26.5. The SLMM Data Record - Slam Multiplying Factor

      2   5  7   11          21
    - --- -- ---- ----- ----- ---------
   |X|   |  |SLMM|     |XXXXX|         |
    - --- -- ---- ----- ----- ---------
       |  |   |     |            |
       |  |   |     |            |_(2) slam Multiplying Factor (Default zero) (F10.0)
       |  |   |     |                 
       |  |   |     |
       |  |   |     |_(1) Structure Number (I5)
       |  |   |
       |  |   |_Compulsory Data Record Keyword (A4)
       |  |
       |  |_Optional User Identifier (A2)
       |
       |_Compulsory END on last data record in Data Category (A3)

(1) The structure number must correspond to one of the structures defined in Data Category 2; in other words, if '1' is input then this will correspond to the structure defined in Data Category ELM1. If '2' is input then this will correspond to the structure defined in Data Category ELM2, etc. Note that this multiplying factor relates to slam coefficient for the structure specified on this data record only.

(2) This value is the multiplying factor for all slam coefficients for MORISON elements on the structure specified (1).

Effect of the Multiplying Factor on Slam Coefficients

If this data record is not input, the default multiplying factor of zero, is assumed by the program. This means that all the slam forces, which only exist for Morison elements, are automatically zero.

The effect of a non-zero multiplying factor is primarily to bring the slam forces into effect, i.e. the values of the slam coefficients for each element are simply multiplied by this factor.

Slam Coefficients and the Time-Step

The value of the slam coefficient, for each element, is based on the premise that the slam force is equal to the rate of change of the added mass tensor (with time) multiplied by the velocity.

This means that the time-step (specified in Data Category 16) must be sufficiently small to accurately represent the added mass at each stage of immersion/emergence. In general this will depend on the geometry of each element and its orientation to the water surface.

In practice, this severe restriction of the size of the time-step means that this facility is only used when specifically investigating the effects of slam forces on individual elements during critical stages of the simulation period, as the momentum change due to slam forces are normally small and have little effect on the overall motion of the structure.