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

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The present problem simulates a fish cage floating on the surface of seawater using the method of Fluid Solid Interaction (FSI) in ANSYS Fluent software.

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Project Description

The present problem simulates a fish cage floating on the surface of seawater using the method of Fluid Solid Interaction (FSI) in ANSYS Fluent software.
Keeping fish in cages is one of the new topics in the field of fish farming. Using this cage, part of the sea water, lake, water behind the dam, etc. can be separated from the surrounding water by various tools such as nets, and then the fish can be enclosed in this area.

The advantage of these cages in comparison with the fish ponds is that they have a lower investment cost, can be expanded or moved, the fish are confined in a limited space and this type of energy is consumed less, as well as the ability to use natural sea water and the use of natural and live food. We can place these cages in a fixed, floating, submerged, etc. position on the sea water flow. We design the floating cages in such a way that we can hold their mesh by a specific ring or frame.
In this simulation, we design a computational domain of ​​seawater, including the air and water with a certain level of water.

Project Description

So that a fish cage is located inside this area. We model the fish cage is cylindrical and design half of it with symmetry boundary condition to reduce computational costs. Due to the fact that this fish breeding cage is floating within the computational area, the flow of sea water collides with this cage, and as a result, a two-way interaction between fluid and solid occurs. 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.

Because the dynamic mesh technique allows changing the mesh structure of the model in a time-dependent manner. In the determination of dynamic mesh methods, we apply Smoothing and Remeshing methods. According to the smoothing method, the number of nodes does not change and only adjusts the mesh of an area by moving or deforming the boundaries. We use the Remeshing, when the displacement of the borders is large relative to the size of the local cells to regenerate the destructive cells of the critical size limit. To define two-way FSI, we should use System Coupling in ANSYS Workbench software.

Project Description

To do this, you must first define the model in each of the Fluent and Transient Structural software and then couple the process of solving them with this system coupling, and also considering that the geometry is the same in both fluid and solid design modes, we should stablish a coupling between the geometry of each of these two softwares. Now it is necessary to define the walls or boundaries of the model that are affected by the FSI in the Fluent and Transient Structural software. To define the desired border in Fluent software, we should activate the Dynamic Mesh model.

Because the meshing around solid boundaries changes over time due to the deformation of solids, and as a result, we must consider the dynamic mesh around these boundaries. To do this, we should define the boundary of the fish cage in the dynamic mesh section in system coupling mode. This means that the instantaneous change in the structure of the mesh is due to coupling with solid analysis. Then, to define the desired boundary in transient structural software, you must define boundary constraints. In this model, we sould define the boundaries of the cage on the symmetry plane as Fixed Support.

Project Description

These boundaries are under the influence of a constant flow of fluid and do not change position or deform. However, the other solid boundaries of the cage all change position due to the collision of the fluid flow, and therefore, we should define these boundaries as Displacement. Also in the settings of this software, the time step size of the simulation process and the duration of the simulation should be defined in accordance with the solution settings in the Fluent software.
Finally, to make a connection or coupling between fluid and solid and to define their effect on each other, Data Transfer must be defined.

In this way, the results of these two solutions in the two mentioned software are transferred to each other. Therefore, two data transfer 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 the flow of fluid around the wall strikes the wall and exerts a force on it.

Project Description

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 sea water and air, the VOF multi phase flow model should be used; So that air is defined 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 sea water, the Wave behavior is defined for 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 enters with an average flow rate of 3.08 m.s-1 in the direction of the horizon (y axis); So that the sea water flow has a bottom at a height of -15 m and a free surface at a height of 0 m. Finally, the air flow is discharged at a pressure equal to atmospheric pressure. Due to the main nature of the model based on the use of dynamic mesh, the simulation process should be defined by transient solver.

In the present model, the simulation process is performed in 0.05 seconds with a time step of 0.01 seconds. Since the simulation process is performed in both fluid and solid software, the same time period must be defined for both software.

fish cage

Geometry & Mesh

We design the present model in three dimensions using Design Modeler software. The model includes a computational domain with seawater and air flow and a circular fish breeding cage within this domain. Due to the fact that the model has a perfectly symmetrical structure, to reduce the computational cost, the model is semi-designed. This computational area has a section called input and a section called output, and the lateral faces of this area have symmetry condition.

fish cage

We carry out the meshing of the model using ANSYS Meshing software, and the mesh type is unstructured. The element number is 4922130. The following figure shows the mesh.

fish cage

CFD Simulation

We consider several assumptions to simulate the present model:

  • We perform a pressure-based solver.
  • The simulation is unsteady, since we are using Dynamic Mesh method with a transient manner.
  • The gravity effect on the fluid is equal to -9.81 m.s-2 along the Z-axis.

The following table represents a summary of the defining steps of the problem and its solution:

Viscous Laminar
Multiphase Model VOF
formulation implicit
interface modeling type sharp
number of eulerian phase 2 (air & water)
Dynamic Mesh Active
mesh methods smoothing & remeshing
Boundary conditions
Inlet Velocity Inlet
averaged flow velocity magnitude 3.08 m.s-1
wave BC for water free surface level to bottom level 0 m to -15 m
wave height 9 m
wave length 9 m
Outlet Pressure Outlet
gauge pressure 0 pascal
Wall of Cage Wall
wall motion stationary wall
Symmetry Surfaces symmetry
Pressure-velocity coupling SIMPLE
pressure PRESTO
momentum second order upwind
volume fraction compressive
Initialization methods Standard
gauge pressure 0 Pascal
x-velocity 0 m.s-1
y-velocity -3.08 m.s-1
water volume fraction 0 (patch)


After the solution process, we obtain the results in both Fluent and Transient Structuralsoftware. All results are related to the final second (0.05 s) of the simulation process. 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 contours of velocity, pressure, water and air volume fraction 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, we made an attempt to obtain the surface of the sea water wave and then represent show two-dimensional contours of pressure and velocity as well as velocity vectors on it.

There is a mesh file in this product. By the way, the Training File presents how to solve the problem and extract all desired results.


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