Data Categories 6 to 8 - the Radiation/Diffraction Analysis Parameters
The data input in these data categories relate to the equation of motion of a diffracting structure or structures in regular waves, for a range of frequencies and directions.
- Data Category 6
Required frequencies Required directions - Data Category 7
Linear hydrostatic stiffness matrix The buoyancy force at equilibrium The Z coordinate of the center of gravity at equilibrium Added mass matrix Radiation damping matrix Diffraction forces Froude-Krylov forces Response motions (or RAOs)
Usually, not all of the above data are required for a particular mode of analysis, in which case, the user may simply omit the data which are not applicable.
Note: Although the Radiation/Diffraction parameters calculated by Aqwa-Line can be transferred to other programs in the Aqwa suite, this is not mandatory. This means that, if the backing file produced by an Aqwa-Line run is not available (i.e. Aqwa-Line has not been run previously) or if the user wishes to input values from a source other than Aqwa-Line, he may do so in these data categories.
The following sections show the required data input for the available modes of analysis:
Input for Aqwa-Line with no previous Aqwa-Line runs
The Radiation/Diffraction parameters (i.e. added mass, wave damping, drift coefficients, etc) are to be calculated.
- Data Category 6
Required frequencies Required directions - Data Category 7
The Z coordinate of the center of gravity at equilibrium
Input for Aqwa-Line adding additional frequencies to a previous Aqwa-Line run
If the whole range of frequencies at which the parameters are defined is specified in a single run, large computer costs can be incurred if the program has to be re-run for any reason. Aqwa-Line therefore permits the user to specify selected frequencies in the initial run and additional frequencies in subsequent runs.
If Aqwa-Line has been run with the data input described in the previous section, the program may be restarted at RESTART Stage 2. This is done by using the RESTART option and RESTART data record, and omitting Data Categories 1 to 5. The user then specifies, in Data Category 6, only the additional frequencies at which the parameters are to be calculated.
- Data Category 6
One or more additional frequencies
Note that the frequencies input in Aqwa-Line runs must differ from those in previous runs or they will be automatically rejected. As all parameters are defined for a unique range of directions, these directions may not be redefined General Description.
Input for Aqwa-Line specifying known values of the radiation/diffraction analysis parameters
The new user is advised to ignore this facility
Known Radiation/Diffraction parameters may be input in Data Categories 7 and 8. This applies when using Aqwa-Line either initially or in RESTART mode (see notes A and B).
This facility may be used to input additional values of the parameters when the user is unable to describe the structure fully with a conventional Radiation/Diffraction model (e.g. additional damping or added mass due to a structural appendage). Additional linear stiffness is often used where the structure contributes to the water plane area but not to the Radiation/Diffraction model.
The user may also wish to extend the lower range of frequencies (where analysis costs can be prohibitive) with experimental data.
Recovery from errors may also be achieved by manual input of previous results where the backing file has been lost.
The data definition keyword (see Wave Frequency Dependent Parameters and Stiffness Matrix - WFS* (Data Category 7) and Drift Force Coefficients - DRC* (Data Category 8)) indicates whether Aqwa-Line will:
(a) OMIT the calculation of those parameters which are input, or
(b) ADD the input parameters to those calculated.
In case (a), input of the linear hydrostatic stiffness matrix and its associated buoyancy force means that Aqwa-Line will omit the calculations of these parameters. Input of the other parameters (which are all frequency dependent) means that Aqwa-Line will omit all calculations at this particular frequency.
- Data Category 6
One or more frequencies
- Data Category 7
Input known parameters
- Data Category 8
Input known parameters
All parameters input, together with those calculated by Aqwa-Line, will be contained in the backing file as input for further runs. The user should refer to the individual Data Category description for details of the actual data record input format.
Input for Aqwa-Drift/Fer/Librium/Naut with results from a previous Aqwa-Line run
The relevant Radiation/Diffraction information will be stored on the backing file created by a previous Aqwa-Line run. No additional input is required.
Input for Aqwa-Drift/Fer/Librium/Naut with results from a source other than Aqwa-Line
All data appropriate to the analysis may be input in Data Categories 6 to 8. The parameters which are input will depend on the type of analysis and the particular structure analyzed.
- Data Category 6
Required frequencies Required directions - Data Category 7
Linear hydrostatic stiffness matrix The buoyancy force at equilibrium The Z coordinate of the center of gravity at equilibrium Added mass matrix Radiation damping matrix Diffraction forces Froude-Krylov forces Response motions (or RAOs)
Input for Aqwa-Drift/Fer/Librium/Naut with results from a previous Aqwa-Line run and a source other than Aqwa-Line
If you wish to APPEND to or CHANGE the parameters calculated by a previous Aqwa-Line run for the current analysis, this is achieved by using the backing file from a previous Aqwa-Line run (i.e. automatically read), together with specifying the parameters to be appended or changed.
To APPEND to the parameters calculated in a previous run, additional frequencies which differ from those existing may be input in Data Category 6, together with values of the appropriate frequency dependent parameters in Data Category 7 and Data Category 8, at these additional frequencies. Note that, as all parameters are defined for a unique range of directions, these directions may not be re-defined.
To CHANGE the parameters calculated in a previous run, these parameters are simply input in Data Category 7 and Data Category 8 and, depending on the type of input, the parameters will be either overwritten with the input values or become the sum of input values and original values.
- Data Category 7
Input appended or changed data
Input for Towed Tethers
Towed tethers require specific data to describe the towing vessel.
- Data Category 7
Nominal linear hydrostatic stiffness matrix for the vessel The depth below the still water level of the center of gravity, which must be the same as the coordinate of the trailing end of the tether, input in Data Category 1 The hydrostatic force on the vessel, which must be equal to the weight (mass * acceleration due to gravity)
Input for Installed Tethers
For installed tethers no additional data is required.