Dynamic Mesh
There are the following three ways to define mesh movement.
- Moving Cell Zone : Moves the cell zone or the entire mesh.
- Moving Boundary : Moves the boundary surface and deform the internal mesh.
- Rigid Body Dynamics : Calculates the 6-degrees-of-freedom motion of multi rigid bodies.
Press the Change button for Motion Type, and the following settings window will appear; select your desired option.
Motion Function
There are five types of motion functions that define the motion of an object: Rotating, Rotating Oscillation, Linear Translation, Linear Oscillation, and Manual Position.
Rotating
Input the coordinates of the center of rotation, the direction of the axis of rotation, and the angular velocity.
Rotating Oscillation
Input the coordinates of the center of rotation, amplitude, and angular velocity.
Linear Translation
Input the velocity in the x, y, and z directions.
Linear Oscillation
Input the amplitude and frequency.
Manual Position
Input the center of gravity coordinates and surge, sway, heave, roll, pitch, and yaw over time into a table.
Moving Cell Zone
It is a motion in which the entire computational domain moves or the cell zone moves along the interface surface, without any deformation of the mesh. Representative examples include the sliding mesh motion of a rotating body and the sloshing motion of a tank.
You can define multiple motions per cell zone. If you add an motion using the Add Motion button at the bottom, it will appear in Motion Definitions.
Motion Definition
Each motion sets three things: Name, Motion Function, and Cell Zones..
Multiple motion functions can be set for a single motion. If nothing is selected in cellZones, it is defined as the motion of the entire domain.
Moving Boundary
The motion of the boundary mesh is implemented using a motion function. As the boundary moves, the internal mesh is also deformed.
Specify the mesh motion method for each boundary surface. You can select from three options: Fixed, Slip, and Prescribed Motion.
Fixed
The mesh does not move.
Slip
The mesh does not move in the normal direction perpendicular to the boundary surface, but moves only in the tangential direction.
The ‘Use Fixed Normal Vector’ option is available. When enabled, a user-specified vector is used as the normal direction vector; when disabled, the normal direction of each surface mesh cell is calculated internally. Using this condition allows for more securely fixing the mesh in the normal direction.
Prescribed Motion
The motion of the boundary surface is set using a motion function.
It can be used only when the boundary condition is set to Wall and the Wall Motion is set to Moving Wall – Mesh Motion.
The motion function can use five types, such as Rotating, Rotating Oscillation, Linear Translation, Linear Oscillation, and Manual Position, as in the Moving Cell Zone.
Rigid Body Dynamics
It is a motion implementation method that can apply various laws of dynamics. It can implement 6 degree of freedom motion of multi-body motion.
Solver
Three time integration methods are supported: Newmark, Crank-Nicolson, and Symplectic.
Newmark
It is a second-order implicit method that uses two integration coefficients for velocity and displacement. If the value of the coefficient used to calculate velocity(gamma) is large, numerical damping increases, and if the value of the coefficient used to calculate displacement(beta) is large, stability is high but accuracy may decrease.
Crank-Nicolson
This is a second-order implicit method that discretizes the time derivative by averaging the values of the current and next time points. It uses two off-centering coefficients to suppress oscillations. The Acceleration Off-Centering Coefficient (aoc) is a value between 0 and 1; 0 represents a completely explicit condition, while 1 represents a backward Euler condition, and a value between 0.5 and 1 is recommended. A larger Velocity Off-Centering Coefficient (beta) reduces numerical oscillations but may compromise time accuracy.
symplectic
It is a second-order explicit method and is widely used in problems where long-term integration is important.
Acceleration Factors
It is a device that ensures numerical stability by controlling acceleration, using two methods: relaxation and damping. Relaxation is a device that mitigates changes by utilizing values from previous times, while damping is a method that reduces values. The following equation is used.
$\ddot{q} = d \cdot (\alpha \cdot \ddot{q} + (1-\alpha ) \ddot{q}_{prev})$ㅁ
$\alpha$ : relaxation factor
$d$ : damping factor
Rigid Bodies
To define the motion of an object, the dependency on a parent object, mesh deformation range, inertia, local coordinate frame, joints, restraints, etc., are set.
Parent
Select the parent rigid body in the hierarchy to which the rigid body belongs.
Mesh Deformation Range
When a rigid body moves, it must deform the surrounding mesh along with it, and this is the reference distance that defines how it deforms.
- Offset from surface : Up to this point, the mesh moves identically to a rigid body; there is no mesh deformation, only movement.
- Outer Distance : The mesh does not deform in the region outside this point. In other words, only the mesh between the offset from surface and the outer distance is deformed.
Inertia
Set the mass, center of mass, and moment of inertia.
Local Coordinate Frame(transform)
Specifies the local coordinate frame of the rigid body. The restraint is defined based on this local coordinate system.
Joint
A rigid body has 6 degrees of freedom, consisting of 3 translations and 3 rotations, and is a function that constrains movement in a specific direction.
Prismatic
Set the direction with the condition that it moves only in a specific direction.
Revolute
Set the axis of rotation as a condition for rotating around a specific axis.
Spherical
It is a condition where it is fixed at a single point but rotates in all directions. There are no separate settings.
Restraint
It is used to inhibit or induce movement by adding force or torque, while maintaining the degrees of freedom.
Damper
A constant damping constant is set.
$F_d = c \cdot v$
Spring
It is a model that connects a rigid body to a specific point using a spring and a damper.
- Attachment point : The location where a spring is connected to a rigid body.
- anchor : a stationary reference point
- Rest Length : Natural length of the spring
- Spring constant : spring stiffness
- Damping constant : A damping coefficient proportional to velocity
Rotational Spring
This is a spring model applied to rotational motion. Input the axis of rotation, spring constant, and damping constant.
