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

Description

In this project, we present a simulation of a Fish Cage floating on the seawater via ANSYS software.

Since the fish cage is floating on seawater, an interaction occurs between the water waves and the cage structure. First, the water flow exerts a volume force on the cage's body by hitting it. Subsequently, displacement or deformation appears on the cage body, which can lead to the water flow being affected. Therefore, we intend to perform a numerical simulation of the fish cage as a Fluid-Structure Interaction (called FSI).

The interaction between fluid and structure can be implemented as:

• One-way FSI

• Two-way FSI

In this project, we aim to analyze both the effect of fluid on the structure and the effect of the structure on the fluid. So, we choose Two-way FSI, which is a more accurate and realistic but more complex approach.

We modeled the geometry via Design Modeler software. The computational domain is a sample space of the marine environment that includes both fluid and solid domains. There is a solid cage structure within the fluid environment, which is water up to a certain level and airflow above it.

We meshed the computational domain via ANSYS Meshing software. The mesh is of an unstructured type, and approximately 490,000 cells have been generated.

Methodology

Fluid-structure interaction can be performed in two general methodologies:

• In the ANSYS Workbench environment, using an external solver (specifically, system coupling)

• Only in the Fluent solver (in the form of an intrinsic FSI).

In this project, we implemented a two-way FSI in the ANSYS workbench environment.

For two-way FSI with an external solver, three main steps are required:

• Simulation of the fluid domain from the model using the Fluent solver

• Simulation of the solid domain from the model using the Transient Structural solver

• Definition of the Data Transfer between the fluid and structural solvers using the System Coupling tool

For utilizing the system coupling, we define two data transfers:

• In the form of Forces to the interface wall (from the fluid solver to the structural solver)

• In the form of Displacements of the interface wall (from the structural solver to the fluid solver)

Since we were analyzing two-way FSI and considering the effect of the structure's displacement on the adjacent fluid, we used the Dynamic Mesh model. In other words, we establish a connection between the fluid and structure calculations with the System Coupling option. Then, for defining a deforming mesh, we enabled the smoothing and remeshing methods.

In addition, we considered the fish cage floating on seawater. Therefore, we used a Multiphase model to define the water level above which the air exists. So, for defining a two-phase flow, we used the volume of fluid (VOF) model.

Results

We analyzed the results in two fluid and solid approaches:

In Fluent, we studied the behavior of water flow. For this, we obtained the distributions of the pressure, velocity, and volume fraction of water. The results show that the water flow collides with the fish cage body in a wave mode and, as a result, exerts a hydraulic force on the cage structure.

 In Structural Transient, we studied the behavior of the fish cage body under the influence of the applied forces of the water flow. For this, we obtained the distribution of the deformation, von Mises stress, and elastic strain. The results confirm that the water flow affects the cage structure and, as a result, parts of it undergo deformations and displacements.

In conclusion, we can claim that we carried out the simulation project of a fish cage correctly and acceptably by using the two-way FSI method.