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Steve March 26, 2008 15:30

CCM+ Outlet BC for Ship Resistance
Hello All

Figured that I would give a shout out here to see if anyone else has come across this.

I've been screwing around with CCM+ to do resistance calcs for a few ships with the VOF, K-e model, with the field functions used to define the free-surface, etc. I've run into an interesting conundrum with the outlet.

A bit of history first, I have used both a pressure outlet with hydro field function and a flow-split outlet with no field function in the past. The best, most nicely-damped resistance results that I have ever gotten has been with the flow-split outlet with an older version. (2.08.006, maybe?) However, I have not been able to mimic this setup with other vessels and newer versions.(I'm using the most current 3.02.003) I have refined mesh regions at the water level that go across the entire domain, and a meter above and below. Also, I've played with time steps (0.2 - 0.02) and inner iterations (20 - 5) with no difference in results. My wall y+ numbers are good, and the Courant numbers are within the range.

Currently, I have been using the hydrostatic field function at the pressure outlet. But this gives me these weird waves that seem to be generated from the inlet. Additionally, the resistance results don't damp out nicely, though I still get reasonable results. Any ideas of where these waves are coming from?

When I have run the sims with a flow-split outlet, the water surface seems to "drain" out the outlet, and settle to some arbitrary level, below the set water level. Absolutely no idea what's going on here.

So any ideas, similar stories, funny jokes,etc would be most appreciated.

Andrew April 4, 2008 15:16

Re: CCM+ Outlet BC for Ship Resistance
This is exactly what I am trying to figure out. I'll post back if I make any discoveries.

Andrew April 9, 2008 01:07

Re: CCM+ Outlet BC for Ship Resistance
I've had some moderate success with time steps of 0.01 sec or lower and 50+ inner iterations on a Series 60 hullform at model scale.

For the pressure outlet, I set the volume fraction to composite, then set the water VF to the field function, "Volume Fraction of Water".

I think with this and the hydrostatic pressure field functions, the effect is pretty good, but the results haven't been checked thoroughly.

Dan May 8, 2008 19:12

Re: CCM+ Outlet BC for Ship Resistance
The water spilling out of the domain you will get if your pressure outlet isn't defined properly, or from poor initialization.

I use the same set-up for volume fraction on outlets as Andrew.

I've encountered the oscillating resistance values before, and at times can't get around this, CFX claims this is due to a segregated solver with VOF, StarCCM+ have very differing opinions on this from CFX, I feel its a combination of the magnitude of wave resistance / total resistance and mesh density/transitions around the hull. I'd recommend trying a different hull first if you're really stuck on the oscillating resistance, or possibly trying porting your mesh to Star CD and playing with a bit of a blending factor to help settle this oscillation out.

Let me know what you're able to find out.

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