Resistance

[chinnanva2003]

Hi,

I am chinna Naval architect from UAE... i am trying to figureout over all resistance experience by a ship sailing through calm water. i used fluent for this analysis, so far i was able to simulate single phase for underwater hull form for domain independent study so now i am facing some problem in using multiphase flow for resolving over all resistance including wave drag. The major issue i am facing was when used unsteady flow the free surface is getting distorted at inlet boundary.

Hope this could be due to some improper inlet boundary condition,Can any body help me regarding this.


Thanks in advance.

[Ad Hoc]

Are you trying to find the total resistance, as an academic exercise, or for a real ship/boat?

If for a real ship/boat, then the software you are using will not give reliable and consistent results. You will need to tank test. BUT, it all depends on the program, what are the limits and whether you are staying "inside" those limits.

[chinnanva2003]

no this is not an academic project. Fluent might not give consistent results but i feel it all depends on the mesh distribution around the body i feel..can please explain in brief what do u mean by limits....

[chinnanva2003]

we are even going for tank test at a later stage but in the mean time we are trying to predict the resistance using CFD techniques.

[chinnanva2003]

Originally Posted by Ad Hoc

Are you trying to find the total resistance, as an academic exercise, or for a real ship/boat?

If for a real ship/boat, then the software you are using will not give reliable and consistent results. You will need to tank test. BUT, it all depends on the program, what are the limits and whether you are staying "inside" those limits.

no this is not an academic project. Fluent might not give consistent results but i feel it all depends on the mesh distribution around the body i feel..can please explain in brief what do u mean by limits....

[Ad Hoc]

Limits....all software programs have "limits" or "boundaries". For example, is Fluent applicable for Froude numbers in your range, or the hull shape or length/displacement ratio, ...does it take sinkage and trim into account does it model transom sterns etc etc....not one program is perfect, nor applicable across all Fn's.

[chinnanva2003]

you are correct fluent might not take these trim and sinkage effects directly because it is not a custom made software for marine application, regarding the non-dimensional numbers and all i feel fluent can handle a wide range of flow simulation as far as CFD is concerned and it has got so many techniques and models to handle wide range of problem. how ever i feel if i can do some basic analysis i can have some preliminary understanding of the resistance estimation ...if possible can u suggest me any other software that can account for all the above issues.....

[Ad Hoc]

C..2003

If all you wish to do is "..some basic analysis i can have some preliminary understanding of the resistance estimation.." then all you need to do is study your text books and learn the theory behind it. Then obtain some data, from university or others and perform the calculations by hand to see if you obtain the same results.

No software will account for all the issue and parameters. But you can buy software that is more representative of the "type" of boat you have, ie L/B and B/T ratios....L/D ratios, similar speed/Fn's, hull/form shape, etc etc. These can give you an "indicative" answer, but it will not be 100% accurate.

[Jenny Giles]

Originally Posted by Ad Hoc

Are you trying to find the total resistance, as an academic exercise, or for a real ship/boat?

If for a real ship/boat, then the software you are using will not give reliable and consistent results. You will need to tank test. BUT, it all depends on the program, what are the limits and whether you are staying "inside" those limits.

Would you be able to give us some ballpark estimates of how much tank tests cost. I know that it will depend on how extensive they are, but how much does building models cost?

Does it cost more to do sea-keeping tests or simple drag tests?

Do you do just one set of tests? Or do you do some on the early design iterations and more when the design is more refined?

Thank you.

[Ad Hoc]

Tanks test depends upon 2 main factors
1) The test tank centre you use.
This can be a small university type of place or a major test centre like MARIN or SSPA. Using an independent company, such as SeaSpeed, they have their own equipment but hire or rent out locations, this keeps their overheads down and so are cheaper.
2) The amount of type of tests.
Simple calm water resistance, or in waves, or both, or LCG chase, RAOs etc etc.

Both have an adverse affect on the costs.

But a basic test tank from an independent, would be around £5~10k, depending upon model size and number of runs. Using a big test house can go from £20~50k.

Years ago we were "forced" by a client to use MARIN, rather than one we selected. They charged £44k, for resistance and RAOs....shocking!!!

We do our own in-house tank testing. We make our own models, and my colleague has a tank in his back garden. I am building mine this winter. We offer this to other clients too, as a quick rough and ready solution/answer.

So, yes simple resistance testing is cheaper. Seakeeping much more expensive, since this requires a tank that can do this and then which spectrums; the size of the tank will dictate how much of the spectrum you can produce too.

I'm lucky, we have many many previous designs we have done to use as data from old tank tests or interpolation between hulls, this is what naval architecture is about...establishing trends. Not absolutes. Also if in doubt, we tank test ourselves.

[baeckmo]

Originally Posted by Ad Hoc

...................... we tank test ourselves.

Hmm, interesting, this is a fella dedicated to his work....How's your resistance today, John? I guess midships section may have changed lately?

[Ad Hoc]

baeckmo

My resistance is good...just been to the gym and then played tennis for 2 hours....not sure about my "middle section" though...

Originally Posted by Ad Hoc

baeckmo

My resistance is good...just been to the gym and then played tennis for 2 hours....not sure about my "middle section" though...

Do´nt have the girls around while testing! You know that obstacles (even when not "rigid") will increase resistance.

[Ad Hoc]

er...um...

[Alik]

Originally Posted by apex1

Do´nt have the girls around while testing! You know that obstacles (even when not "rigid") will increase resistance.

In hydrodynamics this is called appendages, not 'obstacles'!