of videos that illustrates how to slice up a fluid volume to get a structured mesh. You might learn something from them. You can see six-sided bodies being carved out of the volume.  I started thinking about how to do this for your fluid.  I think the basic unit in your geometry looks like this:

From that block, you can mirror it and recreate the full bed.  That is what I would attempt to split to get a structured mesh, but the tet mesh seemed pretty good.

When a single body is sliced, the mesher will mesh them separately and the nodes at the cut face will not necessarily line up and each coincident face will have its own set of nodes, so the bodies are not connected, they are separate. See this example.

In DesignModeler, if you select those two bodies in the Outline, and right click and select Form New Part, they will be put into a common Part.  Now the mesher knows that they should be connected and it creates one set of nodes on the coincident face and elements from each body are hooked up to common nodes on the face.

Kind regards,