The second approach is to formulate the fluid domain as compressible fluid [24–26]. The formulation is equivalent to lossless acoustic wave equations. The lossless acoustic wave equations and structural equations can be combined into a single set of governing equations and subsequently discretized via FEM. As a result, any commercially available finite element solver, such as ansys, can be used. For example, Hengstler [26] employed this approach to consider a fluid domain interacting with an elastic plate. In one of his case studies, the bottom of the fluid domain is coupled with an elastic plate, while the top of the fluid domain is subjected to a free or a fixed (e.g., a wall) boundary condition. This particular case study reveals two marvelous results. First, the free or fixed boundary condition at the top of the fluid domain affects the natural frequency of the plate submerged in the fluid. When the top boundary condition is free, the natural frequency decreases as the depth of the fluid domain increases. In contrast, when the top boundary condition is fixed, the natural frequency increases as the depth of the fluid domain increases. Second, the fluid region that interacts with the plate vibration is limited. When the depth of the fluid domain is large enough, the depth does not affect the fluid–structure interaction significantly.