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-   -   Model for ship resistance calcs. (https://www.cfd-online.com/Forums/star-ccm/92153-model-ship-resistance-calcs.html)

 Surfboy September 3, 2011 18:37

Model for ship resistance calcs.

Hi
I am trying to learn how to use star-ccm+.

my main goal is to know how to calculate resistance force for a hull that i import as a CAD.
i followed the tutorials but there is no one tutorial that does the complete process.

does anyone have a common model used for practice with know results like the Wigley Hull or the DTMB 5415?

or do you think there is a better tutorial i can follow for that?..

thanks,

E.

 lava12005 September 4, 2011 20:58

Have you done the boat DFBI tutorial?
If yes, then you can calculate the resistance from the report -> force coefficient and then specify the hull for the part and also other appropriate parameter.

 Surfboy September 4, 2011 21:06

yes, did the dfbi tutorial

two things that i couldn't figure out yet:

1. how can i import an iges file correctly and build the right region around it (if i import a hull made by surfaces, will it work? do i need to substract it somehow from the surrounding region?, all the part of importing the hull is not clear.

2. i want the stillwater resistance, with the ship fixed(like in a tow tank) so the ship doesn't move, the flow (no waves) goes at some speed and i measur the total force on the hull.

thanks,

E.

 lava12005 September 4, 2011 21:25

If you want the flow to have no wave, you can simply set the wave amplitude to be 0 in VOF wave, and you can also set the current speed.

Regarding the region, I am not so sure with what you said, do you mean that you have the hull surface only? If you do only have the hull surface, you can proceed by creating the outer region and then substract the hull from it, so it will look like similar as the one provided in the tutorial.

 Surfboy September 4, 2011 22:51

Problems with importing the geometry

thanks,

but right now my problem is how to import the geometry...

i tried the simple case of importing a file (iges) which is the region(big box) with the substructed hull(just another box). can't make it work...

is it possible that iges is not a good format (does it import only surfaces or also solid)?

what i tried is to import the geometry, define as part and regions..but gets errors for example when i try to show the symmetry part on scenes.

so don't know whats the problem yet...

:confused:

 alfaben September 5, 2011 03:31

go to representations and then to repair geometrie for an failure check

 lava12005 September 5, 2011 04:33

By the way, I would like to know to simulate the flow around ship hull using VOF, how do we know the size of the mesh? is there any guide? do we need to consider y+?

Since the mesh provided in the boat DFBI looks coarse to me..

 abdul099 September 5, 2011 17:58

lava, the mesh in the tutorial is a coarse one for reasons of computing time. Most people do a tutorial to understand the process - not to wait a week for a useless result on a fine mesh. They want to click their way through the tutorial, wait five minutes, get a nice colored picture, be excited and try the same on their own case.

To get the right mesh size follows the usual way:
1. Experience or
2. Refine it step by step until the result doesn't change any more or more likley
3. Refine it step by step until you find out, you can't handle the model any more or your boss won't pay for the computing time
And yes, you have to consider Y+ and choose the appropriate turbulence model for your mesh. A Y+ of 46983271 for a low Re model isn't the perfect choice...

Surfboy, importing geometry works best with native CAD format. When you don't have any reader licence for a native CAD format, use parasolid. IGES is possible, but not the prefered file format. STL is also possible, it works almost every time. But the surface quality might be not good, so use it as a workaround when nothing else works.
All imported geometry is just a surface. It will always be a (hopefully closed) surface, even after a surface remesh. It gets a "volume" when you create the volume mesh - anything else is only a surface representation. (I think, CAD files and parasolid also store the information on which side of the surface the material will be, but ccm+ might just look for a closed volume)

Just import the ship hull, put a box around it and either subtract or combine and split by surface topology. Both ways should work and you should get an error free surface when your imorted geometry has a good quality.

Like lava said, you can have a look on the tutorial, but choose a flat VOF wave or do it without the VOF wave model. Just don't activate the 6DOF movement, and you'll get the stillwater resistance.
Try to look for available options in the tutorials and play with them - most options are self explaining when you invest five minutes and bother to read the user guide and think about it. I'm happy to help with any questions - except the one you can answer on your own by reading the first five sentences when opening the user guide.

 lava12005 September 8, 2011 00:17

Thanks abdul for the explanation. I have another question regarding hull simulation.

Currently I am trying to simulate a hull sideslipping, and trying to determine the side force developed from it. Since my computing power is super limited for now, I decided to try running Inviscid case.

