Mean Drift Data Mean Drift Data

In most cases, the mean drift database is created as a consequence of creating the pressure database. This is simply the average of the nonlinear forces over time. In general, there are three contributions to this force: diffraction/incident potentials, radiation potentials, and the coriolis acceleration. The mean force due to diffraction/incident potentials is independent of the motion while the other two depend on the motion. Here, the last RAOs computed for the current process will be used to compute these contributions. The diffraction/incident and the radiation mean forces are also used to apply a slowly varying force in the time domain as described earlier. The coriolis acceleration contribution to the mean is not used in the time domain. Instead, it is computed exactly.

One can examine the drift data with the command


     V_MDRIFT, BODY_NAME

which will place one in the Disposition Menu to do whatever one wishes.

One can save a set of drift data for later use by issuing the command:


     E_MDRIFT, BODY_NAME

which writes the drift data currently associated with body BODY_NAME to a file for later use.

The user can input his own drift database. To define mean wave drift response operators, one first enters a submenu with the command:


     I_MDRIFT, BODY_NAME,  PKT_NAME -OPTIONS

and the available options are:


     -HEADING, H(1), H(2), ...., H(n)

     -PERIOD, T(1), T(2), ...., T(n)

     -MD_TYPE, METHOD

Here, PKT_NAME is the name of the set of response operators, the -PERIOD option defines the periods (sec) and the -HEADING option defines the headings (deg) for which drift values will be defined. The option -MD_TYPE specifies the method used to calculate the drift data, this can either be the NEARFIELD (pressure integration) or the FARFIELD (momentum) method. Once in the menu, the components of the mean wave drift force not including motions for the specified headings are defined with the command:


     M_DRIFT, PER,  FXR(1), FXI(1), ...., FYAWI(1), ...  \
     FXR(n), FXI(n), ..., FYAWI(n)

Here PER is the period for which this set of forces is applicable and must be one of the T(i) specified with the -PERIOD option. FXR(1), FXI(1), .. FYAWI(1) are the real and imaginary parts of the mean drift forces and moments per unit of wave amplitude squared (bforce/blength**2 for force, bforce/blength for moments) for the heading HED(1). Likewise, the values FXR(2) ... are for heading HED(2), etc. The corrections to the mean force due to motions are a complex 6x6 matrix defined with the command:


     MD_MOTION, PER,  HED, MDR(1,1), MDI(1,1), ... MDI(6,6)

the units here are bforce/blength**3 for force and bforce/blength**2 for moments) as the mean drift forces due to motions are normalised by the motion displacement. When all of the data has been defined, the menu should be exited with a END_I_MDRIFT command.