wrong values for resistance

Surfboy · October 27, 2011, 20:36

Hi,

I am running a ship's hull fixed (not 6dof) with slow speed current.

i am getting very accurate result for the frictional resistance (about 3% from ITTC formula) but very low pressure drag (half from what was found in tow tank).

what could be wrong?

E.

abdul099 · October 28, 2011, 00:31

That sounds like "Doctor, Doctor, I've got headache. What could be wrong with me?"

Try do give a little more information. I assume, there are not many visionaries here...

Surfboy · October 28, 2011, 00:51

I am running the dtmb 5415 hull (scaled - 5.72 meters).
the hull is fixed (wall, no motion).
the flow is 2.097 m/s.
time step 0.001 sec.
2 iterations per time step (so far converged nicely). residuals seem fine.
Y+ a little bit high especially around the bulbous faces (around 120-130)

the bow wave seems very good by shape and height, the free surface looks really good.

i am at t=4.7 sec now (i let it run until t=20)

i get a good constant value for shear resistance, but the pressure resistant is off by 50%...but it convergerd to almost constant value.

meshed with prism layer (6 layers, growth 1.4)

i am running version 6.02.009.

i attached a pic from the free surface.

what data might help and i missed (physics/mesh/?)

thanks.

E.

any other data that might help ?

abdul099 · October 29, 2011, 06:01

Does every time step converge well with only to inner iterations? It seems to be a very low number. Maybe it's worth to try with 3 or 4 inner iterations, just to see if there's an impact.
How does your mesh look like, is it fine enough? What about courant numbers?

You also said, the hull is fixed. In your first post, you said you compare it to results from a tow tank. Is the hull also fixed in the tow tank? So can it change pitch or heave? What about the extends of the domain, does it match the tow tank extends? Are your boundaries far enough away from the hull?

October 27, 2011, 20:36	wrong values for resistance	#1
Surfboy New Member Eran Join Date: Sep 2011 Location: Massachusetts Posts: 11 Rep Power: 14	Hi, I am running a ship's hull fixed (not 6dof) with slow speed current. i am getting very accurate result for the frictional resistance (about 3% from ITTC formula) but very low pressure drag (half from what was found in tow tank). what could be wrong? E.

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October 28, 2011, 00:31		#2
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	That sounds like "Doctor, Doctor, I've got headache. What could be wrong with me?" Try do give a little more information. I assume, there are not many visionaries here...

October 29, 2011, 06:01		#4
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	Does every time step converge well with only to inner iterations? It seems to be a very low number. Maybe it's worth to try with 3 or 4 inner iterations, just to see if there's an impact. How does your mesh look like, is it fine enough? What about courant numbers? You also said, the hull is fixed. In your first post, you said you compare it to results from a tow tank. Is the hull also fixed in the tow tank? So can it change pitch or heave? What about the extends of the domain, does it match the tow tank extends? Are your boundaries far enough away from the hull?