Mean Drift Data

In most cases, the mean drift database is created as a consequence of creating the pressure database. As with pressures, one can alter the settings of the drift data with a command:

     A_MDRIFT, BODY_NAME,  PKT_NAME -OPTIONS

and the available options are:

     -MD_FORCE, D_FORCE_MULT

     -MD_DAMP, DDMPMU

     -MD_PHASE, PHASE

This command performs in exactly the same manner as A_PRESSURE, except that here, drift data is being operated upon. 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.

Often, one wishes to have drift forces computed via both methods available. This is accomplished with the command:

     MD_CREATE, BODY_NAME,  PKT_NAME -OPTIONS

and the available options are:

     -MD_TYPE, DTYPE

     -MD_PHASE, PHASE

which creates a new drift data packet, PKT_NAME, computed using DTYPE. For example consider:

     G_PRESSURE B1 D10 -MD_TYPE SALVESEN
     MD_CREATE B1 D10_P -MD_TYPE PRES

which creates a pressure database and two drift databases. One of these is named D10 and it is computed with the Salvesen method. The other is named D10_P and it is computed by direct pressure integration. Notice that at the conclusion of these two commands, the packet D10_P is associated with the body.

Drift forces also depend on motion. The drift database contains information to include the effect of a set of response operators. To include these in the forces which will be applied, one should issue the command:

     MD_MOTION, BODY_NAME, FACTOR

which will add FACTOR times the motion effect to the stationary drift force to the drift packet currently associated with body BODY_NAME. Here, the current set of response operators will be used. If one later wants to ignore motion effects, he simply re-issues the command with a factor of zero.

Simple estimates of the mean drift force can be created with the command

     G_MDRIFT, BODY_NAME,  PKT_NAME -OPTIONS

and the available options are:

     -MD_TYPE, DTYPE

     -DIMENSIONS, LENGTH, BEAM, DRAFT

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

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

Here, DTYPE must be either FORMULAE, or SEMI. The -DIMENSIONS option defines the size (feet or meters) of the body used in estimating the drift. If it is omitted, the actual body dimensions will be used. The wave drift data computed here is not sophisticated, and only mean drift is considered.

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_FORCE, D_FORCE_MULT

     -MD_DAMP, DDMPMU

     -MD_PHASE, PHASE

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. Once in the menu, the components of the mean wave drift force 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 per wave amplitude**2 defined with the command:

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

the units here are feet or meters, bforce, seconds, and radians. The mean wave drift damping per unit wave height**2 is defined by

     MD_DAMP, PER,  HED, DMR(1,1), DMI(1,1), ... DMI(6,6)

and again, the units here are feet or meters, bforce, seconds, and radians. When all of the data has been defined, the menu should be exited with an END command.