6.11. The ILID Data Record - Suppression of Irregular Frequencies

This data record is used to define a lid for suppression of irregular frequencies. The lid is modeled by horizontal diffracting elements that must be contained in a specified group. The elements may be defined by the user or they can be generated automatically by Aqwa.

Automatically Generated Lid

The data record shown below is used to request that the lid elements should be generated automatically. Note that the TOTAL number of elements must still be fewer than the maximum permitted, so the user must allow "space" for the lid when creating the model. The lid will be created at a z-coordinate defined on the ZLWL data record (The ZLWL Data Record - Waterline Height), which must also be included in Data Category 2.


      2   5  7   11    16    21
    - --- -- ---- ----- ----- -----------------------------
   |X|   |  |ILID| AUTO|     |(LID_SIZE=????,START_NODE=NNNNN)
    - --- -- ---- ----- ----- -----------------------------
       |  |   |     |     |    |
       |  |   |     |     |    |_(3)Optional parameters
       |  |   |     |     |
       |  |   |     |     |_(2)Group number
       |  |   |     |
       |  |   |     |_(1)Automatic lid generation
       |  |   |
       |  |   |_Compulsory Data Record Keyword (A4)
       |  |
       |  |_Optional User Identifier (A2)
       |
       |_Compulsory END on last data record in Data Category (A3)

(1) This identifier specifies that the lid is to be generated automatically

(2) This is the group number that will be used for the lid elements. If there is more than one structure with a lid a different group must be used for each structure. If a group number is not specified a default value of 999 will be used.

(3) These optional parameters allow the user to specify the nominal size of the lid elements and the starting number for the nodes that will be generated. The default size of the lid elements will be based on the average size of elements in the model. If this results in the maximum allowable number of elements being slightly exceeded, the user may increase the size to reduce the total number of elements. The program will generate new node numbers starting from the last user-defined number. This parameter allows the user to specify a starting node for the lid nodes.

User-Defined Lid


      2   5  7   11    16    21
    - --- -- ---- ----- ----- -----------------------------
   |X|   |  |ILID|     |     |
    - --- -- ---- ----- ----- -----------------------------
       |  |   |           |
       |  |   |           |_(1)Group number
       |  |   |
       |  |   |_Compulsory Data Record Keyword (A4)
       |  |
       |  |_Optional User Identifier (A2)
       |
       |_Compulsory END on last data record in Data Category (A3)

(1) The lid elements must be put into a single group with this number. If there is more than one structure with a lid a different group must be used for each structure. If a group number is not specified a value of 999 will be assumed.

The user must then define a number of elements that should:

be within the vessel
have their normals pointing upwards (i.e. the nodes must be listed anti-clockwise when looking down on the elements)
be at the still water surface

Except for the requirement that the centroids should be below the water surface, the usual modeling checks are still applied, and the TOTAL number of elements must still be fewer than the maximum permitted.