With a few modest tweaks to the default behavior of the "marine template" used in Orca3D Marine CFD, it is possible to simulate the flow around a partially submerged lifting surface. In this example we will be analyzing a tapered foil with a tip bulb at an angle of attack of 5 degrees to the inflow. The geometry was created using the Orca3D Foil Assistant, based on a NACA 0012 foil shape with parameters defined as shown below. The tip bulb was also a NACA 0012 section shape with a "squish ratio" of 0.5 as shown below.
The resulting geometry was translated so that a small portion near the foil root was above the free surface, taken to be the Rhino z = 0 plane. It was also rotated 5 degrees about a vertical axis as shown below. Finally note that the foil was split along a chordwise line slightly below the z = 0 plane. This was done to avoid an overly dense CFD grid, as will be seen in the following.
To begin the analysis run Orca3D > Speed/Power > Marine CFD Analysis from the main menu and select the foil model. In the Orca3D Marine CFD form set the input to "Float Plane" and specify zero sinkage, trim, and heel. You can leave the VCG at 0 because the foil will be simulated with no dynamics (i.e. no heave or pitch) so the mass properties don't really matter. Click the Float button so that Orca computes the mass properties and thinks the model is floating at the specified condition. Use "Planing Hull" for the hull type and set the desired flow speed, 5 kt in this case.
Now click the "Options" button and change the CFD Grid Size to "Coarse." This is one of the rare cases where a coarse mesh may be appropriate to avoid a very high cell count.
It is important that the CAD mesh geometry for the foil is sufficient to capture the flow and high pressure concentration near the leading edge. While the default surface meshing in Orca3D Marine CFD is generally well-suited to the hull analysis problem, it may not be for the foil problem. In the example below we've increased the mesh density to accurately capture the leading edge shape around the foil and the tip bulb.
Finally we apply face attribution as shown below. Note that the top of the foil is defined as a deck face since at least one deck face is required, and that the upper portion of the foil is defined as a hull face but the lower portion of the foil and bulb are classified as "Appendage" faces. This is done to avoid an overly dense CFD grid. Remember to include the small trailing edge on the back of the foil and the tip bulb when assigning the face attribution.
Now generate the CFD simulation files and open them in Simerics MP. Take a look at the grid and make sure it has a reasonable cell count and is accurately capturing the geometry. Select the Marine Symmetry plane in the Geometric Entities pane, and then check the Grid box in the Results tab of the main pane.
Before starting the simulation we want to turn off the dynamics since the foil should be fixed in space other than the forward velocity. To do this select the Marine module in the Model panel and change the Dynamic Option to "No Dynamics" as shown below.
Now you are ready to start the simulation. Below is an example of the dynamic pressure distribution on the foil along with the free surface elevation.
