We have used Orca3D Marine CFD to perform simulations of vessels in shallow water and have validated predictions relative to published model test data. Performing this type of simulation requires several modifications to the default simulation generated by the marine template. This includes modifications to the domain size and boundary conditions, along with several other relatively simple changes to the setup. There are some nuances to properly setting up the simulation to ensure that any numerical artifacts in the simulation results are minimized. Please contact us to work together with you to set up your shallow water simulation.
To perform a resistance or powering analysis in shallow water:
- The fluid domain and CFD mesh requires some adjustment from the defaults generated for a deep water run from Orca3D. In order to make this adjustments:
- Select “Built Meshes” in the Geometric Entities Panel. This will show the current marine mesh settings in the Properties panel as shown below.
- Expand the “Domain Size” field and change the “Depth” field to the appropriate depth. Keep in mind that this Depth input value is non-dimensionalized by the vessel's Maximum Dimension, typically the length of the vessel. Also remember that the Depth measurement starts at the lowest point on the geometry which is already below the free surface. You can use the “Grid and Geometry Information” option in the Mesh panel to determine the surface extents both for the length and lowest point in the geometry. In the example below this was determined to be 0.43.
- In the Domain Size also change the Front and Side values to larger values. Exactly whether this is required and by how much depends largely on how close the simulation is to the Critical depth. This is the depth corresponding to a depth Froude number of 1 and clearly depends on speed as well as depth. The closer one is to the critical depth, the more domain space is needed both in front of and to the side of the hull as the wake behind the hull spreads out and it is possible a solitary wave at the bow or even in front of the bow begins to form. In the example below we are using modest increases for these values.
- Next change the Mesh Size to “User Defined” so that you can increase the wave zone angle to account for the restricted depth. As shown below in this example we’ve increase the Wave Zone Angle from the default of 20 degrees for deep water to 30 degrees to account for shallow water. In the limit as the critical depth is approached this angle will approach 90 degrees.
- Click the Build Marine Mesh button in order to re-mesh the model with these new parameters. The effect on the resulting mesh is shown below.
- After re-meshing the model, you should change the boundary condition on the bottom surface to a Wall boundary condition to simulate the no slip condition of the bottom. Select the “marine_outside_bottom” boundary of the domain and change the Marine boundary condition to Wall as shown below.
- Now start the simulation as you normally would.
Although the process for performing a shallow water analysis in Orca3D Marine CFD is not difficult, it sometimes requires making adjustments to the meshing and simulation parameters to achieve high quality results. To determine if this is required for your analysis, look at the results of the simulation to see if there are any visual anomalies (sometimes these can be observed during the run so you can stop and adjust before waiting for completion). Some things to look for:
- Is the wave zone angle that was specified sufficient to capture the wake behind the vessel? Sometimes the shallow water may cause the wake angle to exceed the meshing angle. If this happens you should stop and remesh with a larger wave zone angle.
- Is there any evidence of numerical wave reflection from the back, sides, or even the front of the domain? If there is, this could indicate that you need a larger domain and/or mesh refinement of the wave damping zones at the boundaries.
- Is there a significant wave forming forward of the vessel? This can happen in very shallow water near the critical depth. If so you will need to refine the mesh at the free surface in front of the vessel.
- Is the vessel heaving/pitching an extreme amount or does it seem to be divergent in heave/pitch? This can sometimes happen due to the larger vertical forces and moments in shallow water. To get around it you can consider freezing the dynamics until the flow field evolves.