Self-Propelled Submarine Motion, Dynamic Mesh (6-DOF)
The present problem simulates the motion of a self-propelled submarine floating on the water surface by dynamic mesh method using ANSYS Fluent software.
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Self-Propelled Submarine Project Description
The present problem simulates the motion of a self-propelled submarine floating on the water surface by dynamic mesh method using ANSYS Fluent software. In this simulation, a computational domain considering air and water with a certain level is designed; So that a self-propelled submarine is floating on the surface of the water. In such models, the location of the cells in the grid changes due to the displacement of their adjacent boundaries over time, and therefore, to define the instantaneous grid shift, we benefit from the dynamic mesh model. Initially, a self-propelled submarine was designed for this simulation, and then three computational zones were created around the submarine.
In the determination of dynamic mesh methods, smoothing and remeshing methods have been used. According to the smoothing method, the number of nodes (cells connections) does not change and only adjusts the mesh of an area by moving or deforming the borders. However, the remeshing method is used when the displacement of the borders is large compared to the size of the local cells to regenerate the destructive cells of the critical size limit. Six degrees of freedom (six-DOF) has also been used to define the type of dynamic mesh behavior; This means that the model has the ability to move and relocate in six degrees.
The properties of six degrees of freedom, including the mass and the moment in different directions for this model, are defined as a UDF. To define areas with dynamic mesh, the body or walls of the submarine itself is defined as Rigid Body, and also a small cylindrical area around the submarine is defined as rigid body. And then around this cylindrical region, a larger cubic area of the Deforming type is defined. This means that the submarine and a limited area around it act as a rigid and integrated body and can be moved in a transitional or rotational manner without any change in meshing.
It should be noted that in the settings of the section related to the Rigid Body, the spatial coordinates of the submarine center of gravity should be defined, as well as its position in the model. The circumference of this region is under a dynamic mesh of rigid body type, and a cubic region with a Deforming state. This means that the meshing in this area is modified or changed depending on the location at the borders of adjacent areas in a time-dependent manner, which is done with the same two methods of remeshing and smoothing. The submarine is defined as floating on the water surface, the volume of fluid (VOF) multi-phase flow model should be used.
So that in the upper part of the computational area, air and in the lower part, water are defined. Both phases flow at a speed equal to 1.62 ms-1 in the horizontal direction (X-axis) and exit at a pressure equivalent to atmospheric pressure. At the output, the open channel condition is used to define the water level. It is assumed that the height of the free water level at the outlet of the computational area is equal to 1.084824 m and the height of the floor is equal to -100 m. Due to the main nature of the model based on the use of dynamic mesh, the simulation solver should be defined as transient. In the present model, the simulation process is performed in 10 seconds with a time step size of 0.001 seconds.
Self-Propelled Submarine Geometry & Mesh
We design the present model in three dimensions using AutoCAD, CATIA, and ICEM software. The horizontal length of the submarine is 16.25 m and at the end of it, there is a number of impellers with a diameter of 0.825 m. Around the submarine, we design a cylindrical computational area inside a cubic area, as the total computational domain.
We carry out the meshing of the present model using ICEM software. The mesh type is hybrid (a combination of structured and unstructured); Thus, meshing in areas close to the submarine hull is unstructured and in general areas of the model is structured. Also, the element number is equal to 2802219.
Self-Propelled 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 using 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:
|near-wall treatment||standard wall function|
|interface modeling type||sharp|
|number of eulerian phase||2 (air & water)|
|mesh methods||Smoothing & remeshing|
|air volume fraction||1|
|Water-Inlet||Mass Flow Inlet|
|water volume fraction||1|
|Pessure specification method||free surface level|
|wall motion||stationary wall|
|momentum||second order upwind|
|turbulent kinetic energy||second order upwind|
|turbulent dissipation rate||second order upwind|
|gauge pessure||0 Pascal|
|y-velocity & z-velocity||0 m.s-1|
|water volume fraction||1 (patch)|
At the end of the solution process, we obtain two-dimensional contours related to the velocity and volume fraction of each of the air and water phases in the areas around the floating submarine. We present these contours in two planes X-Y and Y-Z and in the last second of the simulation process. Also, we show the diagram of the amount of submarine transitional and rotational movement in the direction of all three axes X, Y and Z.
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.