So I'm using VOF flat wave, and a pretty coarse mesh (568k cells only). I run it on implicit unsteady with 0.01s timestep. And here is my question:
1. I notice that the water surface (iso-surface of the heavy fluid volume fraction) is almost fix with and without sideslip, is this because of the inviscid condition?
2. Will the sideforce developed through the inviscid condition is worth to look at?as in for first rough analysis..

 naimishharpal September 9, 2011 17:24

Quote:
Hi... One question for U. When calculating Y+, do you consider Water properties, or Air properties? let's say 1/3 part of boat is immersed in water, and rest 2/3 is in Air. What to do in such scenario?

 alfaben September 10, 2011 03:38

I have calculated the force coefficient from a modelboat and got a negative value.
My reference aera is the projected aera from the ship in the water.
Reference density is the water density.
Reference velocity is the wave velocity (flat wave).
What reference pressure is to choose?
I try it with default 0.0 and 101325 Pa. But in this case we have the hydrstatic pressure of head wave.
Directionvector is 1, 0, 0
Force option pressure + share
Parts ship
Cooridnate system: I tried it with laboratory, inital and boat

Thanks
alfaben

 lava12005 September 10, 2011 11:17

What's your wave direction? 1,0,0 too?
The reference pressure shouldn't be an issue right? since they will integrate the pressure through out the whole body so the constant will just drop out..

 alfaben September 10, 2011 14:18

vertical wave direction is 0,0,1
point on water level is 0,0,0.008
current 0.29,0,0 m/s
wind 0.29,0,0
i get force coeffiecents with 0.35 or -0.035 for the half model
normal force coefficients for ships are 0.06 - 0.08
ok -0.035x2 = -0.07
but negative
projected ship area 144.038mm^2
projected area from ship in water 89.80 mm^2 (value in calculation)

 alfaben September 11, 2011 05:17

News

I think we have a problem with the dimensions.
When I calculate with reference area 89.80mm^2 : 1000000 = 8.98e-5 m^2 the values are to high force coefficient in this case 0.35
When I calculate with reference area 89.80 mm^2 : 100000 = 8.98e-4 m^2; or 898.0 mm^2 : 1000000 the values are ok.
I donīt understand why.
Where is the failure.

 abdul099 September 12, 2011 18:21

Lava,
I might be wrong, but I wouldn't run the ship inviscid. Pretty much of the drag of a ship should be skin friction, therefore you are neglecting an important thing.
Under slip conditions, it should be different. The bigger the slip angle, the more drag will come from pressure differences. But I think, the slip angle will not be big enough to neglect the skin friction. (It might be possible when the ship moves nearly perpendicular to its longitudinal axis, but that's no appreciable operating condition for most ships). It might be worth to run one simulation with slip angle inviscid and one turbulent. Compare the values and you've got a statement whether the values for the sideforce can be used as an estimation when running inviscid or not. Just do a restart from a converged simulation...

What do you mean with "isosurface is nearly fix"? Same for slip and no slip? Inviscid or not, there should be some waves when the ship moves. But this waves might do not neccesarily look different as they are caused by displacement of fluid volumes.
(By the way, it shouldn't make much difference whether you're running the case laminar or inviscid. There will be more computational effort when running turbulent (due to additional equations for tke and tdr, or what turbulence model ever you would be using), but it might be worth to spend the time. I never got a remarkable speedup when running laminar.)
Maybe it's due to your mesh. 568k cells is just a tiny mesh. You should refine it near the free surface, in the region where the volume fraction changes from 1 to 0. I know, it's hard to give oneself a push and increase the cell count when only limited ressources are availabe - but it's not possible to simulate the whole world with five cells. There is a minimum effort you have to invest to get good results.
I'm sure you will find a good balance between computational effort and quality of the results.

naimishharpal,
I would consider Y+ of the liquid phase as the drag from the water will be higher than the one from wind. At least for low wind speeds. When simulating a ship in a storm, try to find a good compromise. Maybe adjust the near wall prism layer thickness to different values for the immersed part and the not immersed part. An having a look on the Y+ values is pretty easy, even without having to calculate anything: Just create a scalar scene and plot wall Y+, and you will see where to adjust near wall prism layer thickness...

alfaben,
a negative force coefficient is either due to a negative force or due to the wrong direction specified in the report. When the direction is right, the HAVE to be a force against the specified direction. For what reason ever, I don't know your simulation.
According to your last post, I think you might have a general problem with your results. The coefficient report will not care about your expected results and decide to divide by a different value to tease you a little bit. Are your forces resonable? Do you have any experiment to compare with?

 sanjay January 12, 2012 01:04

how do I generate a wave so that it hits the star board side of the boat ???:rolleyes:

regards
www.aerosapien.blogspot.com

 sail January 12, 2012 17:48

Quote:
 Originally Posted by sanjay (Post 338941) how do I generate a wave so that it hits the star board side of the boat ???:rolleyes: regards www.aerosapien.blogspot.com
Hi sanJay.

have you gone throught the boat tutorial? in this case it is sufficent to change the wave vector of 90° i.e. assuming that the y axis is your starbord you can put 0,1,0

i'm assuming you want to symulate the roll of the ship.

 sanjay January 12, 2012 23:59

you are right I want to simulate the roll.;)

regards
www.aerosapien.blogspot.com

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