Free surface boat simulation

[Anonymous]

Hi there,

I'm looking to simulate a boat's Total Resistance, which consists of Air resistance, Wave making resistance and Viscous resistances (Skin friction and Viscous pressure). I believe I can simulate Air resistance just fine, but is there any way I can simulate wave making, skin friction and viscous pressure?

I believe the approach mentioned in this video would be sufficient (23:30): https://www.youtube.com/watch?v=CQH5z0qmPYo&t=1372s

A few second earlier this video mentions an article about this setup, which I wasn't able to find: 

My free surface setup has gone as far as this:

As you can see, my waves are nowhere near the waves from the video. What do I need to do?

So far I have only set up Height of fluid to be at the boat's waterline, and I have added velocity in (left) and out (right) boundaries.

Thanks for any hints. 🙂

[Anonymous]

Hi @Anonymous , 

I can tell you will need to have a finer grid, the water free surface looks rough, I can even see the tetrahedral shape of the water.. Try the following: 

- Firstly, you could train on a small CFD model to get an idea of the mesh requirements here if you need to capture a boat wake (eg it can be a small cube floating instead of a big boat)

- You should avoid to include solids in the numerical grid, then you save some mesh count hence cpu time

- More importantly, work on a half-symmetric CFD model, mirroring will do the rest

- My last suggestion would be to refine the grid step by step until you are able to capture a wake.

- Look up in the forum for similar post https://forums.autodesk.com/t5/cfd-forum/free-surface-boat/td-p/6537902

See below the mesh density required to capture some free surface flows

Coarse grids and poor solver setups will lead to numerical divergence or unreal behavior for sure, convergence plot will likely give you an idea of the solution health.

unhealthy

better

Take a look at the files here, 'FS1' is to show you numerical instability due to a poor setup and 'FS2' is to show you the difference between coarse/fine grid (LINK)

[Anonymous]

Thank you very much for such a thorough answer.

I tried the simulation with a smaller mesh and setup, yet still I seem to get bad results. Can you tell from these screenshots whether the mesh is still too coarse or if the problem is possibly elsewhere?