Bandstructure of 2D solid-fluid phononic crystal

Hi all,

I am trying to obtain the band structure for 2 dimesnional phononic crystal composed of liquid (water) cylinders in Al through the acustic-structure interaction model in COMSOL 4.3a. Although in the inverse case of Al circles in water we can obtain band structure correctly we couldn't do the same in the above mentioned case. we couldn't understand what the problem is. we couldn't find a band structure in a 3D either.

any help will be appreciated,

Thank you.