Fish Cage Floating on Seawater: FSI

Fish Cage Floating on Seawater CFD Simulation by FSI Method, ANSYS Fluent

Description

This project simulates a fish cage floating on the surface of seawater using the Fluid-Structure Interaction (FSI) method in ANSYS Fluent.

The 3D geometry was designed in Design Modeler, representing a computational domain containing seawater, airflow, and a circular fish breeding cage. Since the model is perfectly symmetrical, only half of the geometry was modeled to reduce computational cost, with an inlet section, an outlet section, and symmetry conditions applied along the lateral faces.

The domain was meshed in ANSYS Meshing, totaling 4,922,130 elements. Given the nature of FSI problems, a transient solver was used throughout.

Methodology

Since the fish breeding cage floats within the computational domain, seawater flow directly strikes the cage, requiring a two-way fluid-structure interaction to capture the coupled behavior between fluid and solid. This was implemented using the FSI method within the ANSYS Workbench environment.

Because the fluid mesh structure changes around the geometry as the FSI simulation progresses, a Dynamic Mesh was required, using smoothing and remeshing methods to accommodate the time-dependent mesh changes. Two-way FSI was established through System Coupling in ANSYS Workbench, which required defining the model separately in both Fluent and Transient Structural, then coupling their solution processes.

This coupling required two distinct data transfers: first, a Force transfer from the model wall in Fluent to the corresponding wall in Transient Structural — representing the force exerted on the cage as fluid flow strikes it; and second, a displacement transfer from the wall in Transient Structural back to Fluent — representing how the cage's structural deformation, in turn, alters the surrounding fluid flow.

Since the fish cage operates within a two-phase domain (seawater and air), the VOF multiphase model was used, with air occupying the upper region and seawater the lower region. To represent the cage floating in open seawater, wave behavior was introduced via the Open Channel Wave boundary condition — incoming air and seawater entered at an average velocity of 3.08 m/s along the horizontal (y-axis), with the seawater floor set at -15 m and the free surface at 0 m. Airflow was discharged at atmospheric pressure.

Conclusion

Results were obtained from both Fluent and Transient Structural, all corresponding to the simulation's final time step (0.05 s). Transient Structural results include deformation, strain, and stress contours across the fish cage's structural body.

Fluent results include 2D contours of velocity, pressure, and water/air volume fraction on the mid-plane (matching the symmetry plane), along with pressure distribution across the cage's body surface. The seawater wave surface itself was also extracted, showing 2D pressure and velocity contours and velocity vectors along it — clearly capturing the waves generated by the Open Channel Wave boundary condition in the volume fraction results.

Structurally, the maximum deformation was observed in the thin connecting bars linking the cage's top and bottom holders — highlighting these as the most mechanically stressed components under the combined wave and current loading.