GHS BULLETIN

GHS BULLETIN How HMMT WIND /PANEL Works

HMMT WIND /PANEL calculates the heeling moment due to wind and does so using the 3D surfaces created in Part Maker. The surfaces are composed of quadrilateral or triangular panels which wrap the station-based geometry.

For each panel in the surface, the method will compute the centroid, the normal vector, and the panel area. The pressure due to the wind is computed at the panel centroid according to the height of the centroid above the water plane, and the wind speed or pressure, as specified by the WIND command (see HELP WIND for the pressure formula). Together with the normal vector and area, the wind force on the panel is computed. The associated heeling moment is found by multiplying the force on the panel with the lever from the panel centroid to the center of the underwater lateral plane area. The moment is always computed about the current heel axis, which can be changed using the AXIS command.

Because the wind moment calculation depends on the surface panels, a quality surface is important. Inverted normal vectors are especially problematic as they mey result in panels being incorrectly included or excluded from the calculation.

Shielding
Shielding is computed using a shielding distance, which is the distance within which downwind panels will be shielded by upwind panels. The figure below illustrates how the shielding distance works. When a downwind panel is outside the shielding distance there is no effect from the upwind panel. Consequently, a shielding distance of zero results in no shielding (the default), and a shielding distance of infinity results in total shielding (the behavior that occurs when using /PANEL:FULL).

Upwind panels can shield downwind panels only when the centroid of the upwind panel is within the extents of the downwind panel, or the centroid of the downwind panel is within the extents of the upwind panel. The extents of a panel, for shielding calculations, are determined based on the three or four corners of the panel inset by 0.1% of the length of the panel edges. This is done to avoid geometric precision issues when panels are coincident and orthogonal. The figure below shows different scenarios when panels are considered shielding.

All panels from all surfaced parts/components are included in the shielding calculations unless they are deducting components. To omit a part from the shielding calculations, the best way is to not surface the part, or delete the surface from the part using Part Maker SURFACE OFF statement. Parts/components which are not surfaced before invoking HMMT WIND /PANEL will still be shown in the DI STATUS WIND view, but they will be shown in a gray color.

The wind force and moment calculation respects shape factors, which are assigned on a component level using the COMP /SF command. If a shape factor is set to zero, the area (and force, and moment) from the component will be omitted from the calculation, however, it will still be used for shielding calculations.

When panels intersect the waterplane, only the area above the waterplane is used in the calculation. However, because the surface panels are not modified in the process (only the area is modified) the panels may still appear red below the waterline when looking at DI STATUS WIND.

If you would like to see another bulletin created regarding a specific topic, please email Creative Systems, Inc. at support@ghsport.com.