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

Description

The present problem simulates a fish cage floating on seawater’s surface using the Fluid Solid Interaction (FSI) method in ANSYS Fluent software. We perform this CFD project and investigate it by CFD analysis.

The present model is designed in three dimensions using Design Modeler software. The model includes a computational domain with seawater and airflow and a circular fish breeding cage within this domain.

Due to the fact that the model has a perfectly symmetrical structure, half of the model is designed to reduce the computational cost. This computational area has a section called input and a section called output, and the lateral faces of this area have symmetry conditions.

The meshing of the model has been done using ANSYS Meshing software, and the element number is 4922130.

Also, a transient solver has been enabled due to the nature of FSI problems.

Fish Cage Methodology

Due to the fact that this fish breeding cage is floating within the computational area, the flow of seawater collides with this cage, and as a result, two-way interaction between fluid and solid must be considered. Therefore, we should apply the FSI method in the ANSYS Workbench software environment.

When using the FSI method, due to the change in the structure of the fluid flow mesh around the geometric model, it is necessary to define a Dynamic Mesh. The dynamic mesh technique allows changing the mesh structure of the model in a time-dependent manner.

In determining the dynamic mesh method, we apply Smoothing and Remeshing methods. To define two-way FSI, System Coupling should be used in ANSYS Workbench software.

To perform the system coupling, the model must first be defined in each Fluent and Transient Structural software and then couple the process of solving them with this system coupling.

This way, the results of these two solutions in the two software are transferred to each other. Therefore, two data transfers must be defined in the system coupling section; Thus, this data transfer for a specific area or boundary must be defined from a Source to a Target.

To do this, a data transfer from the model wall in Fluent software to the same model wall in Transient Structural software must be defined as Force.

This means that fluid flow around the wall strikes the wall and exerts a force on it. Also, a data transfer from the wall in Transient Structural software to the same wall in Fluent software must be defined as displacement. This means that the wall changes the fluid flow around it.

It should be noted that since the fish cage moves within a computational domain with two phases of seawater and air, the VOF multiphase flow model should be used to define air in the upper part of the computational area and seawater in the lower part.

Since we assume that the fish cage is floating in the seawater, the Wave behavior is defined as the flow of water entering the computational area. To do this, the Open Channel Wave BC option must be activated.

Therefore, the incoming air and seawater flow enter with an average flow rate of 3.08 m/s in the direction of the horizon (y-axis); So, the seawater flow has a bottom at the height of -15 m and a free surface at the height of 0 m. Finally, the airflow is discharged at a pressure equal to atmospheric pressure.

Fish Cage Conclusion

After the solution process, we obtain the results in both Fluent and Transient Structural software. All results are related to the simulation’s final second (0.05 s). In transient structural software, we obtain deformation, strain, and stress contours on the body surface of this fish breeding cage.

In Fluent software, we obtain two-dimensional velocity, pressure, water, and air volume fraction contours on the middle plate (same as the symmetry plate). Also, we present the contour of the amount of pressure on the surface of the body of this fish breeding cage.

In addition, it has been tried to obtain the surface of the seawater wave and then represents two-dimensional contours of pressure and velocity and velocity vectors on it.

Also, as can be seen in volume fraction contours, the waves created by open channel wave BC are observable. Moreover, the maximum deformation can be seen in the thin bars connecting the top and bottom holders.