THE HYDRODYNAMIC MENU

The Hydrodynamics Menu contains commands which allow the user to to build, alter, and maintain databases of hydrodynamic properties which act on body panels. The Hydrodynamic Database provides MOSES with the ingredients used in almost all types of analysis. This database can be input directly in this menu, or it can be computed from the model. The menu is entered with

     HYDRODYNAMICS

and when one has completed his task here, he exits with

     END_HYDRODYNAMICS

In order to fully understand the implications of the pressure database, it helps to have some information about what the program computes and how it uses the intermediate results. Basically, the primitive quantity is a set of velocity potentials on each panel which results from the interaction of the panel with the sea. The first six of these velocity potentials arise due to unit motion of the body, at a given frequency, in each of the degrees of freedom. These are called radiation velocity potentials. The remaining potentials result from a wave being stopped by the body. These are called diffraction potentials. The diffraction potentials depend not only on wave frequency but also on wave heading. All of the potentials are complex numbers ( a real and an imaginary part for each potential ). Collectively, we will refer to these potentials as "diffraction results" since they are normally computed via some type of diffraction analysis. The forces on Morison's Equation elements are not a part of the pressure database, are computed whenever they are needed, and are not considered in this Menu. In addition to the diffraction potentials, the "incident wave potentials" are also required, but since they are easy to compute, they are not stored in the pressure database. Also, for some computations, MOSES needs to have not only the potentials, but their gradient, so they are included in the database.

From these basic ingredients, MOSES then computes a new, or "Total" database which includes:

• The forces which unit amplitude regular waves exert on a stationary body,
• The bodies' added mass and damping matrices.
• The mean drift force which a regular wave exerts on a stationary body,
• The change in mean drift force with a motion of the body, and
• The damping on a body due to time variation of the mean drift force.

These results are a function of wave heading, forward speed, and encounter frequency.

The Hydrodynamic database actually consists of three different types of data for each body: Pressure data and Mean Drift Force Data. Also, the data is stored by "Packet Name". Thus it is possible to have several different sets of hydrodynamic data available for each body. For example, one can have different sets for different draft and trim conditions, or different sets computed with different methods. A packet of data is associated with body whenever the packet is: Generated, Imported, or Altered. In general, the user is free to define the packet name for the data when it is created, but if the name already exists, then a new name will be created. The same name can exist for each of the different types of data, but all names for a given type of data must be unique. A special reserved packet name is NONE. If this name is associated with a body for a given type of data, then it has the same effect as null data.

The data in the Hydrodynamic database is used in both frequency and time domain computations. Since the database consists of frequency domain quantities, its use in the frequency domain is easily inferred. For a time domain simulation, use of this data is not obvious. Here, three things happen. First, an excitation force is created as a cosine series of the frequency domain forces. The periods for the series are those specified with the -P_WAVE option on the &ENV command. The amplitudes are chosen to conform to the specified sea spectrum, and a set of phases are chosen. Next, a mean drift force is computed from the drift force "RAOs" and the sea spectrum. A time varying drift force is created as a cosine series at periods specified with the -P_DRIFT options of the &ENV command and amplitudes which conform to the drift spectrum and a set of phases. Finally, the frequency domain added mass and damping matrices are transformed by an inverse Fourier transform into a convolution kernel, or "retardation function". Thus, the equations of motion are integro-differential equations. Thus, any time domain simulation in a seaway first requires the basic data discussed above.

The options which control mean drift force behavior are:

     -MD_FORCE, D_FORCE_MULT

     -MD_DAMP, DDMPMU

     -MD_TYPE, DTYPE

     -MD_PHASE, PHASE

The -MD_FORCE option defines a scale factor, D_FORCE_MULT, by which the mean drift force will be scaled before it is used. The option -MD_DAMP defines a scale factor for the mean drift damping. -MD_TYPE option is used only when drift data is computed from a hydrodynamic database. There are two methods for using diffraction results to produce drift results: the method MOSES has always used and a new method. If DTYPE is PRES_INTEG, the new method will be used. The traditional method used a representation by Salvesen which employs an assumption of the body being a "weak scatterer". It is selected with a DTYPE of SALVESEN. The new method directly integrates the results over the wetted surface with essentially no assumptions. The new method is, however, much more sensitive to the accuracy of the diffraction solution than the old, and it should not be used with a strip theory solution. The option -MD_PHASE defines a phase angle (deg.) of the wave drift force in the frequency domain. Here, each drift force component will have a phase, PHASE, relative to the wave crest being at the origin. The default for these options is set with &DEFAULT.

In addition to the "tweak" options, there are several other options which appear on more than one command. Thus, they will be discussed here.

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

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

     -SPEED, VR(1), ...., VR(n)

The -HEADING option is used to change the default values of vessel heading for which results will be computed. H(i) is the ith value of heading (deg.) to be considered. Here, heading is measured as an angle from the X axis, positive toward Y. Hence, for a vessel described with the origin at the bow, head seas are 180 degrees. The -PERIOD option is used to alter the default values of encounter period, and T(i) is the ith value of encounter period in seconds. As many of these options as needed can be input, so long as the number of periods does not exceed 200. The defaults for heading and period are set with options on &DEFAULT. If one wishes to include the effects of forward speed, he should include the -SPEED option. Here, VR(i) is a forward speed in knots. When the response operators are computed, they are computed for a single speed.

There are basically two types of data considered in this menu: pressure and mean drift. Each of these will be considered in detail later, but as a general rule commands which generate data begin with G_, those which alter data with A_, those which post-process with V_, those which import with I_, and those which export with E_.

The commands for importing hydrodynamic data are designed to allow the user to completely describe a hydrodynamic data base. Although the commands for doing this are documented in the following sections, the user is encouraged to examine the samples of data provided with this software release. To input your own data, it is helpful to first export a hydrodynamic data base to get an understanding of the MOSES file format. Then, modify this file as desired, and import it to the program.