# Motions

Define body motions in SimLab.

From the menu bar, click nFX Analysis > Boundary Conditions > Motion.

## Introduction

Wall boundary particles can be moved during a simulation with a prescribed motion. The principal concept is to define a motion for a specific phase/body and apply this motion within a given time-frame.

Common parameters:

Start Time / End Time
Specifies the start and finish times which define the window where the motion is active.
Damping Time
This parameter specifies a time window for the acceleration of the body. At the start time, the motion will ramp up from the body’s initial state to the specified value over this time. This minimizes pressure fluctuations and instabilities caused by instantaneous changes in velocity.
A free text box where additional parameters can be specified.

## Motion Types

The following motion types are supported in SimLab. Also, animation is supported for few of the motions.

Translation
motion_type = IMPOSE_VEL. Defines a translation type motion.
Velocity: Imposed velocity defined as a vector in x, y, z directions.
Rotation
motion_type = ROTATE_AXIS. Defines a rotation type motion.
Frequency Unit: Rotational Frequency may be defined using revolutions or radians, per second or per minute.
Initial Frequency: Rotation Frequency at Start Time, usually zero unless creating sequential motions.
Frequency: Rotation Frequency which will be reached after damping time.
Planetary
motion_type = PLANETARY. Allows definition of planetary gears.
Year: The main orbit of the planetary bodies.
Day: The local rotation of the planetary bodies.
Frequency Unit: Rotational Frequency may be defined using revolutions or radians, per second or per minute.
Initial Frequency: Rotation Frequency at Start Time, usually zero unless creating sequential motions.
Frequency: Rotation Frequency which will be reached after damping time.
Note: Planetary motion is normally accompanied by a Rotation motion for the carrier.
Conrod
motion_type = CONROD. Allows definition of a piston and conrod mechanism.
Note: Conrod motion is normally accompanied by a Rotation motion for the crankshaft.
Frequency Unit: Rotational Frequency may be defined using revolutions or radians, per second or per minute.
Crankshaft Axis: Rotation Axis of the crankshaft.
Initial Frequency: Rotation Frequency at Start Time, usually zero unless creating sequential motions.
Frequency: Rotation Frequency which will be reached after damping time.
Piston Axis: Piston sliding axis (vector must point towards crankshaft).
Journal Axis: Location of the conrod journal bearing axis.
Crankshaft Phaseshift: Angle to define Conrod starting position, based on angle from crankshaft_normal (TDC).
Conrod length: Conrod length between wrist pin and journal bearing centers.
Piston Offset: Offset distance from the piston centerline to the crankshaft centerline.
Rigid Body
motion_type = PASSIVE_RIGID_BODY. Defines a 6-DOF rigid body motion for a body freely interacting with the fluid. Constraints, forces, and damping are available for each translation and rotation component.
Coordinate system: If a local coordinate system is defined in SimLab, it can be selected in place of the global coordinate system. If defined, the motion XYZ parameters use the LCS, not the GCS.
Mass, Center, and Mass Moment of Inertia are automatically calculated when a body is picked, using the density from the assigned material.
Initial velocity specifies the body’s velocity at time = 0.
Constraint System: A local coordinate system can be selected specifically for the constraint system.
Hinge point: If the body is to rotate around a different point than the center of mass, the hinge point can be defined (together with constraint system) to define the motion. For both linear and angular motions, there are nine parameters each to specify. These parameters are optional and only need to be specified when needed:
• Locked Displacements: When the direction is ticked, the corresponding DOF will be removed.
• Force: Specify an external force acting on the body (for example, an actuator) in each axis.
• Spring stiffness: Define a spring stiffness resisting displacement in each axis.
• Spring relaxed position: Used when there is pre-deformation of the spring. This specifies the equilibrium point of the spring.
• Damping: Define motion damping in each axis.
• Motion upper/lower limit: These parameters are used to restrict the range of motion (for example, a hard stop) available in each axis.
• Velocity may be specified in each axis either as a constant, or by using a time history file (time, X vel, Y vel, Z vel).
Custom
motion_type = POSITION_FILE. Allows import of a time history file to define any kind of arbitrary motion. The solver will automatically linearly interpolate between the prescribed position data.
Orientation = false: The input .csv file must be of column-format time x y z alpha beta gamma where alpha, beta, and gamma are components of the rotation vector in units of [rot/s]. The Cartesian coordinates of the first row of the input file must be 0 0 0, which corresponds to the initial position of the Center of Mass (COM).
Orientation = true: The input .txt file must be of column-format time x y z phi eta zeta orientation. The phi, eta, and zeta are cosine values of a vector which define the axis of rotation of the body, and the orientation column is body orientation in [rad].
The rotation vector defines the rotation frequency [rotations per second] as a function of time around the COM. Difference in vector positions, divided by the difference in time will determine the wall velocity. If the wall velocity exceeds the reference velocity specified in the configuration file at any point in the simulation, the simulation will stop.
Start time/End time may be used to limit which window from the time history file is used.
Ocillate
motion_type = OSCILLATE. Oscillatory motion is defined as the to and fro motion of an object from its mean position.
Oscillation Amplitude: Oscillation amplitude in global x, y, and z axis directions.
Initial Oscillation Frequency: Initial oscillation frequency in global x, y, and z axis directions.
Oscillation Frequency: Oscillation frequency in global x, y, and z axis directions.
Initial Phase Shift: Initial phase shift angle of the oscillation.

## Other Options

A free text box where additional parameters can be specified.