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-   -   DFBI Analysis of a Sail Boat in Planing Regime (http://www.cfd-online.com/Forums/star-ccm/86772-dfbi-analysis-sail-boat-planing-regime.html)

 ziyasaydam April 1, 2011 08:49

DFBI Analysis of a Sail Boat in Planing Regime

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Dear All,

I am working on 6-dof analysis of a sail boat hull for which I have the experiment results. Drag, sinkage and trim results as well as the wave profile are fine for low speed analysis.

However, as I increase the speed and the boat approaches planing regime, drag results started deviating from the experimental results. I found out that this deviation is due to a rather unphysical and weird air phase being formed on the bottom of the hull, along from the bow to the transom!!! I have attached two pictures from the analyses for better understanding of the problem.

So far, I tried changing time step, turbulence model and grid density, all without success. Has anybody experienced similar issues? Or any ideas why this is happening?

Ziya

 fastwave April 7, 2011 04:55

This is a common problem with some VOF solvers. What is your mesh like near the boat surface?
Also what time step are you using.
Both will affect you air entrapment under the boat.

 ziyasaydam April 7, 2011 10:09

I have a prism layer mesh around the hull to keep Y+ around 400. Time step is 0.02, CFL number is kept <10.

 fastwave April 8, 2011 08:49

I would try a smaller time step. CFL of 1 to see if that helps the problem. The finer y+ you go near your hull the worse the problem gets. But you seem to be coarse. Decreasing the time step might help to prevent the air getting sucked below the hull. Having a CFL more than 1 means that the air at the interface has a chance to travel a few cells and once it is below the hull it keeps going.