CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > STAR-CCM+

wrong values for resistance

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   October 27, 2011, 20:36
Default wrong values for resistance
  #1
New Member
 
Eran
Join Date: Sep 2011
Location: Massachusetts
Posts: 11
Rep Power: 5
Surfboy is on a distinguished road
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.
Surfboy is offline   Reply With Quote

Old   October 28, 2011, 00:31
Default
  #2
Senior Member
 
Join Date: Oct 2009
Location: Germany
Posts: 637
Rep Power: 12
abdul099 is on a distinguished road
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...
abdul099 is offline   Reply With Quote

Old   October 28, 2011, 00:51
Thumbs up ok doc :-)
  #3
New Member
 
Eran
Join Date: Sep 2011
Location: Massachusetts
Posts: 11
Rep Power: 5
Surfboy is on a distinguished road
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 ?
Attached Images
File Type: jpg top_surface.jpg (50.9 KB, 31 views)
Surfboy is offline   Reply With Quote

Old   October 29, 2011, 06:01
Default
  #4
Senior Member
 
Join Date: Oct 2009
Location: Germany
Posts: 637
Rep Power: 12
abdul099 is on a distinguished road
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?
abdul099 is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
It would be wonderful if a tool for FoamToTecplot is available luckyluke OpenFOAM Post-Processing 165 November 27, 2012 07:54
BuoyantBoussinesqSimpleFoam and axial-symmetric results wrong mass flow Thomas Baumann OpenFOAM 6 December 21, 2009 11:31
Cd values for 3D rectangular cylinder karthik FLUENT 1 August 28, 2009 06:32
CFD VALUES AT EXPERIMENTAL POINT J FLUENT 1 September 25, 2005 16:55
Y+ Values for a sperating flow over a hill Dave FLUENT 2 July 9, 2005 23:18


All times are GMT -4. The time now is 02:26.