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Submarine Movement in Water by Dynamic Mesh (1-DOF), ANSYS Fluent

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The present problem simulates the motion of a submarine in water using the dynamic mesh method in ANSYS Fluent software.

This product includes Mesh file and a Training Movie.

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Description

We Project Description

The present problem simulates the motion of a submarine in water using the dynamic mesh method in ANSYS Fluent software. In this simulation, a computational domain including air and water with a certain level of water is designed; So that a submarine is located in this area. Due to the fact that this submarine is moving within the computational domain and thus affects the surrounding grid elements, so there is a need for momentary and time-dependent change in meshing based on the type of displacement at the adjacent boundaries of the mesh. Therefore, the dynamic mesh model is used to define the instantaneous change of meshing.

Initially, a submarine was designed for this simulation, and then a computational domain consisting of two phases of air and water around the submarine was designed. In the determination of dynamic mesh methods, smoothing and remeshing methods have been used. According to the smoothing method, the number of nodes or element connections does not change and only adjusts the mesh of an area by moving or deforming the boundaries. However, the remeshing method is used when the displacement of the boundaries is large compared to the size of the local cells in order to reconstruct the destructive cells of the critical size limit.

Project Description

In the definition of areas under dynamic mesh, the wall part of the submarine is defined as Rigid Body. This means that the hull of the submarine acts as a rigid and integrated body and can be moved in a transitional or rotational manner. This rigid body behavior means that the body itself does not change and only the meshing of the surrounding areas changes over time. Since the submarine has only one degree of freedom and can only rotate around its central axis (x-axis), and in other degrees it is constrained and has no transient or rotational motion, we use a UDF for defining this type of movement, considering a degree of freedom.

The UDF of submarine rotational motion is defined in such a way that at 0 s to 3 s of this modeling, the rotational velocity value changes between +1.5 rad.s-1 and -1.5 rad.s-1. It should be noted that in the settings of the section related to the rigid body, the spatial coordinates of the center of gravity of the submarine as well as the axis of its rotation should be defined. Since the submarine is moving within a computational domain with two phases of water and air, the VOF multiphase flow model must be used; So that air is defined in the upper part of the computational area and water in the lower part.

Project Description

Since we assume that the submarine is moving in seawater, the wave behavior is defined for the flow of water entering the computational domain. To do this, the open channel wave BC option must be activated. Therefore, the incoming water flow enters with an average flow rate equal to 10 m.s-1 in the direction of the horizon (x axis); So that the bottom of the wave is defined at a height of -10.16 m and its peak at a height of 0 m. Inlet air flow also enters the area with the condition of inlet pressure equal to atmospheric pressure (relative pressure zero). 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 in terms of time, the simulation process is performed in 3 seconds with a time step of 0.01 seconds.

Geometry & Mesh

We model the present model in three dimensions using Design Modeler software. The model includes a computational domain with air and water flow and a submarine within this area. This computational area has a section called input and a section called output, and the four faces around this area have a symmetry condition.

submarine

We carry out the meshing using ANSYS Meshing software, and the mesh type is unstructured. The element number is 316846. The following figure shows the mesh.

submarine

CFD Simulation

We consider several assumptions to simulate the present model:

  • We perform a pressure-based solver.
  • The simulation is unsteady, since we are applying dynamic mesh method.
  • The gravity effect on the fluid is equal to -9.81 m.s-2 along the Y-axis.

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

Models
Viscous k-epsilon
k-epsilon model RNG
near-wall treatment standard wall function
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
Air-Inlet Pressure Inlet
gauge total pressure 0 pascal
Water-Inlet Mass Flow Inlet
averaged flow velocity magnitude 10 m.s-1
free surface level to bottom level 0 m to -10.16 m
wave height 1 m
wave length 1 m
Outlet Pressure Outlet
gauge pressure 0 pascal
submarine Wall
wall motion stationary wall
Methods
Pressure-velocity coupling SIMPLE
pressure PRESTO
momentum second order upwind
turbulent kinetic energy first order upwind
turbulent dissipation rate first order upwind
volume fraction compressive
Initialization
Initialization methods Satandard
gauge pessure 0 Pascal
x-velocity, y-velocity, z-velocity 0 m.s-1
water volume fraction 0 (patch)

Results

At the end of the solution process, we obtain two-dimensional contours related to the velocity and volume fraction of each of the water and air phases as well as two-dimensional path lines in the areas around the submarine. We obtain these contours locally on a plane perpendicular to the horizontal axis of the submarine (parallel to the Y-Z plane). We present these contours at different times of the simulation process. You can see that the submarine rotates around its central axis (x-axis) and this rotational motion according to the defined UDF has clockwise and counterclockwise reciprocating motions over time.

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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