In order to ensure numerical convergence of your Orca3D Marine CFD simulations, we always recommend that you perform a grid convergence study in which you examine the effects of varying the number of cells in the grid on the flow field and integrated properties such as heave, pitch, and drag. This is an important and effective way to determine if you have adequately resolved the flow phenomena associated with a given design.
Our experience has shown that when applying Orca3D Marine CFD to planing hulls with transverse steps that give rise to ventilation of the flow field behind the steps, a minimum of 3-5 grid cells are required along the height of the back faces of the steps as well as in the area immediately behind the steps. The default "Normal" grid setting used by Orca3D when setting up a CFD simulation may not be sufficient to achieve this resolution. One way to check this is to examine the grid on the hull geometry as well as on a longitudinal plane cut through the grid as shown in the image below. In this image it appears that we may have a reasonable number of cells on the back face of the front step (see the intersection with the center plane on the step on the left side of the image) this drops below 3 cells right behind the step and below 2 cells shortly behind that.
One approach to increasing the resolution is to set the "CFD Grid Size" in the Orca3D CFD options to a finer value such as "Medium_Fine" or "Fine" as shown below.
As is shown in the image below, this change resulted in an increased number of grid cells (3+) behind the front step, although this still decreases to under 3 a little further back. Furthermore, this grid size control is a "global" control that affects the overall cell size in the fluid domain. While this may be appropriate to capture the global flow field at a finer resolution, there are also ways to add local refinement only where it is needed.
To add local refinement in the area of the steps you can use refinement zones. This can be achieved by first enabling grid refinement zones in the CFD options as shown below, and then defining a box refinement zone around the steps. The required "Cell Size" for the refinement zone will need to be determined through trial and error. The default value of 0.002 represents a non-dimensional cell size where the normalization is by the boat's largest dimension, typically its length. So here it would be 0.2% of the length of the vessel.
In the image below you can observe the local refinement around both steps such that the desired refinement level of 3-5 cells in the region behind the step is maintained for most of the distance behind the step.
