How to simulate a Boat on water surface in Autodesk CFD? Easy!
by 11 September 2017•
Here we have a boat (partially submerged solid) moving through water. As any other aerodynamics analysis, we generally keep the boat still and move the fluid around it.
Follow the steps below, which include the model setup while applying the best practices you learnt from setting up a free surface analysis.
1. CAD Model
In your CAD software, model the boat in a ‘box’:
- Split this
volume into two parts
- Upper part (To capture water/air boundary). This represents the air
- The bottom part. This represents the water
- Leave a
good length for the outlet (5X the diameter of the flow channel)
enough space on the sides of the boat to capture the turbulence around it
- If you can
utilize symmetry, split the whole model into two even parts and use only half of
Image 2: 1/2 CAD Model
Define the solid (boat) as well as the fluid (water) materials.
Assign a solid material to the boat hull
Assign water to the two parts of the box volume (this will make sense later)
Image 3: Materials
Note that the following error message will pop up if you try to assign two touching fluids (do not do this):
Image 4: Touching fluids error
3. Boundary conditions
Since half of the model is considered, a symmetry boundary should be applied on the symmetry plane.
4. Initial conditions
We need to initialize the model by assigning initial conditions to the water volume.
- Change the Selection Mode to Volume
- Assign Height of Fluid (HOF) to the lower water
volume (the upper has nothing assigned and will begin the analysis empty)
- An Initial Velocity equal to that at the inlet
and outlet. This helps initialize the motion state of water
Image 7: Initial Conditions
Manual meshing was used for this model.
Meshing is very important as it controls the model accuracy and also contributes to a good visualization of the results of free surface analyses. Be sure to:
- Refine the mesh in regions where water and air
- Make sure to define a fine and uniform mesh
distribution at the water/air interface
- For regions where water would be turbulent,
apply a good mesh using regions:
- Either in the CAD software while creating a CAD region around the boat and then assigning a small mesh element size to it in CFD
Image 8: Cad'ed Region
2. Or in CFD by adding a Mesh Refinement Region
Image 9: Mesh Refinement Region
- Unless you are running heat transfer analysis,
suppress the boat from the mesh. Its surfaces still can be refined while
switching to surface selection and also displayed in the results
Image 10: Mesh
On the Solve dialog, enable ‘Free Surface’
Image 11: Free surface dialog box
Leave everything the same as default according to the Help documentation of Free surface