By default, simulations generated in Orca3D Marine CFD include heave and pitch dynamics, i.e., they are free to sink and trim. You can turn off either or both of those degrees of freedom as follows:
1. In the "Model" windows select the "Marine" module.
2. In the "Properties" window click on the dropdown list for "Dynamic Options" and choose the desired degrees of freedom as shown below.
If you want to set a particular value of heave or pitch, there are two approaches, each with advantages/disadvantages. For example, if you want to set a fixed Heave value:
- Set the heave value in Orca3D through the sinkage input value when setting up the simulation and choose the Pitch Only or No Dynamics option in Simerics as shown above. Advantage: simple, Disadvantage: it assumes this is equilibrium and computes the weight and CG based on that input, which might not be the right mass properties; if you are using No Dynamics it doesn’t matter but if you are allowing Pitch to be free then the pitch moment due to mass/cg may not be right
- Set the Heave dynamics module Motion Type to Prescribed and set the Displacement value as per below. Advantage: mass properties are correct, Disadvantage: the model instantaneously jumps from initial static equilibrium to whatever heave you specify. This causes all of the water around the boat to “jump” as well, but this is more of a numerical behavior and should dissipate as the simulation continues.
Depending on your settings, this can sometimes cause the error "inner box has moved out of the outer box." The size of the “box” that accommodates the morphing of the mesh is determined by the mesh settings shown below, Max Pitch Angle and Max Heave (the Max Heave value is ignored unless there is an incident wave included in the simulation). One to resolve the problem is to increase the max pitch angle to something like 45 deg. That’s not saying the vessel will really pitch to 45 deg, just that the mesh morphing box is sized to accommodate that. Of course after changing that value you will need to rebuild the mesh.