TIME DOMAIN SIMULATION TIME DOMAIN SIMULATION

To initiate a time domain simulation, use the command:


     TDOM, -OPTIONS

where the available options are:


     -NO_CAPSIZE, YES/NO

     -EQUI

     -NEWMARK, YES/NO, BETA, ALPHA

     -CONVERGE, NUMB, TOL

     -RESTART, RESTART_TIME

     -RESET, RESTART_TIME

     -STORE, STORE_INCREMENT

When a time domain simulation is requested, MOSES will convert the frequency domain hydrodynamic pressure data for use in the time domain. This will results in a constant added mass, a convolution kernel, and a set of forces. The total time domain force is obtained from the frequency domain force by using a Fourier series for each heading and by interpolating a total force from the heading ones and the current vessel heading. If there is no pressure data available, null data will be used for the conversion. This will result in MOSES using the input added mass and damping matrices for the hydrodynamic interaction, and there will be no wave exciting force due to the vessel.

Normally, if a body has a roll or pitch angle greater than 90 degrees, the program will stop the simulation anticipating that something occurred that was not intended. If the -NO_CAPSIZE, YES option is used, no checking for capsizing will be performed.

The -EQUI option is useful for finding equilibrium solutions for particularly difficult or complex situations. With this option, MOSES will enter the time domain with only the mean forces applied. This is conceptually the same as using the &EQUI command, except that here, the user can examine the results for each time step.

Two methods are available for integrating the equations of motions: a Predictor/Corrector method, and a Newmark method. The default is to use the Newmark Method. If, however, -NEWMARK, NO is specified, then the Predictor/Corrector method will be used. This method has been around for years, and works well in many cases. Its primary shortcomings are that for "stiff" systems, very short time steps must be used, and for larger time steps, considerable numerical damping is induced. The Newmark method ameliorate these difficulties. The two parameters BETA and ALPHA are the two Newmark parameters. If they are omitted, the defaults of BETA = .25 and ALPHA = .5 will be used. The -CONVERGE option defines the convergence parameters when a Newmark method is used. Here, convergence is defined as the change in location between two iterations at the same time step. MOSES will take at most NUMB iterations until the norm of the change in location is less than TOL. The default for NUMB is 5 and TOL is 5e-2. The Predictor/Corrector method does not iterate.

In general, a time domain simulation will begin at time equal zero, with current locations and velocities. The environment to which the system will be subjected is that specified on the last &ENV command. Events will be computed every DELTA_TIME seconds until time TOBSERV is reached, where DELTA_TIME and TOBSERV are also specified on the &ENV command.

At the conclusion of the time domain simulation, no results are automatically reported. Instead, they are stored in the database for further use. To obtain reports, graphs, or other types of information about the simulation, one should issue the PRCPOST command to enter the Process Post-Processing Menu.

When one issues a TDOM command, the results of any previous simulation for this process name will be lost, unless one wishes to "restart" the simulation. This is accomplished by adding either the -RESTART or -RESET option to the TDOM command. In either case, RESTART_TIME is the time at which the previous simulation will be restarted. When one restarts a process, the events up to the restart event of the previous process will be saved, and a new process will be computed for events afterwards. Normally one restarts a time domain so that a different time step can be used or to extend the observation time.

In some cases, however, one wished to change the model at some time. Here one uses -RESET. Suppose, for example, that at time 100, one wishes to activate a set of connectors. This can be accomplished with:

     &INSTATE -EVENT 100
     &CONNECTOR C100@ -ACTIVE
     TDOM -RESET 100

This first sets the initial state to that at event 100, activates the connectors and then "resets" the time domain at 100. With -RESET the connector and compartment settings at the initial state will be used at the reset time.

During the computation of a time domain, the data is stored in the database. The user has some control over this data storage with the option -STORE. The state of a configuration will be written to the database at STORE_INCREMENT increments of computed steps. If the option -STORE is omitted, the data will be written every computed time step. In other words, a default of STORE_INCREMENT equal 1 will be used.

The string function


     &SLAM(:PAR_SELE, E(1), E(2), DE )

is useful for determining "slam events." This function looks at event between E(1) and E(2) in increments of DE in the current process and finds the events where any part of the parts selected by the part selector :PAR_SELE are submerged. If E(1), E(2), and DE are omitted, then all the events in the database will be used. The result returned is a set of pairs (e1, e2) where some of the selected parts is submerged between e1 and e2.