Appendix: Non-Inertial Reference Frame

A non-inertial reference frame can be specified in two ways: using a file-based input or through constant parameters. The framework allows users to define the rotation axis, center, speed, and acceleration of the non-inertial frame to account for dynamic effects in computational simulations.

File-Based Specification

Non-inertial reference frame data can be provided by specifying a filename in the nonInertialReferenceFrame run control file variable. This file contains the time-dependent center of gravity, acceleration, and angular velocity. An example specfication is shown below.

File-based non-inertial reference frame specification in the run control file (.vars file)
  nonInertialReferenceFrame: <file=reference_frame_data.dat>

The file must contain a series of time points with corresponding values for acceleration, center of gravity, and angular velocity. The first time value must be zero. The format for each line in the file should follow this pattern:

File-based non-inertial reference frame data format
  time accel_x accel_y accel_z cg_x cg_y cg_z omega_x omega_y omega_z

The framework automatically computes angular acceleration based on the provided angular velocity values using a backward difference scheme.

Constant Parameter Specification

Alternatively, the non-inertial reference frame can be specified using constant parameters such as the rotation axis, center, speed, and linear acceleration. A summary of the required parameters is provided below.

Parameter	Description
`rotAxis`	The axis of rotation as a 3D vector.
`rotCenter`	The center of rotation as a 3D vector.
`rotSpeed`	The angular speed of rotation (in radians per second).
`accel`	The linear acceleration vector (in meters per second squared).

A sample of the constant parameter specification as it would appear in the run control file is shown below.

nonInertialReferenceFrame:
<
  rotAxis= [0.0, 1.0, 0.0],
  rotCenter= [0.0 m, 0.0 m, 0.0 m],
  rotSpeed= 0.1 rad/s,
  accel= [0.0 m/s/s, 0.0 m/s/s, -9.81 m/s/s]
>

Important Constraints

Only one specification method (either file-based or constant parameters) can be used at a time. Specifying both will result in an error.
For the file option, at least two time points must be provided, and the first time point must be zero.
For the constant parameters, rotational acceleration is assumed to be zero.