Floating Vessel Motion in Water by Dynamic Mesh, ANSYS Fluent Training

Project Description

The present problem simulates the motion of a floating vessel in the water by dynamic mesh method using ANSYS Fluent software. In this simulation, a computational domain of water with a certain height level is designed with a floating vessel on the surface of the water. In such models, we need momentary and time-dependent change in meshing based on the type of displacement at adjacent boundaries of grids, and therefore, to define the instantaneous change of meshing, the Dynamic Mesh model is used. Initially, a floating vessel was designed for this simulation, and then three computational zones were generated around it. In the determination of dynamic mesh methods, Smoothing and Remeshing methods have been used.

According to the smoothing method, the number of nodes or cell 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 boundary is large compared to the size of the local cells in order to reconstruct the destructive cells of the critical size limit. Six degrees of freedom (6-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.

Project Description

To define areas with a dynamic mesh, a small rectangular cube area (around the floating vessel) is created as a Moving area and a slightly larger area around the moving section is defined as a Deforming area. Finally, a large general area around all of these areas is designed to be defined as a Stationary area. The floating vessel part and the moving area are defined as Rigid Body. This means that the floating vessel 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.

Since the vessel has only two degrees of freedom and can only move up and down in the vertical direction (z axis) or rotate around its central axis (y axis), and has a constraint in other directions (It does not have any transitional or rotational motion), a UDF is used to define this type of motion with two degrees of freedom. 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 floating vessel and also the axis of its rotation around itself must be defined. Deforming area is located around these two parts.

Project Description

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. Finally, the large area includes all the sections in a static manner and has no movement and no grid changes. The vessel is defined as a floating object on the water surface, the VOF multiphase flow model should be used; So that air is defined in the upper part of the computational domain and water in the lower part. Both phases of the air and water flow at a speed equal to 1.44 ms-1 in the horizontal direction (x-axis) and exit at a pressure equivalent to atmospheric pressure.

At the outlet boundary of the model, the Open Channel condition is used to define the water level. 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 (transient solver). In the present model, the simulation process is performed in 7 seconds with a time step of 0.01 seconds.

Floating Vessel Geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. The mentioned floating vessel is designed in the middle of three cubic computing domains. For ease of simulation, the floating vessel should be designed so that its center of gravity is at the center of the vertical direction (z-axis).

The meshing of the model has been done using ANSYS Meshing software. The mesh type is unstructured near the floating vessel, and structured in the other parts. The element number is 902808. The following figure shows the mesh.

Floating Vessel CFD Simulation

We consider several assumptions to simulate the present model:

• We perform a pressure-based solver.
• The simulation is unsteady, since we are going to use 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	standard
	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
	Six DOF	active
Boundary conditions
Air-Inlet		Velocity Inlet
	x-velocity	-1.44 m.s-1
	air volume fraction	1
Water-Inlet		Mass Flow Inlet
	x-velocity	-1.44 m.s-1
	water volume fraction	1
Outlet		Pressure Outlet
	Pressure specification method	free surface level
float		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		Standard
	gauge pressure	0 Pascal
	x-velocity	-1.44 m.s-1
	y-velocity & z-velocity	0 m.s-1
	water volume fraction	0 or 1 (patch)

Results & Discussion

At the end of the solution process, two-dimensional contours related to the pressure on the floating vessel and two-dimensional contours related to the velocity and volume fraction of each of the air and water phases are obtained in the areas around the floating vessel. These contours correspond to the last second of the simulation process. Also, the graph of the amount of displacement of the vessel in the vertical direction (z-axis) in terms of time passage and the graph of the amount of rotation angle of the floating vessel around its central axis (y-axis) in terms of time are obtained.

These graphs show the angular and transitional spatial changes over a period of 7 seconds, and as the amplitude of the oscillation of the changes in the two graphs is clear, the changes reach their minimum in the seventh second, and in fact we can say that the rotational and the transition motion are damped after this time. Also, the figure shows that the floating vessel position in the seventh second at the position z = 0.021 and the angle Y_theta = -1.338 reaches stability.

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.