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 two 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, or Imported. 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 -S_PERIOD 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 -MD_PERIOD 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", and the equations of motion are integro-differential equations. Thus, any time domain simulation in a seaway first requires the basic data discussed above.

The basic method of computing mean drift force from the hydrodynamic pressures is to integrate the results over each submerged panel. If, however, one has used strip theory, then this results in zero surge drift force (the surge diffraction potential is ignored). In this case, a representation by Salvesen which employs an assumption of the body being a "weak scatterer" is used to estimate a surge component.

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 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.