[Angela Wang]

Hi foamers,

I wanna share my two cases with respect to free surface flows to see if anyone have same problems as mine. The 1st is the Duncan's Experiment, with a submergence of the hydrofoil as 1.034. The 2nd is the wigley hull case with Froude Number =0.316. Both of them are run with interFoam, laminar+Euler mesh, non-dimensionalized, OpenFoam 1.7.

The problems I have for the Duncan's Experiment:

there is a wake (an area where velocity jumps down) behind the trailing edge, see fig attached. I tried to run it as inviscid, and with different domains and meshes. It still didn't go away. Is it physically reasonable? Or is it caused by the numerical viscosity?

The wave elevation compares well with the experimental data with a little bit under-prediction.

The problems I have for the Wigley hull case:

While the wave elevation along hull compares well with the experimental data and the wave pattern seems good, the force calculated oscillates with time and the force coefficient shifts away off the experimental data. For C_w, 0.0015 is expected from the experimental measurement. However, my value oscillates around 0.0022. Does any one have similar experience? see fig attached.

** Cw=Np/(0.5*rho*U^2*S_w) where Np is the first term written in the forces.dat file (pressure_x), S_w wet area of half hull [the domain is set as symmetry].

[daveatstyacht]

Angela,
I am a bit confused about your first issue with a speed drop in the foil wake. It is normal for the wake of an object to be slower than free stream in viscous flow though from what I can discern your issue is a region aft of the trailing edge slowing down sharply. Have you tried running the foil in a single fluid run (simpleFoam) to see if the behavior occurs without the influence of the free surface?

As for the Wigley hull, that is a good place to start for still water resistance validation. An issue that you need to be careful about for sub-critical Fn is wave reflections off of the outlet (and sides). The reflected waves can lead to oscillating forces. Also be aware that your choice of mesh and fvSchemes can have a very strong impact on the wave making (numerical dampening and diffusion). Good luck in your studies.

Regards,
Dave

[flowris]

Angela,

If you send your files, I will take a look at them.

I have done a scaled Wigley simulation and also had oscillating forces. Their average compared well to experimental values from a towing tank. I believe this is what really happens in flows around hulls.

I also did simulations on a hydrofoil in water only, and now I am trying to move forward to foils near the free surface.

[Rafael_Coelho]

Hello I am quite new on OpenFoam and I am trying to calculate drag forces on the Wigley tutorial. Can anyone tell me what should I change?

Thanks you!