Although in the default workflow used with Orca3D Marine CFD there is little if any data entry required in the SimericsMP CFD tool's GUI, there are times where location-dependent information must be input by the user. Examples include changing the location of the tow point for a resistance analysis, adding force vectors as propulsion sources, and defining the locations of monitoring points in the flow field. Entering location-dependent information in SimericsMP requires knowledge of the coordinate system origin location and axis orientation. In general, the coordinate system used in SimericsMP will NOT be the same as that in your Rhino model. This is because as part of the CFD simulation setup in Rhino/Orca3D we transform the model internally so that the origin of the transformed coordinate system lies on the flotation plane associated with the static equilibrium condition and the vertical coordinate axis (usually the z-axis) is perpendicular to the equilibrium flotation plane. If the static equilibrium condition has no trim or heel relative to the Rhino coordinate system, then this is a simple translation of the model equal to the sinkage. In the general case, however, there will be some trim (and possibly heel for an asymmetric simulation) so the internal transformation will contain rotation as well. Although this transformation is performed internally and is not apparent in the Rhino model, the exported geometry used in the CFD analysis is transformed so that the SimericsMP model will start in static equilibrium at the beginning of the simulation.


This internal transformation results in a different coordinate system, both in terms of the origin location as well as in the axis orientation, in SimericsMP relative to the Rhino model. This difference can be problematic when location-dependent input is required in SimericsMP, because of course the input must be specified in the SimericsMP coordinate orientation. When specifying the location of a monitoring point for example, it would be nice if you could take advantage of the simple measurement and constructive geometry capabilities in Rhino rather than have to figure out the coordinates in SimericsMP. The good news is that you can, using a simple technique. In Orca3D, you have the option to transform the Rhino model to the resultant static equilibrium flotation plane, which is exactly what we do internally for CFD export. To accomplish this, follow these steps:


1. It is advised that you first make a copy of your Rhino model geometry that the transform will be applied to so that you don't affect the original geometry.


2. Next, select Orca3D > Stability > Compute Hydrostatics and Stability from the main menu in Rhino.


3. After selecting the copied model geometry, enter the hydrostatics input condition (either sinkage, trim, heel, or weight and CG) just as you did in the OrcaSimericsAnalysis command, and make sure to click the checkbox that says "Transform Model to Resultant Condition" as shown below.



4. Click the Calculate button. After the computation is complete, the selected geometry will be transformed so that the z=0 plane corresponds to the equilibrium flotation plane. This should match the coordinate system of the model in SimericsMP and should allow you to transfer measurements made in your Rhino model over to SimericsMP.