Aqwa-Drift and Aqwa-Naut can accept forces calculated at each timestep by a user-defined routine. This routine may be accessed through a compiled dynamic link library (DLL) or from a process running on an external server. Details of both methods are given in the following sections, but the effect of each is similar.
Either of both forms of external force calculation can be used. Aqwa sums the forces from each and adds them to any other external forces (for example, those specified in a .XFT file). The combined values utilized by Aqwa can be output to the listing and .PLT files by requesting the output of external forces using the PRNT data record. These may also then be plotted in Workbench or the Aqwa Graphical Supervisor.
The information that Aqwa sends to the external routine at each calculation stage [1] of each timestep is:
An external file name, for the external server only.
A mode flag:
0: Initialization. The routine is called once with mode 0 before the simulation.
1: Called during the simulation. Force and added mass output expected.
99: Termination. Signifies that the simulation is complete.
A set of optional integer control parameters (all zero if not defined).
A set of optional real control parameters (all zero if not defined).
The number of structures in the model.
The instantaneous analysis time (not clock time).
The time step size.
The calculation stage (1 or 2).
The position of each structure in 6 DOF [2].
The velocity of each structure in 6 DOF [3].
The position of the center of gravity when the structure is in the definition position.
The information that Aqwa expects from the external routin at each calculation stage of each timestep is:
A force to be applied at the center of gracity of each structure in 6 DOF [3].
An added mass in 6 DOF to be added to the existing added mass.
An error flag. The run will stop if this flag is non-zero.
Aqwa integration stages: Aqwa time integration is based on a 2-stage predictor corrector method. This routine is called twice at each time step, once with calculation stage = 1 and once with calculation stage = 2. On stage 2, the position and velocity are predictions of the position and velocity at the next time step. For instance, if the initial time is 0.0 and the step 1.0 second then the calling sequence is as follows for the first 3 integration steps:
Time = 0.0, time step = 1.0, stage = 1 Time = 0.0, time step = 1.0, stage = 2 Time = 1.0, time step = 1.0, stage = 1 Time = 1.0, time step = 1.0, stage = 2 Time = 2.0, time step = 1.0, stage = 1 Time = 2.0, time step = 1.0, stage = 2 The position uses the fixed reference axes (FRA). Aqwa defines the orientation of a structure using Euler angles. These are the angles which are printed in the listing file output and passed to the external routine. They are the angle by which the structure has to be rotated about the FRA to reach its given position, applied in the order RX, RY, RZ (roll, pitch, yaw). Euler_to_LSA.mcd is a PTC Mathcad worksheet which illustrates the transformation from LSA to FRA and vice versa. Conversion of Euler Angles to LSA is a text version of the Mathcad worksheet Euler_to_LSA.mcd.
The axes used are parallel to the FRA and centered at each structure’s center of gravity.